Overview of batteries for future automobiles

2 Overview of batteries for future automobiles P. Kurzweil 1, J. Garche 2 1

University of Applied Sciences, Amberg, Germany 2 Fuel Cell and Battery Consulting, Ulm, Germany

2.1 General requirements for batteries in electric vehicles Today’s batteries for automotive applications require both high energy and high power in order to have a large range and to manage lively acceleration and preferably the recovery of braking energy. In hybrid vehicles, the battery has to deliver brief peak loads, while the internal combustion energy supplies the basic power and maintains the battery in an almost constant state-ofcharge (SoC). Great efforts have been focused on the development of reliable and safe batteries. In smart grids, series combinations of many automotive batteries (plug-in hybrids) might play an additional role in future virtual storage systems. In urban traffic, the estimated range d of a small electric vehicle, having an empty mass of 1500 kg and a drive battery of 300 kg, depends linearly on specific energy of the battery W: d 2W z km Wh kg
1 Present (Li-ion) traction batteries of 80e120 Wh kg
1 allow distances of 160e240 km. Long-distance trips at a constant speed of 80 km h
1 expand the theoretical range to d ¼ 4.5$W. The development goals of the United States Advanced Battery Consortium (USABC) for advanced automotive batteries read: 350 Wh kg
1 (C/3), 750 Wh L
1 (C/3), 300 W kg
1 (10 s peak load), 700 W kg
1 (30 s), life 1000 chargeedischarge cycles, ambient operating temperatures
30 C to þ52 C, charging time <7 h, boost charge up to 80% state-of-charge (SoC) within 15 min, self-discharge < 1% per month. Present 3.6-V lithium-ion technology provides specific energies (energy densities) of up to 220 Wh kg
1

LeadeAcid Batteries for Future Automobiles. http://dx.doi.org/10.1016/B978-0-444-63700-0.00002-7 Copyright © 2017 Elsevier B.V. All rights reserved.

27

LeadeAcid Batteries for Future Automobiles

(450 Wh L
1), lithium polymer delivers 250 Wh kg
1 (400 Wh L
1), and thinfilm lithium-ion battery provides 250 Wh kg
1 at the cell level. The Argonne National Laboratory estimates the specific energy (energy density) of future lithium battery systems at 200 Wh kg
1 (375 Wh L
1) until the year of 2020, and 300 Wh kg
1 (550 Wh L
1) after 2030; i.e., in the next two decades the present performance data have to be improved by a factor of 2.5. Current lithium-ion vehicles include the Daimler ‘Smart’, BMW ‘i3’, Renault ‘Zoe’, Fiat ‘500’, Nissan ‘Leaf’, Mitsubishi ‘i-MEV’, Honda ‘Fit’, Coda ‘EV’ and Tesla ‘Model S’. Leading automakers such as, e.g., Toyota are currently forcing new developments of lithium batteries with respect to solid-state, lithiumesulfur and lithiumeair technologies. Although most of the lithium-ion battery effort is aimed at plug-in hybrid electric vehicles (PHEV) and battery electric vehicles (BEV) the use of the lithium-ion battery technology is also discussed for micro- and mild-HEVs, where lithium-ion batteries can impress with high specific energy/power and long lifetime. If their drawbacks such as costs and low and high-temperature behaviour could be overcome, lithium-ion batteries will be serious competitors for leadeacid batteries. The requirements for different electric vehicle classes and appropriate battery systems are summarized in Fig. 2.1. An overview of the different battery systems with respect to performance, cost, life and safety is given in Table 2.1 and Fig. 2.2. This chapter compiles the advantages and challenges of present battery technologies for automotive applications in comparison with the leadeacid battery technology. Also future battery technologies and their possible market introduction are discussed.

Figure 2.1 Battery requirements depending on vehicle class and battery technology.

28

LeadeAcid Batteries for Future Automobiles

Table 2.1 Specific battery parameters for electric vehicles (rough estimations on cell level)

Battery type

Specific energy (Wh kgL1)

Specific power (W kgL1)

Cycle-life

Efficiency

Self-discharge % per month (25 C)

Approximated cost per battery at present ($ kWhL1)

Leadeacid

35

150

400

80%

3e5%

80 SLI battery 200 industrial battery

Nickelecadmium

50

400

1500

70%

20%

450

Nickelemetalhydride

90

300

1000

75

30%

650 industrial battery 200 consumer cell

Nickelezinc

75

500

500

70%

20%

350

ZEBRA (Na/NiCl2)

160 cell 90 battery

150

2000

90%

Only thermal self-discharge

800

Lithium-ion

200

400

1500

93%

2e3%

350 industrial battery 170 consumer cell

2.2 Energy storage in leadeacid batteries Since the nineteenth century, the robust leadeacid battery system has been used for electric propulsion and starting-lighting-ignition (SLI) of vehicles [1e3]. Recent applications comprise dispatching power, bridging power and stabilizing power in power grids. Extensive information on leadeacid batteries is given in this book.

2.3 Alkaline batteries Alkaline batteries [4] supply a mature storage technology for mobile applications (Table 2.2). However, nickelecadmium (NiCd), nickelemetal-hydride, nickelezinc, nickeleiron and silverezinc systems have gradually been replaced by lithium-ion batteries. The nickeleiron and the silverezinc systems are not discussed in this section because of low importance for automotive use.

2.3.1 Nickelecadmium batteries In 1901, E.W. Jungner described nickelecadmium batteries, and T. A. Edison nickeleiron batteries (also called steel accumulator). Jungner’s accumulator with plate-electodes prevailed over Edison’s battery for use in electric

29

LeadeAcid Batteries for Future Automobiles

supercapacitor

10000

Specific power–cell level (W kg–1)

(Very) High Power Li-Ion (spiral wound) 1000

NiMH

Li-Polymer

NiCd

100

NaNiCl2 (Zebra)

High Energy Li-Ion

10

lead-acid

0 0

20

40

60

80

100

120

140

160

180

200

Specific energy–cell level (Wh kg–1)

Figure 2.2 Ragone Diagram for different battery systems. After S. Scharner, P. Lamp, E. Hockgeiger, AABC 2010, May 17the21st, Orlando. Courtesy: BMW.

vehicles. Ackermann and Schlecht (1928) introduced sintered nickel electrodes providing a large surface area especially for aircraft batteries. G. Neumann (1947) discovered the recombination mechanism for sealed alkaline cells, which enabled the portable batteries in the 1950s. In the 1970s, polymer-bound negative electrodes were introduced. M. Oshitan in Japan by end of the 1980s discovered nickel foams for the positive electrode. Saft’s sealed batteries in 2005 employed polymer-bound positive electrodes in sinter and foam technology.

2.3.1.1 Automotive applications At present, the NiCd technology for electric vehicles is less important despite some advantages compared with leadeacid. This is due to relatively high costs and the legal restriction of the use of cadmium especially in the European Union. In the past, NiCd batteries were used as SLI batteries for motorbikes because of their easy storage in the off-use time (winter). In the pre-lithium era, NiCd was used as a traction battery. Some 2,800 Peugeot 106 E cars were produced from 1993 to 2003 with a 120 V/100 Ah NiCd battery consisting of 20 liquid-cooled Saft STM-5MRE modules providing a specific energy of 54 Wh/kg (C/3). Advantages. Compared with leadeacid batteries, the NiCd system tolerates deep discharge and storage in the discharged state for longer time, offers

30

LeadeAcid Batteries for Future Automobiles

Table 2.2 Overview of alkaline storage batteries System Nickele cadmium

Cell reaction for discharge

Rated data and properties

(
) Cd þ 2OH
/ Cd(OH)2 þ 2e


Theory: 1.2-1.3 V; 244 Wh kg
1

(þ) NiO(OH) þ H2O þ e
/ Ni(OH)2 þ OH


Practice: 35e49 Wh kg
1 (5 h), 32 Wh kg
1 (1 h), 134 Wh L
1 (5 h); max. 700 W kg
1, 8000 cycles, -40 to þ60 C. Challenges: Memory effect, energy efficiency 65%, selfdischarge 15e20% per month

Nickeleiron

(
) Fe þ 2OH
/ Fe(OH)2 þ 2e


Theory: 1.36 V, 265 Wh kg
1.

(þ) NiO(OH) þ H2O þ e
/ Ni(OH)2 þ OH


Practice: 30 Wh kg
1 (5 h), 23 Wh kg
1 (1 h), 70 Wh L
1, 100 W kg
1, 2000 cycles, low-cost. Challenges: energy efficiency 50%, rapid self-discharge, H2 evolution. Obsolete.

Silverezinc

Nickelezinc

Nickelemetalhydride

Nickele hydrogen

(
) Zn þ 2OH
/ Zn(OH)2 þ 2 e


Theory: 1.85 V

(þ) AgO þ H2O þ 2 e
/ Ag þ 2 OH


Practice: 120 Wh kg
1 (5 h), 800 W kg
1, z100 cycles.

AgO denotes silver(I,III)-oxide. Cathode: Hydroxidozincate and ZnO.

Challenges: Rapid self-discharge, H2 evolution, expensive. Formerly used for satellites, moon buggies, torpedoes.

(
) Zn þ 2OH
/ Zn(OH)2 þ 2 e


Theory: 1.73 V, 323 Wh kg
1.

(þ) NiO(OH) þ H2O þ e
/ Ni(OH)2 þ OH


Practice: 80 Wh kg
1 (5 h), 60 Wh kg
1 (1 h); 200 W kg
1; 200 cycles, energy efficiency 55%. Non-commercial.

(
) MH þ OH
/ H2O þ M þ e


Theory: 1.3 V

(þ) NiO(OH) þ H2O þ e
/ Ni(OH)2 þ OH


Practice: 1.2 V (5 h); 76 Wh kg
1 (5 h), 275 Wh L
1 (5 h); 210 W kg
1 (20 min); 700 cycles;
20 to þ60 C; selfdischarge 20% per month, minor memory effect.

(
) NiO(OH) þ H2O þ e
/ Ni(OH)2 þ OH
(þ) ½ H2þ OH
/ H2O þ e


1.32 V; rapid self-discharge. Complicated by H2 electrode (Pt/Ni/PTFE) and pressure vessel (30e40 bar). Cathode: b-NiOOH by pre-charging from NiO þ5 Co(OH)2. Obsolete.

Overcharge: (
) 2 OH-/ 2e
þ ½ O2 þ H2O (þ) 2H2O þ 2e
/ 2OH
þ H2 Zinceair

(
) Zn þ 2 OH
/ Zn(OH)2 þ 2 e
(þ) ½ O2 þ H2O þ 2 e
/ 2 OH


Paste anode (ZnO, PTFE, PbO, cellulose) and bifunctional oxygen cathode (Ni/Pt/C). Secondary battery not yet realized.

31

LeadeAcid Batteries for Future Automobiles

longer cycle-life,higher specific energy and energy density, higher specific power and power density, and a very good deep temperature behaviour. Challenges. Compared with leadeacid, NiCd is more expensive, uses toxic cadmium, has significant faster self-discharge and a negative cell voltage temperature coefficient, that may lead to thermal runaway at constant voltage charge. Furthermore, the memory effect is worth mentioning after repeated partial discharge and/or storing incompletely charged batteries, the available capacity after charge is lost dramatically.

2.3.1.2 Cell chemistry The nickelecadmium battery consists of a nickel-positive electrode (cathode) and a cadmium-negative electrode (anode) in potassium hydroxide solution. With charging, thermodynamically instable nickel(III)-hydroxide and higher hydroxides are formed by protonation of nickel(II)-hydroxide. The equilibrium potential of the positive nickel electrode (0.49 V SHE) lies above the oxidation potential of water at pH 14 (0.401 V SHE), and therefore above the stability window of water. With charging, cadmium hydroxide is reduced to metallic cadmium at the positive plate (
0.81 V SHE). The cadmium electrode is thermodynamically stable, because the reduction potential of water equals
0.83 V SHE at pH 14.

Discharge processes ð
Þ Anodic oxidation: ðþ Þ Cathodic reduction: Cell reaction:

Cd þ 2OH


NiOðOHÞ þ H2 O þ e
Cd þ 2 NiOðOHÞ þ 2 H2 O

/ CdðOHÞ2 þ 2e


/ NiðOHÞ2 þ OH
/ CdðOHÞ2 þ 2 NiðOHÞ2

The real electrode processes are complicated and run across dissolved species (cadmium hydroxide, higher nickel hydroxide). The mass of hydroxide remains constant during charging and discharging, whereas the water quantity (0.67 mL Ah
1) and electrolyte concentration change without a large impact on volume.

Thermodynamic data DE 0 ¼ Ecathode
Eanode ¼ ½þ0:49
ð
0:81Þ V ¼ 1:30 V q ¼ 2F=M ¼ 165 Ah kg
1 w ¼ q$DE 0 ¼ 215 Wh kg
1 In practice, specific energy yields 50 Wh kg
1, i.e. about 23% of the thermodynamic value, due to excess active-material (necessary for sufficient

32

LeadeAcid Batteries for Future Automobiles

service life), incomplete active-mass utilization and the weight of inactive cell components. Negative electrode: Finely crystalline CdO powder, which forms cadmium hydroxide in the alkaline solution. Thermodynamic potential
0.809 V SHE, and capacity 477 Ah kg
1. Positive electrode: Nickel(II)-hydroxide mass with conductivity improvers (graphite, nickel felt) between perforated steel sheets (pocket plates or tubular plates) or on porous sinter nickel. Thermodynamic data: þ0.450 V SHE, 294 Ah kg
1. The support material is soaked with nickel and cadmium nitrate solution, respectively; the hydroxides are precipitated by the help of alkali lye or by electrolytic formation, whereby the pH value is shifted due to the cathodic hydrogen evolution. Electrolyte: usually 21% potassium hydroxide (density 1.17 g cm
3). LiOH and NaOH show a lower conductivity than KOH. However, the extended hydrate spheres of [Li(H2O)3]þ and [Na(H2O)2]þ, in contrast to [K(H2O)]þ, improve the performance of the nickel electrode (þ) in LiOH and NaOH at elevated temperatures. High alkali concentration supports the capacity of the a/g-nickel electrode at the cost of reduced life by volume changes. The hydroxide concentration and density are no measure for the SoC of the battery. Water that is consumed by electrolysis during continuous charging must be refilled. Due to vents the battery should not be overturned. Separator: Non-woven polyolefin separators with improved wettability. The battery must be sufficiently filled with electrolyte (1.5e2.5 mL Ah
1) to avoid the drying out of the separator. Overfilled batteries reveal explosion risks by gaseous oxygen that cannot diffuse from the positive to the negative electrode and escape through the safety valve in a flooded cell.

2.3.1.3 Nickel electrode Nickel hydroxide appears to be the best positive material for alkaline batteries [5] due to its high reversibility and cycle stability. b-Ni(OH)2 exhibits a sheet structure of edge-bridged NiO6-octahedra; hydrogen atoms reside between the sheets in the tetrahedral environment of the oxygen atoms lying above or below. With charging, a proton migrates out of the hexagonal host lattice.
ðþ Þ Charge: H2 NiII O2 or NiðOHÞ2 þ OH
/ HNiIII O2 ðor NiOOHÞ þ H2 O þ e
The thermodynamic capacity of 289 Ah kg
1 (nickel hydroxide) is nearly reached in practice.

33

LeadeAcid Batteries for Future Automobiles

In reality, complex solid state reactions take place between each two phases of a-Ni(OH)2 and b-Ni(OH)2 (discharged state), and b-NiOOH and g-HxKyNiO2, zH2O (charged) with embedded water. With overcharging, the thermodynamically stable, potassium-rich g-phase swells by incorporation of water leading so that the separators dry out in practical cells. On discharge, the g-phase is reduced to an unstable a-phase, which forms b-Ni(OH)2 after dissolution and precipitation. Doping Co-precipitated Co(III) improves 1000-fold the electronic conductivity of the nickel hydroxide lattice in the discharged state thanks to proton vacancies and stacking faults. Each percent of cobalt lowers the potential of the Ni(OH)2 electrode by 5 mV in favour of the capacity-forming electrolyte decomposition. Moreover, a high-oxygen overpotential is desired. At 60e70 C an undoped nickel electrode cannot be charged in practice because of the competing oxygen evolution. Co-precipitated, bivalent metal ions (cadmium, zinc and magnesium) prevent the formation of the g-phase by reducing the electrostatic repulsion of the layers and improving oxygen overpotential.

2.3.1.4 Cadmium electrode Metallic cadmium, as negative electrode, exhibits a hexagonal lattice (a ¼ 298 pm, c ¼ 562 pm) in the charged state. The thermodynamic capacity of 366 Ah kg
1 (Cd(OH)2) or 477 Ah kg
1 (Cd), respectively, is not achieved in practice due to a parasitic oxygen recombination and a 50% loss by the negative precharge of cadmium metal, which generates conductive centres in the nonconducting hydroxide. Oxidation and reduction follow a dissolution-precipitation mechanism involving complex cadmium hydroxide ions [Cd(OH)4]2
, which determine the discharge current of the cell at low temperatures (
20 to
40 C). Nevertheless, nickelecadmium exhibits a better low-temperature performance than leadeacid, nickelemetal-hydride and lithium batteries. Crystallization inhibitors. With overcharging and storage, the hydroxide electrode ages by recrystallization of the active-mass involving a loss of active surface area. Nuclei-forming agents such as Ni(OH)2, Mg(OH)2 and Y2O3 limit the unwanted crystal growth. Cadmium needles and dendrites, piercing through the separator, increase self-discharge and cause short-circuit currents. Charging by rapid pulses and extended temperature changes limit the service life of nickelecadmium batteries seriously.

2.3.1.5 Open nickelecadmium cells At the end of charging, the overcharging by the electrolysis of water occurs, whereby 0.34 g Ah
1 is decomposed.

34

LeadeAcid Batteries for Future Automobiles

ð
Þ Cathodic reduction: ðþ Þ Anodic oxidation:

4 H2 O þ 4 e
/ 2 H2 þ 4 OH
4 OH
/ O2 þ 2 H2 O þ 4 e


Cell reaction:

2 H2 O

/ 2 H2 þ O2

Maintenance-free alkaline batteries exhibit a too-small dimensioned positive electrode, which is coated during charging by oxygen from electrolysis. Molecular oxygen diffuses to the negative electrode and recombines with water there.

2.3.1.6 Gas-tight nickelecadmium cell The cadmium electrode (
) exhibits a larger capacity than the nickel electrode (þ) for three reasons: 1. With overcharging, the oxygen evolution at nickel starts before the hydrogen evolution on cadmium. ðþ Þ Regular charge:

NiðOHÞ2 þ OH


/ NiOðOHÞ þ H2 O þ e


Overcharge:

4 OH


/ O2 þ 2 H2 O þ 4 e


ð
Þ Regular charge:

CdðOHÞ2 þ 2 e


/ Cd þ 2 OH


Oxygen reduction:

1 O2 þ H2 O þ 2 e
/ 2 OH
2

2. Dissolved oxygen is reduced at the cadmium electrode. 3. Hydrogen evolution on cadmium starts no earlier than at 90% of SoC, which is not achieved in closed cells (negative charging reserve). Typical rated data: Charging voltage 1.4e1.5 V; end-of-charge voltage: about 1.7 V. Maximum capacity: 50 Ah, due to waste heat during overcharging. The danger of explosion by gas pressure during pole change was solved by technical measures inside the cell. For the cell design of NiCd batteries see the nickelemetal-hydride (NiMH) section (Section 2.3.2.5).

2.3.1.7 Operating behaviour and heat management The open-circuit voltage of half-charged alkaline batteries amounts to about 1.30 V (25 C). The temperature coefficient of
0.2 to
0.7 mV K
1 is independent of the SoC. The charging characteristics U(t) ascends continuously until the rated voltage is reached. With overcharging, especially at high currents, oxygen evolves and partly recombines to water, so that the voltage slightly drops and the pressure inside the cell increases. The discharge characteristics U(t) proceeds rather flat at an average voltage of 1.25 V (at C/5) or 1.2 V (1C ¼ discharge at the current given by the

35

LeadeAcid Batteries for Future Automobiles

rated capacity), because the two-phase reaction at the positive electrode (between Ni(OH)2 and NiOOH) allows the system only one degree of freedom. Heat management: A NiCd battery cools down due to the endothermal reaction (26 kJ mol
1) during charging below the thermoneutral voltage of En ¼
DH/(zF) z 1.43 V; above 1.43 V heat is released. The temperature jump following overcharge and O2-recombination can be used for SoC control. Discharge (below 1.43 V) is always exothermal. NiCd batteries provide high currents in a temperature range between 10 and 50 C. Down to
30 C, the performance of the cadmium electrode (
) declines.

Charging methods 1. Open alkaline batteries are charged at constant current (CC) first, until cell voltage exceeds a certain value, then constant voltage (CV) is applied until the leakage current is reached. 2. Maintenance-free, closed batteries overheat during CC/CV charging, because cell voltage drops abruptly at the end of charging. Therefore, a small CC (C/10 to C/5) is used to charge up to 150% of the rated voltage. 3. Boost charge at CC (C/2 to 2C) requires an end-of-charge detection by use of the voltage drop (
DU z 0.015 V or DU/U z 0.1%) or the heating of the cell (DT/Dt). Overcharge up to 105e115% cannot be avoided and limits battery life. This method fails with slow charging <0.5C and at temperatures above 40 C for lack of marked voltage changes. Self-discharge: The initial self-discharge of 20% per week at 40 C by gradual oxygen evolution and reduction improves in the course of time. 1 ðþ Þ 2 NiOOH þ H2 O / 2 NiðOHÞ2 þ O2 2 1 ð
Þ Cd þ O2 þ H2 O / CdðOHÞ2 2 In a late stage, self-discharge is controlled by nitrogen impurities in the electrolyte (details see NiMH-battery). ð
Þ NO3
þ 2e
þ H2 O / NO2
þ 2OH
/ NO3
þ 2e
þ H2 O ðþ Þ NO2
þ 2OH


ðat cadmiumÞ ðslowly at nickelÞ

Ageing: See NiMH battery. For NiCd batteries, especially reversible ageing (memory effect): Repeated partial discharge (flat cycling) leads to a loss of capacity. The battery ‘bears in mind’ the uncomplete discharge processes and delivers about 0.05e0.1 V

36

LeadeAcid Batteries for Future Automobiles

less than the full voltage. The exact reasons for this behaviour are unknown; presumably the proton rearrangement in the nickel lattice involves bad conductivity. Therefore, modern batteries should be fully discharged and recharged occasionally. Fortunately, the memory effect is reversible: By multiple discharge cycles down to SoC ¼ 0, and full charges with low current, the effect can be removed. Storage: NiCd and NiMH batteries can be stored at 30e50 C without permanent loss of performance. Large particles of cadmium hydroxide formed during long storage, increasing the charge voltage so that the NiCd battery should be charged at low currents in order to avoid unwanted hydrogen evolution. The loss of capacity during storage is reversible.

2.3.2 Nickelemetal-hydride batteries (NiMH) This modern power source is based on a hydrogen storage electrode in hydroxide solution [4]. In the late 1960s, researchers at Philips in the Netherlands discovered the alloy LaNi5, which is able to store hydrogen by the formation of hydrides. By end of the 1980s, storage alloys in NiMH batteries achieved a 30e40% larger energy density than NiCd. In 2004, metal alloys of the type AB3e4 were developed (using magnesium, rare earth and transition metals).

2.3.2.1 Automotive applications In the past, NiMH batteries were used in BEVs such as General Motors ‘EV1’, Honda ‘EV Plus’, and Ford ‘Ranger EV’. While large NiMH battery systems suffer from high costs, small 1 kWh-NiMH-batteries have been used successfully, e.g., in the hybrid vehicles Toyota ‘Prius’, Honda ‘Insight’, Ford ‘Escape Hybrid’, Chevrolet ‘Malibu Hybrid’, and Honda ‘Civic Hybrid’. The first Toyota ‘Prius’ was launched in 1997. Up to 2015, Toyota sold 8 million hybrid vehicles, most of them running on NiMH batteries. An overview of the development of the Prius’ NiMH battery is given in Table 2.3. As Toyota announced, the new Prius generation will use Li-ion batteries as well. A combination of NiMH and Li-ion is under consideration for 12-V dual microhybrid battery systems, e.g., LIB (10 Ah)/NiMH (15 Ah) [6]. Advantages compared with leadeacid: tolerates deep discharge, tolerates storage in the discharged state for longer time, offers longer cycle-life, higher specific energy and energy density, higher specific power and power density (lower than NiCd). Challenges compared with leadeacid: higher cost, significantly faster selfdischarge, negative cell voltage temperature coefficient, which, however, is compensated during charge by MH-formation heat. A memory effect has been observed, but was less severe than that with the NiCd battery and no longer appears in modern NiMH cells.

37

LeadeAcid Batteries for Future Automobiles

Table 2.3 Characteristics of four generations of NiMH-batteries in the Toyota ‘Prius’ 1997 Prius (Generation I Japan only)

2000 Prius (Generation II)

2004 Prius (Generation III)

2010 Prius (Generation IV)

cylindrical

Prismatic

Prismatic

Prismatic

Cells (modules)

240 (40)

228 (38)

168 (28)

168 (28)

Nominal voltage

288.0 V

273.6 V

201.6 V

201.6 V

Nominal capacity

6.0 Ah

6.5 Ah

6.5 Ah

6.5 Ah

Specific power

800 W kg
1

1000 W kg
1

1300 W kg
1

1310 W kg
1

Specific energy

40 Wh kg
1

46 Wh kg
1

46 Wh kg
1

44 Wh kg
1

Module weight

1.09 kg

1.05 kg

1.045 kg

1.04 kg

35(oc)  384(L)

19.6  106  275

19.6  106  285

19.6  106  285

Form factor

Module dimension

2.3.2.2 Cell chemistry The negative H2-storage electrode (LaNi5, NiTi2, alloys of Ni, Co, Ce, La, Nd, Pr, Sm) absorbs atomic hydrogen during discharge, and releases hydrogen from the lattice during charging.

Discharge processes ð
Þ Anodic oxidation: ðþ Þ Cathodic reduction: Cell reaction:

MH þ OH


/


NiOðOHÞ þ H2 O þ e MH þ NiOðOHÞ

/ /

H2 O þ M þ e


NiðOHÞ2 þ OH
M þ NiðOHÞ2

Thermodynamic data: Cell voltage, DE0 ¼ Ecathode
Eanode ¼ [0.49
(
0.82)] V z 1.31 V, is similar to the NiCd system. Specific charge q ¼ F/ M ¼ 165 Ah kg
1, and specific energy w ¼ q , DE0 ¼ 216 Wh kg
1, exceed the practical values (80 Wh kg
1). Charging: The applied hydrogen pressure of 0.01e10 bar generates an equilibrium potential of 0.84 to 0.77 V SHE at the metal-hydride electrode. ð
Þ 2 M þ H2 /2 MH Overcharge: With overcharging hydrogen and oxygen are generated by the electrolysis of water. In maintenance-free NiMH-cells, the hydride electrode (
) is dimensioned larger than the nickel electrode (þ), so that oxygen released on the nickel electrode can migrate through the electrolyte to the hydride electrode, and recombine to form water there. Overall, the energy

38

LeadeAcid Batteries for Future Automobiles

of overcharge is converted into heat; the slight voltage drop during the O2 reduction is used by charging devices as a criterion for cut-off. ðþ Þ Regular charge:

NiðOHÞ2 þ OH


/

NiOðOHÞ þ H2 O þ e


Overcharge:

4 OH


/

O2 þ 2 H2 O þ 4 e


ð
Þ Regular charge:

M þ H2 O þ e


/

MH þ OH


Oxygen reduction:

1 O þ H2 O þ 2 e
2 2

/

2 OH


Positive electrode (cathode): Nickel foam. See NiCd system (Section 2.3.1.2). Electrolyte: 30% KOH in polymer non-woven fabric. Hydroxide concentration and electrolyte volume remain unchanged during cycling.

2.3.2.3 Negative metal-hydride electrode Lanthanumenickel storage alloys of the type AB5 such as LaNi5 consist of hexagonal layers of lanthanumenickel and nickel. With charging, hydrogen dissolves in a solid solution forming the a- and b-phase, whereby the host lattice expands by 25%. The volume changes during cycling cause an unstable surface which tends to corrosion and decomposition into very small particles. Therefore, pure LaNi5 is not suitable for batteries. Doping with aluminium, manganese or cobalt improves the intermetallic cell volume and reduces the required equilibrium pressure for charging from p(H2) ¼ 1.7 bar to about 0.1 bar. Larger atoms (Al, Mn) replace nickel on its lattice sites. Cobalt, although expensive, improves the ageing stability during cycling by forming an intermediary g-hydride phase (3e3.5 H mol
1), which lowers the mechanical stress caused by volume changes a / g / b. Replacement of lanthanum. Intermetallic compounds are stoichiometric compositions such as LaNi3, La2Ni7, and La5Ni19 having 75e90 at-% nickel. Different metals are able to replace lanthanum completely. The overstoichiometric phase LaNi4.85 to LaNi5.4 is stable up to 1270 C. Commercial alloys such as LaNi3.55Mn0.4Al0.3Co0.75 (320 Ah kg
1) provide a good lifetime. Magnesium in the layered structure La1
yMgyNix (3 < x < 4. 0 < y < 1) improves specific capacity up to 400 Ah kg
1 and relieves lattice stresses. Alloys of the type A2B7 and A5B19 are stacks of AB5 and A2B4 units, which provide high capacities and long lives.

2.3.2.4 Operating behaviour and heat management Charge/discharge behaviour and charging methods: see NiCd battery. Heat management: In a NiMH battery, charging and discharging are exothermic reactions having a thermoneutral voltage of En ¼ 1.28 V. The cooling down during the endothermal charge (reversible heat 36 kJ mol
1)

39

LeadeAcid Batteries for Future Automobiles

is overcompensated by the exothermal MH formation. The hydride electrode (
) limits at temperatures below 0 C, and the nickel electrode (þ) tends to unwanted oxygen evolution above 60e70 C. Self-discharge. The initial self-discharge of 20% per week at 40 C is due gradual oxygen evolution but improves in the course of time. 1 ðþ Þ 2 NiOOH þ H2 O / 2 NiðOHÞ2 þ O2 2 1 ð
Þ 2 MH þ O2 / 2 M þ H2 O 2 About 20% self-discharge per month at room temperature is caused by sidereactions at the charged nickel electrode by the nitriteeammonia redox couple and desorption of hydrogen: 1 ð1Þ 6 NiOOH / 2 Ni3 O4 þ 3 H2 O þ O2 2

ð2Þ 6 NiOOH þ NH3 þ H2 O þ OH / 6 NiðOHÞ2 þ NO2 NO2
þ 6 MH ð3Þ 2 NiOOH þ H2

/ NH3 þ H2 O þ OH
þ 6M / 2 NiðOHÞ2

In a late stage, self-discharge is controlled by nitrogen impurities in the electrolyte, which are the result of the fabrication process of sintered-type Nielectrodes, which are mostly prepared via electrochemical reduction of Ni(NO3)2. With the nitrate a nitrate/nitrite shuttle is formed in the electrolyte. At the negative electrode nitrate is reduced to nitrite: (-)NO-3 þ H2O þ 2e- ¼> NO-2 þ 2OH-MH þ OH- ¼> M þH2O þ e- At the negative electrode produced nitrite ions can diffuse to the positive electrode where they are oxidized to nitrate ions again: (þ) NO-2 þ 2OH- ¼> NO-3 þ H2O þ 2e-NiOOH þ H2O þ e- ¼> Ni(OH)2 þ OH- The consequence of this nitrate/nitrite shuttle is the selfdischarge of the cell. The self-discharge rate increases at rising temperatures. Ageing. Nickelemetal-hydride batteries as well as nickelecadmium batteries provide a lifetime of 10e15 years. Ageing proceeds slowly by four effects: 1. Metallization of the separator by electroplating results in self-discharge and finally short-circuits. 2. At the end of float charging, the discharge characteristics shows a 70 mV high step caused by the formation of the g-Ni phase (þ), which disappears again after several cycles. At high temperatures, lithium nickelate (LiNiO2) is irreversibly formed in LiOH electrolytes, which reduces the cell voltage by120 mV (compared with NiOOH). 3. NiMH batteries tend to corrosion of the (
)-hydride electrode. In LaNi3.55Mn0.4Al0.3Co0.75 alloy, the elements corrode in the row: rareearth metal > aluminium > manganese > nickel, cobalt. The balance is: 2:15 La þ 1:15 H2 O / LaðOHÞ1:15 þ 1:15 LaH

40

LeadeAcid Batteries for Future Automobiles

The decomposition of water causes that the separators dries out. Corrosion consumes storage alloy and generates a discharge reserve (LaH) at the expense of charging capacity. 4. Reversible ageing (memory effect) occurs partially also at NiMH batteries e see also NiCd ageing section.

2.3.2.5 Cell design NiMH and also NiCd batteries (nickelecadmium and nickelemetal-hydride) have been commercialized using cylindrical, flat, wound, stacked, bipolar, maintenance-free, vent regulated and open cell designs. In cylindrical cells of the sizes AAA, AA, A, Cs, C, D and F, the positive electrode, the separator and the negative electrode are rolled forming a spiral-wound package, which is assembled in a nickel-plated steel case (
). Electrical contacts are welded on, and electrolyte is filled in according to the pore volume of electrode and separator. A cap having a safety valve and an insulating O-ring seal is pressed in. The active-materials are formed by formation. In rectangular parallelepiped cells, tailored electrodes and separators are packed in a case. After filling in the electrolyte solution, the cells are sealed by a safety valve. In future bipolar modules, the single cells are electrically connected in series wall-by-wall.

2.3.3 Nickelezinc batteries The nickelezinc (NiZn) system is known for more than 100 years based on T. A. Edison’s patent in 1901. NiZn batteries that were used in the beginning of the twentieth century in railcars failed after a limited number of discharge/recharge cycles. In the 1960s, nickelezinc batteries were utilized as alternative to silverezinc batteries for military applications. In the 1970s and 1980s, electric vehicles were realized. Until 2004, Evercel, Inc. improved the NiZn system. PowerGenex is producing commercial advanced NieZn batteries in the United States and China, e.g., size AA cells for consumer applications.

2.3.3.1 Automotive applications In the 1980s, General Motors demonstrated vehicle propulsion with a 115 V/ 150 Ah NiZn battery (339 kg, 206 L, 50 Wh kg
1) in a car (curb mass 1150 kg). In 1998, a 11.2 kWh NieZn battery of Evercel was built in a Pivco Citi Bee. An electric vehicle (EV) made by Trapos at nearly the same time was tested at 40 km/h with both nickelezinc and leadeacid batteries. A 205 kg (12 kWh) nickelezinc battery provided a range of 172 km, whereas a 280 kg (7.0 kWh) leadeacid battery stopped at 69 km. EagleePicher manufactured a

41

LeadeAcid Batteries for Future Automobiles

18 kWh (90 V/200 Ah) NieZn monoblock, which was tested in a Solectria vehicle. EagleePicher’s NieZn electric boat set the international water speed record in the 1970s. Later, PowerGenex developed also the NieZn system for micro- and mild-hybrids. France PSA and German automotive supplier HELLA have expressed their interest in that system. As an alternative to a 12 V/80 Ah AGM battery, a 13.2 V/55 Ah NiZn battery provides 72 Wh/kg at a weight of 10.1 kg and a service life of five years [7]. As well, the NiZn system was proposed for use in 48-V mild-hybrids [8]. Advantages: High dynamic charge-acceptance (DCA) over service life, wide temperature range, good cold start capability, tolerance to high temperatures, voltage stability during engine start, simple battery management and control. Costs between leadeacid and NiCd, about 300 $/kWh. Challenges: Compared with leadeacid: higher cost, and significant faster self-discharge.

2.3.3.2 Cell chemistry Discharge reaction

ð
Þ Anodic oxidation: ðþ Þ Cathodic reduction:

Zn þ 2OH
2NiOðOHÞ þ 2H2 O þ 2e


/ /

ZnðOHÞ2 þ 2 e
2NiðOHÞ2 þ 2OH



1:24 V 0:49 V

Overall:

2NiOðOHÞ þ 2H2 O þ Zn

/

2NiðOHÞ2 þ ZnðOHÞ2

1:73 V

Charge reaction Overall:

2 NiðOHÞ2 þ ZnðOHÞ2

Overcharge reaction:

H2 O

/ 2NiOOH þ 2H2 O þ Zn

1 / H2 þ O2 2

Self-discharge reaction ðþÞ 2 NiOOH þ H2 O ð
Þ Zn þ H2 O 1 Zn þ O2 2

1 / 2 NiðOHÞ2 þ O2 2 / ZnO þ H2 / ZnO

Thermodynamic data: Cell voltage, DE0 ¼ Ecathode e Eanode ¼ [0.49
(
1.24)] V z 1.73 V, i.e., higher than NiCd and NiMH systems (requiring a special charger). Specific energy w ¼ q , DE0 ¼ 326 Wh kg
1.

42

LeadeAcid Batteries for Future Automobiles

Practical data: 75 Wh kg
1, i.e., 23% of the thermodynamic value. In contrast to nickelecadmium, the specific power of nickelezinc (500 Wh kg
1), is gradually lost during the lifetime by the increasing internal resistance due to electrolyte loss and redistribution by excessive heating at high-current loads (P ¼ I2R). Self-discharge about 20% per month at 25 C. Storage above 50 C and storage for more than three years is not recommended. The lifetime of about 500 cycles (low compared with NiCd and NiMH) is due to the irreversible dissolution of zinc hydroxide in the electrolyte. The zinc electrode suffers shape changes and dendrites. Negative electrode (zinc): Zinc oxide is mixed with additives (zinc metal, zinc alloys, carbon, conductive polymers, etc.) to improve conductivity and corrosion stability. Up to 25% additional calcium oxide forms calcium zincate, that is a less soluble discharge product than the regular zinc hydroxides. Unfortunately, the additive decreases cell energy density, and kinetic alloys restricts discharge and charge rates. Zinc electrodes are fabricated similar to MH electrodes using compressed powders or pastes and various substrates (perforated foil, foam or expanded metal), typically made of copper or copper-plated materials. An incorporated surface layer inhibits hydrogen evolution at the zinc potential and is stable under conditions where the negative electrode undergoes polarization [9]. Positive electrode: Nickel hydroxide cathodes; see Section 2.3.1.2. Electrolyte: Owing to the growth of zinc dendrites and the solubility of zinc compounds, NiZn batteries require electrolyte blends (KOH/NaOH/LiOH), buffering agents, and cycle-life improvers (such as fluorides, borates and silicates). Separator: Providing an electrolyte reservoir, and retarding zinc migration, microporous polypropylene, e.g., CELGARD of 0.025 mm thickness, is commonly used. This material is treated with a surface additive in order to reduce pore size and to improve the resistance to zinc penetration.

2.4 High-temperature sodium batteries Batteries using liquid sodium in oxide ceramic solid electrolytes [10] have been developed since the 1980s for mobile and stationary applications in the kilowatt range as the NaNiCl2 system (Fig. 2.3A) and the NaS system (Fig. 2.3B).

2.4.1 Automotive applications ZEBRA batteries were employed in demonstration projects as in BEVs, e.g., Renault ‘Twingo’, ‘Smart’ and ‘Panda’; ‘Think City’, 3.5 t Iveco ‘Daily Electric’

43

LeadeAcid Batteries for Future Automobiles

(A)

(B)

Ceramic seal

Current collector (anode) NiCl2 + NaAlCl4

Disc springs Aluminum seal Sodium

Insulator

β-alumina electrolyte

solid electrolyte Sulfur/carbon liquid sodium metal case (cathode)

Sodium wick

Metal case (cathode)

Figure 2.3 High-temperature batteries: (A) sodiumenickel chloride accumulator, (B) sodiumesulfur battery and shamrock design.

delivery vehicle; in hybrid and full battery buses, e.g., Autodromo Electric Bus, Cito Electric Bus, EVO Electric Hybrid Bus, MAN Electric Bus and LARAG Wil Bus, Daimler MB410E or Nova RTS [11]. At present, the importance of the NaNiCl2 system for automotive applications is relatively low besides few research and development activities. However, EUROBAT’s future trends analysis (till 2025) for automotive batteries favours the ZEBRA technology besides leadeacid, nickelemetal-hydrid and lithium-ion [12]. The sodiume sulfur battery no longer plays a role for automotive applications. The complex thermal management system of high-temperature sodium batteries excludes application for small SLI relevant systems. Advantages over leadeacid batteries: higher specific energy and energy density, no temperature problems caused by thermal management, and highenergy efficiency. Drawbacks are mainly caused by the high operating temperatures (w300 C) of both sodium batteries (NaeS, NaeNiCl2): need of thermal management, as only few thermal cycles are accepted; thermal selfdischarge limits the parking time (‘airport effect’); safety concerns due to the reaction of sodium with sulfur or, less severe with the liquid electrolyte (NaAlCl4); relatively high costs for the time being.

44

LeadeAcid Batteries for Future Automobiles

2.4.2 Sodiumenickel chloride battery (ZEBRA) ZEBRA [13] denotes the Zeolite Battery Research Africa or the Zero Emission Battery Research Activities (since 1985), which are based on a liquid sodium negative electrode and a solid NiCl2 electrode (þ) at high temperatures. ZEBRA batteries are currently manufactured by FZ Sonick (joint venture between Italian FIAMM and Swiss MES-DEA) and are directed for stationary and automotive applications. General Electric pursues stationary applications under the trade name Durathon (2012). In 2015, the company abandoned the project.

2.4.2.1 Cell chemistry The liquid sodium joins to a b-alumina solid electrolyte. The nickel chloride electrode immerses into molten sodium tetrachloroaluminate NaAlCl4. With charging, chloride reacts with nickel, giving nickel chloride; from sodium chloride, metallic sodium is formed.

Discharge reactions ð
Þ Anodic oxidation: ðþÞ Cathodic reduction:

2 Na NiCl2 þ 2e


NiCl2 þ 2Naþ þ 2e


Cell reaction:

/ 2 Naþ þ 2 e
/ Ni þ 2 Cl
/

Ni þ 2 NaCl

ð2:58 VÞ

Negative electrode (anode): Metallic sodium (melting point 98 C) resides between solid electrolyte and a steel wall. The active-materials (sodium and metal chloride) are generated electrochemically after integration of the cell by excess nickel, without producing poisonous chlorine. 2 NaAlCl4 þ Ni/NiCl2 þ 2 Na þ 2 AlCl3 Positive electrode (cathode): A mixture of nickel powder and sodium chloride resides in a beaker of solid-electrolyte, which is coated by the liquid sodium ion conductor NaAlCl4 (300 C). Sodium, nickel and chlorine exist in three phases (NiCl2, NaCl, Ni), so that temperature and pressure may vary as the degrees of freedom, whereas the potential has to remain constant according to Gibbs’s phase rule. A variation of the ZEBRA battery reduces the internal resistance by both nickel chloride and iron chloride. In a parallel combination of nickel and iron cell, the nickel cell is discharged first, until the cell voltage reaches the value of the iron cell, which is discharged then until exhaustion. The discharge characteristic reveals two fixed voltage plateaus. At high current, when the cell voltage drops below the iron level, nickel and iron deposition occur simultaneously. Additional reaction:

FeCl2 þ 2Naþ þ 2e
/ Fe þ 2 NaCl

ð2:35 VÞ

45

LeadeAcid Batteries for Future Automobiles

Solid electrolyte: The brittle b-alumina (sodium-containing Al2O3) is formed as a beaker that conducts sodium ions with sufficient conductivity at temperatures >250 C. Besides electrolyte function the b-alumina has also separator function. In case of emergency, when liquid sodium comes into contact with sodium aluminate, the reactions products close the cracks in the solid electrolyte again. Large cracks lead to short-circuits by undesired aluminium that is formed. NaAlCl4 þ 3 Na/4 NaCl þ Al Materials and cell design. A sodiumemetal chloride battery consists of 18% nickel, 16% iron, 4% copper, 26% halide salt, 16% b-alumina, 10% stainless steel, 4% construction steel, 4% thermal insulation and 2% of other parts. During fabrication, the positive electrode is filled with nickel powder and sodium chloride. The air-sensitive active-materials are generated electrochemically during the first charge of the sealed battery. A porous wick of carbon felt accomplishes the contact to the solid electrolyte, because the nickel particles tend to form larger particles in the course of time. Additional sodium fluoride and aluminium powder improves the conductivity. Iron sulfide blocks the agglutination of the nickel particles. A steel shell in the negative electrode presses the sodium against the b-alumina electrolyte. Recently, shamrock-shaped b-alumina tubes are used which allow higher currents thanks to their improved surface area.

2.4.2.2 Operating behaviour ZEBRA batteries perform less powerfully than lithium-ion batteries (Table 2.4). Efficiency: Coulomb efficiency reaches nearly 100%, because side-reactions do not exist. Energy efficiency amounts to 90%, excluding thermal losses. Performance data: Thermodynamic cell voltage: 2.58 V, practical voltage 2.3e2.5 V. The energy/power ratio (z2 h
1) exceeds that of the NaeS battery (z0.5 h
1). Life: 2500e5000 cycles. Self-discharge: Electrochemical reactions play no role. The waste heat flow of a 20-kWh battery amounts to 120 W, which must be provided by the discharging the battery as long as no external heating is employed. The thermal self-discharge is w15% per day. Battery management: The production technology of the ceramic electrolyte tube limits cell capacity to about 30 Ah, so that several single cells have to be connected in parallel and in series to realize high capacities and voltages. The failure of a single cell can lead to a short-circuit, so that a cell monitoring is required. ZEBRA batteries are charged at CV under current limitation, followed by a final period under CC. A 32-Ah cell required 15 A

46

LeadeAcid Batteries for Future Automobiles

Table 2.4 Performance data of high-temperature batteries Sodiumenickel chloride (ZEBRA)

Sodiumesulfur

(
) 2 Na / 2 Naþ þ 2 e


(
) 2 Na / 2 Naþ þ 2 e


(þ) NiCl2 þ 2 e
/ Ni þ 2 Cl


(þ) 3 S þ 2 Naþ þ 2 e
/ Na2S3

Cell voltage (V)

<2.5 V; 300 C

2.1 V; 350 C

Specific energy (Wh kg
1)

90e120

110e145 (thermodynamically 790)

Energy density (Wh L
1)

140e170

80e120

Specific power (W kg
1)

150e250

60e80

Power density (W L
1)

250e300

40e60

Cell reactions on discharge

and 2.67 V as an optimum. The end-of-charge is reached after about 5 h, when the measured amp-hours meet the calculated capacity, and the current drops to 0.5. Boost charge at 2.85 V badly affects the cell resistance, so that it should be used for only 80% of the rated capacity within 75 min. Life: About 4500 cycles at 80% depth of discharge and 14 years of float-life were reported. Some 2000e5000 chargeedischarge cycles are available depending on the stability of the solid electrolyte and the gradual coarsening of the nickel particles, which involves a loss of active surface area during charging and discharging of the positive electrode. Already at 265 C, a surface layer of sodium chloride grows on the nickel grains. Conducting additives such as NaBr, NaI, S, Al, Fe and FeS in the positive electrode improve cycle-life. Aluminium forms zones of NaAlCl4; sulfur suppresses grain growth by poisoning the nickel surface. Iron improves current density and protects from overcharge by its lower potential. Safety: During cooling down, problematic mechanical stresses occur in the cell, which may result in a failure by short-circuits through cracks in the solid electrolyte. The liquid second electrolyte NaAlCl4 is able to absorb the dangerous sodium during unwanted overcharge and deep discharge. ð
Þ Overcharge ð3.05VÞ: Ni þ 2 NaAlCl4 / 2 Na þ 2 AlCl3 þ NiCl2 ð
Þ Deep-discharge ð1.58 VÞ: 3 Na þ NaAlCl4 / Al þ 4 NaCl During overcharging, the aluminate creates a sodium reserve. The first charge of the battery after fabrication generates a sodium excess at the negative electrode, which can be consumed at low voltage.

47

LeadeAcid Batteries for Future Automobiles

ZEBRA cells reside in a double-walled container with heat insulation between inner and outer case. The mechanical stability in electric vehicles was proven by crash tests. If, however, the solid electrolyte is broken as consequence of an accident, then reacts the sodium with the liquid NaAlCl4 electrolyte with a relatively low reaction energy.

2.4.3 Sodiumesulfur battery The sodiumesulfur (NaeS) battery was developed between 1980 and 1995 for vehicles by Asea Brown Boveri and Silent Power Ltd. By safety reasons, however, this development was stopped. At present, NGK (Japan) is producing NaeS batteries for stationary applications. To the reactions of the NaS battery see Table 2.4.

2.5 Lithium-ion batteries Lithium batteries are considered as the state-of-the art for various portable applications, power tools, electric vehicles and more and more also for stationary storage systems. Two technologies can be distinguished. 1. Systems with metallic lithium anode in liquid electrolyte (for coin cells only) or polymer electrolyte (small activities of lithium-metal polymer for electric vehicles, e.g., by Bolloré) 2. Lithium-ion batteries with lithium intercalation or lithium alloy electrodes (no metallic lithium) in liquid or polymer electrolytes, currently used for nearly all application areas, including automotive. Today’s lithium-ion batteries [14e16] utilize so-called intercalation electrodes made of graphite and metal oxides in order to avoid the aggressive reactions of metallic lithium with the liquid or polymer electrolytes and the formation of lithium dendrites. J.B. Goodenough and coworkers founded in 1979 the metal oxide family LixMO2 (M ¼ Co, Ni, Mn). In the late 1980s, D.W. Murphy, B. Scrosati and coworkers invented the rocking chair technology. Sony in 1991 commercialized the first lithium-ion battery based on graphite and lithium cobalt oxide. Later, Moli Energy Ltd. in Canada developed lithium nickel oxide, and the Bell Communications Research Laboratory found lithium manganese spinel. Goodenough, in 1996, patented lithium iron phosphate. New 5-V cathode materials and high capacity anodes are currently under worldwide investigation, which might enable more powerful batteries for electric vehicles in near future. Advantages over leadeacid batteries: higher specific energy and energy density, higher specific power and power density and longer lifetime. Challenges: higher costs, reduced deep temperature and high-temperature behaviour, complicated recycling processes and safety risks.

48

LeadeAcid Batteries for Future Automobiles

2.5.1 Automotive applications Li-ion technology has conquered stationary storage and consumer applications in all automotive areas. Li-ion is competitive to leadeacid batteries in micro-/mild-hybrids as well as to nickelemetal-hydride (NiMH) batteries in full-hybrid vehicles. Battery vehicles and PHEV require high specific energy for long range, and Li-ion batteries offer more than 1/3 higher specific energy in contrast to NiMH. Full-hybrids, requiring high specific power, may run both on NiMH or Li-ion, whereby decreasing prices favour the Li-ion battery. Mild-/ micro-hybrids welcome the power, dynamic charge-acceptance, energy and lifetime of the Li-ion batteries. Challenges, however, are prices as well as low and high-temperature behaviour. For applications the cell chemistry of Li-ion batteries plays an important role. Power applications (SSV/micro-/mild-hybrids, full-HEV) prefer lithium titanate anodes (LTO) and lithium iron phosphate (LFP) cathodes. Energy applications (PHEV, BEV) require graphite anodes and cathodes of lithium manganese spinel (LMO), nickel-manganese-cobalt oxides (NMC), and nickelcobalt-aluminium oxides (NCA). LTO systems require more cells in return to the low voltage of the anode (w1.5 V vs Li/Liþ). LFP cells offer more or less constant cell potential during charge and discharge, which makes SoCdetermination difficult via the cell voltage. See Table 2.5A.

2.5.1.1 Battery electric vehicles There are already some full BEVs with lithium batteries commercially on the market. An overview about the most important BEVs is given in Table 2.5B. Most cars use lithium-ion batteries but there is also a Li-metal solid polymer battery on the market (Batscap battery of Bolloré in the Bluecar).

2.5.1.2 Stopestart vehicles/micro-/mild-hybrid electric vehicles Stopestart vehicles and micro-hybrids are able to use the classic board net architecture, but only LFP-graphite chemistry can be directly adopted to a 12-V system.  LFPegraphite is compatible to the 12-V grid. A series combination of four cells may be charged up to 15 V, and operates down to 12 V. Six LFPeLTO cells may be charged above 16 V.  Three cells of NMCe and NCAegraphite yield a maximum of 12 V. Four NMCe or NCAegraphite cells work between 14 and 16 V, so that the battery cannot be fully charged. Five cells having LTO anodes can be charged up to 16 V. LTO anodes, however, make all conventional cathode materials compatible to the board net requirement [17]. From these LTO-systems, the LTO/LFP is of high interest, due to its high power and high safety behaviour [17].

49

LeadeAcid Batteries for Future Automobiles

Table 2.5A Application and performance of Li-ion battery materials NCA

NCM

LMO

LFP

LTO

þ

þ

þþ

þþþ

þþþ

Full-HEVs

þþ

þþ

þþ

þþþ

þþþ

PHEV

þþ

þþ

þþ

þþ

þþ

BEV

þþþ

þþþ

þ

þ

þ

Capacity

þþþ

þþ

þ

þþ

þþ

Power

þ

þþ

þþ

þþ

þþþ

Thermal stability




þ

þþ

þþþ

þþþ

Safety




þ

þþ

þþþ

þþþ

Cycle-life

þ

þþ

þ

þþþ

þþþ

Cost




þ

þþþ

þþþ

þ

SSV, micro-/mildhybrid


: poor; þ, fair; þþ, good; þþþ, excellent.

Nevertheless caused by cost reasons the ‘standard’ system for micro-hybrid applications is the graphite/LFP system. In 48-V applications, typical choices are series connections of high-power cells consisting of 12e13 NMCegraphite or NCAegraphite cells, or 14 LFPe graphite cells, or 20e22 NMCeLTO cells could be used. A 12-V Li-ion (Gr/LFP) and a 12-V absorptive glass-mat (AGM) battery are compared in Table 2.5C. This graphite/LFP system is commercially already used in some premium cars (see Table 2.5D).

2.5.1.3 Challenges Li-ion batteries, in comparison to leadeacid batteries, exhibit specific energy/power advantages. Drawbacks for automotive applications are discussed in the following.

Low temperature behaviour There are solutions via chemistry choice (e.g. LTO/LFP) and via heating of the battery [18] as well as self-heating [19]. With self-heating using an inert Ni

50

LeadeAcid Batteries for Future Automobiles

Table 2.5B Li-ion cells used in current battery electric vehicles (BEV) Car company

Model

Cell producer

Cathode

Cell energy Cell specific energy density (Wh LL1) (Wh kgL1)

Anode

Capacity (Ah)

Cell type

60

Prismatic

237

126

Prismatic

364

228

BMW

i3

Samsung

LMO þ NMC

Gr

Bolloré

Bluecar

Batscap

LFP

Li

Coda

EV

Lishen

LFP

Gr

16

Prismatic

226

116

Daimler

Smart

LG Chem

NMC

Gr

50

Pouch

270

140

Fiat

500

Samsung

NMCLMO

Gr

64

Prismatic

243

132

Honda

Fit

Toshiba

NMC

LTO

20

Prismatic

200

89

Mitsubishi

i-MEV

Li energy Japan

LMONMC

Gr

50

Prismatic

218

109

Nissan

Leaf

AESC

LMO-NCA

Gr

33

Pouch

309

155

Renault

Zoe

LG Chem

NMCLMO

Gr

36

Pouch

275

157

Tesla

Model S

Panasonic

NCA

Gr

3.1

Cylindric

630

233

Gr, graphite.

foil integrated into the cell (All-Climate battery of EC Power) it is possible to heat-up the cell, e.g., from
20 C to 0 C in 12.5 s using 3% of the cell energy or from
40 C to 0 C in 30 s using 5% of the cell energy [20].

High-temperature behaviour Caused by lifetime and safety reasons, 40e50 C are the maximal operating temperature; therefore, it is challenging to have the battery in the combustion engine compartment HeV better than at location in the trunk. Safety Li-ion systems with high safety are reached by using more stable activematerials (see Table 2.5A) and active/passive safety devices such as a battery management system (BMS), which is integrated into the battery container and controlled the key operational parameters. Costs A detailed cost analysis [37] forcasts about 150 $kWh
1 beyond the year of 2020. But this optimistic view still differs from the leadeacid battery cost of

51

LeadeAcid Batteries for Future Automobiles

Table 2.5C Comparison of a 12-V Li-ion (Graphite/LFP of LG) and an AGM battery [76,77] Graphite/LFP 60 Ah

AGM, 90 Ah

Capacity at 25 C, 0.1 C (Ah)

61

90

Capacity at 25 C, 1.0 C (Ah)

60

65

Nominal voltage (V)

13.2

12.6

CCA at 100% SoC, 18 C (A)

880

e

CCA at 100% SoC,-25 C (A)

820

w700

Life (Years)

>10

3e5

Weight (kg)

10

24

w80 $ kWh
1. The higher costs, however, can be compensated by higher life time and lower weight of lithium-ion battery. A market share increase of Li-ion batteries in the field of micro-/mild-hybrid and SLI batteries of 5%/year is expected [21].

2.5.2 Cell chemistry Due to its density of 0.534 g cm
3 at 20 C and its standard potential of E0 ¼
3.045 V, lithium is the lightest of all metals for powerful batteries. The lithium metal electrode tends to the deposition of nonuniform dendrites after many chargeedischarge cycles, which may cause short-circuits, fire and explosions. In order to reduce this safety risk, so-called intercalation

Table 2.5D Examples of commercial micro-hybrid vehicles with a Li-ion batteries Vehicle manufacture

Vehicle model

Launch year

Estimated quantity of units in service

Battery manufacturer

1

Mercedes Mercedes-AMG

S-class SLS AMG Coupe SLS S63 AMG 365 AMG Coupe

2013

12K

A123 systems (LFP)

2

BMW

M3

2014

<1K

GS Yuasa (LFP)

52

LeadeAcid Batteries for Future Automobiles

electrodes are employed, which are able to store and release lithium-ions reversibly in liquid or solid electrolytes, but at a lower specific energy (Fig. 2.4). Usually, graphitic carbon is used as the negative electrode (discharge: anode), and metal oxides as the positive electrode (discharge: cathode). The electrolyte is a solution of a lithium salt in an organic solvent mixture, mostly carbonates. The graphite active-material reacts slowly with the solvent by forming a protective layer on the electrode surface, the solid electrolyte interface (SEI) [22]. This SEI is lithium-ion conducting and reduces the reaction rate of the graphite with the electrolyte. During discharge, lithium ions penetrate the positive electrode and leave the negative electrode. During charge, lithium insertion (intercalation) and deintercalation are reversed, why the Li-ion technology is compared with a rocking-chair e a swing effect. ðþ Þ Intercalation during discharge: Li1
x MIV O2 þ x Liþ þ x e
/ LiMIII 1
x O2 þ
ð
Þ Deintercalation during discharge: Lix C6 / xLi þ xe þ C6 The open-circuit voltage of about 3.7 V is caused by the change of Gibbs’s free enthalpy of the lithium ions in both electrode materials, which directly depends on the mole fraction x. DG ¼ DG0 þ RT ln

x ¼
x FDE 1
x

At the end of lithium insertion into the positive electrode, and complete emptying of the negative electrode, cell voltage drops to the cut-off voltage of 2.7 V, which should not be underrun in order to prevent damage of the material.

( ) Anode: graphite

Separator

(+) Cathode: Metal oxide Currebnt collector (aluminum)

Current collector (copper)

Discharge: Li+ + e + C6 LixC6

Elektrolyte LiPF6/solvent

Discharge: Li+ + e- + CoO2 LiCoO2

Figure 2.4 Basic cell design and electrode reactions of a lithium-ion battery.

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2.5.3 Negative electrode materials (discharge: anodes) Most commercial cells use carbon as the negative electrode [23], at which lithium intercalation proceeds during charging (Table 2.6). Promising new materials comprise titanates, lithium alloys and Li-oxides.

2.5.3.1 Graphite Natural graphite. Since 1990, low-crystalline graphitic carbon of high capacity has been used [24]: a low-cost material, which can be charged with lithium-ions in organic carbonates up to a thermodynamic composition of LiC6 corresponding to a specific capacity of 372 Ah kg
1. In practice, 0.9 Liþ per six carbon atoms and about 335 Ah kg
1 (graphite) or 0.5 Liþ per 6 C (petrol coke) are achieved. The intercalation takes place in the small voltage window of 0.20e0.05 V versus LijLiþ. The galvanostatic chargeedischarge characteristic shows three steps; see Fig. 2.5. During charge (intercalation), the distances between the graphite layers expand by about 10%. At the first charge, the organic solvent (propylene carbonate) is decomposed inevitably, whereby the SEI is formed at the expense of an irreversible loss of cell capacity of about 80 Ah kg
1. The lithium-ion conducting SEI consists of lithium salts (Li2O, LiF, Li2CO3 etc.) and polyolefins, which are able to compensate volume changes in the graphite material during cycling without cracks. The SEI is growing according to a t1/2 law during the life of the cell. At negative electrode potentials (0 V vs Li/ Liþ), lithium metal can be plated from high Liþ concentrations at the phase boundary after high-current charges or slow Liþ intercalation at low temperatures. Artifical graphite also called synthetic graphite consists mainly of graphitic carbon that has been obtained by graphitization, heat treatment of nongraphitic carbon, or by chemical vapour deposition from hydrocarbons at temperatures above 2100 K. Mesocarbon microbeads (MCMB) are spheroidal graphite particles, which are won by heat treatment below 1000 C. The high specific energy (300e900 Ah kg
1) is in contrast to poor reversibility and moderate cyclelife. Spherical particles in coating masses flow better and require less binder. Amorphous carbon. Furan and phenol resins, cellulose and sugar stay amorphous during heat treatment at 1000 C, as they are nongraphitizable hard carbons (used as activated carbons). Although cycle-life is good, the capacity is lower than that of graphite (Table 2.7).

2.5.3.2 Lithium titanate (LTO) Li4Ti5O12 or Li[Li0.33Ti1.67]O4 or 2Li2O,5TiO2 is a powerful anode material with thermal stability and a capacity of 170 Ah kg
1 (thermodynamic 175 Ah kg
1).

54

Graphite (graphitizable, soft carbon)

Amorphous (hard Carbon)

Spinel Li4Ti5O12 (LTO)

Metal oxide (SnO2, SiO2)

Tin composite (TCO)

Silicon alloy (Li4.4Si)

Lithium metal

0.05e0.2

0.1e0.7

1.56

0.05e0.6

0.05e0.6

0.05e0.6

0

372

e

175

e

w1000

4000

3830

w350

w200

w170

w750

w600

e

e

Fair

Good

Very good

Good

Good

Good

Bad

Cycle-life

Good

Fair

Very good

Bad

Bad

Bad

Bad

Life time

Very good

Good

Very good

Bad

Bad

Bad

Bad

Commercial

Commercial

Commercial

Mid-term

Mid-term

Long-term

Obsolete

Good

Fair

Fair

Fair

Good

Good

Good

Potential versus LijLiþ: (V) Specific capacity: a) thermodynamic (Ah kg
1) b) practical (Ah kg
1) Specific power: (W kg
1)

State of the art Costs

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Table 2.6 Negative electrode materials (anodes) for lithium-ion batteries

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Figure 2.5 Lithium-ion battery: (A) Chargeedischarge characteristics of a graphite electrode.

Due to its three-dimensional defect-spinel structure, this zero strain material allows lithium-ion intercalation without volume changes. ð
Þ Discharge:

Li2.33 Ti1.67 O4 / Li1.33 Ti1.67 O4 þ Liþ þ e


Unfortunately, the potential is as high as þ1.562 V versus LijLiþ; but it remains constant within a plateau between 0 and 170 Ah kg
1, so that no SEI is formed. Therefore, the SEI related disadvantages of graphite, such as the bad low temperature behaviour and the limited charge current, do not occur. The specific energy, however, reaches only half of that of graphite anodes, because about 1 V of cell voltage is missing. Insufficient conductivity requires a coating with nanoparticles and carbon. The material is suitable for powerful

Table 2.7 Properties of graphitic anode materials for lithium-ion batteries

Raw material Specific capacity (Ah kg
1)

Natural graphite

Artificial graphite

Soft carbon

Hard carbon

e

Coke

Pitch

Polymer resin

375

325

250

200

Low order

Disordered

Good

Very good

Structure Use at high currents

56

Regular layer lattice Low

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batteries with good deep temperature behaviour in electric vehicles but with reduced specific energy. Reducible transition metal oxides. Today’s metal oxides as anode materials in lithium-ion batteries are instable and expensive (w750 Ah kg
1, 0.8e1.6 V vs LijLiþ). Transition metal vanadates LiMVO4 (M ¼ Mn, Fe, Co, Ni, Cu, Cd, Zn) and MgV2O6 form nanoparticles at the first discharge, which allow the formation of lithium oxide in subsequent cycles. Metal deposition: ð
Þ Charge:

MVO4 þ 4 Li þ




M þ x Li þ x e

/ 2 Li2 O þ M þ VO2 / Lix M

2.5.3.3 Lithium alloys Metal alloys are compact, light, cheap and powerful in a potential range of 0.05e0.60 V versus LijLiþ. Specific capacities of 1000 Ah kg
1 (tin) and >4000 Ah kg
1 (silicon) are in contrast to poor stability and safety concerns.     Li4:4 Si 4; 212 Ah kg
1 > Li 3; 861 Ah kg
1 > Li2 Si5 > Li3 As > Li2 Sn5 zLiAl   > Li3 Sb > Li3 Bi; LiC6 339 Ah kg
1 . Lithium alloys suffer from unwanted volume changes during lithium insertion and deintercalation, whereby the electrode is gradually destroyed e ‘electrochemical milling’. Volume expansion at charge from 312% (Li2Si5) to 10% (LiC6) drops in the row: Li2 Si5 > Li2 Sn5 > Li3 Bi; Li3 As > Li3 Sb > LiAl; LiZn[LiC6 Lithium-tin alloys have reached a commercial state (Fuji 1997: SnB0.56P0.4Al0.42O3.6), although the production was discontinued in the meantime. Lithium-silicon alloy Li4.4Si (0,047 V vs LijLiþ) promises 4212 Ah kg
1 at a reduced cycle-life. Silicon-graphite composites achieve about 1500 Ah kg
1 at above 0.6 V vs LijLiþ. In 2015, first commercial cells with Sicomposite anodes were introduced (LG Chem 18650 e INR18650MJ1, 3.5 Ah; Panasonic 18650 e NCR18650GA, 3.45 Ah, Samsung SDI e INR18650-35E, 3.5 Ah).

2.5.4 Positive electrode materials (discharge: cathodes) The energy density of present lithium-ion batteries is mainly determines by the capacity of the positive electrode (Fig. 2.6). At the positive electrode (the cathode during discharge), the electrochemical reduction takes place, whereby lithium cations intercalate into the positive electrode material (M).

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Kathode (+)

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LiMn2O4 and doped compounds LiCoO2 and doped compounds Li(Ni,Co)O4 and doped compounds LixFePO4, LiVOPO4 LixMnO2 and doped compounds Vanadium oxides

Anode (—)

Cell voltage

Transition metal oxides

SnOx LiSn Carbon

Nitrides

Graphite

Figure 2.6 Electrode potential and specific electric charge of different materials for lithium-ion batteries.

ðþ Þ Cathode:

M þ Liþ þ e


/ MðLiÞ

The 4-V materials of the first and second generation are expensive and pose safety problems, except for LFP (Table 2.8). The structure of the cathode material defines the mobility of the inserted lithium-ions, and therefore the ionic conductivity.  The olivine lattice of LixFePO4 allows a linear ion mobility (1D) only. Safety and cycle stability are good, the price is moderate; but specific capacitance and voltage are challenges for electric propulsion (Fig. 2.7).  Layered oxides such as LiCoO2 and Li1
x(Ni0.33Mn0.33Co0.33)O2 (in short: NMC) offer a two-dimensional mobility for lithium cations (2D) . High specific capacity and moderate safety face rather high cost (Fig. 2.7).  The spinel lattice of LixMn2O4 guarantees a three-dimensional conductivity (3D) in all space directions. Despite excellent capacity and a moderate price, the stability of this commercial material in common electrolyte solutions requires further improvement (Fig. 2.7).

2.5.4.1 Lithium cobalt oxide (LCO) The first commercial rechargeable lithium-ion battery produced by Sony in 1991 introduced lithium cobalt oxide LiCoO2 ¼ Li1
xCoO2 (LCO), which has become the most prominent cathode material since then. An overview about LCO systems is given in Table 2.9.

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Table 2.8 Present lithium-ion batteries with different cathode materials and graphite anodes Positive Electrode

LiCoO2

LiMn2O4 spinel

LiNi0.8 Co0.15 Al0.05O2

LiNi0.33 Mn0.33 Co0.33O2

LiNiO2

LiFePO4

Abbreviation

(LCO)

(LMO)

(NCA)

(NMC)

(LNO)

(LFP)

Cell voltage (V)

3.7

3.8

3.7

3.7

3.2e4.2

w3.5

Potential versus Li/Liþ: (V)

3.0e4.4 (3.9)

3.0e4.5

e

e

3.8

e

Capacity thermodynamic (Ah kg
1) Ah L
1

274

296

e

e

192

168

706

634

e

919

e

Practical (Ah kg
1)

w140

w120

w190

w160

w170

e

Specific energy (Wh kg
1)

90e180

160

140

<180

High

80e120

Energy density (Wh L
1)

220e350

270

e

e

Specific power (W kg
1)

Small

High

High

Moderate

Moderate

Moderate

Cycle-number (#)

w1000

>1000

Moderate

Poor

Moderate

>2000

Safety

Moderate

Good

Moderate

Moderate

Moderate

Safe

Price

Expensive

Cheap

Expensive

Expensive

Moderate

Moderate

e

e

Table 2.9 Overview about the LCO system [15] Wh kgL1

Wh LL1

3.90

534

2723

LixC6 / x Liþ þ x e
þ 6 C

0.15

e

e

1LiC 6 2

3.75

375

1432

3.7e3.75

140e210

340e580

Discharge reactions and thermodynamic data (þ) Cathode:

Intercalation (electrochemical reduction): Li1
xCoIVO2 þ x Liþ þ x e
/ LiCoIIIxCoIV1
xO2 Li0.5CoO2 þ ½ Li / LiCoO2

(
) Anode:

Cell reaction:

E0 (V)

versus Li:

Deintercalation (electrochemical oxidation)

þ Li0.5CoO2 / 3 C þ LiCoO2

Typical data of commercial batteries:

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Performance data: rated cell voltage 3.7 V; operating voltage 3.5e4.3 V versus Li/Liþ; long-time stability above 500 cycles at a capacity retention of 80e90%; specific capacity 140e160 Ah kg
1 (thermodynamic 274 Ah kg
1). State-of-the art: mature and safe system for consumer electronics, although cobalt is considered harmful to health and the environment. Electrode reactions: During discharging, the cobalt oxide electrode incorporates lithium-ions by intercalation, which is released again during charging. Overcharge above 5.2 V generates cobalt dioxide (LiCoO2/ Liþ þ e
þ CoO2), which tends to decompose to lower oxides under oxygen release. There is a fire hazard and explosion risk due to the organic electrolyte! Improved electrodes: Doping by charge-transporting holes improves the electrical conductivity and storage capacity of LCO. Aluminium improves the stability against oxygen release. LiAl0.15Co0.85O2 provides a higher cell voltage and a specific capacity of 160 Ah kg
1. Doping by magnesium is harmful, because it creates unwanted secondary phases.

2.5.4.2 Lithium nickel oxides (LNO and NCA) By replacing the expensive cobalt by lower cost nickel, the layer lattice of lithium nickel oxide LiNiO2 (LNO) provides a 0.25 V less negative reduction potential (3.6e3.8 V versus LijLiþ) and 30% more specific capacity (w170 Ah kg
1) than LiCoO2. LNO is less thermally stable than LCO, whereby a certain

Figure 2.7 Linear, two- and three-dimensional lithium-ion mobility in (A) olivine, LiFePO4 (C Li), between PO4 tetrahedrons and FeO6 octahedrons, (B) layered structure of LiCoO2 (C Li) with Cobalt in the octahedron centres, (C) LiMn2O4-spinell (C Li) with manganese in the octahedron centres.

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safety risk exists in organic electrolytes, when the delithiated LNO structure collapses. Lithium-nickel-cobalt-aluminium oxide LiNi1
x
yCoxAlyO2 (NCA) has been commercialized in 3.7-V cell by Saft, Panasonic EV Energy, Matsushita and Lithium Technology, although the synthesis is laborious (3.7 V vs LijLiþ, 190e200 Ah kg
1). Doping with lithium fluoride improves reversible capacity and cycle stability. The moderate temperature stability of NCA can be improved by a coating of aluminium fluoride.

2.5.4.3 Solid solutions of manganese oxide Li(Ni,Mn,Co)O2 (NMC) The formation of mixed crystals by inert cations (Al, Ga, Mg, Ti) stabilizes the layered structure [25] of the lithium metal oxides LiMO2 (M ¼ Co, Ni, Mn). In order to utilize the redox couples FeIV/FeIII and MnIV/MnIII, soft synthesis methods are required using stacked LiFeO2 and LiMnO2 phases. Lithium manganese oxide Li1
xMnO2 (LMO) is decomposed in the attempt to charge is completely, because the inactive orthorhombic modification is more stable than the electrochemically active-material. During discharge (delithiation) below Li0.5MnO2, the spinel phase LiMn2O4 is formed, accompanied by a voltage drop. Lithium nickel-manganese-cobalt oxide LiNi1-x-yMnxCoyO2 (NMC) is generally liked for electric vehicles, because NMC cells provide high capacity and high voltage (3.2e4.2 V). However, at above 4.4 V cell voltage, accelerated ageing by a growing internal resistance is observed. Instead of the thermodynamic capacity of 274 Ah kg
1, only 66% of the available lithium in the NMC is used, providing 160 Ah kg
1, 3.7 V and about 592 Wh kg
1. The volume changes during charging and discharging amount to moderate 1e2%. Cobalt improves the conductivity; manganese is good for the stability even during charging above 4.4 V. Doping by aluminium works against oxygen release. Normally is used NCM333 (LiNi0.33Mn0.33 Co0.33O2). NCM532, NCM622, and NMC811 are also under development with higher capacity but lower stability. Despite decade-long research, NMC suffers from the irreversible capacity by the mixed occupation of nickel on the lithium sites. To avoid any collapse of the lattice, lithium cannot be completely extracted during charge. Novel approaches reduce manganese and cobalt below 25%, increase the mass fraction of nickel aiming at 190 Ah kg
1. NMC anodes can be combined with LMO cathodes. Lithium excess LiMnO3 for 5-V batteries [26]. Future solid solutions of Li2MO3eLiMO2 (M ¼ Ni, Mn, Co, Cr), mainly investigated by the Argonne National Laboratory, utilize inert placeholders of Li2MnO3 in order to stabilize the LMO lattice: Layeredelayered composites, xLi2MnO3 $ (1
x)LiMO2, and layered-spinel composites, xLi2MnO3,(1
x)LiM2O4. Overlithiated materials such as 0.3Li2MnO3, 0.7LiMn0.5Ni0.5O2 yield 250 Ah kg
1 at moderate power capability but poor cycle stability.

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2.5.4.4 Lithium manganese spinel LiMn2O4 (LMO) Lithium manganese oxide LiMn2O4 or Li1þxMn2
xO4 (x  0.8) forms a spatial network, in which manganese exhibits the thermodynamic valence of þ3.5, because Mn(III) and Mn(IV) coexist. During charging (delithiation), more Mn(IV) is formed. The phase changes can be observed as plateaus in the charge characteristics, until metastable g-MnO2 is reached above 4.4 V. Even at potentials above 5 V versus LijLiþ, lithium is not completely removed from the Mn2O4 lattice, so that fortunately no manganese dioxide is formed. Charge process :

LiMn2 O4

/ Li1
x Mn2 O4 þ x Liþ þ x e


Table 2.10 Overview about the LMO system [15] Wh kgL1

Wh LL1

4.00

474

2044

LixC6 / x Liþ þ x e
þ 6 C

0.15

e

e

0.8 LiC6 þ Li0.2Mn2O4 / 4.8 C þ LiMn2O4

3.85

346

1225

Discharge and thermodynamic data (þ) cathode:

Intercalation (electrochemical reduction) 0.8 Li þ Li0.2Mn2O4 / LiMn2O4

(
) anode:

Cell reaction:

E0 (V) versus Li:

Deintercalation (electrochemical oxidation)

An overview about the LMO system is given in Table 2.10. The working voltage during discharge amounts to 4.1 V versus LijLiþ; specific capacity is 100e120 Ah kg-1 lies 10e20% below that of LiCoO2. Preferably, the LMO spinel can be discharged at high currents (>5 C). LMO spinels are chemically and thermally stable, less expensive, inherently safety, nonpoisonous and environmentally friendly. Lithium-ions intercalate and deintercalate rapidly in the spatial network, i.e., the current capability is high. Solvent molecules cannot penetrate in the lattice. The volume change during cycling is the lower than that of layered structures. But LMO spinels do not provide high cycling and storage stability at elevated temperatures. The solid Mn3þ disproportionate: In short:

2 Mn3þ ðsÞ /

Mn2þ ðlÞ þ Mn4þ ðsÞ

Mn2þ in the liquid phase could migrate to the anode and poison it. Doped manganese spinels: The stability of Mn(III) is improved by a larger fraction of Mn(IV) by excess lithium; doping with aluminium, niobium or zirconium; replacing oxide by fluoride; surfaces coated with acid scavengers, an electrolyte additives that reduce Mn(II) solubility.

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Table 2.11 Overview about the LFP system [15] Wh kgL1

Wh LL1

3.45

586

2110

LixC6 / x Liþ þ x e
þ 6 C

0.15

e

e

LiC6 þ FePO4 / 6 C þ LiFePO4

3.30

385

1169

3.3

55e160

120e290

Discharge reactions and theoretical data (þ) Cathode:

Intercalation (electrochemical reduction): Li þ FePO4 / LiFePO4

(
) Anode:

Cell reaction:

E0 (V) versus Li:

Deintercalation (electrochemical oxidation)

Performance of commercial batteries

Spinels for 5-V batteries: Current research activities aim at higher currents, voltages and energies, whereby stable electrolytes turn out as challenges. Li1
x(M0.5Mn1.5)O4 yields 5.1 V versus LijLiþ using an additional discharge level. The spinel Li1
x(Ni0.5Mn1.5)O4 (LMNS) can be paired with graphite (4.8 V) or LTO (3.2 V). Li[Ni0.5Mn1.5]O4 (148 Ah kg
1) works at an average voltage of 4.75 V versus lithiated graphite, and forms Ni(IV) during discharge.

2.5.4.5 Lithium iron phosphate (LFP) Lithium transition metal phosphates based on J.B. Goodenough, A. Manthiram, A.K. Padhi and K.S. Nanjundaswam (1989, 1997) are considered as environmentally friendly and less expensive electrode materials for larger cells. LiFePO4 exhibits the spatial lattice of olivine which is known form the mineral triphylite. Lithium is able to move along a linear diffusion pathway (D1) between edge-linked, distorted octahedrons. LiFePO4 is prepared from LiOH, H3PO4 and Fe(NO3)3 at 700 C. During charging (deintercalation) the LiFePO4 forms FePO4, whereby the phases propagate by diffusion. During discharge, LiFePO4 provides an excellent flat plateau at about 3.4 V. Despite the thermodynamic capacity of 168 Ah kg
1, the material performs at least 14% less powerful than LCO. Favourably, the mechanical stresses in the olivine structure remain low during cycling. LiFePO4 behaves chemically and thermally stable, proves cycle stability, is inexpensive and nonpoisonous. It withstands temperatures up to 300 C without decomposition, and endures overcharge and short-circuits. It does not release oxygen and does not tend to thermal runaway at temperatures < 300  C. An overview about the LFP system is given in Table 2.11. Improvement of conductivity: The low conductivity of the olivine (<10
9 S cm
1) is improved by shorter diffusion pathways and coatings: highly conducting carbon black or carbon nanoparticles, surface layers of FePx or Fe3O4, or LiFePO4 nanoparticles.

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Doped lithium iron phosphate for 5-V batteries: The incorporation of manganese, cobalt or nickel improves working voltage and energy density. The voltage plateaus of Co3þ/Co2þ at 4.8 V and Ni3þ/Ni2þ at 5.1 V versus LijLiþ require novel electrolyte systems. Li1
xMnPO4 (LMP) yields 4.1 V versus LijLiþ with an excellent flat voltage plateau at the expense of moderate conductivity. The manganese-rich material LiFe0.15Mn0.85PO4 (LFMP) proved 150 Ah kg
1, 4.0/3.4 V, and about. 590 Wh kg
1. Olivine fluorides for 5-V batteries. Lithium-rich materials are able to store more energy, if they are able to use two redox-active lithium ions per molecule. The natural mineral tavorite LiFeIII(PO4)(OH) suggests isostructural compositions of the kind LiM(YO4)X, which combine a redox-active metal M, and a p-block element Y ¼ P, S, and X ¼ O, OH or F. The material Li2FePO4F yields 3.6 V and 115 Ah kg
1. Li2Mn(PO4)F impresses with thermodynamic values of 1218 Wh kg
1 and 3708 Wh L
1. Iron-manganese phosphate Li2MP2O7 (M ¼ Fe, Mn, Co) might theoretically be capable of cycling the seconds lithium atom at voltages above 5 V, although no appropriate electrolytes are known at present. The natural II II mineral arrojadite, AI3BCIII 2 Al(Fe , Mn )14(PO4)12, x (H2O,OH) with A ¼ Na, K; B ¼ Ca, Sr, Ba, Pb, Fe; C ¼ Al, Fe, is able to ingest lithium ions in parallel anionic channels of 50 pm widths.

2.5.4.6 Advanced materials in development The development of new materials [27] faces higher energy and power at longer service life and lower cost. Future 5-V electrode materials require completely new electrolytes with low resistance. Electrode potentials above þ4.3-V (cathode) and below þ1-V versus LijLiþ (anode) lie outside the stability window of conventional electrolytes. Electrolytes for future 5 Vbatteries are, however, still unknown. Thin-film electrodes with high surface area and improved lithium-ion intercalation will allow more power and higher capacity. Stabilizing additives are intended to improve the passivation layer between electrode and electrolyte (SEI).

2.5.5 Electrolyte 2.5.5.1 Organic liquid electrolytes Present lithium-ion battery electrolytes are mixtures of 20e50% ethylene carbonate and organic carbonates (DMC, DEC, EMC) or esters (EA, MB), and a conducting salt, and preferably few additives. Conducting salt. Mostly, complex salts such as LiPF6, LiBF4 and LiClO4, having stable anions are used to provide lithium ions and to guarantee the ionic conductivity of the electrolyte. Lithium sulfonylimide (LiTFSI) and

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oxalatoborate (LiBOB) have been known as more recent alternatives for the common LiPF6. Fluororalkylphosphates promise advantages for 5-V batteries. But still LiPF6 is the optimal conduction salt. Lithium hexafluorophosphate (LiPF6), used in commercial lithium-ion cells, tends to decompose in the presence of traces of water: HF þ LiF þ POF3; and LiPF6 þ 4H2O / 5 LiPF6 þ H2O / 2 HF þ LiF þ H3PO4. As well, the complex anion forms PF3 and PF5, which corrode the aluminium current-collector, whereby a protective layer of aluminium fluoride is formed. 0.75 mol L
1 LiPF6 in EC/EMC (1:1) conducts 9.7 mS cm
1 at 25 C, and is useful between
20 C and 60 C. 1 mol L
1 LiPF6 provides 5.8 mS cm
1 (in propylene carbonate (PC)) and 10.7 mS cm
1 (in EC/DMC 1:1), is oxidized above 4.65 V versus LijLiþ and is thermally decomposed at 70 C. Solvents support the dissociation of the conducting salt by reducing the Coulomb forces within the salt. Mainly at the negative electrode the solvent is decomposed during the first charge (formation process), whereby a passivation layer is formed, the so-called SEI [28]. Viscous solvents decompose slower than mobile liquids, but their conductivity is bad. 1. Ethylene carbonate (EC) e in contrast to PC e forms a protective layer of presumably lithium ethylene dicarbonate on the graphite surface. It is useful in the potential window between þ0.9 V versus LijLiþ (at glassy carbon) or þ0.8 V (at graphite) and þ6.2 V versus LijLiþ (at glassy carbon). Lithium hexafluorophosphate (LiPF6) provides good lithium intercalation in organic carbonates. 2. PC is oxidized above þ6.6 V versus LijLiþ, but it tends to co-intercalate into graphite, whereby the material is destroyed by volume changes. PC is used in combination with electrodes of petrol coke and amorphous carbon. Electrolyte additives [29] are designed to prevent the unwanted evolution gases by electrolysis and the loss of capacity at the first chargeedischarge cycle. 1. SEI improvers as vinylene carbonate and vinylethylene carbonate passivate the anode (
) by forming a layer of polyvinylidene carbonate, which additionally includes inorganic components. 2. Protection of the cathode (þ) by additives which capture water and acid impurities and prevent that the oxidation of the carbonate solvent and the evolution of CO2. Tributylamine, silicones and dimethylacetamide neutralize acid impurities. Ethers such as 12-crown-4 dissolve preferably lithium-ions and prevent the co-intercalation of PC in graphite. 3. Stabilizing agents retards the thermal decomposition of the LiPF6 salt and weaken the reactivity of PF5 in organic solvents.

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4. Overcharge protectors bring about the highly reversible self-discharge at oxidation potential shortly below the decomposition voltage of the electrolyte. Unfortunately, this works to the disadvantage of cell power. Anisole derivates such as 2,5-di-tert-butyl-1,4-dimethoxybenzene (oxidation >3.9 V vs LijLiþ) and 1,4-Difluoro-2,5-dimethoxybenzene (>4.2 V) work as redox systems of poor long-time stability. 5. Shutdown additives such as biphenyl or cyclohexylbenzene isolate the positive electrode by a polymer layer, which grows up at unwanted high voltages, while hydrogen is simultaneously evolved at the negative electrode. 6. Fire-retardant additives produce a heat insulating carbon layer on the battery and terminate the radical reactions that are necessary for combustion. 7. Lithium deposition improvers prevent the growth of needle-shaped dendrites and spongy lithium at the passivation layer (SEI) during the deposition of lithium on the graphite electrode (
). Fluroroethylene carbonate is useful, because it is decomposed to vinylene carbonate and hydrogen fluoride; unfortunately at the expense of cycle efficiency and selfdischarge.

2.5.5.2 Ionic liquids The conductivity of ionic liquids e salts that are molten at room temperature e is at present not good enough for the use in commercial lithium-ion batteries. The 1-ethyl-3-methylimidazolium cation (emim) and the sulfonylimide anion (TFSI) provide 15 mS cm
1 at 25 C. Ionic liquids are viscous, sensitive to water and little stable during chargeedischarge cycling. By adding a conducting salt, viscosity and conductivity are further deteriorated by the formation of crystalline mixed salt phases. The salt LiTFSI achieves 0.1 mS cm
1 at 50 C in methylimidazolium-(CH2)3SO3. Ionic liquids are too expensive at present. Up to now systems with ionic liquids have not been commercialized.

2.5.5.3 Solid electrolytes Inorganic solid electrolytes utilize Liþ ions as the solely mobile species, i.e., the transference number is close to 1, when they move through vacant or interstitial sites in a crystalline or glassy matrix. Unfortunately, the ionic conductivity of these materials is such low that they are not yet widely used for powerful all-solid-state batteries (see 2.6.3). Lithium lanthanum zirconate, Li7La3Zr2O12 (LLZO), achieves a conductivity of 10
4 S cm
1. Garnets Li5La3M2O12 (M ¼ Nb, Ta) show high stability against lithium metal at voltages up to 6 V. Amorphous lithium phosphorous oxynitride (LiPON, Li2.9PO3.3N0.46 made by sputtering of LiPO4 in N2 atmosphere) is an electronic insulator and a glassy electrolyte, which exhibits a lithium ionic conductivity of 3.3,10
6 S cm
1 at 25 C and a good electrochemical stability at cell

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potentials up to 5.5 V versus lithium. The sulfide Li3.25Ge0.25P0.75S4, the most stable member of the thio-LISICON (Lithium super ionic conductor) family, has an ionic conductivity of 0.0022 S cm
1 at 25 C. Polymer electrolytes [30] are mostly based on solvent-free polyethylene oxide (PEO) and a highly conductive salt (such as LiN(CF3SO2)2, LiTFSI) with a large electrochemical window that forms an eutectic composition. The ionic conductivity of the ‘high-molecular-weight dry polymer’ (10
6.10
4 S cm
1 at 25 C) is directly linked to the degree of amorphicity and the lowest possible glass transition temperature controlling the ion mobility. Side chains with solvating groups increase the degree of freedom and improve ionic conductivity, but compromise the mechanical properties. Aligning or organizing the polymer chains enhances ionic conductivity; liquid crystalline chain polymers reach liquid-like conductivity values on heating (or when kept under polarization) that are retained upon cooling to room temperature. The poor ionic conductivity of crystalline complexes PEO:LiXF6 (X ¼ P, As, Sb) can be raised by a factor of 100 by partial replacement of the XF6 ion with other mono- or divalent anions. Hybrid polymer electrolytes contain plasticizers to act as chain lubricants in the solid polymer: Gels contain 60e95% liquid electrolyte, and are 2e5 times less conductive than liquid solutions. PEO-based gels achieve ionic conductivities of more than 0.001 S cm
1. Nanoparticle fillers such as aluminium oxide (Al2O3), titanium dioxide or silica (SiO2) to PEO increase the conductivity several fold, prevent crystallization for several weeks, and increase apparent lithium transport number (PEO20LiClO4/8% Al2O3: from 0.31 to 0.77; PEO20LiBF4/ZrO2 film: from 0.32 to 0.81).

2.5.6 Separator The separator [31] in liquid electrolyte batteries has to prevent electronic contact of the electrodes and enable free ionic transport. It must guarantee the safe insulation of the electrodes, even under abusive conditions. The separator material must be chemically stable, about 25 mm in thickness, and exhibit a porosity below 1 mm, in order to hold sufficient liquid electrolyte and to prevent penetration of particles from the electrodes. To ensure a uniform current distribution and to prevent the growth of dendrites on the negative electrode, the permeability of the separator must be uniform. The resistance of a separator to ion transport depends on the thickness, porosity and tortuosity. Polyolefin separators made by the dry process (e.g., CELGARD, PPePEePP) have an open and straight pore structure, whereas separators from the wet process (e.g., Exxon Mobile ‘Tonen’, PE) show a tortuous pore structure. Inorganic fillers may favour wettability and liquid retention. Separators as thin as 10 mm are being used to optimize cell energy density.

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LeadeAcid Batteries for Future Automobiles

Non-woven separators are mats of numerous natural or synthetic fibres bonded together by resins or thermoplastic fibres. As thicknesses of 20 mm and less cannot be easily produced and owing to the rough surface, nonwoven separators have only been used as the supporting framework to make gel polymer electrolyte batteries. Inorganic composite separators, containing ultrafine metal oxide or carbonate particles in a polymer matrix, provide improved wettability for alkylene carbonates, g-butyrolactone and other organic solvents. They show good thermal stability. For safety reasons, more and more inorganic layers (Al2O3, SiO2, TiO2 etc. with a binder) are coated on one or both sides of the polyolefin or nonwoven separators. Degussa’s ‘Separion’ combines a flexible perforated nonwoven polymer mat polyethylene terephthalate (PET) with porous ceramic coatings (Al2O3/SiO2) on both sides. As an inorganic binder sol to suspend aluminium oxide powders on a PET substrate.

2.5.7 Cell and battery design Cylindric cells (18650) 18 mm diameter and 65 mm length contain a cylindrical sleeve of anode, separator, cathode and separator. Advantages: fast production, relatively cheap, high-energy density, high safety due to low cell energy. Drawbacks: low volume utilization (w50%), high connection expense, failure probability. There is a trend towards larger standard cylindrical cells: 21700 (Samsung SDI), 20700 (Panasonic) and 20650 (LG Chem). Prismatic cells: contain a flat sleeve or a stack of single units in a rectangular case or a lamination. They offer good mechanical stability due to the container, good volume utilization (w75%) but relative expensive manufacturing. Pouch cells (coffee-bag): The flat embodiment provides high specific energy due to a light container, good volume utilization (w75%), flexible and relative cheap production. Drawbacks: low mechanical stability, safety issues (membrane, current interruption), difficult integration into the cell. Pouch cells are called as well ‘Li-polymer (LiPo)’ cells, whereas ‘polymer’ is related to the case material and not to a polymer electrolyte. For EV batteries all designs are used. Tesla is using >7000 of cylindrical 18650 cells in its Tesla S vehicle to save costs because the 18650 cells are commodity. Most automakers prefer the prismatic and the pouch type because of higher energy density. High specific energy lithium-ion polymer electrolyte batteries (180e210 Wh kg
1) have been used in hybrid electric vehicles (HEVs), and deliver

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moderate rates of up to 10 C pulses. Saft has demonstrated very high power (VHP) lithium-ion technology capable of 8 kW kg
1 (for 2 s) and 12 kW kg
1 for a millisecond) in the 270 V emergency battery for an F-35 aircraft. Recently, Lithium Technology Corporation (LTC) has developed lithium-ion cells with a capacity of 500 Ah and an energy content of 1.8 kWh in a single cell. In the all-solid-state thin-film battery, solid films of negative electrode, solid electrolyte and positive electrode are constructed sequentially on a substrate. Both electrodes are capable of reversible lithium insertion. When a discharging current flows, the lithium ions in the negative electrode migrate to the positive electrode. Lithium metal is deposited mostly by vacuum thermal vapour deposition; solid electrolytes and oxide electrodes are prepared by radio frequency (RF) sputtering, RF magnetron sputtering, chemical vapour deposition (CVD), electrostatic spray deposition (ESD), pulsed laser deposition (PLD), and solegel processes. The electrolyte may be LiPON or a lithium glass (e.g., Li3.6Si0.6P0.4O4, Li6.1V0.61Si0.39O5.36, Li2SO4/ Li2O/B2O3, Li2S/SiS2/P2S5, LiI/Li2S/P2S5/P2O5).

2.5.8 Performance data, life and ageing Lithium-ion batteries offer specific advantages over conventional batteries, which, however, are in contrast to some shortcomings (Table 2.12). Electric vehicle propulsion requires high-energy density and temporary power capability, whereby the batteries should be able to be charged within short time. Specific energy. The thermodynamic value Wm of the cell is related only to the active-materials: Wm ¼

W
DG zFU0 ¼ ¼ ¼ Q $ DE m M M

Q denotes the specific electric charge, which is delivered by the molar masses M of electrode materials. In practice, roughly only one fourth of the thermodynamic value is obtained because of the incomplete mass utilization, and additional weight of electrolyte, separators, collectors, leads, cell container and other passive components. Specific power is limited by the internal resistance of the cell, comprising electrolyte resistance and the overpotential of the electrode reactions. and internal resistance, Electric power, P ¼ UI ¼ U0
IRi ,
Ri ¼ Re þ I hþ þ h
, as a measure of the overpotential h at the anode (
) and the cathode (þ), give with reference to the mass m of the cell: Pm ¼

P UI I 2 Ri ðU0
hþ
jh
j
IRel Þ$I ¼ ¼ ¼ m m m m

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Table 2.12 Specific characteristics of lithium secondary batteries Advantages

Shortcomings

Performance data 

   

High cell voltage (3.0.4.2 V), specific energy (90e240 Wh kg
1, 200e500 Wh L-1 on cell level) and specific power (up to 500 W kg
1) High discharge rate (40C); fast charge (<3 h); useful power >80% DoD More than 1000 cycles; deep cycling possible; coulombic efficiency almost 100%. Low self-discharge rate (5e10% per month, 20 C) No memory effect; no reconditioning needed; tolerates micro-cycles.

  

 

Chemical reactivity; stability of the chemicals Higher internal impedance than nickelecadmium Degradation at high temperatures and at discharge <2 V; capacity loss or thermal runaway when overcharged Temperature range:
20. þ 60 C Venting and thermal runaway when crushed

Cell design   

Low weight; very small batteries and high-capacity available. Can be optimized for capacity or rate. No free liquid electrolyte; gelled electrolytes and solid state chemistry available

  

Safety precautions and protective circuitry Stricter regulations on shipping Preferred charge method: Constant voltage, constant current

Applications and cost From consumer electronics to electric vehicles

More expensive than leadeacid

High-power cells are realized by using extra thin electrode layers (50.100 mm) and separators (<25 mm). High specific energy is obtained by electrodes having up to 150e200 mm thickness. The internal resistance Ri of a lithium-ion battery is nearly constant at medium values of state of charge, but increases at the fully charged and fully discharged state. As Ri increases at low temperatures, conventional Li-ion systems and especially polymer systems cannot be used for power applications at deep temperatures. Power-to-energy ratio. Typical lithium-ion cells have an average cell voltage of 3.6 V; therefore, a single lithium cell is able to replace three nickelemetal-hydride cells. The specific energy exceeds 3e4 times that of a leadeacid battery. Commercial lithium-ion cells for portable applications offer specific energy up to 250 Wh kg
1 (650 Wh L
1) [32] and specific power up to 1500 W kg
1 (for 20 s); a power-to-energy ratio of around

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LeadeAcid Batteries for Future Automobiles

Wm/Pm z 6. The Ragone plot (see Fig. 2.1) illustrates specific energies of 160e190 Wh kg
1 for lithium-ion cells in applications that require low specific power at small currents. At medium to high-power demands, specific energy declines to 50e90 Wh kg
1 for a single cell. These values worsen further in battery modules by about 25%e33%. Some high-power cell designs allow charging in less than 5 min to 80% (SoC; available energy for discharging divided by the total stored energy), i.e., at a C-rate of 10 Ce15 C. Furthermore, discharge at high rates (40 C) is able to provide boost power for accelerating hybrid vehicles. In practice, the SoC range used, determines the energy of the battery that is actually available; e.g., between SoC 90%e25%, only 65% of the stored energy is used, but also SoCs between 95% and 15% are possible. Efficiency. The electrical efficiency is defined as the product of voltage efficiency and coulombic efficiency, hel ¼ hU hC. The low internal resistance and the lack of significant side-reactions result in coulombic efficiencies (capacity discharged over capacity charged) of almost 100%. The voltage efficiency (average voltage during discharging over average voltage during charging) determines the electrical efficiency, which increases with rising temperature from about 90% (at 0 C) to about 98% (at 40 C). With rising discharge current, electric efficiency falls caused by increasing electrode polarization. Efficiency depends on the charge factor (reciprocal of coulombic efficiency), depth-of-discharge, current, time on float charge, time of discharge and other quantities. Lithium secondary cells can be deepcycled close to 100% capacity, and full power can be delivered down to 80% depth-of-discharge (DoD; discharged electrical energy divided by the total stored energy) e in contrast to 50% for leadeacid. Hence, the stored energy can be used more effectively than in any other battery. Round-trip energy efficiency (energy supplied on discharge over energy used to charge) is more than 90%. Chargeedischarge characteristics. Fig. 2.8A reveals four typical regions: 1. Lithium-ion batteries are charged at CC preferably. After the initial voltage jump, which is mainly caused by the electrolyte resistance, cell voltage increases continuously. Charge rates of 1C are usual, as long as the negative electrode is not plated with lithium at too high rates, which leads to rapid cell capacity fade. 2. At the end of charging, the flowing current turns to almost zero (leakage current), and the cell voltage reaches a constant value at medium SoC. The final charging up to the end-of-charge voltage is done at CV preferably. 3. The discharge of the completely charged battery at CC is coined by an initial voltage drop, which is mainly caused by the electrolyte resistance (IR drop, UR ¼ I,Re), followed by a more or less exponential voltage decay.

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LeadeAcid Batteries for Future Automobiles

(A)

(B) Charge

4.2

Overcharge Discharge

4.0 Cell voltage (V)

E vs. Li|Li+ (V)

4.4 4.2 4.0

3.8

0.5 C 1C 2C 3C 5C 8C

3.6 3.4 3.2 3.0

3.8

2.8 2.6

3.6 0

5

10 t (h)

15

0

2

4

6 8 10 12 14 16 18 Capacity (Ah)

Figure 2.8 (A) Chargeedischarge characteristics of a lithium-manganese spinel half-cell in the presence of a redox shuttle additive (Source: ZSW). (B) Practical discharge characteristics of a lithium-ion battery (KOKAM 17 Ah, 3.7 V) at different C-rates.

The maximum allowed discharge rates are generally higher than the charge rates. 4. At the end of discharge, the load current turn to nearly zero, and cell voltage arrives at a constant value that corresponds to the lower SoC. The cut-off voltage of about 2.3 V must prevent irreversible damage of the active-material and dangerous reactions during recharge. The discharge characteristics (cell voltage vs discharged capacity) trends towards lower cell voltages at higher C-rates (current ¼ C-rate  rated capacity). Available capacity is lost at very high discharge rates and at low temperatures (Fig. 2.8B). Self-discharge is in the same range as for leadeacid batteries. Lithium-ion batteries retain a single charge for up to several weeks: about 0.5% per week (at 20 C), 2% per week (at 40 C), and 4...10% per week (at 60 C). The percentage of capacity retention versus storage time follows approximately an exponential function, i.e., the fully charged cell discharge more rapidly than partly discharged cells. In ‘smart’ lithium-ion batteries, stored for long periods, self-discharge is caused by the small constant drain of the built-in voltage-monitoring circuits that run constantly. At lower SoC the selfdischarge rate is reduced (see below the storage section). Life. The self-discharge reactions reduce both calendar-life (shelf-life) and cycle-life by the growth of an SEI on the negative electrode of lithium-ion cells. Regardless of whether and how it was charged and discharged, an

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old battery will deliver less energy than a virgin battery. Lithium-ion batteries degrade at high temperatures, and they lose capacity when they are overcharged. Ageing [33,34] causes the internal resistance to rise, i.e., the terminal voltage drops under load and less high currents can be withdrawn from the battery. Nevertheless, cycle-life of lithium-ion batteries exceeds that of most other commercial batteries. Sometimes the degradation curve displays a more or less clearly visible change point, where the rate of degradation begins to increase more rapidly (whereby several ageing mechanisms of different velocity converge). There are five main ageing processes that result in reduced capacity as well as power loss: 1. The SEI, the necessary surface film on the anode, grows excessively by consuming capacity determining lithium ions. 2. Solvent molecules may intercalate, together with the lithium ions, into the electrode material and damage the crystal structure and prevent further intercalation of lithium ions. 3. The electrolyte may decompose at a rate depending on temperature and terminal voltage. 4. During cycling, structural changes might occur in the electrode material and the integrity of the electrodes might be compromised owing to the continuous expansion and contraction of the active-materials. 5 Li-plating at deep temperatures and high charge currents. The optimum operating temperature for minimum ageing is in the region of 20 Ce30 C [35]. Storage. Lithium-ion batteries should ideally be stored below ambient temperature, but they should not be frozen (w
40 C) and should not be stored fully charged. A fully charged lithium-ion battery will lose approximately 6% capacity at 0 C, 20% at 25 C, 35% at 40 C and 40% at 60 C per year. When stored at about 50% charge level, the capacity loss is 2%, 4%, 15%, and 25% per year at 0, 25, 40 and 60 C, respectively. Furthermore, high and low state-of-charge must be avoided. In the fully charged state chemical energy might be converted to thermal energy in case of short-circuits. At very low SoC the anode potential can rise up to the dissolution potential of the copper current-collector. The Cu2þ ions migrate into the separator where they form metallic copper during the next charge, leading to shortcircuits. Therefore, for storage a ‘self-discharge reserve’ of about 35% is necessary.

2.5.9 Cost It is difficult to give a reliable number for the different types of industrial batteries [36]. The leading manufactures offer lithium-ion packs

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for electric vehicles at prices between US$300 and US$600 (Fig. 2.9). The goal of US$150 per kWh, which is commonly expected beyond the year of 2020, will bring a bright commercialization of battery vehicles [37].

2.6 Power sources after Lithium-ion 250 Wh kg
1 (battery) or 350 Wh kg
1 (cell) might be the practical upper limit of the lithium-ion technology, which is required for a sustainable market penetration of electric vehicles (DoE). New approaches are necessary. Some research activities reach back to the oil crisis in the 1970 and 1980s, which promise visionary concepts for future storage batteries. Independent experts expect the commercialization of post lithium-ion systems such as LieS not before 2025, and of Li-Air after 2030. Therefore, the concepts described in the following reflect more or less mature ideas of experimental batteries. All post Li-ion systems suffer from relatively low specific power, which prevent direct use in battery and hybrid vehicles. Supercapacitors are discussed as power boosters in battery hybrid systems. A material road map for Li-ion and post Li-ion developments is given in Fig. 2.10.

2,000

95% conf interval whole industry 95% conf interval market leaders Publications, reports and journals News items with expert statements Log fit of news, reports, and journals: 12 ± 6% decline Additional cost estimates without clear method Market leader, Nissan Motors, Leaf Market leader, Tesla Motors, Model S Other battery electric vehicles Log fit of market leaders only: 8 ± 8% decline Log fit of all estimates: 14 ± 6% decline Future costs estimated in publications
2014 US$ per kWh

1,500

1,000

500

0 2005

2010

2015

Year

2020

2025

2030

Figure 2.9 Estimated cost projection for lithium-ion batteries . After B. Nykvist, M. Nilsson, Nat. Climate Change 5 (2015) 329.

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2000

2005

2010

2015

2020

NMC/NCA

LCO

CATHODE

2025

LiNiPO4, 5v LiNiMMO2 High voltage LiCoPO4, 5v

LMO

2030

5v spinal Sulfur

Air

LiMnPO4, 4v

LFP

C/Alloy Composite Graphite

ANODE

Hard Carbon

ELECTROLYTE

LiPF6 + Org.solvents

Soft Carbon Li4 Ti5O12

Li Metal Si Alloys

Non Si Alloys

LiPF6 free Gel-polymer electrolyte electrolyte

5v electrolyte Polymer Solid membrane Electrolyte

SEPARATORS

Polyolefin

Polyolefin + Cellulose ceramic coating Non-woven

Figure 2.10 Material development road map for lithium batteries according to Ch. Pillot (Avicenne Energy, France).

2.6.1 Lithiumesulfur battery Lithium sulfide (Li2S8) provides much higher energy density than lithium-ion metal oxide chemistries. Specific energy is estimated at 2600 Wh kg
1 (thermodynamically) and about 350 Wh kg
1 (in practice). But the energy density is in the same order w350 Wh/l and therefore not larger as lithiumion systems. Cell chemistry. The lithiumesulfur battery [38] consists of a lithium anode (
), and a sulfur cathode (þ), Fig. 2.11A. During discharge soluble lithium sulfides are formed, and Li2S is deposited on the carbon matrix of the positive electrode. During charging, Li2S does not bring back sulfur, but forms polysulfide anions [Sx]2
which diffuse through the electrolyte as a shuttle and leads to self-discharge. ðþ Þ cathode:

S þ 2 Liþ /Li2 S þ 2 e
ðsulfur is reduced during dischargeÞ S8 /Li2 S8 /Li2 S6 /Li2 S4 /Li2 S3 /Li2 S2 /Li2 S

ð
Þ anode: 2 Li/2 Liþ þ 2 e
Self-discharge S8 /Li2 S8 /Li2 S4 /Li2 S3

At the sulfur cathode, between S8 (fully charged) and the formation of Li2S, different reduced species occur depending on the depth of discharge: Li2S8 at 12.5% DoD (2.4 V), Li2S4 at 25% DoD (2.2 V), Li2S2 at 50% DoD, and finally Li2S at 100% discharge (2.05 V). The chemical reaction proceeds more and more into the sulfur grain with rising DoD. The products and their potential

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LeadeAcid Batteries for Future Automobiles

(A)

(B)

Charge

1

2

E vs. Li/Li+ / V

2.4

Discharge

e–

Li2S

Lithium

+

S2–

Discharge

[Sn]2–

Li2Sn Li+ Charge

S8

S8 Li2S8 Li2S4

Li2S2

Li2S

0% 12.5% 25%

50%

100%

Sulfur / graphite

+

– Li

3

2.1

Depth-of-discharge (%)

S8

S8

Li2Sx

Li2S to Li2S4

Li2S

Figure 2.11 Lithiumesulfur battery: (A) cell design and electrode reactions causing volume changes during cycling. (B) Potential steps during discharge.

are given in Fig. 2.11B. The cell voltage equals only 2.1 V, but lithiumesulfur cells tolerate overvoltage. Challenges [39]. As sulfur is an insulator (5,10
30 S cm
1 at 25 C), it must be incorporated into an electronically conducting structure such as carbon (powder or multi-walled nanotubes) with the help of a polymer binder. The sulfides Li2S2 and Li2S are insoluble in the electrolyte, and cause a passivation layer on the electrode surface. Therefore, the depth of discharge must be limited in practice. Technical measures, such as protecting layers on the lithium electrode, membranes instead of porous separators, gel electrolytes and solvents with reduced solubility for sulfides, represent attempts to reduce self-discharge. Lewis acids such as BF3 have been found to suppress polysulfide formation. Concerning the electrolyte [40,41], a mixture of tetraethylene glycol dimethylether (TEGDME) and 1.3-dioxolane has been found to be appropriate to form a stable SEI. The dramatic capacity fade at low temperatures (w1300 Ah kg
1 at 20 C to 360 Ah kg
1 at
10 C) can be alleviated by adding 5% methyl acetate. Cycle-life is determined by the electrolyte volume, which must be prevented from drying-out at the expense of specific energy. Only at an electrolyte/ sulfur ratio E/S  3, high-energy density cells are possible but with low lifetime. An increase of the E/S ratio leads to higher lifetime but reduced energy density [42]. The metallic lithium anode poses still a problem of dendrite growth. Research activities focus novel lithium tinecarbon alloys. Prototypes [43]. Lithiumesulfur batteries by Sion Power (2.2 Ah) have provided specific energies above 350 Wh kg
1 (350 Wh L
1) but with low cyclelife and high self-discharge.

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2.6.2 Lithiumeair battery A future option of energy storage is given by the lithiumeair system [44] in an organic or aqueous electrolytes [45]. The specific energy is estimated to thermodynamic values of 3505 Wh kg
1 (nonaqueous electrolyte) and 3582 Wh kg
1 (aqueous electrolyte) and about 400e500 Wh kg
1 for practical values [46]. The technology can be divided into aprotic lithiumeair, aqueous lithiumeair, solid lithiumeair and mixed aprotic/aqueous submersible lithiumeair systems (Fig. 2.12). Cell reactions. The anode consists of lithium metal, the cathode is a gas diffusion electrode made of porous carbon and coated with an oxygen reduction catalyst. The discharge reactions read: ð
Þ anode: ðþÞ cathode:

APROTIC

Lithium Metal Artificial Interface

O2

Li2O2 Reaction Products Air Cathode

Lithium Metal

Air Cathode

O2

MIXED AQUEOUS/APROTIC

Lithium Metal Solid-Electrolyte Interface

Polymer-Ceramic A

Polymer-Ceramic B

Aqueous Electrolyte + Soluble Reaction Products Air Cathode

SOLID STATE

Glass-Ceramic

O2 þ 2e
þ H2 O/OH
þ HO
2 2 Liþ þ 2 OH
/2 LiOH

AQUEOUS

Lithium Metal Solid-Electrolyte Interface Aprotic Electrolyte

2 Li/2 Liþ þ 2 e


O2

Li+ Conducting and Hydrophobic Membrane Aqueous Electrolyte + Soluble Reaction Products Air Cathode

O2

Figure 2.12 Different Lithiumeair battery Systems. Modified from G. Girishkumar, B. McCloskey, A.C. Luntz, S. Swanson, W. Wilcke, J. Phys. Chem. Lett. 1 (2010) 2193e2203.

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Thermodynamically, the oxygen reduction runs at a potential of þ1.229 V SHE, and lithium oxidation at
3.045 V SHE, so that open cell voltage equals DE0 ¼ E0cathode
E0anode z 4.27 V. In bold visions, dissolved oxygen might be fed from seawater, providing 3.79 V at pH 8.2. 1 2 Li þ O2 þ H2 O/2 LiOH 2 1 2 Li þ O2 þ 4 Hþ /4 Liþ þ 2 H2 O 2

ð3:45 V; pH 14Þ ð4:27 V; pH 0Þ

In nonaqueous electrolyte, the oxidation of dry lithium runs as the main reaction. The hydrolysis of water is usually unwanted, although 2.56 V are obtained in a seawater battery (pH 8.2). 2 Li þ O2 1 2 Li þ O2 2 2 Li þ 2 H2 O

/

Li2 O2

/

Li2 O

/

2 LiOH þ H2

ð2:97 VÞ

ð2:22 V; pH 14Þ

In practice, the kinetically inhibited reactions at the oxygen electrode require high charging voltages, but delivers too low discharging voltages, i.e., efficiency, specific energy and power are limited. At carbon, oxygen is reduced rather inefficiently, even at low current densities, e.g., Uout/Uin ¼ 2.5 V/4.5 V z 55% at 0.04 mA cm
2). A carbon-supported PtAu catalyst improves efficiency to Uout/Uin ¼ 2.7 V/3.7 V z 73% [47,48], although commercial cells will not employ such expensive material. Water-stable electrodes. Recent ideas suggest solid electrolyte protected lithium electrodes (PLE) in protic or aprotic solvents. Aqueous electrolytes avoid clogging of the oxygen electrodes, but require a Liþ conducting interlayer on the lithium metal electrode, and additionally a protective solid electrolyte of lithium nitride, lithium metal phosphate or sulfide glasses. Challenges [49]. Aqueous lithiumeair chemistry has been designed so far for primary batteries. The lithiumeair system requires detailed research to increase reversibility and to reduce dendrite growth. Present lithiumeair prototypes withstand about 100 chargeedischarge cycles, whereby the capacity fade is tremendous. Yet unsolved problems remain: (1) the evaporation of the solvent at the air electrode, (2) the water-input caused by humid oxygen, (3) the irreversible formation of lithium carbonate by CO2, (4) the slow oxygen transport to the air electrode and (5) clogging of the air electrode pores by lithium peroxide. The already mentioned low specific power and efficiency amounted to 60% are further challenges.

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2.6.3 All-solid-state lithium batteries Solid-state batteries are an emerging option for next-generation traction batteries promising low cost, high performance and high safety [50,51]. Liquid electrolytes with high ionic conductivity (w10
3 S cm
1 at room temperature) and practically no electronic conductivity, perform effectively over a wide temperature range (from few tens of degrees below 0 C to about 100 C). But they pose disadvantages: high flammability, highly resistive SEI at the electrodes leading to capacity loss, electrolytic decomposition at high voltages limiting the use of high voltages cathode materials, formation of HF at thermal runaway, and risk of leakage. Solid electrolyte lithium-ion cells do not show these drawbacks and allow higher operating temperatures due to better thermal stability. Due to higher electrochemical stability, high potential cathodes and even metallic Li may be used as anode leading to a higher specific energy. However, lithium melts at w180 C. State-of-the-art. At present, the solid state technology concentrates on small cells due to the production costs. The coin cell (20 mm diameter, 1 mm thick, 85 mAh) of Infinite Power Solutions, Inc., and Sakti3 reached energy densities above 1000 Wh L
1 [52]. About 20 companies worldwide succeeded in prototype manufacturing. The solid-state technology offers opportunities for large cells and EV applications. The ‘Batscap’ of Bolloré uses a Li-metal anode, a V2O5 cathode and a PEO-LiTFSI polymer electrolyte; the 2.7 kWh module yields 31 V, 25 kg, 25 L, Pmax: 8 kW, 110 Wh/kg. Ten modules form the 27 kWh battery in the ‘Blue Car’, having a range of ca. 250 km and a recharge time of 6 h. About 3000 cars are in use [53]. Since solid systems do not require any cooling system, they weigh less and require less space than lithium-ion batteries for powering electric automobiles. Volkswagen acquired a 5% stake in US QuantumScape, and Bosch purchased US Seeon; both US companies are developing polymer systems. Toyota is working at all-solid cells with ceramic electrolytes. The 2 Ah model cell (C/Li2S-P2S5/NCM) reached about 400 Wh L
1 and 250 W L
1. Materials. The cell chemistry of all-solid state cells is in general the same as of liquid electrolyte cells. Anode materials comprise carbon, titanates, Lialloys and metallic lithium; cathode materials are Li-based oxides (LCO, NCA), and phosphates (LFP), vanadium oxide [51] and future microstructural 5 V materials. As polymer electrolytes, mainly PEO with conducting salts such as [LiCF3SO2)2N] (LiTFSI) is used. As ceramic electrolytes especially LiPON, Li10GeP2S12 or Li2S-P2S5 are possible. Challenges. The ionic conductivity of polymer electrolytes (10
6.10
5 S cm
1) at room temperature is poor (Fig. 2.13); moderate Li-ion conductivity is reached at 60e100 C. Solid ceramic electrolytes (10
4 to 10
3 S cm
1) come close to liquid organic electrolytes but suffer from the poor contact between solid electrolyte and solid electrode interphase, and the grain boundary resistance, which often dominates the bulk resistance. To

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10–2

200

100°C

10–3 σ

25°C 50°C

10–4

–25°C

0°C liquid

glass

10–5

A ⎡ – Eb ⎤ σ = —— — exp ⎢—————⎥ K(T– To) √T ⎣ ⎦

polymer

10–6 103/T

10–7 2.5

3

3.5

4

Figure 2.13 Arrhenius plot of specific conductivity versus reciprocal temperature of polymer, glass and liquid organic Liþ conducting electrolytes. After M. Armand, The future of lithium-metal batteries, Munich Battery Discussions, March 17e18, 2014.

overcome these effects, thin-film cells are developed with about 0.1 mm thickness, which is one tenth of the thickness of the thinnest prismatic liquid electrolyte Li-ion cell. Hybrid electrolytes consist of solid-state electrolyte and a small amount of liquid electrolyte. Specific power density of all-solid cells is low compared with liquid electrolytes. Cost for small cells in the range of 25,000 result from expensive processing by atomic layer deposition (ALD). Cheap nonvacuum manufacturing processes are under development, promising 100 US$ kWh-1.

2.6.4 Metaleair batteries Metaleair and metal-ion batteries [54] based on zinc, magnesium, calcium, and aluminium, promise more energy per volume in the bi- and trivalent metals, compared with the lithium and sodiumeair battery. However, efficiency (w60%) and power are low due to the high overpotential of the air electrode. The low power capability of the oxygen electrode might enable hybrid systems with a power battery; due to the elaborate recharging, metal-air systems will be not used for SLI and micro-/mild-hybrid batteries. Automotive applications Zinceair Gulf General Atomic started the development of Zn-air batteries already in 1960 and they demonstrated a mini-moke jeep with a 20 kWh battery. General Motors conducted tests in the 1970s with a mechanically rechargeable 35 kWh battery in a 1350 kg vehicle. Since 1997 the Slovenian Miro Zoric has developed rechargeable Zn-Air batteries for small and midsized buses in Singapore together with the Singapore Polytechnic. The same batteries were later used in the Renault R5. Several 6 V/300 W modules were developed at Lawrence Livermore National Laboratory (LLNL) for small

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LeadeAcid Batteries for Future Automobiles

buses in Santa Barbara in 1995. End of the 1990th Zn-air batteries of the German company ZOXY were used in a van in Utah and reached 760 km at temperatures <0 C. At the same time an electric car in California reached 1650 km at high temperatures. These batteries were rechargeable, for only 10 times. Sixty-four vans of the German Post were equipped with 650 kg Zneair batteries of Electric Fuel Ltd. (EFL) in 1995. The range was about 300 km. The recharge was mechanically; the Zn electrode was changed and electrolytically regenerated in a central station. Aluminiumeair The Israeli company Phinergy is developing Al-air batteries. They claimed 250e400 Wh/kg and 15e130 W/kg [55]. In 2013 this battery, activated by water, ran a car for about 2000 km. The recharge took place mechanically by replacement of the aluminium anodes. Cell chemistry. Metal-air batteries combine the cathodic oxygen reduction with the anodic metal dissolution. Typical cell reactions end at oxides or hydroxides. 4 M þ n O2 / 4 MOn=2 4 M þ n O2 þ 2n H2 O / 4 MðOHÞn The inexhaustible oxygen electrode [56] exhibits a thermodynamic capacity of 3350 Ah kg
1, which does not account for the calculation of cell capacity. Challenges. Base metals more negative than 0.4 V SHE are spontaneously oxidized in alkaline solutions, so that aluminium must be used in neutral solution, and alkali metals in organic solution. The cell voltages obtained at present are far below that of lithium batteries. The reversibility of the charge and discharge processes is yet poor. Specific energy Wh kg
1 Þ: Li > Al > Mg [ Na [ Zn > Fe > Cd [ Pb Energy density Wh L
1 Þ:

Al [ Fe[Zn > Mg; Cd > Pb > Li > Na

Cell voltage ðVÞ:

Li [ Mg; Na > Al [ Zn > Cd; Fe > Pb

Sodiumeair battery [57,58]. This air-breathing battery suffers of high overpotential at the air electrode and low efficiency, because sodium peroxide Na2O2 is formed in organic carbonate electrolytes. Nevertheless, the Na-O2 battery can be recharged at 2.9 V without side-reactions known from the LieO2 system. Cell voltage during discharge achieves a poor 1.8 V.  2:263 V; 1;105 Wh kg
1Na2 O; 2; 643 Wh kg
1Na Naþ þ O2 þ e
/NaO2 2 Naþ þ O2 þ 2 e
/Na2 O2 ð2; 330 VÞ 4 Naþ þ O2 þ 4 e
/Na2 O ð1:946 VÞ

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LeadeAcid Batteries for Future Automobiles

Zinceair battery [59]. Based on the dissolution of zinc in potassium hydroxide solution, and the oxygen reduction at carbon, the battery provides, in practice, 1.2 V, 150e200 Wh kg
1 and 100e200 Wh L-1. Despite volume changes during cycling and dendrite growth, the Zneair system has been considered as a promising candidate for future traction batteries, which can be mechanically recharged.

2.6.5 Metal-ion batteries beyond lithium Metal-ion batteries using low-cost materials open up a most speculative option to present lithium-ion systems. Present performance data are yet disappointing. Sodium-ion battery [60,61]. Future low-cost batteries might use sodium ions in aqueous or nonaqueous electrolytes. At present, the cell design and the passivation of the negative electrode pose challenges. ð
Þ anode:

NaðCÞ # C þ Naþ þ e


ðþ Þ cathode:

Host þ Naþ þ e
# NaðHostÞ

The currently low values of capacity and potential of 116 Ah kg
1 and 2.7 V SHE are complicated by the slow intercalation of the large sodium-ion (radius: 980 pm) into the graphite host lattice, and the threefold molar mass of sodium (23 g mol
1) compared with lithium. Because metallic sodium is dangerous as the negative electrode material, novel intercalation electrodes are wanted. In the presence of a solvation shell of glycol ethers (glymes) sodium-ions co-intercalate into graphite. þ  ð
Þ Discharge: NaðdiglymeÞ2 C20 /20 C þ NaðdiglymeÞ2 þ e
Experimental positive electrode materials use layered and three-dimensional materials of pure and doped manganese oxide, cobalt oxide, iron phosphate and iron sulfate. Magnesium-ion battery [62,63]. Pioneer work of a rechargeable battery using MgxMo6S8 electrodes in a solution of magnesium organochloroaluminate in tetrahydrofuran or polyethers (glyme) proved 60 Wh kg
1 for more than 2000 chargeedischarge cycles. During discharge, magnesium is dissolved, and magnesium ions intercalate into the sulfidic host lattice (1.1 V vs MgjMg2þ). ð
Þ anode:

Mg # Mg2þ þ 2 e


ðþ Þ cathode:

Mg2þ þ 2 e
þ Mo6 S8 # Mgx Mo6 S8

Alternatively to the metallic magnesium anode, bismuth, tin and magnesium alloys were proposed. Unfortunately, magnesium cannot be deposited from aqueous solutions, because magnesium oxide and hydroxide are formed

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LeadeAcid Batteries for Future Automobiles

irreversibly. Grignard-compounds RMgX (R ¼ Alkyl; X ¼ Br, Cl) in ethers avoid the passivation of magnesium. The complex electrolyte Ph2Mg,3AlCl3 (Ph ¼ phenyl) in THF provides a voltage window of 3.0 V and almost 100% chargeedischarge efficiency. Alternatively, chloroaluminates, borates, organic silicon complexes, and Mg2þ conducting polymer electrolytes are under investigation. Magnesium-sulfur conversion electrodes [64] store energy not by intercalation, but by the chemical reaction Mg (anode) þ S (cathode) / MgS. Although the system promises 1.77 V, 1,671 Ah kg
1 and 3459 Ah L
1, a suitable electrolyte is not yet known.

2.6.6 Halide battery Rechargeable batteries that use the intercalation of anions are just known as laboratory concept. The fluoride battery [65,66] combines base metals, such as calcium, lithium or lanthanum, with the stand and potential of fluorine (2.87 V SHE) in order to realize cell voltages of up to 6 V. Instead of aggressive fluorine, fluoride ions are used as the mobile species between the electrodes. ð
Þ anode: ðþ Þ cathode:

M þ x F
# MFx þ x e
M0 Fx þ x e
# M0 þ x F


Cell reaction: M0 Fx þ M# M0 þ MFx

The system CajCoF3 yields a thermodynamic cell voltage of 3.59 V and a capacity of 456 Ah kg
1. Cycle-life and power capability are yet poor, and appropriate fluoride conducting electrolytes have to be developed. Dual-ion batteries use complex anions such as Bis(trifluormethane sulfonyl) imide (TFSIe), which simultaneously intercalates both into the anode and the cathode material during charging.

2.6.7 Redoxflow batteries Redoxflow batteries [67e69] use soluble reagents, which are oxidized and reduced at the electrodes and cycled between storage containers (Fig. 2.14). At present, the energy and power densities are small compared with leade acid batteries. Future systems require long-time stable, low-cost organic redox systems in order to meet the demands of electric vehicles. Automotive applications. For electrotraction, the concept of future flow batteries might allow to refuel the liquid active-mass at a filling station mechanically without any time effort for electric charging. A generally recognized problem of flow cells is their low specific power. NanoFlowcell, a Lichtenstein start-up, claimed 600 Wh kg
1 and 6000 Wh kg
1 for the flow battery in their ‘Quant F’ car, providing 1000 km range e the figures have not

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LeadeAcid Batteries for Future Automobiles

Figure 2.14 (A) Vanadium redox flow battery; (B) zincebromine battery.

been confirmed so far. A golf cart running on a 36 V/5 kW vanadium redox flow battery and two 40 L-tanks of 1.85-M vanadium electrolyte has shown a range of 17 km; two 60 L-tanks with 3-M electrolyte achieved about 40 km. The Studiengesellschaft für Energiespeicher und Antriebssysteme (SEA) in Muerzzuschlag (Austria), has developed zinc/bromine batteries for electric vehicles since 1983, e.g. a 45-kWh/216-V battery (700 kg) in a Volkswagen van of the Postal Service with a maximum range of 220 km at 50 km/h. Also a Fiat Panda was equipped with a 18 kWh zinc-bromine battery (72 V, 250 Ah) 360 kg, and energy efficiency 80%). Hotzenblitz, a German company, designed an electric vehicle with a 15 kWh/114 V zincebromine battery. Toyota Motor Corporation has also been developing zincebromine batteries for electric urban transportation vehicle ‘EV-30’; this two-seated vehicle is designed for the transport of people in buildings, shopping centres, small communities and train stations. For small SLI and micro-hybrid batteries, the flow system appears to be too complex. Ironechromium redox flow battery [70]. Since the 1970s the NASA has developed redox-active salt solutions, which can be oxidized and reduced reversibly at polymer-bound graphite electrodes in half-cells separated by a membrane. For the discharge:  ð
Þ Anodic oxidation: Cr2þ #Cr3þ þ e
E 0 ¼
0; 41 V  E 0 ¼ þ0; 77 V ðþ Þ Cathodic reduction: Fe3þ þ e
#Fe2þ  Cell reaction: Fe3þ þ Cr2þ #Fe2þ þ Cr3þ DE 0 z1; 18 V

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LeadeAcid Batteries for Future Automobiles

Despite cheap chemicals, the system suffers from low energy density, high self-discharge and hydrogen evolution during charging. Stationary storage systems were realized in Japan and in the United States. Vanadium redox flow battery [71]. This most developed system employs vanadium ions of different oxidation states in sulfuric acid, which are supplied to the membrane cell from storage tanks. The discharge reactions at carbon felt electrodes, as shown below, run just opposite to reactions during charging.

ðþ Þ Cathodic reduction:

VO2 þ þ 2 Hþ þ e
#VO2þ þ H2 O

E 0 ¼ þ1:00 V

ð
Þ Anodic oxidation:

V2þ #V3þ þ e


E 0 ¼
0:26 V

Membrane diffusion:

Hþ ð
Þ#Hþ ðþÞ

 

In practice, cell voltages of 1.4 V at 50% SoC and 1.6 V at full charge have been achieved. Specific energy amounts to poor 25e30 Wh kg
1. The colours of the catholyte and anolyte reveal the oxidation state of vanadium: V(II) violet, V(III) green, V(IV) blue, V(V) yellow and turquoise shadings at medium SoC. Formation at the first charge in a sulfuric-acid electrolyte of V(III) and V(IV) (1:1) in both half-cells generates V(II) and V(V) (Fig. 2.14A). Zincebromine battery. A solid zinc layer is dissolved during discharge, and deposited again during charge. Bromine, which is supplied at polymer-bound carbon, forms water-soluble oligobromides [Br3]
and [Br5]
, which are bound by organic bases and separated in the storage tank by gravity, so that they cannot diffuse to the zinc anode. Additionally, anode and cathode space are separated by a polymer membrane. Simplified, the discharge reactions read: ð
Þ Anodic oxidation:

Zn #Zn2þ þ 2 e


E 0 ¼
0; 76 VÞ

ðþ Þ Cathodic reduction:

½Br3 
þ 2 e
#3 Br


E 0 ¼ þ1; 09 VÞ

Cell reaction:

Zn þ ½Br3 
#Zn2þ þ 3 Br


DE 0 z1; 85 VÞ

For the working principle see Fig. 2.14B. Thermodynamically, the cell voltage corresponds to an energy of 440 Wh kg
1. Unfortunately, the technology suffers from low peak power, moderate expensive materials, zinc dendrites, corrosion, high self-discharge, hydrogen evolution during overcharge and leakage currents in the electrolyte system. Premium Power realized bipolar stack (30 kW, 45 kWh). Austrian researches run an electric vehicle on a 45 kWh battery for over 80,000 km.

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LeadeAcid Batteries for Future Automobiles

Polysulfideebromine redox flow battery. A regenerative fuel cell (trade name Regenesys) was realized using redox active anions in two half-cells, which contained an electrolyte of sodium polysulfide and sodium tribromide, and are separated by a cation-exchange membrane. The discharge reaction supplies an open-circuit voltage of 1.36 V, and a chargeedischarge efficiency of 75%: ð
Þ Anodic oxidation:

2 ½S2 2
#½S4 2
þ 2 e


E 0 ¼
0:27 VÞ

ðþ Þ Cathodic reduction:

½Br3 
þ 2 e
#3 Br


E 0 ¼ þ1:09 VÞ

Cell reaction:

Zn þ 2 ½S2 2
#Zn2þ þ ½S4 2


DE 0 z 1:36 VÞ

The system suffers from bromine and hydrogen sulfide vapours, low current densities and complexity for monitoring, so that the application in vehicles has not been proved so far. Soluble leadeacid flow battery. This simple experimental system employs soluble lead salts such as the methane sulfonate in methane sulfonic acid. During discharge at carbon electrodes, lead ions pass into the solution.

ð
Þ Anodic oxidation:

Pb#Pb2þ þ 2e


ðþÞ Cathodic reduction: PbO2 þ 4Hþ þ 2e
#Pb2þ þ 2H2 O Cell reaction:

Pb þ PbO2 þ 3Hþ #z2Pb2þ þ 2H2 O

E 0 ¼
0:13 V E 0 ¼ þ1:46 V  DE 0 z1:59 V

 

Technical problems are low power, appropriate bipolar electrodes, lead dendrites during charging and concentration changes of acid and Pb(II). No membrane as required, and a single electrolyte reservoir is enough. Visionary concepts employ the deposition and dissolution of metals and metal oxides as well as solideliquid phase transitions. Metal-free cells were proposed based on quinones.

2.7 Supercapacitors So-called double-layer capacitors or supercapacitors [72e74] utilize the electric capacitance at the phase boundary between two carbon electrodes in a liquid organic electrolyte, which results both from dielectric properties and battery-like faradaic reactions. The high-power capability during pulse discharge (up to 10 kW kg
1) is in contrast to the moderate energy density (typically 5 Wh kg
1). As a link between classical capacitors and batteries, however, such energy storage devices can be charged and discharged very

86

LeadeAcid Batteries for Future Automobiles

quickly, so that they are discussed with respect to power boosters and braking energy recovery in HEVs and hybrid batteries. The activated carbon technology is commercially available from a number of companies worldwide, whereas metal oxides and conducting polymers play no role at present.

2.7.1 Automotive applications In 2010, PSA Peugeot-Citroën demonstrated its second-generation microhybrid system ‘e-HDi’, in Citroën C4 and C5. A 70-Ah sealed leadeacid battery is supported by a Continental Automotive-sourced ‘e-booster’ system with power electronic and supercapacitor (Maxwell Boostcap 600 F/5V). The capacitor overcomes the need for a 100-Ah battery, which usually enables the restarting of diesel engines. The capacitor voltage drops by around 0.5 V during the engine restart, i.e., far less compared with a leadeacid battery at a partial SoC, and can be recharged at the rate of 0.1 V s
1 within 5 s. The Mazda i-ELOOP system in ‘Mazda6’, ‘Mazda3’ and ‘CX-5’ cars comprises a 25-V supercapacitor, a 5-kW alternator, and a 12-V step-down DC/DC converter. The supercapacitor extends the cycle-life of the leadeacid battery and its durability in the engine compartment, which results in higher fuel economy extension of the battery service interval [78].

2.7.2 Carbon technology Cell chemistry. Commercial double-layer capacitors are based on the high surface area of carbon materials, whereby additionally battery-like redox reactions (faradaic capacitance) take place. Approximately, the equation for the capacitance C (in Farads) and the energy W of a plate-capacitor holds for supercapacitors as well, C ¼ ε0 εr

A 1 ; and W ¼ CU2 d 2

where d denotes the nanometre-thin thickness of the double-layer (electrode) or the distance between the electrodes (cell), A is the real electrode cross-sectional area, ε the apparent permittivity of the electrodee electrolyte interface. Unfortunately, capacitance and internal resistance depend strongly on frequency, voltage and temperature, so that a pseudocapacitance C(u,U,T) is measured in practice instead of a constant capacitance value. C ðUÞ ¼

dQ Im Y ðuÞ
ImZ ðuÞ ; and Cu ¼ ¼ dU u u$ ðIm Z ðuÞÞ2 þ ðRe Z ðuÞÞ2

Herein Q electric charge, U voltage, Z is the impedance (Re real part, Im imaginary part), Y admittance, u circular frequency.

87

LeadeAcid Batteries for Future Automobiles

State-of-the art. Cell voltage 2.3e3 V, capacitance per unit cell 10e5000 F, internal resistance (ESR) some Milliohms, specific energy 3 to 10 Wh kg
1, specific power 1e10 kW kg
1, peak power typically 8e80 kW kg
1. Full discharge and deep discharge: allowed. Coulombic efficiency: > 99%. Cyclelife: 1 Mio. and more. Calendar-life: 10 years and more. Materials. Commercial band electrodes in spiral-wound unit cells are mostly based on activated carbon, bonded by fluoropolymers, and carbon black as an additive, on aluminium support in an organic solution (quaternary ammonium salts in acetonitrile or PC). Different materials such as carbonaceous fibres, metal oxides and conducting polymers, as well as aqueous solutions and solid electrolytes, play no commercial role at present. The separator is made of a non-woven fabrics of microporous polyolefin, or stretched films of polytetrafluoroethylene (PTFE) resulting from a hydrophilic rendering treatment. Polyethylene-polypropylene (PEePP) three-layer separators and expanded PTFE have been produced for several years. Advantages. Nanoporous carbon provides high surface area (up to 2000 m2g
1), tailored pore geometry and pore size distribution, wettability and conductivity. Assuming an average value of 15e25 mF cm
2 and a specific surface area of 1000 m2 g
1 for activated carbons, the achievable capacitance would be 150e250 F g
1. Typically, capacitances around 0.15 F cm
2 (100e180 F g
1, and 70e130 F cm
3) are yielded in solutions based on acetonitrile or PC. Novel graphene materials provide even far better value. The strength of supercapacitors is their power capability, in that they respond in the kilowatt range immediately after opening the discharge circuit. High safety is given, because the electrolyte volume is small; failure terminates in a high resistance (no short-circuit). Challenges. Compared with batteries, the energy density is small. The cell voltage is strongly limited to the decomposition voltage of the electrolyte (<3 V) or even lower at aqueous systems. Supercapacitors age by a gradual destruction of the polymer-bound electrode and accidental electrolysis due to improper operating voltage and overtemperature, whereby the resistance increases and capacitance drops.

2.7.3 Hybrid systems Leadeacid capacitor hybrid. Electric propulsion demands high currents from the battery. Deep-discharges, i.e., high permanent currents over a wide voltage window until the battery is nearly empty, seriously limit battery life. The hybrid technology of battery and supercapacitor e using combinations of PbO2 electrodes, spongy lead and carbon electrodeseimproves cycle-life and dynamic performance of a leadeacid battery under partial SoC duty as

88

LeadeAcid Batteries for Future Automobiles

required in hybrid electric vehicles. For the already commercialized UltraBatteryTM see chapter 12. Lithium-ion capacitor. Subaru Technical Research and Fuji Heavy Industries presented a cell with an activated carbon positive electrode, and a carbonaceous lithium-intercalation electrode (negative). The capacitance of the hybrid capacitor is determined by the active-mass m of the positive electrode, C ¼ Cþmþ/(mþ þ m
), because C
[ Cþ. Cell voltage lies at 3.8e4.0 V, to which the intercalation electrode contributes a near constant potential, and the carbon electrode reverses its charge to negative below w3 V. The electrolyte is based on lithium hexafluorophosphate in PC. The lower cell voltage amounts to 1.9e2.2 V, and the internal resistance is 0.24e0.43 U cm2. Specific energy of 10 Wh kg
1 and specific peak power of w1.3 kW kg
1 were measured at a 2000 F/3.8 V device.

2.8 Fuel cells Fuel cells utilize the cold combustion of hydrogen and oxygen by electrochemical reactions, and can therefore be considered to be oxyhydrogen batteries. Fuel cells have been employed in trucks, trains/locomotives, motorcycles, boats, submarines, racing cars and airplanes. At present, the most advanced technology for electric vehicles is the polymer electrolyte fuel cell (PEMFC).

2.8.1 Automotive applications In contrast to classical batteries, fuel cells can be designed individually for a given power (stack) and energy/capacity (fuel tank). For long-distance travelling, electric vehicles require a huge battery, whereas the fuel cell of a given power is able to extend it energy content by a large fuel tank at a moderate system mass. The linear relationship between range d and mass m shows a steep slope b for the battery (d ¼ b m þ m0), whereas the less steep fuel cell line start at a higher system mass m0. This means: Battery systems are advantageous at short ranges, and fuel cells win at long distances. Electric vehicles. In 1959, Allis-Chalmers powered a tractor with a 15 kWalkaline fuel cell (AFC). Space missions with alkaline cells (Apollo) and PEM cells (Gemini) awakened the interest of the automakers. AFCs were used in 1966 by GM in a Chevrolet Electrovan with 150 kW (32 units á 5 kW), providing a range of 200 km. In 1967, Karl Kordesch built a hybrid AFC/NiCd motorcycle with hydrazine as the fuel. At the same time, TU Dresden/BAE (Germany) developed a 3-kW forklift with a hydrazineeair AFC. In 1970, Union Carbide demonstrated a hybrid car with a 6-kW AFC and leadeacid battery running for a 200 km range. In Japan, in 1972, the National Institute of Advanced Industrial Science and Technology (AIST), Panasonic and Daihatsu

89

LeadeAcid Batteries for Future Automobiles

together manufactured a pickup truck with a 5.2-kW hydrazineeair AFC, consuming 1 L of hydrazine for 80 km. In 1998, Zero Emission Vehicle Company (ZEVCO) launched a taxi with 5-kW AFC in London. Although the AFC is relatively cheap by using non-platinum catalysts, the difficult air supply (CO2 cleaning) has hindered the commercialization up to now. The focus for EVs is on PEMFCs. The first PEMFC vehicle was presented in 1993 by Energy Partners Consulier; three 15 kW PEMFCs on the loading area powered the car over a 90 km range. Daimler-Benz started their fuel cell development in the early 1990s. Different car manufacturers released demonstration vehicles [75]. In 2016, three different fuel cell vehicles were commercially available: 1. Hyundai ‘Tuscon ix35’, since 2014, range 425 km, 100 kW PEMFC and Li-ion battery, 0.95 kWh. Leasing price US$600. 2. Toyota ‘Miray’, since 2015, range 500 km, 114 kW PEMFC and NiMH battery 1.6 kWh. Price: US$58,500 (United States), EUR79,000 (Europe). 3. Honda ‘FCX Clarity’, since 2016, range 480 km, 100 kW PEMFC, Li-ion battery: 1.2 kWh. Price: US$67,300. For light vehicles, the direct methanol fuel cell (DMFC) is used, which is ableto consume methanol, but provides poor power only. Buses. Fuel cell buses were realized, e.g., A330 FC Hybrid Bus of VanHool N.V. with 120 kW PEMFC and 24 kWh Li-ion battery (LTO). A 50 kW Phosphoric acid fuel cell (PAFC) powered a vehicle by Fuji Electric, and a 100 kW PAFC was used by UTC. These phosphoric acid systems had to be run at the operating temperature of about 200 C all the time. Fuel cell auxiliary power unit (FC-APU). With respect to CO2 reduction, inefficient components, such as the electric generator, are under consideration. The efficiency of belt-driven generators ranges from 50% to 30% (at maximum RPM), and 80% with an integrated starter-generator (ISG). Near-future concepts such as drive-by-wire and break-by-wire will increase the average electric power consumption up to 2e4 kW. Just to generate electricity, a fuel consumption of 1.5e3 L per 100 km is required, and additional effort is necessary for air conditioning and heating. Truck engines consume some 3 L fuel per hour, or 30 kW thermal power input, when idling. A fuel cell, as an auxiliary power generator (APU), would combine at least 40% efficiency and appropriate power output e compared with the less efficient ICE/generator chain. The fuel cell, as a secondary system, should preferably run on diesel or gasoline, which requires a reforming process and complicates the PEMFC system. Solid oxide fuel cell (SOFC) allow internal reforming. Prototypes of such systems were developed by Delphi in cooperation with BMW in 2001. They fitted a 5 kW SOFC working model into the trunk of a 7-series BMW. Eberspächer (Germany) presented a 3 kW SOFC system with a 30% efficiency, very low NOx emissions, no diesel particles and a noise level of

90

LeadeAcid Batteries for Future Automobiles

58 dB (A) outside the vehicle and less than 40 dB (A) inside. Compared with engine idling of a heavy-duty truck, the CO2 emissions could be reduced by 71%. The diesel consumption is 1.0 L h
1 at an electrical net power of 3 kW.

2.8.2 Cell chemistry and cell design Fuel cells utilize polymer, liquid, solid or molten electrolytes, which coins the abbreviation of the technology. The fuel, usually pure hydrogen, is oxidized at the anode (negative electrode); oxygen from air is reduced at the cathode (positive electrode). The electrode reactions are exactly the reverse of the electrolysis of water. Polymer electrolyte fuel cell (PEMFC): A thin proton exchange membrane separates two carbon electrodes, which are coated with finely dispersed, carbon-supported platinum, and are feed with humidified gases. Thermodynamic cell voltage: 1.23 V; in practice about 0.8 V. ð
Þ 2 H2 ðþÞ O2 þ 4 Hþ þ 4 e


/ 4 Hþ þ 4 e
/ 2 H2 O

Advantages: The compact membrane-electrode assembly, which can be stacked using bipolar plates, enables high specific power (z1 kW kg
1). High electric efficiency 50e68% (cell), and 43e50% (system based on natural gas). Challenges: Traces of CO in the hydrogen feed poison the platinum catalyst . Drying-up and freezing of the membrane; still expensive. Direct methanol fuel cell (DMFC): This variant of the PEM fuel cell running on liquid or gaseous methanol provides a cell voltage of about 0.5 V (in theory 1.186 V). ð
Þ CH3 OH þ H2 O

/

CO2 þ 6Hþ þ 6e


3 ðþÞ O2 þ 6 Hþ þ 6 e
2

/

3 H2 O

Advantages: not fuel processing required. Challenges: Methanol penetrates the membrane by diffusion and electroosmosis, and is oxidized at the anode. Relatively inactive catalysts. Low power and efficiency 20e30% (cell). Alkaline fuel cell (AFC): A 30% potassium hydroxide solution resides between two nickel electrodes. ð
Þ H2 þ 2 OH


/ 2 H2 O þ 2 e


1 ðþÞ O2 þ H2 O þ 2e
2

/ 2 OH


Advantages: low-cost catalysts, high efficiency, fast oxygen reduction kinetics, low operating temperature. Challenges: pure oxygen is required in order to prevent the formation of K2CO3 in the electrolyte; corrosion

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LeadeAcid Batteries for Future Automobiles

problems at the electrodes and in the gas spaces caused by the alkali hydroxide. Phosphoric acid fuel cell (PAFC): Hot phosphoric acid (190 C) separates the carbon electrodes with platinum catalyst, which are feed with hydrogen (by steam reforming from natural gas) and air. ð
Þ ðþÞ

2 H2 O2 þ 4 Hþ þ 4 e


/4 Hþ þ 4 e
/2 H2 O

Advantages: Prototypes in the Megawatt range demonstrated the cogeneration of power and heat. Challenges: Moderate electrode life and electrolyte conductivity. Molten carbonate fuel cell (MCFC): A high-temperature technology, using molten alkali carbonates at in a heat-resistant matrix at about 630 C, which has not been used for electric road vehicles so far due to technological challenges. There are, however, demonstration projects on larger vessels. ð
Þ H2 þ ½CO3 2


/ H2 O þ CO2 þ 2 e


ðþÞ O2 þ 2 CO2 þ 4 e


/ 2 ½CO3 2


Advantages: Cell voltages of 0.75 V were demonstrated (thermodynamic 1.04 V). Low-cost electrodes based on nickel-chromium and oxidized sinter nickel; CO tolerant; hydrogen generation by internal reforming near the anode; cogeneration of electricity and heat. Challenges: CO2 cycling; shortcircuits caused by electrode corrosion; sensitive to sulfur. Solid oxide fuel cell (SOFC): A plane or tubular solid electrolyte of ZrO2/ Y2O3 (YSZ), which conducts oxide ions, is coated with the electrodes: nickel, and doped perovskites such as LaSrMnO3. ð
Þ 2H2 þ O2


/

2H2 O þ 4e


ðþÞ O2 þ 4e


/

2O2


Advantages: Cell voltage 0.88e0.93 V (in air and pure oxygen); CO tolerant materials; internal reforming of natural gas directly at the fuel electrode; high electric efficiency of 60e65% (cell), and 55e65% (natural gas system). Challenges: temperature stability of the materials; electric interconnection between adjacent cells by ceramic conductors.

References Leadeacid [1] J. Garche, et al., Encyclopedia of Electrochemical Power Sources, in: Secondary Batteries: Lead Acid Batteries, vol. 4, Elsevier, Amsterdam, 2009, pp. 550e864.

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LeadeAcid Batteries for Future Automobiles

[2] P. Kurzweil, Gaston Planté and his invention of the leadeacid battery. The genesis of the first practical rechargeable battery, J. Power Sources 195 (2010) 4424e4434. [3] D.A.J. Rand, P.T. Moseley, Energy storage with leadeacid batteries, in: P.T. Moseley, J. Garche (Eds.), Electrochemical Energy Storage for Renewable Sources and Grid Balancing, Elsevier, Amsterdam, 2015 (Chapter 13).

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