Development of a benchmarking model for lithium battery electrodes

Journal of Power Sources 320 (2016) 286e295

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Development of a benchmarking model for lithium battery electrodes Timm Bergholz a, *, Carsten Korte a, Detlef Stolten a, b a b

Forschungszentrum Jülich, Institute of Energy and Climate Research, Electrochemical Process Engineering (IEK-3), Germany Chair for Fuel Cells, RWTH Aachen University, Germany

h i g h l i g h t s
A top-down benchmarking model for lithium battery electrodes is developed.
For HEVs, state of the art electrodes (graphite, LTO and LFP) are most suitable.
TiO2 and LFP are most interesting for stationary batteries due to cost and lifetime.
New chemistries have to be optimized for cells in BEVs due to the energy density.
Li2MnO3$LiNi0.5Co0.5O2 or LiNi0.5Mn1.5O4 with silicon anodes can reach 330 Wh/kg.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 January 2016 Received in revised form 26 March 2016 Accepted 17 April 2016

This paper presents a benchmarking model to enable systematic selection of anode and cathode materials for lithium batteries in stationary applications, hybrid and battery electric vehicles. The model incorporates parameters for energy density, power density, safety, lifetime, costs and raw materials. Combinations of carbon anodes, Li4Ti5O12 or TiO2 with LiFePO4 cathodes comprise interesting combinations for application in hybrid power trains. Higher cost and raw material prioritization of stationary applications hinders the breakthrough of Li4Ti5O12, while a combination of TiO2 and LiFePO4 is suggested. The favored combinations resemble state-of-the-art materials, whereas novel cell chemistries must be optimized for cells in battery electric vehicles. In contrast to actual research efforts, sulfur as a cathode material is excluded due to its low volumetric energy density and its known lifetime and safety issues. Lithium as anode materials is discarded due to safety issues linked to electrode melting and dendrite formation. A high capacity composite Li2MnO3$LiNi0.5Co0.5O2 and high voltage spinel LiNi0.5Mn1.5O4 cathode with silicon as anode material promise high energy densities with sufficient lifetime and safety properties if electrochemical and thermal stabilization of the electrolyte/electrode interfaces and bulk materials is achieved. The model allows a systematic top-down orientation of research on lithium batteries. © 2016 Elsevier B.V. All rights reserved.

Keywords: Lithium battery Electrode material Benchmarking model Fuel cell hybrid Battery electric vehicle Stationary application

1. Introduction Lithium Ion Batteries (LIBs) are of special interest with respect to fuel cell battery hybrid systems for light traction (0.5e5 kW) and individual transport (5e200 kW), as well as for battery electric vehicles (BEVs) and stationary applications [1e3]. The batteries in these systems must fulfill multiple requirements. An increased volumetric EV and gravimetric energy density Em is essential to enabling the achievement of higher operational ranges by BEVs and

* Corresponding author. Landshuter Allee 150, 80637 München, Germany. E-mail address: [email protected] (T. Bergholz). http://dx.doi.org/10.1016/j.jpowsour.2016.04.085 0378-7753/© 2016 Elsevier B.V. All rights reserved.

reducing the mass and volume of fuel cell power trains. The power density Pm of the battery determines the power capability of BEVs and the possible peak load of hybrid fuel cell systems. Increased cyclic and calendaric lifetime, improved safety characteristics, the usage of more abundant raw materials and low specific costs are further important factors for all considered applications. Conventional LIBs must be modified to reach these goals. For this reason, the introduction of novel cell chemistries is required. This paper outlines a benchmarking model that draws on the fundamental material characteristics of lithium battery electrode materials so as to systematically and quantitatively identify their potential for application in different battery and fuel cell hybrid power trains.

T. Bergholz et al. / Journal of Power Sources 320 (2016) 286e295

1.1. Battery requirements for different power trains Table 1 lists the requirements for the battery in a hybrid electric vehicle (HEV), a BEV and a stationary energy system, as communicated by the Japanese New Energy and Industrial Technology Development Organization (NEDO) [4] and the US Department of Energy (DOE) [5] for the years 2020 and 2030. Additionally, the derived target values for the battery of an in-house, 1 kW directmethanol fuel cell (DMFC) hybrid system for light traction electric vehicles (LHEVs) are listed [6]. The characteristics of a commercial LIB comprised of a carbon-based anode, a LiNi0.8Co0.15Al0.05O2 cathode and an organic solvent-based electrolyte from Gaia are opposed [6]. Based on the discrepancy of the target values and LIBcharacteristics, state-of-the-art systems must be optimized with different prioritizations. Table 1 specifies the respective priorities from 0 for a low priority to þþ for a high priority. BEV batteries place a high demand on volumetric and gravimetric energy density and costs. The power densities of present systems fulfill the demand of BEVs, while the cyclic and calendaric lifetimes must be increased. In contrast, HEV and LHEV batteries have high priorities in terms of power density and lifetime. The gravimetric energy density of the exemplary LIB has to be increased by 20% and 36% for HEV and LHEV batteries, respectively. The higher value required for LHEVs arises from the lower limits for battery mass and volume in the comparably smaller systems. Due to the high volumetric energy density of conventional batteries, they attain the requirements of the hybrids considered. State-of-the-art LIBs reach the desired energy and power densities needed for stationary applications, while their calendaric and cyclic lifetime, as well as their costs, must be significantly optimized. According to the derived prioritization for the additional parameters of raw materials and safety, all applications must fulfill high safety standards, whereas the utilized resources are more important for large-scale BEV and stationary batteries.

2. Development of a benchmarking model In order to evaluate electrode materials for the deployment in HEVs, BEVs and stationary applications, a benchmarking system originally developed by Camp et al. is applied [7]. The required material characteristics are the following: energy density, power

287

density, safety, lifetime, costs and raw materials, which are defined in the following part to enable quantitative benchmarking. The process is based on the calculation of valuation factors for the different characteristics F(char), which are arranged on linear scales. Eq. (1) assigns the factors from the minimum value char1, F(char1) ¼ 1 to the maximum value char10, F(char10) ¼ 10, respectively.

FðcharÞ ¼ 1 þ ðcharðiÞ  char1 Þ=ððchar10  char1 Þ=9Þ

(1)

2.1. Energy density The theoretical values for the gravimetric E0m and volumetric E0V energy density generally discussed in the literature [8] are based on the open circuit potential against Li/Liþ, the theoretical specific capacity and 100%-dense electrodes without considering porosity or the addition of passive materials. To determine the values of the real electrodes, Em and EV, the integral mean potential UCCV of a complete discharge at C/10 at room temperature after the formation procedure [9], together with the respective specific capacity Qm and tap density rtap of the real electrodes are extracted from literature. Additionally, the active material fractions of anode %an, cathode %cat and the passive material content %passive, illustrated in Fig. 1 for a 18650 model cell have to be considered [10]. This results in Eq. (2) and Eq. (3) for Em and EV respectively:

.  Em ¼ UCCV ðQmcat $%mcat Þ1 þ ðQman $%man Þ1 , 1   %mpassive

(2)

.  EV ¼ UCCV ðQVcat $%Vcat Þ1 þ ðQVan $%Van Þ1 $ 1   %Vpassive

(3)

Additionally a mean value of different silicon electrodes cited in the literature is used to calculate the specific energy of potential cathodes. Metallic Li is not considered, due to its severe safety concerns [11]. The mean value for the silicon anode is: UCCV ¼ 0.44 V, Qm ¼ 1400 A h/kg, rtap ¼ 0.52 g/cm3, %m-an ¼ 0.65, %Van ¼ 0.87 [12e14]. For the benchmark of anodes, the averaged parameters of a high capacity composite cathode Li2MnO3$LiNi0.5Co0.5O2 serves as the counter electrode (UCCV ¼ 3.61 V, Qm ¼ 239 A h/kg, rtap ¼ 1.89 g/cm3, %m-cat ¼ 0.87, %V-cat ¼ 0.87) [15e17].

Table 1 Requirements for the battery in a HEV, BEV and a stationary PV system as communicated by the NEDO and DOE, together with the values of an in-house LHEV, the characteristics of a commercial LIB and derived prioritizations. LHEVa 1

Em/Wh kg EV/Wh L1 Pm/W kg1 g Cyclesh,i Yearsh Costs/$ kWh1 Raw materialsa Safetya a b c d e f g h i

f

150[þ] 150[þ] 2400[þþ] 5000[þþ] 15[þþ] n.a.[0] [0] [þþ]

HEVb

BEVc

Statd

LIBe

133[þ] n.a.[þ] 2500[þþ] 4000[þþ] 15[þþ] n.a.[0] [0] [þþ]

250[þþ] n.a.[þþ] 1000[þ] 2000[þ] 13[þ] 170[þ] [þþ] [þþ]

100[0] n.a.[0] 1000[0] 6000[þþ] 20[þþ] 120[þþ] [þþ] [þþ]

110 290 2000 1000 10 1000

Derived requirements. DOE requirements for 2020 [5]. NEDO requirements for 2020 [4]. NEDO requirements for 2030 [4]. Characteristics of a commercial Gaia cell [6]. Prioritization scale: high priority [þþ], medium priority [þ], low priority [0]. Nominal discharge power. At room temperature and a mean state of charge of 60%. 1C charge and discharge current for 80% depth of discharge.

Fig. 1. Composition of a commercial LIB with mass fractions according to Ref. [10] and calculated volume fractions by considering the respective material densities.

288

T. Bergholz et al. / Journal of Power Sources 320 (2016) 286e295

2.2. Power density According to the literature, the transport of Liþ in conventional LIBs is primarily limited by the diffusion process in the active electrode materials [18]. The Li diffusion coefficient DLi, together with the diffusion length r, is an indicator of the possible rate of charge and discharge reaction. It is proportional to the electronic De- and ionic DLiþ conductivity of the material [18]. Fig. 2 shows the Liþ-concentration gradient along the particle radius of a cathode particle before (left, t0) and after (right, t1) a discharging pulse. Assuming that the Li-transport is only limited by diffusion in the active material and taking into account a constant Li-source at the particle surface, the Li-concentration cLi can be described by the error function in Eq. (4), as long as the diffusion length is small compared to r [19]. The influence of the electrodes microstructure and the electronic conductivity of the electrode composite are neglected. The symmetry factor sym is 2 for one dimensional transport.

cLi ðr; tÞ ¼ ðvcLi =vrÞt ¼ cLi;0

.pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  .  sym$pDLi $t $exp r 2 sym$DLi $t (4)

By integrating the Li concentration cLi along the particle radius r at a given time t, the depth of discharge DoD during a current pulse can be determined according to Eq. (5):

ZR DoD ¼

cLi ðrÞt dr

(5)

r¼0

In order to benchmark different electrode materials, the particle size (dHEV) for a DoD of 10% during a current pulse of 36 s (10 C) is numerically determined. As is shown in Fig. 3, particle sizes rise with increased Li diffusivities. Graphitic anode materials with high values for DLi in turn result in larger particles compared to the layered intercalation cathode LiCoO2 with an order of magnitude reduced diffusion coefficient. The indicated cathode with a spinel LiNi0.5Mn1.5O4 or olivine structure LiFePO4 needs markedly smaller particles to enable the DoD demanded. The data for DLi were extracted from the literature as mean values of at least three different studies for the completely lithiated materials at room temperature. The parameter dHEV correlates with the square root of DLi, which is in agreement with the function for the mean square displacement (Dx ~ √Di t). Thus, LiFePO4 needs 20 reduction in particle size compared to LiCoO2, with a four orders of magnitude higher value of DLi.

Fig. 2. Li concentration gradient cLi in dependency of time t and particle radius r before (left) and after (right) a discharging pulse of the duration t1 for a cathode material.

Fig. 3. Depth of discharge in dependency of the particle radius during a current pulse (36 s) for different electrode materials.

Referring to various literature studies, DLi respectively the resulting value of dHEV does not solely reflect the power capability of a material [20]. The possible formation of a resistive Solid Electrolyte Interface (SEI) resulting from the electrode-electrolyte reaction further limits power capability. Thus, the stability of the electrode against the utilized electrolyte must be taken in account. The electrochemical stability window of a conventional electrolyte composed of a 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with LiPF6 (1 M) with a cathodic stability of 0.4 V [21] and an anodic stability of 3.6 V [22] is considered. The integrated difference of the values against the electrode potentials for a C/10 discharge after the formation phase is indicated in Fig. 4 for a graphite anode and a Li2MnO3$LiNi0.5Co0.5O2 cathode. The parameters DUred, Eq. (6) and DUox, Eq. (7) enable a comparable benchmark for the extent of anode and cathode SEI formation [23].

DUred ¼

100% Z

½0:4 V  4an ðSoCÞ$DQ ðSoCÞ=Q0 dSoC

(6)

½4cat ðSoCÞ  3:6 V$DQ ðSoCÞ=Q0 dSoC

(7)

SoC¼4SoC ¼0:4V

DUox ¼

100% Z

SoC¼4cat ¼3:6V

Fig. 4. Extent of electrolyte oxidation DUox and reduction DUred of a EC, DMC, LiPF6 electrolyte against a graphite anode (red) [24] and a Li2MnO3$LiNi0.5Co0.5O2 composite cathode (blue) [15] for the C/10 discharge voltage profiles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

T. Bergholz et al. / Journal of Power Sources 320 (2016) 286e295

2.3. Safety characteristics Fig. 5 depicts an overview of possible mechanisms for the initiation of a thermal runaway in LIBs based on various literature reviews [22,25,26]. If the enthalpy of side reactions delivered is larger than the dissipated heat, a fire, explosion or outlet of gas can occur, which represents a major drawback for Li-based batteries. Thermal side reactions in conventional LIBs are normally initiated by the decomposition of the conducting salt of the electrolyte, displayed in Fig. 5-1. The onset temperature of the thermal degradation of LiPF6 in conventional electrolytes amounts to Tonset ¼ 60  C [22]. This process is followed by the degradation of the anode SEI, as in Fig. 5-2) in a temperature range of between Tonset ¼ 80e140  C. This leads to a further reduction of the exothermic electrolyte at the anode. The generated reaction enthalpy DH causes (see Fig. 5-3a) cathodic SEI degradation between Tonset ¼ 140e240  C and (3b) the decomposition of the bulk cathode material with the liberation of O2. The decomposition temperature Tdecomp and enthalpy DH of the bulk cathode decomposition strongly depends on the nature of the utilized cathode material and its state of charge SoC [27]. A state-of-the-art cathode material for BEV batteries, LiNi1/3Mn1/3Co1/3O2 has a Tdecomp of 250  C and a DH of 283 J/g at a Li content of xLi ¼ 0.55, whereas the values significantly decrease (Tdecomp ¼ 80  C) e and respectively increase (DH z 700 J/g) e at a lower degree of lithiation xLi ¼ 0 [28]. Both steps lead to the electrochemical (Fig. 5-3a) and chemical oxidation (3b) of the electrolyte at the cathode, which proceeds exothermically. If the temperature rises above 220e300  C, the binder in the electrode composites exothermically decomposes (Fig. 5-4). Thus, the different values for Tonset, Tdecomp or DH vary depending on the composition of the electrolyte and the passive materials utilized, as well as the active electrode materials. Their SoC, purity, particle morphology and size are further parameters which affect the electrodes' safety specifications. Additionally, the melting of the composed electrodes at the melting temperature Tmelt can significantly enhance the side reaction rate and lead to a thermal runaway [29]. Safety issues also arise due to electrically-initiated side reactions

289

[30]. Charging above the defined end-of-charge voltage can lead to Li-plating on the anode which results in an increased reduction rate of the electrolyte [31]. A further problem is the potential formation of Li-dendrites and the resulting internal short circuiting of the cell [32]. During overcharge, the delithiated metal compound at the cathode increasingly oxidizes the electrolyte. Thus, the value of Tonset for electrolyte oxidation is markedly decreased, while the respective value for DH is increased. Furthermore, overcharging can lead to the decomposition of the bulk cathode or to a decreased decomposition temperature Tdecomp, if the minimal Li content xLi-rev, where the material is stable, is reached. This has been demonstrated, for instance, with CoIV or NiIV-containing materials [33]. Deep discharges cause the dissolution of the anodic Cu current collector [34]. Subsequent charging then leads to the formation of Cu dendrites and thus to an internal short circuit. Taking into account the reaction enthalpies of the different LIB components, the overall heat of reaction during a thermal runaway of an 18650-type cell divides according to the chart in Fig. 6. All reactions assigned to the cathode account for around 70% of the overall DH, and thereby cathodic chemical electrolyte oxidation accounts for the largest part (50%). Correspondingly, the electrolyte reduction at the anode, which initiates the thermal runaway (see Fig. 5), only contributes 10% to the overall DH. In order to benchmark the safety properties of different electrode materials, the following parameters are considered:
The value of Tonset of the SEI degradation at an anode or a cathode material is extracted from thermal stability measurements of the completely charged materials in a conventional organic solvent-based electrolyte with LiPF6.
The corresponding values of DH reflect the extent of a possible thermal runaway.
The decomposition temperature Tdecomp of the pure, fully charged cathode material is an indicator of when the most exothermic reaction is occurring in the cells.
The minimal Li content xLi-rev, where the bulk cathode material is stable, reflects the stability of the cathode against decomposition during overcharge.

Fig. 5. Possible mechanism of the initiation of a thermal runaway in LIBs.

290

T. Bergholz et al. / Journal of Power Sources 320 (2016) 286e295

2.5. Costs

Fig. 6. Calculated components of the generated reaction enthalpy of the thermal runaway of a 18650-type cell according to literature values of the single components (2.5 A h, graphite anode/LiCoO2: 1/2, carbon/PVdF/SuperP: 88/7/5, LiCoO2/PVdF/ SuperP: 80/10/10, EC/DMC/LiPF6: 1/1/1 M), all ratios in weight percentage.

Several studies about the present and future costs of commercial LIBs assign a large portion to the raw material costs [40]. Percentages of 60e70% have been calculated for the raw materials and their manufacture on the cell level for 18650 type LIBs [10]. The price of the transition metal-containing cathode material thereby constitutes the largest component in conventional cells [41]. Its price strongly depends on the materials-specific capacity Qm, the resulting cell voltage, as well as the conductivity and complexity of processing the material [40]. In contrast to the cost model developed by the Argonne National Laboratory [40], which simulates the overall cell costs, the evaluated specific material costs $kWh in $/kWh defined in Eq. (9) consider the specifications of the pure active material without material processing or passive components. The cell voltage UCCV reflects the closed circuit potentials against the standard electrodes defined for the evaluation of the energy density. Every component i of the electrode material (e.g., Li and Co in LiCoO2) is included with its mass fraction Mi/M and mass-specific costs Km,i.

$kWh ¼ 1000 Wh=UCCV $

1=Qm $Mi =M$Km;i

(9)

i


Melting of the electrode materials at Tmelt can additionally lead to thermal runaways.

2.6. Raw materials

2.4. Lifetime An increase of the internal resistance DR and a decrease of the rated capacity DQ cause aging in LIBs, induced by cyclic and calendaric load. There are several factors that affect the degradation rate, such as: temperature, DoD, SoC, C-rate, material preparation and cell design [35]. The characteristics of the active and passive cell components are also important. For the active electrode materials, the volume effect DV during cycling causes mechanical degradation. This is defined as the percentage change of the volume between charge Vch and discharge Vdis in proportion to Vdis, as indicated in Eq. (8) [36]. The volume is extracted from the respective densities of pure lithiated and delithiated active materials.

DV ¼ Vch  Vdis =Vdis  100%

X

(8)

Many studies of cell aging reveal a correlation between the rate of electrolyte-electrode reaction, leading to SEI formation and the overall degradation of the LIB [37]. As for the evaluation of the power density, the difference of the cathodic (0.4 V) and anodic stability (3.6 V) of a conventional electrolyte (EC/DMC/LiPF6: 1/1/ 1 M) against the anode, Eq. (6) and cathode, Eq. (7) electrode potentials DUred and DUox provides comparable parameters to benchmark the extent of SEI formation (see Fig. 4). The degradation of the bulk cathode material under the liberation of O2 is a further important degradation mechanism, significantly affecting the lifetime of LIBs with cathodes, which decompose below a minimum degree of lithiation (e.g., LiCoO2, LiNiO2) [38]. The minimal Li content xLi-rev, already defined for the safety evaluation, is used to benchmark the extent of bulk cathode decomposition. The aging rate also significantly increases at an upper temperature limit of around 60e70  C in present LIBs [39]. This phenomenon is attributed to SEI decomposition at the anode and an increased reduction of the electrolyte, which also affects the safety evaluation (see Fig. 5) [22]. Thus, the onset temperature Tonset of the SEI degradation at the completely charged anode in a conventional organic solvent-based electrolyte with LiPF6 is a further parameter considered in the benchmarking process.

An increasing market share of LIBs for stationary and transport applications with large storage energies leads to an increased significance of the utilized reserves of raw materials. Besides the existing quantity of minable resources, their annual production plays a major role in evaluation, if elevated LIB production is possible. Additionally, the specific capacity of the resulting electrode Qm is considered by calculating the capacity from the product of Qm, the minable resources, reservesi according to the US mining report of 2010 [42] and the inverse mass fraction (M/Mi) for every element contained in the evaluated electrode (e.g., Li and Co in LiCoO2). According to Eq. (10), the assessable capacity Ahres represents the minimum value of all incorporated raw materials.

Ahres ¼ minðQm $M=Mi $reservesi Þ

(10)

By comparison, the accessible capacity Ahprod/a from the annual production (production/a) is calculated in Eq. (11):

  Ahprod=a ¼ min Qm $M=Mi $ðproduction=aÞi

(11)

3. Benchmarking of electrode materials Based on the evaluated material characteristics, evaluation factors are calculated according to Eq. (1). The respective minimum and maximum values (benchmarks) are identified in an extensive literature study, revealing the characteristics of numerous electrode materials. The exemplary scale of the gravimetric energy density of cathodes F(Em) ranges from 1, for the minimum value of Em ¼ 219 Wh/kg for lithium manganese spinel cathodes (LiMn2O4), up to a total of 10 for the maximum value of Em ¼ 529 Wh/kg for sulfur cathodes (Li2S). The characteristics of the evaluated anode and cathode materials are listed in Table 2 and Table 3. Spider diagrams are used to compare the valuation factors of different electrode materials.

T. Bergholz et al. / Journal of Power Sources 320 (2016) 286e295

3.1. Benchmarking of anodes The minimum and maximum values of the derived valuation factors with the defined benchmarking scale for anodes are listed in Table 4. The maximum gravimetric and volumetric energy density is attained by elementary Li, while the values for Li4Ti5O12 represent the minimum for both factors, which is due to its low specific capacity and high potential [43]. Carbon fibers have the largest diffusion coefficient of the evaluated anodes (DLi z 107 cm2 s1). Thus, the parameter for the particle size to enable a DDoD of 10% during a current pulse of 10 C, dHEV is maximal. The value of Li4Ti5O12 considerably decreases due to a lower DLi z 1011 cm2 s1 of the spinel type material [64]. An opposing trend can be observed for the electrochemical stability parameter against conventional electrolytes DUred. Whereas anodeelectrolyte side reactions are not considered for Li4Ti5O12 [65], other anodes form an SEI layer, which limits possible particle size [21]. Li anodes with a constant potential of 4an ¼ 0 V show the largest integrated potential difference with respect to the cathodic stability of the electrolyte. The same DUred scale is implemented for the lifetime evaluation. Together with a maximal F(DUred), Li4Ti5O12 also features a minimal volume effect DV and maximal onset temperature for thermal electrolyte decomposition Tonset [66]. Li uptake into Si under alloy formation is accompanied by markedly increased volume effects of DV ¼ 312%. This value represents the minimum F(DV) factor. The increased thermal reactivity against the electrolyte of Sibased electrodes leads to minimum valuation factors F(DH) and F(Tonset) within the evaluation of the safety [50]. Melting must be considered for the metals Li and Sn below Tmelt < 250  C. For the other anodes, no melting occurs within the normal temperature range of Tmelt > 800  C. The benchmarks for the evaluation of the material costs and reserves are specified by intercalation cathodes and Li. The annual raw material production, Ahprod/a and minable resources, Ahprod/a are both limited by the electrodes' Li content. Fig. 7 contains the resulting benchmark diagram for exemplary anode materials: graphite (mesocarbon microbeads MCMB, blue), Li4Ti5O12 (green), TiO2 (light green), Sn (orange), Si (black) and Li (red). As is shown in Table 1, energy is an important valuation factor for the application in BEVs. It declines in the order: Li > Si > Sn > graphite > TiO2 > Li4Ti5O12 for both EV and Em. Compared to conventional graphite, Si anodes promise a significant gain in the gravimetric and volumetric energy density at the cell level of about ~100 Wh/kg and ~130 Wh/L. The increase for Sn electrodes is comparably low (~50 Wh/kg and 20 Wh/L), which is due to the considerably smaller specific capacity with respect to silicon (Qm ¼ 600 vs 1400 A h/kg). Introduction of the elementary Li as an anode allows a doubling of Em and an 83% increase of EV. The introduction of Ti-based anodes leads to a decrease of around 50% for Em and EV compared to graphite. TiO2 offers an enhanced energy density with respect to Li4Ti5O12, because its specific capacity is

291

significantly higher. The calculated EV and Em characteristics amount to about one fourth to one half of the theoretical values E0V and E0m, normally discussed in literature. This is due to a decrease in the capacity, voltage and tap density, as well as the introduction of passive materials, considered in Eq. (2) and Eq. (3). In comparison to real cells, the values represent upper limits due to the low current rate C/10, where the capacity Qm and the voltage UCCV are defined. Furthermore, an ideal ratio between anode and cathode and no aging reserve is considered. Thus, graphite, Li, Si and Sn anodes theoretically achieve the energy requirement for BEVs of Em > 250 Wh/kg. The Ti-based oxides, Li4Ti5O12 and TiO2, reach the significantly reduced requirement of the stationary PV system and marginally fail the target values of HEV (Em(Li4Ti5O12) ¼ 109 Wh/kg) and the LHEV systems (Em(TiO2) ¼ 134 Wh/kg), respectively. Graphite offers a high power evaluation. The comparable low factor for F(DUred) can be balanced by a large particle size, F(dHEV) ¼ 10, due to the high DLi. Furthermore, the anode promises low cell costs and utilizes abundant raw materials. Its drawbacks are a low Tonset and high DUred, indicating that electrochemical and thermal electrolyte degradation limit the safety and lifetime of the resulting cells. The literature consistently focuses on the stabilization of the electrolyte to optimize graphite-based cells [67], which enable reasonable lifetimes and safety properties. Elementary Li obtains a high power, cost and raw material evaluation. Issues are mainly connected to its safety, i.e., a low Tmelt together with dendrite formation, which lead to possible thermal runaways. Additionally, the lifetime of resulting cells is limited due to high DV and DUred, as well as dendrite formation. Melting and dendrite formation can only be overcome with the application of solid state electrolytes [68]. Together with the high DV, the outlook of this approach seems to be questionable. Thus, lithium as an anode material is excluded for all applications. Other than high energy densities, Si electrodes promise low cost cells on the basis of abundant raw materials. Due to the high DV, along with a low Tonset ¼ 80  C, the lifetime evaluation compared to graphite is decreased. Additionally, F(dHEV) for Si is considerably lower. Both factors, DV and the low DLi, lead to the necessary utilization of nano-sized materials. This contradicts the comparable low factors for thermal (DH, Tonset) and electrochemical (DUred) electrolyte reactivity, which limit the safety and lifetime evaluation. In conclusion, the implementation of nano-composites [69] and stable electrolyte systems [49] could lead to the desired breakthrough of Si anodes. Nevertheless, safety and lifetime are expected to be lower by comparison to graphite-based cells. Sn electrodes promise only slightly enhanced energy compared to graphite. The markedly decreased predicted lifetime, caused by a high DV ¼ 258%, as well as the lowered cost and raw material evaluation factors, are arguments against the material. Furthermore, possible melting downgrades the safety evaluation compared to graphite, in spite of the higher F(Tonset) and F(DH) values.

Table 2 Characteristics for the evaluation of anode materials based on a literature survey. Anodes MCMB (graphite) Li4Ti5O12 TiO2 Li Si Sn a b

Em/Wh/kg EV/Wh/L dHEV/mm DLi a/cm2/s 263 109 134 520 371 314

488 183 233 897 621 510

1630 3.1 13.5 n.a. 12.3 146

1.1  1.2  2.2  n.a. 1.9  2.6 

6

DUred/V Tonset/ C

0.19 10 1011 <0 10 10 <0 0.4 1010 0.012 108 0.014

80 [45] 125 [43] 125b >125 [48] 80 [49] n.a.

Tmelt/ C DH/J/g

DV/%

$kWh/$/kWh [44] Ahres/PAh [42] Ahprod/a/TAh [42]

n.a. n.a. n.a. 180 e 232

10 [46] 0 [47] 3 100 312 [51] 258 [52]

<2.5 23.8 14.5 5.6 <2.5 11.6

1500 [45] 325 [43] 325b 2900 [48] 5100 [50] n.a.

Averaged value of DLi based on at least three different literature studies at room temperature for completely lithiated materials. Based on thermal reactivity of Li4Ti5O12 electrodes.

>67 49 >67 64 >67 5

>98 71 >98 93 >98 250

292

T. Bergholz et al. / Journal of Power Sources 320 (2016) 286e295

Table 3 Characteristics for the evaluation of cathode materials based on a literature survey. Em/Wh/ kg

EV/Wh/ dHEV/ L mm

DLi a/cm2/s DUox/ Tonset/  V C

DH/J/g

267

534

1.6  108 0.37

760 [53] 290 [54] n.a.

Li2MnO3$LiNi0.5Co0.5O2 384

516

Cathodes LiCoO2

116 4.6

2.6  1011 0.19 11

LiNi0.5Mn1.5O4

332

516

6.4

9.6  10

LiFePO4

258

370

1.8

2.5  1012 0.01

Li2S

529

329

0.03 1  1015

a

0.93

<0

180 [53] 198 [56] 80 [58] 250 [58] 115 [29]

Tdecomp/  C

1015 214 [15] [56] 630 [58] >300 [59] 147 [53] >300 [60] n.a. n.a.

Tmelt/ C



n.a.

xLi-rev

DV/%

0.5 [53] 0

1.9 [55] 62.1

$kWh/$/kWh [44]

0.2 [57] 12.9

Ahres/PAh [42] 1.6

Ahprod/a/TAh [42] 20

44

64

8.2

67

97

n.a.

0 [58] 2.3 [59] 0 [61] 6.8 [62]

4.1

67

98

115

0

7.1

64

93

n.a.

80 [63]

Averaged value of DLi based on at least three different literature studies at room temperature for the completely lithiated materials.

Table 4 Scaling of the valuation factors for the benchmarking of anodes with minimum (char1) and maximum (char10) values.

Em/Wh/kg EV/Wh/L dHEV/mm DUred/V Tonset/ C DUred/V DV/% Tonset/ C Tmelt/ C DH/J/g $KWh/$/kWh Ahres/PAh Ahprod/a/TAh

char1

char10

category

109 (Li4Ti5O12) 190 (Li4Ti5O12) 9.9 (Li4Ti5O12) 0.4 (Li) 80 (graphite, Si) 0.4 (Li) 312 (Si) 80 (graphite, Si) 181 (Li) 5100 (Si) 62.1 (LiCoO2) 1.6 (LiCoO2) 20 (LiCoO2)

520 (Li) 897 (Li) 270 (graphite) 0 (Li4Ti5O12) 125 (Li4Ti5O12) 0 (Li4Ti5O12) 0 (Li4Ti5O12) 125 (Li4Ti5O12) 800 325 (Li4Ti5O12) 4.0 (LiMn2O4) 67 (Li) 98 (Li)

Energy Power Lifetime

Safety

Costs Raw materials

Li4Ti5O12 offers the highest lifetime and safety evaluation due to negligible reactivity with the electrolyte (F(DUred) ¼ F(Tonset) ¼ F(DH) ¼ 10) and a high bulk stability (F(DV) ¼ F(Tmelt) ¼ 10). Thus, its low DLi can be balanced by the reduction of the particle size. This results in a high predicted power density, which corresponds well to the literature [70]. Despite the low energy density, the anode is an interesting material for HEV applications, which do not emphasize the criteria for the available battery space and mass. According to Roland Berger, 26% of the total cell costs are

defined by raw material costs [71]. Taking into account a share of 50% for the active anode material, the raw material costs must be lower than 15.6 $/kWh to reach the target value of 120 $/kWh for the stationary system (see Table 1). In conclusion, cells based on Li4Ti5O12 ($KWh ¼ 23.8 $/kWh) cannot reach the desired cost targets. Activation of inactive Li equivalents in TiO2 (LixTiO2, x ¼ 0.15e1) [72] result in a cost reduction, an increased energy density and raw material valuation factor. The power evaluation of the material is increased compared to Li4Ti5O12, due to a higher DLi. In contrast, its lifetime evaluation is lowered, connected to a comparable high DV ¼ 3%. Thus, TiO2 resembles a suitable anode for HEVs and stationary systems. 3.2. Benchmarking of cathodes The respective benchmarking scale for cathodes is listed in Table 5. The maximum gravimetric energy density is reached by sulfur (Em ¼ 520 Wh/kg), while its low communicated tap density (rtap ¼ 0.79 g/cm3) [73] is the reason why LiCoO2 cathodes offer considerably higher volumetric energy densities (EV ¼ 329 vs. 534 Wh/L). LiMn2O4 cathodes represent the minimum for both factors. LiCoO2 also has the largest DLi, i.e., the highest F(dHEV). The electronic isolator Li2S resembles the minimum for the parameter, while the scale for F(DUox) varies from the value of Li2S, which is electrochemically stable against the electrolyte to the value of the high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) with an electrode potential of 4.7 V. Due to its high reactivity against the electrolyte,

Fig. 7. Benchmark of exemplary anode materials.

T. Bergholz et al. / Journal of Power Sources 320 (2016) 286e295 Table 5 Scaling of the valuation factors for the benchmarking of cathodes with minimum (char1) and maximum (char10) values.

Em/Wh/kg EV/Wh/L dHEV/mm DUox/V Tonset/ C DUox/V DV/% Tonset/ C Tmelt/ C Tdecomp/ C DH/J/g xLi-rev $KWh/$/kWh Ahres/PAh Ahprod/a/TAh

char1

char10

category

219 (LiMn2O4) 289 (LiMn2O4) 0.03 (LiFePO4) 0.93 (LNMO) 80 (LNMO) 0.93 (LNMO) 80 (Li2S) 80 (graphite) 181 (Li) 120 (LiMnPO4) 1600 (LiNiO2) 0.5 (LiCoO2) 62.1 (LiCoO2) 1.6 (LiCoO2) 20 (LiCoO2)

529 (Li2S) 534 (LiCoO2) 116 (LiCoO2) 0 (Li2S) 250 (LiFePO4) 0 (Li2S) 0 (Li4Ti5O12 125 (Li4Ti5O12) 800 385 (LiMnO2) 147 (LiFePO4) 1 (LiFePO4) 4.0 (LiMn2O4) 67 (Li) 98 (Li)

Energy Power Lifetime

Safety

Costs Raw materials

charged LNMO also represents the minimum for F(Tonset). The respective maximum is reached by LiFePO4, corresponding to the high value for F(DUox). For the same reason, the material shows the highest F(DH) [53]. The respective minimum is resembled by charged LiNiO2 cathodes [53]. The parameter F(xLi-rev), which limits both lifetime and safety, is minimal for LiCoO2 and maximal for LiFePO4 and LNMO. The scale for the decomposition temperature of the bulk materials, F(Tdecomp) starts at 120  C for the high voltage olivine-type material MnPO4 [53] and lasts up to 385  C for Li0.5MnO2 with a layered structure [30]. The range for F(DV) for cathodes is markedly decreased compared to the anode materials. The minimum F(DV) is resembled by Li2S (DV ¼ 80%) and the maximum parameter by the zero strain material Li4Ti5O12, which shows no volume effect during cycling. Fig. 8 illustrates the resulting benchmark of exemplary cathode materials: LiCoO2 (blue), LiNi0.5Mn1.5O4 (orange), LiFePO4 (green), Li2MnO3$LiNi0.5Co0.5O2 (red), Li2S (black). The Em values decline in the order: LiFePO4 > LiCoO2 > LiNi0.5Mn1.5O4 > Li2MnO3$LiNi0.5Co0.5O2 > Li2S. All considered cathodes theoretically reach the Em target value of BEVs. If graphite is used as the anode instead of Si, LiNi0.5Mn1.5O4, Li2MnO3$LiNi0.5Co0.5O2 and Li2S attain a higher value than 250 Wh/ kg. The transition from intercalation-based cathodes to Li2S brings

293

about an increase of Em around 100% for second generation cathodes (LiFePO4, LiCoO2) and of 40e60% for third generation systems (LiNi0.5Mn1.5O4, Li2MnO3$LiNi0.5Co0.5O2). Due to the variations of rtap, the order for EV is changed: LiCoO2 > LiNi0.5Mn1.5O4 z Li2MnO3$LiNi0.5Co0.5O2 > LiFePO4 > Li2S. In contrast to Em, the EV of Li2S is reduced by 40 Wh/L compared to LiFePO4. Therefore, the potential of sulfur cathodes for transport applications (BEV, LHEV and HEV), where the installation space for energy storage is limited, is low compared to most of the intercalation-based cathodes. Further arguments against Li2S are safety and lifetime issues relating to the low Tmelt, high DH, low Tonset and high DV. Thus, sulfur as a cathode material is excluded for all applications. Due to its high energy density, the high capacity cathode Li2MnO3$LiNi0.5Co0.5O2 is a promising material for BEV batteries. The evaluation of lifetime, costs and raw materials show medium values compared to other cathodes. The drawback of a high DH compared to the other intercalation-based electrodes can perhaps be solved through the stabilization of the electrolyte/electrode interface by optimizing the electrolyte. This approach also decreases DUox and thus increases the power density evaluation, which is otherwise reduced, due to the comparably low DLi of Li2MnO3$LiNi0.5Co0.5O2. The stability of the electrolyte and the resulting low evaluation factors for F(DUox), F(Tonset) and F(DH) also represents the main drawback of the high voltage spinel LiNi0.5Mn1.5O4. It shows a higher bulk stability (F(Tdecomp) ¼ F(xLi-rev) ¼ 10) compared to Li2MnO3 LiNi0.5Co0.5O2, as well as lower costs and more abundant raw materials. Therefore, LNMO represents an interesting cathode for BEVs if the electrolyte/electrode interface can be stabilized. Both cathodes offer higher safety evaluation factors compared to conventional LiCoO2. The latter entails problems related to a low bulk stability, F(xLi-rev) ¼ 1, F(Tdecomp) ¼ 6.7 and a comparably high thermal reactivity with conventional electrolytes (Tonset ¼ 180  C, DH ¼ 760 J/g). LiCoO2 would be an interesting cathode for HEVs due to its high F(dHEV) ¼ 10. The safety issues could probably be overcome by adopting Li4Ti5O12 as an anode because the higher safety evaluation compared to graphite hinders the initiation of a thermal runaway (see also Fig. 5). The resulting energy densities amount to 107 Wh/kg and 200 Wh/L. These do not reach the target values of HEVs. Due to the high specific material costs and limited Co

Fig. 8. Benchmark of exemplary cathode materials.

294

T. Bergholz et al. / Journal of Power Sources 320 (2016) 286e295

reserves, aside from the safety issues, LiCoO2 is excluded for stationary applications and BEVs. The same argumentation used for Li4Ti5O12 anodes can explain the high potential of LiFePO4 cathodes for HEV batteries. The low DLi can be balanced by a reduction of particle size, due to negligible reactivity with the electrolyte (F(DU) ¼ 9.9). Unfortunately, the required energy density for HEVs or the stationary application is not fulfilled with Li4Ti5O12 anodes on the cell level (Em ¼ 84 W/kg, EV ¼ 186 Wh/L). Therefore, LiFePO4-based LIBs for HEVs or stationary systems must be constructed with graphite anodes, which results in energy densities of Em ¼ 209 W/kg and EV ¼ 338 Wh/L. A combination with a TiO2 anode fulfills the energy density requirement of the stationary system. In addition to a high forecasted lifetime and safety of LiFePO4, its cost and raw material evaluation is maximal. Thus, it represents the most interesting cathode for stationary systems. 4. Conclusion The benchmarking model presented in this study enables a systematic selection of electrode materials for lithium batteries in future hybrid fuel cell electric vehicles, battery electric vehicles and stationary systems. Based on the evaluation of accessible material characteristics from the literature, it was possible to implement comparable scales for the quantitative benchmarking of anode and cathode materials. The model considers parameters for energy density, power density, safety, lifetime, raw material costs and the accessibility of reserves. In contrast to theoretical thermodynamic values, as are normally discussed in the literature, the accessed input parameters represent the mean values of various material tests under real cell conditions. In order to reach the comparably high requirements for the power density, lifetime and safety of hybrid electric vehicles, a combination of graphite-based anodes and LiFePO4 cathodes is most suitable. Li4Ti5O12 anodes offer the highest lifetime and safety evaluation, together with a high predicted power density. Despite its low energy density, the anode is an interesting material for HEV applications, which do not prioritize the criteria for available battery space and mass. Stationary applications require batteries which electrode materials exhibit lower energy densities, together with an increased lifetime, cost and raw material evaluation. Despite the high predicted lifetime of Li4Ti5O12, cells based on this material fail the cost target value of 120 $/kWh and have comparably low raw material valuation factors. TiO2 anodes promise cells with lower costs, increased energy density and raw material evaluation. In contrast to a decreased lifetime connected to a higher volume effect during cycling, TiO2 also promises an increased power density compared to Li4Ti5O12. It is therefore the most suitable anode for stationary systems, as well as for hybrid electric vehicles. In combination with LiFePO4, an energy demand of 100 Wh/kg for the stationary system can be reached. The favored combinations resemble state-of-the-art materials, whereas novel cell chemistries have to be optimized for cells in battery electric vehicles. In contrast to actual research efforts, sulfur as a cathode material is excluded due to its low volumetric energy density and its known lifetime and safety issues. Lithium and tin as anode materials are also discarded due to safety issues linked to electrode melting and dendrite formation. A high capacity composite Li2MnO3$LiNi0.5Co0.5O2 and a high voltage spinel LiNi0.5Mn1.5O4 cathode with silicon as an anode material promises energy densities greater than 330 Wh/kg. As the lifetime and safety properties are significantly reduced compared to graphite, the desired breakthrough of Si anodes is questionable. The input quantities for the benchmarking model provide a set

of standardized material parameters with their basic influence on resulting lithium batteries, including reference values for a broad range of state-of-the-art and novel electrodes. In conclusion, it is possible to perform an integrated comparison of next generation electrode materials with state-of-the-art electrodes and to identify the main research areas for material optimization. Acknowledgements We would like to thank Thomas Grube and Christopher Wood for proofreading our work and providing input regarding the requirements of the applications considered. References [1] E. Karden, S. Ploumen, B. Fricke, et al., Energy storage devices for future hybrid electric vehicles, J. Power Sources 168 (2007) 2e11. [2] R.T. Doucette, M.D. McCulloch, A comparison of high-speed flywheels, batteries, and ultracapacitors on the bases of cost and fuel economy as the energy storage system in a fuel cell based hybrid electric vehicle, J. Power Sources 196 (2011) 1163e1170. [3] A. Chatzivasileiadi, E. Ampatzi, I. Knight, Characteristics of electrical energy storage technologies and their applications in buildings, Renew. Sustain. Energy Rev. 25 (2013) 814e830. [4] S. Wakabayashi, Recent Activities and Topics on Battery Technology in Japan, New Energy and Industrial Technology Development Organization, Japan, 2011. [5] D. Howell, Hybrid Electric Systems, Energy Efficiency & Renewable Energy, Department of Energy, USA, 2011. [6] N. Kimiaie, K. Wedlich, M. Hehmann, et al., Results of a 20,000 h Lifetime test of a 7 kW Direct methanol fuel cell (DMFC) hybrid system - degradation of the DMFC stack and the energy storage, Energy Environ. Sci. (2014) 1e12. [7] R.C. Camp, Benchmarking, Hanser, Camp, R. C., München, 1994. [8] O.K. Park, Y. Cho, S. Lee, et al., Who will drive electric vehicles, olivine or spinel? Energy & Environ. Sci. 4 (2011) 1621e1633. [9] A.J. Smith, J.C. Burns, S. Trussler, et al., Precision measurements of the Coulombic efficiency of lithium-ion batteries and of electrode materials for lithium-ion batteries, J. Electrochem. Soc. 157 (2010) A196eA202. [10] L. Gaines, R. Cuenca, Costs of Lithium-ion Batteries for Vehicles, Argonne National Laboratory, Argonne, 2000. [11] X.H. Liu, L. Zhong, L.Q. Zhang, et al., Lithium fiber growth on the anode in a nanowire lithium ion battery during charging, Appl. Phys. Lett. 98 (2011) 1e3. [12] L.-F. Cui, Y. Yang, C.-M. Hsu, et al., CarbonSilicon CoreShell nanowires as high capacity electrode for lithium ion batteries, Nano Lett. 9 (2009) 3370e3374. [13] A. Magasinski, P. Dixon, B. Hertzberg, et al., High-performance lithium-ion anodes using a hierarchical bottom-up approach, Nat. Mater. 9 (2010) 353e358. [14] Y.-L. Kim, Y.-K. Sun, S.-M. Lee, Enhanced electrochemical performance of silicon-based anode material by using current collector with modified surface morphology, Electrochim. Acta 53 (2008) 4500e4504. [15] J. Zheng, X. Wu, Y. Yang, Improved electrochemical performance of Li [Li0.2Mn0.54Ni0.13Co0.13]O2 cathode material by fluorine incorporation, Electrochim. Acta 105 (2013) 200e208. [16] S.H. Park, S.H. Kang, C.S. Johnson, et al., Lithiumemanganeseenickel-oxide electrodes with integrated layeredespinel structures for lithium batteries, Electrochem. Commun. 9 (2007) 262e268. [17] Y.-S. Hong, Y.J. Park, K.S. Ryu, et al., Charge/discharge behavior of Li [Ni0.20Li0.20Mn0.60]O2 and Li[Co0.20Li0.27Mn0.53]O2 cathode materials in lithium secondary batteries, Solid State Ionics 176 (2005) 1035e1042. [18] M. Park, X. Zhang, M. Chung, et al., A review of conduction phenomena in Liion batteries, J. Power Sources 195 (2010) 7904e7929. [19] H. Mehrer, Diffusion in Solids, Springer-Verlag, Berlin, 2007. Springer. [20] X. Rui, X. Zhao, Z. Lu, et al., Olivine-type nanosheets for lithium ion battery cathodes, ACS Nano 7 (2013) 5637e5646. [21] G. Ning, B.N. Popov, Cycle life modeling of lithium-ion batteries, J. Electrochem. Soc. 151 (2004) A1584eA1591. [22] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chem. Rev. 104 (2004) 4303e4418. [23] S.K. Martha, E. Markevich, V. Burgel, et al., A short review on surface chemical aspects of Li batteries: a key for a good performance, J. Power Sources 189 (2009) 288e296. [24] A. Funabiki, M. Inaba, Z. Ogumi, et al., Impedance study on the electrochemical lithium intercalation into natural graphite powder, J. Electrochem. Soc. 145 (1998) 172e178. [25] R. Spotnitz, J. Franklin, Abuse behavior of high-power, lithium-ion cells, J. Power Sources 113 (2003) 81e100. [26] S.M. Rezvanizaniani, Z. Liu, Y. Chen, et al., Review and recent advances in battery health monitoring and prognostics technologies for electric vehicle (EV) safety and mobility, J. Power Sources 256 (2014) 110e124.

T. Bergholz et al. / Journal of Power Sources 320 (2016) 286e295 [27] S.K. Martha, O. Haik, E. Zinigrad, et al., On the thermal stability of olivine cathode materials for lithium-ion batteries, J. Electrochem. Soc. 158 (2011) A1115eA1122. [28] W. Haiyan, T. Aidong, H. Kelong, Thermal behavior investigation of LiNi1/3Co1/ 3Mn1/3O2 based Li ion battery under overcharged test, Chin. J. Chem. 29 (2011) 27e32. [29] Y.V. Mikhaylik, I. Kovalev, R.N. Schock, et al., High rechargeable Li-S cells for EV application. Status, remainig problems and solutions, ECS Trans. 25 (2010) 23e34. [30] P. Arora, R.E. White, M. Doyle, Capacity fade mechanisms and side reactions in lithium-ion batteries, J. Electrochem. Soc. 145 (1998) 3647e3667. [31] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, et al., Li-O2 and Li-S batteries with high energy storage, Nat. Mater. 11 (2012) 19e29. [32] C.-K. Huang, J.S. Sakamoto, J. Wolfenstine, et al., The limits of low-temperature performance of Li-Ion cells, J. Electrochem. Soc. 147 (2000) 2893e2896. [33] T. Sasaki, T. Nonaka, H. Oka, et al., Capacity-fading mechanisms of LiNiO2based lithium-ion batteries, J. Electrochem. Soc. 156 (2009) A289eA293. [34] S.-T. Myung, Y. Hitoshi, Y.-K. Sun, Electrochemical behavior and passivation of current collectors in lithium-ion batteries, J. Mater. Chem. 21 (2011) 9891e9911. k, M.R. Wagner, et al., Ageing mechanisms in lithium-ion [35] J. Vetter, P. Nova batteries, J. Power Sources 147 (2005) 269e281. [36] S.D. Beattie, D. Larcher, M. Morcrette, et al., Si electrodes for Li-Ion batteries e a New way to look at an old problem, J. Electrochem. Soc. 155 (2008) A158eA163. [37] S. Sankarasubramanian, B. Krishnamurthy, A capacity fade model for lithiumion batteries including diffusion and kinetics, Electrochim. Acta 70 (2012) 248e254. [38] G. Dahlin, K. Strom, Lithium Batteries, Nova Science Publishers, Inc., New York, 2010. [39] J. Wang, P. Liu, J. Hicks-Garner, et al., Cycle-life model for graphite-LiFePO4 cells, J. Power Sources 196 (2011) 3942e3948. [40] S.-H. Kang, W. Lu, K.G. Gallagher, et al., Study of Li1þx(Mn4/9Co1/9Ni4/9)1-xO2 cathode materials for vehicle battery applications, J. Electrochem. Soc. 158 (2011) A936eA941. [41] C. Pillot, The Worldwide Battery Market 2011-2025, Batteries 2012, avicenne Energy, Nice, 2012. [42] USGS, Mineral Commodity-Summaries 2011, 2011, pp. 1e201. [43] I. Belharouak, Y.-K. Sun, W. Lu, et al., On the safety of the Li4Ti5O12/LiMn2O4 lithium-ion battery system, J. Electrochem. Soc. 154 (2007) A1083eA1087. [44] D.U. Sauer, Elektrische Energiespeicher Kondensatoren Supraleitende Spulen, Seminar für Kraftfahrzeug- und Motorentechnik, ISEA, Berlin, 2009. [45] D. Saito, Y. Ito, K. Hanai, et al., Carbon anode for dry-polymer electrolyte lithium batteries, J. Power Sources 195 (2010) 6172e6176. [46] M.R. Wagner, P.R. Raimann, A. Trifonova, et al., Dilatometric and mass spectrometric investigations on lithium ion battery anode materials, Anal. Bioanal. Chem. 379 (2004) 272e276. [47] C.Y. Ouyang, Z.Y. Zhong, M.S. Lei, Ab initio studies of structural and electronic properties of Li4Ti5O12 spinel, Electrochem. Commun. 9 (2007) 1107e1112. [48] K. Sato, I. Yamazaki, S. Okada, et al., Mixed solvent electrolytes containing fluorinated carboxylic acid esters to improve the thermal stability of lithium metal anode cells, Solid State Ionics 148 (2002) 463e466. [49] I.A. Profatilova, C. Stock, A. Schmitz, et al., Enhanced thermal stability of a lithiated nano-silicon electrode by fluoroethylene carbonate and vinylene carbonate, J. Power Sources 222 (2013) 140e149. [50] Y.-S. Park, S.-M. Lee, Thermal stability of lithiated silicon anodes with electrolyte, Bull. Korean Chem. Soc. 32 (2011) 145e148. [51] L.Y. Beaulieu, K.W. Eberman, R.L. Turner, et al., Colossal reversible volume

295

changes in lithium alloys, Electrochem. Solid-State Lett. 4 (2001) A137eA140. [52] B.A. Boukamp, G.C. Lesh, R.A. Huggins, All-solid lithium electrodes with mixed-conductor matrix, J. Electrochem. Soc. 128 (1981) 725e729. [53] G. Chen, T.J. Richardson, Thermal instability of olivine-type LiMnPO4 cathodes, J. Power Sources 195 (2010) 1221e1224. re s, et al., On safety of lithium-ion cells, J. Power [54] P. Biensan, B. Simon, J.P. Pe Sources 81e82 (1999) 906e912. [55] H. Wang, Y.-I. Jang, B. Huang, et al., Tem study of electrochemical cyclinginduced damage and disorder in LiCoO2 cathodes for rechargeable lithium batteries, J. Electrochem. Soc. 146 (1999) 473e480. [56] F. Amalraj, M. Talianker, B. Markovsky, et al., Study of the lithium-rich integrated compound xLi2MnO3$(1-x)LiMO2 (x around 0.5; M ¼ Mn, Ni, Co; 2:2:1) and its electrochemical activity as positive electrode in lithium cells, J. Electrochem. Soc. 160 (2013) A324eA337. [57] W. Lu, Screen Electrode Materials & Cell Chemistries and Streamlining Optimization of Electrode, Vehicle Technologies Program, Argonne National Laboratory, Washington, 2012. [58] F. Zhou, X. Zhao, A. van Bommel, et al., Comparison of Li[Li1/9Ni1/3Mn5/9]O2, Li [Li1/5Ni1/5Mn3/5]O2, LiNi0.5Mn1.5O4, and LiNi2/3Mn1/3O2 as high voltage positive electrode materials, J. Electrochem. Soc. 158 (2011) A187eA191. [59] A. Bhaskar, W. Gruner, D. Mikhailova, et al., Thermal stability of Li1DM0.5Mn1.5O4 (M ¼ Fe, Co, Ni) cathodes in different states of delithiation D, RSC Adv. 3 (2013) 5909e5916. [60] D.D. MacNeil, Z. Lu, Z. Chen, et al., A comparison of the electrode/electrolyte reaction at elevated temperatures for various Li-ion battery cathodes, J. Power Sources 108 (2002) 8e14. [61] S.K. Martha, J.O. Kiggans, J. Nanda, et al., Advanced lithium battery cathodes using dispersed carbon fibers as the current collector, J. Electrochem. Soc. 158 (2011) A1060eA1066. [62] M.S. Islam, D.J. Driscoll, C.A.J. Fisher, et al., Atomic-scale investigation of defects, dopants, and lithium transport in the LiFePO4 olivine-type battery material, Chem. Mater. 17 (2005) 5085e5092. [63] Y. Yang, G. Zheng, Y. Cui, Nanostructured sulfur cathodes, Chem. Soc. Rev. 42 (2013) 3018e3032. [64] K.T. Fehr, M. Holzapfel, A. Laumann, et al., DC and AC conductivity of Li4/3Ti5/ 3O4 spinel, Solid State Ionics 181 (2010) 1111e1118. [65] K. Zaghib, M. Dontigny, A. Guerfi, et al., Safe and fast-charging Li-ion battery with long shelf life for power applications, J. Power Sources 196 (2011) 3949e3954. [66] I. Belharouak, Y.-K. Sun, W. Lu, et al., On the safety of the Li4Ti5O12/LiMn2O4 lithium-ion battery system, J. Electrochem. Soc. 154 (2007) A1083eA1087. [67] G.G. Eshetu, S. Grugeon, G. Gachot, et al., LiFSI vs. LiPF6 electrolytes in contact with lithiated graphite: comparing thermal stabilities and identification of specific SEI-reinforcing additives, Electrochim. Acta 102 (2013) 133e141. [68] H. Kim, G. Jeong, Y.-U. Kim, et al., Metallic anodes for next generation secondary batteries, Chem. Soc. Rev. 42 (2013) 9011e9034. [69] H.M. Jeong, S.Y. Lee, W.H. Shin, et al., Silicon@porous nitrogen-doped carbon spheres through a bottom-up approach are highly robust lithium-ion battery anodes, RSC Adv. 2 (2012) 4311e4317. [70] Z. Chen, I. Belharouak, Y.K. Sun, et al., Titanium-based anode materials for safe lithium-ion batteries, Adv. Funct. Mater. 23 (2013) 959e969. [71] C. Kruger, Value Chain, Kraftwerk Batterie, Roland Berger, Aachen, 2011. [72] A.G. Dylla, G. Henkelman, K.J. Stevenson, Lithium insertion in nanostructured TiO2(B) architectures, Acc. Chem. Res. 46 (2013) 1104e1112. [73] X. Tao, F. Chen, Y. Xia, et al., Decoration of sulfur with porous metal nanostructures: an alternative strategy for improving the cyclability of sulfur cathode materials for advanced lithium-sulfur batteries, Chem. Commun. 49 (2013) 4513e4515.