Nickel-based batteries for medium- and large-scale energy storage

Nickel-based batteries for medium- and large-scale energy storage

4

Z. Huang1, G. Du2 1 University of Wollongong, North Wollongong, NSW, Australia; 2Baosteel Group Corporation, Shanghai, China

4.1

Introduction

Nickel-based batteries include nickel-cadmium (commonly denoted by Ni-Cd), nickel-iron (Ni-Fe), nickel-zinc (Ni-Zn), nickel-hydrogen (Ni-H2), and nickel metal hydride (Ni-MH). All these batteries employ nickel oxide hydroxide (NiOOH) as the positive electrode, and thus are categorized as nickel-based batteries. Their performance, and consequently their application in the energy storage market, however, varies greatly. Among them, only Ni-Cd and Ni-MH were once routinely used for portable electronic devices, power backup, and vehicle propulsion applications. This chapter will thus focus on Ni-Cd and Ni-MH, with a brief description of Ni-H2, Ni-Fe, and Ni-Zn. The chemical reaction occurring in Ni-H2 can be expressed as 2NiOOH þ H2 Ð 2NiðOHÞ2

(4.1)

During discharge, NiOOH is reduced to Ni(OH)2 and H2 is consumed. A reverse reaction occurs during charge (Shukla et al., 2001). The Ni-H2 battery is exclusively used for aerospace applications such as satellites, because it features a cycle life longer than any other maintenance-free secondary batteries, high gravimetric energy density, high power density, and good tolerance to overcharge and reversal. The intrinsic problems, however, such as low volumetric energy density due to gaseous hydrogen, high self-discharge rate, and high cost, preclude it from being widely used in other fields. Ni-Fe relies on the following reaction to store and deliver energy. 2NiOOH þ Fe þ 2H2 O Ð 2NiðOHÞ2 þ FeðOHÞ2

(4.2)

Discharge leads to the reduction of NiOOH and oxidization of Fe (Shukla et al., 2001). Ni-Fe is a very robust battery that features a very long cycle life, high tolerance of abuse, and environmental friendliness. It, however, suffers from disadvantages that include very low energy density, poor power density, and poor charge retention. Advances in Batteries for Medium- and Large-scale Energy Storage. http://dx.doi.org/10.1016/B978-1-78242-013-2.00004-2 Copyright © 2015 Elsevier Ltd. All rights reserved.

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Traditionally Ni-Fe has been used in very limited applications, such as in mining, but recently it has been revisited for storing energy from wind and solar installations, where density is not highly critical. Ni-Zn operates on the following reaction, 2NiOOH þ Zn þ 2H2 O Ð 2NiðOHÞ2 þ ZnðOHÞ2

(4.3)

which is very similar to that of Ni-Fe. The replacement of Fe by Zn, however, causes a short cycle life due to the growth of Zn dendrites, which lead to electrical shorting (Shukla et al., 2001). By improving the electrolyte, this problem has now been eliminated (PowerGenix, 2014). Coupled with advances in both the positive and negative electrode compositions, Ni-Zn now is commercially available. Because it is competing directly with mainstream Li-ion and Ni-MH batteries, its future is uncertain. Ni-Cd batteries offer excellent cycle life, good low-temperature performance, and exceptional tolerance of high discharge rates, combined with versatility in size, ranging from small sealed types to large vented cells. Ni-Cd batteries were once the dominant choice for both portable and standby power supplies. The toxicity of Cd, however, plus direct competition from Ni-MH and lithium ion batteries, has decreased their popularity since the 1990s. Today, although highly robust, the Ni-Cd battery is mainly limited to specialty applications. Ni-MH batteries have a close resemblance to Ni-Cd in configuration, and they both use basically the same positive electrode materials and electrolyte, with the key difference being in the negative electrode materials. The Ni-MH battery has several key advantages over the Ni-Cd, such as minimal environmental impact and higher capacity. After decades of research, key technical improvements have been achieved in terms of both cycle life and charge retention. Since the 1990s, the Ni-MH battery has been the primary choice for portable electronics and power tools. It is now routinely used in commercial hybrid electric vehicles (HEVs), because it offers very desirable attributes, including high energy and power, excellent performance over a range of operating temperatures, good tolerance of abuse, and proven safety. Toyota has been using Ni-MH batteries for its Prius, which has received wide acclaim for its fuel efficiency. The later advent of the rechargeable lithium-ion battery has been very challenging to both Ni-Cd and Ni-MH batteries. Due to its high gravimetric energy density and excellent cycling performance, markets such as computers and cell phones that were once dominated by Ni-MH and Ni-Cd have been largely taken over by Li-ion batteries. The proven robust performance of both Ni-MH and Ni-Cd under various conditions, however, makes these batteries still the preferred choice for certain applications. Continuous performance improvement and cost reduction would likely see these batteries accepted as very good alternatives to Li-ion batteries, which have yet to put their performance under stringent testing over decades.

Nickel-based batteries for medium- and large-scale energy storage

4.2

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Basic battery chemistry

4.2.1

Ni-Cd battery

The operation of the nickel-cadmium battery is based upon the redox reaction between nickel oxide hydroxide and cadmium. The key active units in a fully charged cell include a positive nickel oxide hydroxide electrode, a negative cadmium electrode, a separator, and an alkaline electrolyte that is normally potassium hydroxide. During discharge, the following reaction occurs at the positive electrode: NiOðOHÞ þ H2 O þ e ! NiðOHÞ2 þ OH

(4.4)

The corresponding oxidation reaction at the negative electrode is Cd þ 2OH ! CdðOHÞ2 þ 2e

(4.5)

The net reaction during discharge is 2NiOðOHÞ þ Cd þ 2H2 O ! 2NiðOHÞ2 þ CdðOHÞ2

(4.6)

The reverse reaction takes place during charge (Berndt, 2003). When Ni-Cd batteries are overcharged, oxygen evolution takes place at the nickel electrode: 4OH ! 2H2 O þ O2 þ 4e

(4.7)

In the sealed Ni-Cd batteries, oxygen migrates to the cadmium electrode and reacts with Cd, producing cadmium hydroxide (Figure 4.1). Cd þ 1=2O2 þ H2 O ! CdðOHÞ2

(4.8)

Oxygen can also be released in a vented cell (also known as a wet cell or flooded cell) when large capacities are required. This, however, means loss of electrolyte over time, which necessitates periodic maintenance. To ensure long cycle life, the Ni-Cd battery is designed to be positive limited, that is, the capacity of the negative electrode is larger than that of the positive. The extra capacity reserve of the negative electrode Figure 4.1 Internal oxygen cycle in sealed Ni-Cd battery.

O2 Oxygen evolution 4OH– ® O2+2H2O+4e–

Oxygen reduction O2+2H2O+4e– ® 4OH–

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can not only consume oxygen generated at the positive electrode but also prevents hydrogen evolution by suppressing polarization of the negative electrode to a more negative potential (Berndt, 2003).

4.2.2

Ni-MH battery

Similar to the Ni-Cd battery, the positive electrode of the Ni-MH battery is nickel oxide hydroxide that is converted to nickel hydroxide during discharge. NiOðOHÞ þ H2 O þ e ! NiðOHÞ2 þ OH

(4.9)

The negative electrode is different, in that a hydrogen-absorbing alloy is used. During discharge, metal hydride (MH) is oxidized to metal alloy. MH þ OH ! M þ H2 O þ e

(4.10)

The overall reaction upon discharge is MH þ NiOOH ! M þ NiðOHÞ2

(4.11)

The reverse reaction takes place during charge (Berndt, 2003). The electrolyte in the Ni-MH battery bears a similarity to that in the Ni-Cd battery, which is often 30% potassium hydroxide in water. Lithium hydroxide is routinely added to promote charging efficiency at the nickel hydroxide electrode by suppressing the formation of oxygen. The “metal hydride” in many ways determines the performance of the Ni-MH battery. The “metal” M represents an intermetallic compound, and, based on the stoichiometric composition, the alloys are typically in the form of AB5 and AB2 (Kirchheim et al., 1982; Ovshinsky et al., 1993; Sapru et al., 1986). AB5 is represented by LaNi5, where La is often replaced by mischmetal (Mm), that is, a mixture of rare earths, predominantly Ce, La, and Nd, while Ni is partially replaced by Al, Si, Fe, Mn, Sn, etc. AB2 includes MgNi2, ZrV2, ZrCr2, TiV2, TiMn2, TiCr2, ZrNi2, etc. For both forms, substitution is often necessary to obtain the following desired properties (Anani et al., 1994): l

l

l

l

l

excellent reversibility of hydrogen absorption and desorption low hydrogen equilibrium pressure favorable kinetics for high-rate charge and discharge high electrochemical reactivity good corrosion resistance in alkaline solutions.

Similar to the Ni-Cd battery, when the Ni-MH battery is overcharged, oxygen is produced at the nickel electrode 4OH ! 2H2 O þ O2 þ 4e

(4.12)

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Figure 4.2 Internal oxygen and hydrogen cycles in Ni-MH batteries.

which will pass through the separator and recombine at the surface of the metal hydride electrode: 4MH þ O2 ! 4M þ 2H2 O

(4.13)

Upon overdischarge, the positive electrode is reversed to a lower potential, and hydrogen is produced 2H2 O þ 2e ! H2 þ 2OH

(4.14)

The metal alloy at the negative electrode will absorb the hydrogen (Figure 4.2). Overdischarge often occurs in cells connected in series, because the capacities of the individual cells often vary slightly. Similarly to Ni-Cd batteries, to maximize the cycle life of Ni-MH batteries, the negative electrode possesses higher capacity than the positive. This design can promote effective internal oxygen recombination during overcharge and hydrogen recombination during overdischarge. Furthermore, this configuration can safeguard the negative electrode by suppressing oxidation and corrosion.

4.3

Battery development and applications

The past century has seen crucial developments and tremendous success for both Ni-Cd and Ni-MH batteries. The development history features improvements in materials fabrication, the discovery of new electrode materials, and better battery design and assembly. Japanese battery manufacturers have been the major contributors. The strong electronics industry in Japan produces a large proportion of world’s

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portable electronics, such as digital cameras, mobile phones, computers, and camcorders, which directly drive the production of compact rechargeable batteries. These two types of batteries have been important players in the energy storage market.

4.3.1

Ni-Cd

The earliest Ni-Cd battery was reported by Waldmar Jungner in 1899 and was soon proven to be a robust system for electricity storage. Compared to the market leader back then, the lead-acid battery, the Ni-Cd battery shows better performance, because it has higher capacity, better long-term storage, more charge/discharge cycles, and a greater power-to-weight ratio.

4.3.1.1

Positive and negative electrodes

The positive electrode of an Ni-Cd battery is nickel hydroxide, the same as for NiMH batteries. Over the years, advanced processing has dramatically improved its capacity, power, discharge rate capability, and cycle life. The most common type, high-density spherical nickel hydroxide, is produced via precipitation. Nickel salts react with a caustic such as NaOH in the presence of ammonia. This method yields high-density spherical nickel hydroxide featuring highly suitable particle size, purity, crystallinity, tap density, and surface area, all of which are critical for capacity, utilization, power, and discharge rate capability. There are two dominant types of nickel electrodes, depending on the fabrication method, sintered or pasted. Sintered nickel electrode technology was developed in the 1920s and has been dominant for several decades. It features a porous nickel plaque of sintered high surface-area nickel particles impregnated with nickel hydroxide. Sintered electrodes have excellent rate and power capability, but are heavy and bulky (Puglisi, 2000). The pasted nickel electrodes, which were developed later, feature nickel hydroxide particles in close contact with a high-surface-area conductive network or substrate. Compared with sintered electrodes, pasted electrodes incur lower cost and have higher energy density. To tailor the electrode for specific applications, the electrode formula needs to be specially modified. For example, additives such as Ca(OH)2, CaF2, or Y2O3 can be introduced to suppress premature oxygen evolution for operation above 35  C (Ohta et al., 1998). Cobalt metal or oxides can be introduced to modify the conductive network. For ultra-high-power discharge, metallic nickel fibers can be added to the paste formula to enhance conductivity, but these will increase cost and reduce the capacity and specific energy. The basic active materials of the negative electrode are cadmium oxide or cadmium hydroxide. To improve the conductivity, nickel or graphite is blended in. Repeated charging and discharging will cause gradual cadmium crystal growth, which leads to a decrease in the active surface area and a consequent drop in capacity. This dissolution/precipitation may also lead to dendrites that can penetrate the separator and cause a short circuit. Furthermore, as the dendrites build up, the Ni-Cd battery will

Nickel-based batteries for medium- and large-scale energy storage

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Figure 4.3 Schematic diagram of Ni-Cd button design.

hold less and less of a full charge. To suppress the growth, additives such as polyvinyl alcohol and methyl cellulose are introduced (Shukla et al., 2001).

4.3.1.2 Classification Ni-Cd batteries are routinely constructed in button, cylindrical, and prismatic forms, very similar to the configurations adopted for sealed Ni-MH batteries. The button design is shown in Figure 4.3. Button cells are designed for small devices where there is very limited room. Coin cells are button cells that are very thin. The positive and negative electrodes are in the form of circular discs, with a separator in between. The assembly is kept in a nickel-plated cup to which electrolyte is added. The cylindrical and prismatic designs for the Ni-Cd battery are very similar to those for the Ni-MH and are shown in Figures 4.6 and 4.7. Ni-Cd cells can also be classified into two types: vented and hermetically sealed. In the sealed type, to enable effective gas recombination through the separator and prevent pressure buildup, the cells are not completely filled with electrolyte. In the vented design, a pressure valve is employed to release oxygen and hydrogen when the pressure is too high. Vented cells are mainly used for industrial applications, including railways, communications, emergency lighting, etc. This design requires periodic maintenance, because electrolyte will be lost over time. One advantage of the vented cell is its long-term reliability. By introducing a special fiber-mat separator and a large electrolyte reserve, Saft has been able to produce Ni-Cd batteries featuring long life and ultra-low maintenance, which results in much lower life-cycle costs than those for valve-regulated lead-acid (VRLA) batteries (Saft, 2005).

4.3.1.3 Application Ni-Cd batteries once had a dominant share of the market for rechargeable batteries. They were widely used for cameras, toys, flashlights, cordless telephones, and emergency lighting, among other uses. They were routinely used in power tools, because they

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can supply surge currents due to their low internal resistance. Being robust over decades also makes the Ni-Cd battery a good choice for aircraft, electric cars, and boats. Starting in the 1990s, due to direct competition from Ni-MH and lithium ion batteries, the market share of Ni-Cd batteries has dropped tremendously. Another reason for the drop, arguably the decisive one, is the toxicity of cadmium. Now, in most countries, the European Union in particular, sales of Ni-Cd batteries for portable use have been greatly restricted (Directive of the European Parliament and of the Council, 2006). The Ni-Cd battery still has a certain share of the market in industrial applications, however, due to its demonstrated reliability and competitive costing over a life cycle that can last for over 20 years. It is a preferred choice for railway signaling, telecommunications, emergency and security systems, aircraft, and other technologies. A temporary voltage drop and capacity loss may occur when a sealed Ni-Cd battery is partially discharged and recharged over many cycles (Barnard, 1981). The term “memory effect” has thus been coined, because the battery seems to remember how much capacity is drawn from previous discharges. Full capacity can be recovered after deeply discharging and recharging the battery again (Figure 4.4). This temporary drop in voltage has been ascribed to the physical changes in both the cadmium and the nickel hydroxide electrodes, the former in particular. It has been observed that upon partial charge and discharge, large Cd crystals tend to form, reducing the surface area of the active materials, and this consequently leads to a voltage drop (Buchmann, 2011). Another reason is the formation of intermetallic phases such as Ni2Cd5 and Ni5Cd21 in such an environment (Barnard, 1981). Experiments have also found that the formation of different types of nickel oxide-hydride may cause a temporary drop in voltage (Sato et al., 1996). Figure 4.4 Memory effect of a Ni-Cd battery. Temporary voltage drop (curve B) and recovery (curves C and D) after normal discharge-charge cycling. The inset is an enlargement of the indicated region (Sato et al., 1996). Reproduced by permission of The Electrochemical Society.

1.5 1.4 1.3

A

B C D

E (V)

1.2 1.1 1 1.3

0.9

1.25 1.2

0.8

1.15

0.7 0.6

0

A B 0

10

10

20

C D 20

30 40 Time (min)

50

60

70

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A highly repetitive partial charging and discharging to the same voltage over many cycles needs to occur before the memory effect becomes noticeable. The memory effect thus rarely occurs in practice. It has been blamed, however, for reduced battery performance that is actually caused by inadequate charging, overcharge, or exposure to high temperatures. Advanced electrode fabrication techniques developed for modern Ni-Cd batteries reduce the susceptibility to voltage depression. Therefore, most users are unlikely to experience degraded performance caused by the memory effect.

4.3.2

Ni-MH battery

Research on the Ni-MH battery started in the 1960s. A major breakthrough was made in the 1980s by Philips Research Laboratory, where it was discovered that substitution or addition dramatically improved the performance of metal hydrides (Willems, 1984). Another key contributor to the development of Ni-MH was Ovonic, which effectively improved the performance of the alloys by modifying their structure and composition (Fetcenko et al., 1996; Ovshinsky and Young, 2002). The Ni-MH battery was quickly accepted as the primary choice for portable energy storage, and its prevalence only diminished with the introduction of Li-ion batteries.

4.3.2.1 Negative electrode Initial development of Ni-MH lagged due to the instability of the metal hydrides. The later discovery of new compounds incorporating rare earth metals by Philips Research Laboratories opened the door to the modern Ni-MH battery. These batteries are based on the AB5 formula, where A and B are actually mixtures of different elements. Economically viable alloys containing mischmetal are commonly used for modern NiMH cells. Over the years, AB5 compounds have shown continuous improvement in structural integrity, corrosion resistance, cycling stability, and cost. One key method to achieve these is through substitution. Table 4.1 shows the profound effects of Table 4.1 Effects of substitution with other rare earths on capacity in LaNi5-based alloy Alloys

C(0) (mAh/g)

C(100)/C(0) (%)

LaNi5 LaNi2Co3 LaNi2Co2.9Al0.1 LaNi3.55Co0.75Mn0.4Al0.3 La0.8Ce0.2Ni3.55Co0.75Mn0.4Al0.3 La0.5Ce0.5Ni3.55Co0.75Mn0.4Al0.3 Mm(1)Ni3.55Co0.75Mn0.4Al0.3 Mm(2)Ni3.55Co0.75Mn0.4Al0.3

371 292 289 316 327 278 283 231

45 90 98 87 93.6 94.6 92.2 96.5

C(0): The capacity extrapolated to the zeroth cycle, C(100): Capacity after 100 cycles. Mm(1): Synthetic mischmetal [La:26%, Ce:52%, Nd:16%, Pr:6% (in at%)]. Mm(2): Bastnasite (natural mischmetal) [La:18-28%. Ce:50-55%, Nd:1218%, Pr: 46%, and others: 2% (in at%)]. Source: Kuriyama et al., 1996.

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substitution on the electrochemical durability of the LaNi5 electrode. A typical AB5 electrode has a capacity of around 290 to 320 mAh/g. The ratio of La to Ce can be used to obtain the desired cycle life and power; Co, Mn, and Al significantly affect the ease of activation and formation (Kuriyama et al., 1996). AB2 alloys are also suitable negative electrode choices. They have a higher specific capacity of around 400 mAh/g, but poor stability. The common elements for A are Ti and V; common B-site elements are Zr and Ni. Similar to AB5, substitutions may be made for A and B to improve battery performance. For example, modification of B sites with Cr, Co, Fe, or Mn has enhanced electrode performance (Berndt, 2003). Sanyo recently developed a new Co and Mn-free alloy, consisting of rare earth metals, magnesium, nickel, and aluminum (Yasuoka et al., 2006). This alloy has a superlattice structure consisting of two different sub-cells, one having the AB5 structural characteristics of CaCu5, while the other is a C36 Laves phase with the characteristics of MgNi2. The two sub-cells are ordered with long-range ordering, similar to that in Ce2Ni7 (Figure 4.5). The new alloy has a higher capacity and longer cycle life than the conventional Mm-Ni alloys with the CaCu5 structure. For the negative electrode materials, sufficient oxidation and corrosion resistance are the key to long cycle life. Meanwhile, effective catalytic activity at the surface is also critical for good discharge over many cycles. These seemingly contradictory properties are achieved by optimization of oxide thickness, porosity, and catalysts. Porosity is important to allow ionic access to the metallic catalysts and therefore promotes high-rate discharge. Fine metallic nickel particles dispersed within the oxide have shown excellent catalytic activities (Young et al., 2000). The negative electrode materials are normally supplied by manufacturers who specialize in alloy processing through a series of melting, cooling, and annealing.

Rare Earths Mg Ni

— (120)projection

CaCu5

Ce2Ni7 Superlattice alloy

MgNi2 (C36 Laves)

Figure 4.5 Crystal structure of Mm0.83Mg0.17Ni3.1Al0.2 alloy—a type of “superlattice” alloy (Young et al., 2000).

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4.3.2.2 Electrolyte and separator The electrolyte of Ni-MH batteries is essentially 30% potassium hydroxide in water. Lithium hydroxide is often added to promote charging efficiency at the nickel electrode by suppressing the evolution of oxygen. Sodium hydroxide is sometimes introduced to form a ternary system, which promotes high-temperature charging but leads to shortened life due to the increased corrosion. Ni-MH batteries are mostly fabricated using the “sealed and starved” electrolyte design. To allow for efficient gas diffusion and recombination, the separator is partially saturated. The electrodes are nearly saturated with electrolyte to obtain high charge and discharge efficacy. The separator, seemingly a simple part in the Ni-MH battery, also has a profound impact on the performance. In addition to electrically isolating positive and negative electrodes and providing channels for ionic transport, the separator also helps improve cycle life. Over the years, the separator has been improved to obtain the desired attributes. The traditional nylon separator is susceptible to attack by oxygen and hydrogen. The decomposition products can poison the nickel electrode, which, in turn, facilitates oxygen evolution. Modern Ni-MH batteries use what is labeled as “permanently wettable polypropylene,” which is actually a composite of polypropylene and polyethylene. By using acrylic acid and sulfuric acid, the surface of the composite can be effectively treated to obtain the desired wettability (Reddy, 2010). After hundreds of cycles, the separators still maintain their surface wettability and effectively absorb enough electrolyte.

4.3.2.3 Construction Similar to Ni-Cd, Ni-MH cells are constructed in cylindrical, button, and prismatic forms. Prismatic cells are normally compact and slim, offering better utilization of space in devices such as cell phones that are thin and long. One prismatic design is displayed in Figure 4.6. The electrodes are flat and rectangular in shape. The positive electrodes are welded to a vent-cap assembly. The nickel-plated can is used to hold both electrodes and the electrolyte. The can also serves as the negative terminal. This design contains a safety vent similar to the one used in cylindrical cells. The cylindrical configuration has been widely used, and there are many types of cylindrical cells intended for specific applications. One typical design is shown in Figure 4.7. The main feature of this design is that both positive and negative electrodes are wound together with the separator. A nickel-plated steel can is used to enclose the electrode group and the electrolyte. A gas release vent is fitted to release any excessive gas due to battery abuse. The performance of cylindrical cells can be tailored to meet specific demands. For instance, for motor drives, conducting elements can be incorporated to offer high current; for emergency lighting, high temperature tolerance is necessary, and this is achieved by using a heat resistant separator and increasing the cobalt content.

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Figure 4.6 Schematic drawing of prismatic design. Courtesy of Panasonic.

Cylindrical type + Contact Gas release vent Gasket

Negative electrode

Figure 4.7 Schematic drawing of cylindrical design. Courtesy of Sanyo.

Insulation washer Cover plate

Can Separator Positive electrode

− Contact

4.3.2.4

Ni-Cd versus Ni-MH batteries

Ni-MH can provide higher specific energy than Ni-Cd and lead-acid batteries and has much less environmental impact due to the absence of toxic metals such as Cd and Pb. A comparison between Ni-Cd and Ni-MH batteries based upon “typical” performance is laid out in Table 4.2.

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Table 4.2

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Comparison between Ni-Cd and Ni-MH batteries

Gravimetric energy density (Wh/kg) Cycle life (to 80% of initial capacity) Overcharge tolerance High-rate performance Operating temperature Memory effect Impact on environment

Ni-Cd

Ni-MH

45-80

60-110

1500

300-500

Moderate Excellent Good low-temperature performance Yes Uses toxic cadmium

Low Good Good high-temperature performance Moderate/negligible Minimal environmental problems

4.3.2.5 Low self-discharge Ni-MH batteries Compared to Ni-Cd cells, Ni-MH cells historically have had a higher self-discharge rate, which varies proportionally with temperature, i.e., a higher temperature leading to a higher discharge rate. Reasons for the self-discharge are normally believed to be (1) decomposition of the positive active material (NiOOH); (2) disaggregation of the negative electrode; and (3) nitrogen-containing redox shuttle reactions (Figure 4.8). Self-discharge needs to be limited in order to improve the energy efficiency of Ni-MH batteries. In 2005, Sanyo introduced Eneloop, a type of low self-discharge battery that has received wide acclamation. Sanyo has further improved the performance of its Eneloop, by (1) improving the crystalline structure and composition of the superlattice alloy used for the negative electrode; (2) developing new additive and new coating technology to protect the surface of the superlattice alloy; and (3) optimizing the separator and electrolyte. Today’s Eneloop can be cycled up to 1800 times and maintains up to 90% of its charge after one year (Panasonic, 2014). The advent of

(1) (+) Electrode active substance decomposition

(2) (-) Electrode precipitation of conductive compound

Negative

Positive OH– O2

Separator

NiOOH

NO2– Co, Mn Electrolyte

(3) Nitrogen compound shuttle reaction

Figure 4.8 Schematic drawing of self-discharge. Courtesy of Sanyo.

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low self-discharge Ni-MH batteries is pivotal for HEVs, which can have long rests between driving.

4.3.2.6

Applications

With up to three times the capacity of an Ni-Cd battery of the same size, the Ni-MH battery offers much longer run time between charges. It has been the dominant choice of battery for numerous portable electronic devices such as mobile phones, digital cameras, and laptop computers. The rapid development of lithium-ion technology, especially from the beginning of the twenty-first century, however, has dampened enthusiasm for the Ni-MH battery. Due to their outstanding gravimetric capacity, Li-ion batteries have largely taken over many markets that historically belonged to the Ni-MH battery. In Japan, the market share of Ni-MH in the portable rechargeable battery market dropped from around 50% in 2000 to about 28% in 2012 (by units) (Battery Association of Japan, 2014). The established advantages of Ni-MH batteries in contrast to Li-ion batteries, however, mean that Ni-MH batteries will still be highly competitive in several fields for the foreseeable future. One of the main areas is the automobile. More than 4 million vehicles with Ni-MH batteries have been sold to date, and there have been no significant recall or safety incidents involving batteries. Toyota’s Prius has been the best-selling HEV using Ni-MH batteries. Since its debut in 1997, the Ni-MH battery has demonstrated exceptional reliability, safety, and performance. Tests on the 2001 Prius by Consumer Reports found that the battery was not degraded after 10 years of driving (Fisher, 2014). Ni-MH is expected to last the lifetime of the vehicle. In automotive application, Ni-MH has several advantages over Li-ion batteries. The first is safety. Unlike Li-ion batteries, Ni-MH batteries contain no highly flammable chemicals, which make them much safer in case of accident or abuse. Secondly, Ni-MH has a wide operating temperature range, from 30 to 75  C, which is crucial for a vehicle driven in hot summers and freezing winters. In addition, the robust performance under stringent conditions means that the Ni-MH does not need complex battery management systems, which effectively reduces manufacturing costs. Even with the increasing price of rare earth metals (typically mischmetal) and nickel, after deploying more than 4 million units, HEVs can still be offered at very competitive prices.

4.4

Future trends

During the development of battery technology from lead-acid to Ni-Cd, to Ni-MH, and finally to Li-ion, the following factors have played instrumental roles: capacity, materials cost, cycle life, environmental impact, and safety. The market share of NiCd will shrink further due to its negative environmental issues. It is expected that improvement is still possible for Ni-MH, and performance enhancement will help increase its competitiveness in the future.

Nickel-based batteries for medium- and large-scale energy storage

4.4.1

87

Ni-Cd batteries

The retreat from the consumer market of the Ni-Cd battery is almost exclusively due to environmental concerns. Cadmium that is incinerated or put into landfill severely contaminates the soil, water, and air. In Europe, sales of Ni-Cd batteries to consumers for portable use have been banned. In the United States and Europe, the producers need to collect cadmium at the end of the battery lifetime, which can push up the manufacturing costs of Ni-Cd; this makes Ni-Cd less competitive than Ni-MH and Li-ion batteries. The Ni-Cd battery has yet to disappear from the market completely, because it outperforms other batteries in certain aspects. Compared with the proven Ni-Cd, Li-ion technology is still under development and yet to be subjected to tough testing over the timeframe of decades. Li-ion batteries are known for their high volumetric and gravimetric density, so they hold great promises for aviation application. Saft, however, is sticking to Ni-Cd batteries for SSJ100 and ARJ-21 regional aircraft after “trade-off studies concerning development risk, weight savings and life cycle cost” (Saft, 2012). In early 2013, the Boeing 787 encountered problems with its Li-ion batteries. Airbus subsequently announced that they will use “the proven and mastered nickelcadmium main batteries” when the new Airbus XWB airplane enters passenger service.1 Again, its exceptional performance, demonstrated over decades of application in many fields, positions Ni-Cd well in certain fields where the attributes of high durability, minimum to no maintenance, and long cycle life (up to 20 years) are necessary. Ni-Cd is still highly competitive in stationary power supplies for applications such as railroad signaling, offshore applications, switching and transmission functions, emergency and security systems, etc. If effective recycling of Cd, especially by the manufacturers, can be achieved to minimize environmental impact, the Ni-Cd battery will continue to be a key choice for certain power backup and supply applications.

4.4.2

Ni-MH batteries

Ni-MH has been retreating slowly from the mainstream market, primarily due to the direct competition from the Li-ion battery. The high energy density currently makes Li-ion batteries a favorite choice for portable devices. To make Ni-MH competitive, further improvements in cost, energy density, cycling performance, and design are necessary. Cost reduction has always been the focal point during development. An impressive improvement has been achieved by optimizing the production of nickel hydroxide and nickel foam substrate and using a low-cost pasted metal hydride electrode. Further potential cost reductions involve novel metal hydrides with higher hydrogen storage capacity (from 320-385 mAh/g active materials to 450 mAh/g), but this approach could be highly challenging. Costs associated with inactive cell components can be reduced via novel design and modified fabrication. The future supply of rare earth metals (typically mischmetal) and nickel could be a concern, 1

http://www.airbus.com/presscentre/pressreleases/press-release-detail/detail/airbus-activates-plan-b-forthe-a350-xwb-batteries/. Accessed 24/03/2014.

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especially considering the large amounts used inside the batteries for HEVs. A good recycling program needs to be developed. Compared with Li-ion battery technology, the Ni-MH battery has been sufficiently validated and enjoys a reputation as a trustworthy energy storage medium. It is still highly competitive in certain areas and will further expand into new markets that have been exclusively occupied by alkaline primary cells. The key to the expansion is good charge retention, which has been significantly improved to a remarkable 90% after one year. The new low-self-discharge and high-capacity Ni-MH batteries are the preferred choice for photo strobe flashlights, wireless keyboards, radio-controlled toys, etc. Furthermore, the Ni-MH battery is still of significance for vehicle application, because it has demonstrated high charging/discharging over an extended period, tolerance of abuse, and, last but not least, safety. In 2013, Panasonic announced that it would supply Ni-MH battery systems for Fuji Heavy Industries Ltd.’s first-ever HEV.2 Using Ni-MH batteries that perform well at high temperatures, Panasonic has recently introduced a 12-V Energy Recovery System for idle-stop vehicles, which will improve fuel efficiency and extend the service life of the main lead-acid battery.3 In areas where power density is not so critical, Ni-MH can play even bigger roles. For example, Ni-MH can be used to store enormous amounts of electricity produced intermittently by wind or solar power. Another big market exists for Ni-MH where substantial amounts of electricity need to be stored and supplied as requested. These include telecommunications and uninterruptible power supply systems, which are traditionally dominated by the lead-acid battery. Although lead-acid has low upfront costs, it requires routine maintenance that will drive up the cost eventually. NiMH’s high power, minimal environmental impact, excellent cycle life, and good durability give it a great advantage over the lead-acid battery.

4.4.3

Recycling

The future of Ni-Cd and Ni-MH is also dependent on the efficiency of recycling. Cadmium has to be recycled due to its severe environmental impact, and its recycling requires a special recovery system due to its toxicity. Rare earth metals in the NiMH batteries need to be recycled more efficiently due to their limited reserves and also because of their essential role in today’s high technology products such as jet engines and electronic products. Given that HEVs are sold by the millions and each Ni-MH battery stack can contain up to 10 kg of rare earth metals, only cost-effective recycling will make the Ni-MH battery a sustainable choice for HEVs. Other valuable metals such as nickel and cobalt also demand effective reuse. Recycling basically involves a series of processes, such as removing combustible materials, melting metals, and extracting elements according to their specific physical properties such as density and volatility. To help maintain the price competitiveness of both Ni-Cd and Ni-MH batteries, the recycling technology needs to be energy efficient and cost effective. 2

http://news.panasonic.net/archives/2013/0402_21676.html. Accessed 24/03/2014.

3

http://panasonic.co.jp/corp/news/official.data/data.dir/2013/02/en130208-3/en130208-3.html. Accessed on 24/03/2014.

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Ever-increasing environmental concern across the globe has made governments and professional organizations pass many rules and regulations. Proper disposal and recycling are necessary for not only Ni-based batteries, but also lead-acid and Li-ion batteries, because these batteries contain heavy metals and/or toxic or dangerous chemicals that cause serious soil contamination and water pollution. The future of Ni-Cd and Ni-MH batteries, in this context, depends on efficient recycling of both toxic and valuable metals, in addition to further improvement in performance and reductions in cost.

4.5

Sources of further information and advice

Ni-Cd and Ni-MH battery technologies are mature relative to the Li-ion battery, and there are many technical books devoted to them. This chapter only gives an overview of their development, basic chemistry, and applications. Information on charge and discharge characteristics, charging methods, the impact of temperature, shelf life, storage and disposal, and standards and regulations, can often be found in technical books. For example, Ni-Cd and Ni-MH have different responses to heat, and thus their charging algorithms are different, the details of which can be found in most technical books. Most manufacturers have launched very useful websites containing both the fundamentals of batteries and detailed information regarding specific products, and these enable a quick and easy search for the right type of battery. Professional associations such as Battery University and research organizations such as the U.S. Argonne National Laboratory also offer information related to Ni-based batteries (Sullivan and Gaines, 2010).

References Anani, A., Visintin, A., Petrov, K., Srinivasan, S., Reilly, J.J., Johnson, J.R., Schwarz, R.B., Desch, P.B., 1994. Alloys for hydrogen storage in nickel/hydrogen and nickel/metal hydride batteries. J. Power Sources 47, 261. Barnard, R., 1981. Cadmium in alkaline solution. J. Appl. Electrochem. 11, 217. Battery Association of Japan, 2014. http://www.baj.or.jp/e/statistics. Accessed 26/03/2014. Battery University. http://batteryuniversity.com. Accessed 25/03/2014. Berndt, D., 2003. Maintenance-Free Batteries: Lead-Acid, Nickel/Cadmium, Nickel/Metal Hydride, third ed. Research Studies Press Ltd., England. Buchmann, I., 2011. Batteries in a Portable World—A Handbook on Rechargeable Batteries for Non-Engineers, third ed. Cadex Electronics Inc., Vancouver. Directive of the European Parliament and of the Council, 2006. Amendment to Directive 2006/ 66/EC, PE-CONS 55/13. Fetcenko, M.A., Ovshinsky, S.R., Chao, B., Reichman, B., 1996. US Patent 5536591. Fisher, J., 2014. Consumer Reports, http://www.consumerreports.org/cro/news/2011/02/the-200000-mile-question-how-does-the-toyota-prius-hold-up/index.htm. Accessed 26/03/2014. Kirchheim, R., Sommer, F., Schluckebier, G., 1982. Hydrogen in amorphous metals—I. Acta Metall. 30, 1059.

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Kuriyama, N., Sakai, T., Miyamura, H., Tanaka, H., lshikawa, H., Uehara, I., 1996. Hydrogen storage alloys for nickel/metal-hydride battery. Vacuum 47, 889. Ohta, K., Okada, Y., Matsuda, H., Toyoguchi, Y., 1998. EP 0853346 A1. Ovshinsky, S.R., Young, R., 2002. US Patent 6413670. Ovshinsky, S.R., Fetcenko, M., Ross, J., 1993. A nickel metal hydride battery for electric vehicles. Science 260, 176. Panasonic, 2014. http://www.eneloop.info/home/technology/self-discharge.html. Accessed 26/03/2014. PowerGenix, 2014. http://powergenix.com/?q¼technology-in-depth. Accessed 24/03/2014. Puglisi, V., 2000. Proceedings of 17th International Seminar and Exhibit on Primary and Secondary Batteries, Ft. Lauderdale, FL, March 6-9, 2000. Reddy, T.B., 2010. Linden’s Handbook of Batteries, fourth ed. McGraw-Hill Professional, New York. Saft, 2005. SPL Ni-Cd battery Reliable trackside power. Doc: 21083-2-0405. Edition: April 2005. Saft, 2012. Saft Interview—Article: Aircraft Technology Engineering & Maintenance, 10/07/2012. Sapru, K., Reichman, B., Reger, A., Ovshinsky, S.R., 1986. US Patent 4,623,597. Sato, Y., Ito, K., Arakawa, T., Kobayakawa, K., 1996. Possible cause of the memory effect observed in nickel–cadmium secondary batteries. J. Electrochem. Soc. 143, L225. Shukla, A.K., Venugopalan, S., Hariprakash, B., 2001. Nickel-based rechargeable batteries. J. Power Sources 100, 125. Sullivan, J.L., Gaines, L, 2010. A review of battery life-cycle analyses: state of knowledge and critical needs. Technical Report for Center for Transportation Research, Energy Systems Division, Argonne National Laboratory. Willems, J.J.G., 1984. Metal hydride electrodes stability of LaNi5-related compounds. Philips J. Res. 39, 1. Yasuoka, S., Magari, Y., Murata, T., Tanaka, T., Ishida, J., Nakamuraa, H., Nohma, T., Kihara, M., Baba, Y., Teraoka, H., 2006. Development of high-capacity nickel-metal hydride batteries using superlattice hydrogen-absorbing alloys. J. Power Sources 156, 662. Young, K., Fetcenko, M. A., Reichman, B., Mays, W., Ovshinsky, S. R., 2000. Proceedings of the 197th Electrochemical Society Meeting, Toronto, Canada, May, 2000.