Performance-enhancing materials for lead–acid battery negative plates

7 Performance-enhancing materials for leadeacid battery negative plates K. Peters 1, D.A.J. Rand 2, P.T. Moseley 3 1 Glen Bank, Worsley, Manchester, United Kingdom CSIRO Energy Flagship, Clayton South, VIC, Australia 3 The Advanced LeadeAcid Battery Consortium, Durham, NC, United States 2

7.1 Introduction The fundamental electrochemistry of the leadeacid battery is described in Chapter 3. The abiding use of the battery in many automotive applications 150 years after it was first invented can be largely attributed to progressive improvements in the performance of the negative plate. Over the years, the technology has been successfully adapted to meet new performance requirements. For instance, the engine-starting power of batteries has been increased substantially and the trouble-free service life more than doubled, all at little extra cost. Much of this improvement can be credited to the discovery last century that small amounts of specific materials, when added to the negative plate, not only promote much higher capacities (particularly under high-rate discharge and at low temperatures), but also reduce the build-up of sulfate, which otherwise is a cause of early failure. Shortly after the introduction in 1912 of Charles Kettering’s electric self-starter for automobiles and the consequent growth in the numbers of batteries for startinglighting-ignition, various chemical solutions were proposed to overcome the tendency of the sponge-like lead active-material of the negative plate to densify during service and thereby lower battery performance. Generally, a combination of three additives has been used, and they are known, collectively, as expanders.

7.2 Expanders At one time, the presence of finely -divided amorphous carbon, also known as lampblack (or gas carbon black), was thought to improve the conductivity of the discharge product and to assist in the formation of pasted plates during manufacture. The use of organic additives was introduced later after

LeadeAcid Batteries for Future Automobiles. http://dx.doi.org/10.1016/B978-0-444-63700-0.00007-6 Copyright © 2017 Elsevier B.V. All rights reserved.

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those battery manufacturers who had switched from wood to rubber separators detected a tendency of negative plates to lose capacity at a comparatively faster rate. The addition to the negative plate of small amounts of wood flour, and subsequently lignin extracts from the papermaking industry, not only mitigated the decrease in capacity, but also markedly increased performance under engine-starting conditions [1]. Early experimental work during the last century [2,3] indicated that the lignin and barium sulfate were the agents responsible for the improved capacity and reinforced each other when used together, as shown in Fig. 7.1. At that time, it was thought that the carbon additive (lampblack) had no direct effect on the capacity. It was concluded that the lignin has surfactant properties and is adsorbed on the spongy lead, as well as on the surface of lead sulfate crystals that are formed during discharge. This action has the effect of inhibiting crystal growth during the charge and discharge processes. The barium sulfate particles act as nuclei for the formation of lead sulfate (with which it is isomorphous) during discharge, and the end result is a more uniform distribution of lead sulfate throughout the spongy lead active-mass. The actions of lignin and barium sulfate are considered to be synergistic [4].

Figure 7.1 Effect of additive on battery capacity and life. A: no expander; B: lampblack; C: barium sulfate; D: organic expander; E: barium sulfate and organic expander. Reproduced from E. Willihngantz, National Battery Manfacturers Association Meeting, May 1940, White Sulphur Springs; W. Virginia, USA.

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More recently, in a study of 17 different organic expander materials [5], it was concluded that all of the substances adsorbed on metallic lead and that the capacitance of the lead | sulfuric acid interphase, measured at potentials below the value of the Pb‒PbSO4 couple, markedly decreased in the presence of the organics. Adsorption affected both hydrogen evolution and the formation of lead sulfate. Impedance measurements showed that the presence of adsorbed organic substances increased the hydrogen overvoltage. It was argued that adsorption of the expanders caused a decrease in the active area on which protons could be discharged, thus decreasing the apparent exchange-current density. From the analysis of oxidation transients, it was deduced that expanders retard the solid-state process by which a thin compact film, or passivation layer, can form at less negative potentials and favour a dissolutioneprecipitation mechanism. It has also been reported [6] that carbon has a lower hydrogen overpotential than spongy lead and thus, during charge, hydrogen gas accumulates around lampblack particles to cause an advantageous, mild expansion of the plate material. The fact that this observation was overlooked, not only at that time but also during the years that followed, is remarkable given present-day recognition and exploration of the beneficial effect of carbon on negativeplate performance in modern automotive batteries. The function of carbon additions to the negative active-material has been the subject of much debate. It is only recently, however, that the importance of the carbon component has become clear, and its function is discussed further in Section 7.5. There now exists a wide range of proprietary organo-sulfonates that are used as additives to maintain the expansion and health of negative activematerial throughout extensive life duties. For automotive leadeacid batteries, it has become standard practice to add barium sulfate, lignosulfonate and carbon with a combined weight that is usually less than 1 wt% of the negative mass, with the carbon content less than 0.2 wt%. Today, the battery is an important component of engine designs in vehicles with stopestart features, which are necessary to meet environmental standards. The essential parameter for reliable service is the speed and efficiency of recharge, and the electrochemical response of the negative plate during charge is the main controlling influence, particularly where brake-energy recuperation is used to maximize electrical generation during deceleration. Consequently, most recent developments centre once more on the composition, design and performance of the negative plate.

7.3 Structural influences The active-mass in negative plates is ductile and highly porous. Conventional manufacturing processes consist of coating the conducting lead grid with a

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paste formed from a mixture of lead and lead oxide powder, additives and appropriate amounts of acid and water to produce the required density, followed by reduction of the sulfated mix to a porous lead mass. Decreasing the particle size of the lead and lead oxide powder by grinding can promote a small but significant increase in the effective utilization of the activematerial. When plates are new and fully charged, the porosity approximates to 55% with pore sizes between 0.1 and 10 mm in diameter and an active-mass surface-area of w0.5 m2 g
1 [7]. During extended cycling, there is an increase in porosity and a decrease in surface-area. The lead sulfate that is formed during discharge has a larger molar volume than the charged mass (sponge lead), as shown in Fig. 7.2. Furthermore, over a period of time and particularly during rest periods, preferential growth of large crystals can occur at the expense of smaller ones via a process known as Ostwald Ripening. The distribution of lead sulfate during discharge at various rates has been determined using electron probe microanalysis [8]. During the early part of the discharge, the sulfate distribution is moderately uniform at all rates, but on deeper discharges and particularly at high rates, the discharge product is concentrated in the regions near the plate surfaces. The discharge capacity of plates of varying thickness follows the Peukert relationship, i.e., a linear relationship between log I and log t, where I is the discharge current and t is the duration time of the discharge. On the other hand, the Faradaic efficiency increases as the plate thickness is reduced [9].

Figure 7.2 Negative active-material: (A) before discharge; (B) after discharge. Reproduced from P. T. Moseley, D. A. J. Rand, K. Peters J. Power Sources 295 (2015) 268-274.

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Since the surface-area of the negative active-material (w0.5 m2 g
1) is much less than that of the positive counterpart (4e5 m2 g
1), the proportion of the negative surface covered by sulfate during discharge is much higher than that on the positive. For a full discharge, it has been estimated [10] that a uniform covering of sulfate would have a thickness of 0.3 and 0.03 mm on the negative and positive plate, respectively, with attendant consequences for restricted electrolyte flow within the porous structure. Leadeacid batteries in future automotive electrical systems will be confronted with duty cycles that exacerbate the accumulation of lead sulfate on the negative plate (see Chapters 3 and 12), and if the situation is left unchecked, batteries will quickly fail. This problem and its potential resolution are discussed in more detail in the remainder of this chapter.

7.4 Challenge of high-rate partial state-of-charge duty The emerging application of leadeacid batteries for the storage of energy from regenerative braking in various types of battery-electric (BEVs) and hybrid electric vehicles (HEVs) requires the best possible recovery of charge during the high-rate partial-state-of-charge (HRPSoC) duty that is an essential aspect of the operation of such vehicles. Micro-hybrid vehicles, which are fitted with stop‒start features both to improve fuel economy and to reduce emissions, employ batteries that operate under PSoC conditions and are charged by energy recuperation from the braking system. The schedule is a micro-cycle of short bursts of discharge and charge, each at a high rate. The PSoC amplitude is just a few percent. The current accepted by the battery during the charge periods should be sufficient, when used efficiently, to convert discharge product to the active state and thereby ensure availability of stop‒start and other essential functions. This charge current has been defined [11] as the dynamic charge-acceptance, DCA, and is the average current over the initial charging period, which is usually between 3 and 20 s. The parameter is quoted in terms of amperes (A) per ampere hour (Ah) of battery capacity, i.e., A Ah
1. Standardized industry specifications are in the course of preparation (CENELEC Standard EN50342-6: Lead‒acid Starter Batteries for Micro-Cycle Applications). The DCA of new leadeacid batteries currently lies between 0.5 and 1.5 A Ah
1, but emerging automotive applications may well require higher values because a 14-V alternator can generate up to 3 kW. For effective fuel savings and low-emission features, DCA should be sustained through the operational life of the battery. After relatively short service under PSoC conditions, however, recent work [12,13] has found the DCA to decrease to around 0.1 A Ah
1 and, after brief periods of parking, to even lower levels.

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Similar problems of poor charge-acceptance and inefficient charging have been reported after open-circuit periods [11]. Resolution of this problem is essential if the intended benefits are to be achieved. Factors that influence these inconsistencies in battery performance have been investigated, namely: local current distribution, state-of-charge, temperature, and variable acid strength due to stratification [14]. Given that healthy negatives can be charged efficiently and effectively from deep discharge over a wide range of temperatures and current densities [15] and that early service behaviour is satisfactory, the progressive decrease in DCA under HRPSoC duty can be attributed to rate-limiting processes that include the accumulation of lead sulfate in the negative plates with subsequent effects on concentration and potential gradients within the porous matrix. A team at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia [8] has used electron probe microanalysis to demonstrate that, under the stope start/regenerative braking schedule, the discharge product is mostly located in the outer regions of the plate with areas in the centre relatively undischarged. The surface sulfate is never completely recharged, and recrystallization substantially increases the average crystallite size of the deposit. It was also shown that the discharge product can progressively build up to 60 wt% of the negative mass in a relatively short time. The plate potential becomes increasingly negative and eventually reaches the gassing stage with hydrogen evolution consuming an increasing proportion of the current. The sulfate concentration at the positive plate remains close to expected values. Consequently, battery failure occurs as a result of sulfation of the negative plate. The discharge and charge reactions at the negative plate of a lead‒acid cell are given in Fig. 7.3.

Figure 7.3 Discharge and charge reactions at the negative plate of a lead‒acid cell. Reproduced from P. T. Moseley, D. A. J. Rand, K. Peters J. Power Sources 295 (2015) 268-274.

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New leadeacid batteries can be recharged effectively at high rates of charge because the freshly-discharged product, lead sulfate, has a small crystallite size, which facilitates rapid dissolution, a requirement that is fundamental to subsequent recharge via the so-called solution‒precipitation mechanism, as expressed by reaction (3) in Fig. 7.3. On the other hand, if the battery is left in an open-circuit state at a PSoC for a significant length of time after discharge from top-of-charge, the lead sulfate crystals have the opportunity to grow via the Ostwald Ripening process. Consequently, the chargeacceptance of the battery declines, particularly at the negative plate, which, as noted previously, offers less surface-area than the positive. The immediate history of the battery affects the charge-acceptance quite markedly when micro-cycling is performed over a narrow range of PSoC, with no excursions to top-of-charge [16,17]. A shallow discharge produces some new (small) lead sulfate crystals, which can support a high rate of recharge so that a relatively healthy DCA can be achieved. Immediately after a charge event, however, the small crystals of lead sulfate will have been consumed so that only material of low surface-area remains and a poor DCA is observed. Lacking a construction that is purpose designed for HRPSoC duty, valveregulated lead‒acid (VRLA) batteries typically lose at least 50% of their initial capacity after operating in an HEV mode for a relatively short time. Since the battery is not brought to a full state-of-charge in PSoC duty, there is no routine method available to combat the aforementioned phenomenon of irreversible sulfation of the negative plate, especially at the bottom. On charging a leadeacid cell, the fundamental reaction at the positive electrode may be accompanied by oxygen evolution and that at the negative by hydrogen evolution. All four reactions are independent. The only requirement is that the currents at the two electrodes are equal. The end-point of a completed charge is always water electrolysis, during which gas evolution predominates. The rates of other possible side-reactions, such as grid corrosion, ozone formation and the decomposition of organic additives, are low and usually neglected. With freshly-formed cells, negative electrodes can be charged efficiently over a range of current densities and temperatures with little gas evolution, whereas under similar conditions, positive electrodes evolve oxygen from the outset [15]. At high rates of charge, it becomes difficult to sustain the mass and charge balances that are necessary for the solution‒precipitation charging mechanism to proceed. While the lead sulfate crystals are small, the fluxes of ions to the reaction sites may be rate-determining [18]. With increasing crystallite size, however, the discharge product may become resistant to the charging, and as a consequence, the potential rises and charge current is diverted to the parasitic reaction of hydrogen evolution. Ultimate battery failure occurs when the negative plate remains in the discharged state. It has been

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found [19] that some forms of carbon, when present in or on the negative active-material, can be very effective in minimizing the irreversible formation of lead sulfate.

7.5 Addition of carbon The benefits of including additional carbon in the negative active-material beyond the level that is normal for the expander function (v.s.) were demonstrated by the work of Nakamura and Shiomi [20,21], who made negative plates that contained up to 10 times the customary level of carbon. The actual amounts were not disclosed, but, based on typical practice, were believed to be about 2.0 wt% of the negative mass. The trials, which were directed towards both electric vehicle and photovoltaic power applications, were undertaken with VRLA batteries that operated under PSoC conditions to minimize overcharge effects. Each of the duty schedules was likely to have entailed relatively short bursts of charge at high current densities. Batteries with standard levels of carbon failed quickly due to the buildup of lead sulfate in the negative plate. By contrast, the associated positive plate was fully charged. Batteries with extra carbon enjoyed appreciably longer operating lives. More recent developments have revealed that the additional carbon enhances charge efficiency under high-rate charging conditions, such as occurring in vehicles with regenerative braking. Whereas the advantages have been demonstrated, an unequivocal understanding of the means by which carbon assists the recharge of leadeacid batteries has yet to be reached. Such knowledge will provide a deeper appreciation of the functional relationship between product and required duty, which is essential for the future development and design of cells for new and emerging applications of lead‒acid technology. Possible mechanisms for the extra-carbon effect are discussed as follows in Section 7.7.

7.6 Types of battery configuration The configuration of the negative plate in a conventional leadeacid battery is compared in Fig. 7.4 with alternatives in which additional carbon has been incorporated in various ways, as follows. 1. Conventional leadeacid batteries (see Fig. 7.4A) with no additional carbon in the negative plate, i.e., above that typically included in the expander formulation, exhibit a sharply declining DCA during HRPSoC operation. Individual lead‒acid cells in long strings are also likely to suffer a divergence in state-of-charge during PSoC cycling at any charge/discharge rate. 2. The simplest way to incorporate additional carbon in a conventional leadeacid battery is to mix it with the basic ingredients of the negative

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(A)

Lead grid

Lead grid

(B)

Spongle lead Spongle lead

+ carbon

Conventional lead–acid

Conventional lead–acid

with extra carbon

(C)

(D) Lead grid

Lead grid

Carbon

Sponge lead

Carbon

PbC®Axion

UltraBattery™

(E)

Carbon

Sponge lead

ArcActive

Figure 7.4 Schematics of negative-plate configurations without and with additional carbon. Reproduced from P. T. Moseley, D. A. J. Rand, K. Peters J. Power Sources 295 (2015) 268-274.

plate and then paste in the normal way (see Fig. 7.4B), although it must be acknowledged that supplementary water is required to maintain a satisfactory rheology. Batteries with such plates exhibit a somewhat improved DCA, but small carbon particles may become isolated and lose efficacy, as described below.

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3. Axion Power International Inc. [22] offers the PbC battery that has a negative plate with carbon as the sole active-material; see Fig. 7.4C. All other components are similar to those found in a conventional leadeacid battery. Given that there is no lead sulfate to limit charge-acceptance at the negative plate (the reaction involves the storage of protons, Hþ), the technology sustains DCA well. The carbon acts as a capacitor to provide not only a high degree of DCA, but also to enable self-balancing of series-connected cells during PSoC cycling. The PbC battery does, however, suffer from two disadvantages, namely, a specific energy lower than that of the conventional counterpart and a voltage that varies with the state-of-charge, typical for capacitors. 4. The UltraBatteryTM, a CSIRO invention [23,24] that is under commercial development by Furukawa Battery Co. (Japan), Ltd., East Penn Manufacturing (USA), and Ecoult (Australia) has a compound negative plate in which one section consists of the usual sponge lead active-material, and the other is composed of supercapacitor-grade carbon. As with the Axion PbC design, all other components of the battery are conventional; see Fig. 7.4D. In addition to providing a sustained DCA during HRPSoC operation, the UltraBatteryTM provides self-balancing of individual cells in long series-connected strings in the same manner as does the Axion PbC. A road test [25] of a Honda Insight moderate-hybrid in which the original nickelemetal-hydride (Ni‒MH) battery had been replaced by an UltraBatteryTM of the same voltage (144 V) was continued for 100,000 miles (160,000 km) at the Millbrook Proving Ground in the UK. At the end of the test, the battery was still fully functional. Moreover, the 12 individual 12-V modules were matched together better than at the start of the test and, remarkably, without the intervention of any external equalization [25]. A similar project, which involved the replacement of a Ni‒MH battery in a Honda Civic with an UltraBatteryTM, ran for 150,000 miles (240,000 km) in Arizona [26,27], and again, the individual modules remained fully-balanced without any electronic support. Self-balancing of the UltraBatteryTM has also been observed in stationary energy-storage applications by the Ecoult Company. Comprehensive details of the design and performance of the UltraBatteryTM are given in Chapter 12. 5. A concept evolved by ArcActive Limited [28] has the lead grid replaced by a porous carbon material, which has been activated by an electric-arc process; see Fig. 7.4E. The result is a configuration that might be viewed as analogous to the UltraBatteryTM, but with the carbon layer under the negative active-material instead of above. Similar designs have been proposed by Kirchev et al. [29], Czerwinski et al. [30] and Firefly [31], respectively. The ArcActive cell sustains DCA well, as shown in Fig. 7.5, but its behaviour in long strings has yet to be reported.

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Figure 7.5 Dynamic charge-acceptance (DCA) of the ArcActive cell is sustained through partial-state-of-charge cycling in contrast to that of a conventional valve-regulated leadeacid (VRLA) cell (absorptive glass-mat technology). Reproduced from P. T. Moseley, D. A. J. Rand, K. Peters J. Power Sources 295 (2015) 268-274.

7.7 Understanding the carbon effect Various empirical studies have provided evidence that the addition of certain forms of carbon can invest the negative plate with improved chargeacceptance and/or can accommodate the electrodeposition of lead while sustaining a healthy kinetic hindrance for the evolution of hydrogen. The amount of carbon addition is limited to around 2 wt% of the negative activematerial, largely due to the electrode processing parameters. In recent years, there has been much discussion over the mechanism by which the carbon component can enhance performance. The effectiveness of any particular form of carbon in this role is likely to be influenced by a number of factors that include:        

the presence of metal contaminants at the carbon surface surface functional groups electronic conductivity the size of any pores in the carbon the affinity of the carbon for lead interaction with the organic component of the expander mix wettability by the aqueous electrolyte specific surface-area.

The challenge of the optimization process is to identify which properties are the important ones, and this can only be achieved through a full understanding

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of the mechanism(s). As many as eight different functions have been proposed [32], but the growing body of evidence points to just three candidates that are the most likely to have a significant individual effect. The situation remains complicated, however, because all three can be operative simultaneously. The first function of carbon is to serve as a capacitive buffer to absorb charge current in excess of that which can be accommodated by the Faradaic (i.e., redox ) reaction; see Fig. 7.6. A conventional negative electrode will itself have an attendant double-layer, but the capacitive function (normally in the range 0.4e1.0 F per Ah) only becomes noticeable when the surfacearea is magnified appreciably by the addition of an appropriate form of carbon. The second effect of carbon is to extend the area of the electrode microstructure on which the electrochemical charge and discharge reactions can take place. During the charge reaction, lead can be deposited on the additional surface, as shown in Fig. 7.7 [33]. For carbon to perform either of these two functions to best effect, it should be in an sp2 hybridized form, for example, graphite (v.i.). The third way in which carbon can modify the behaviour of the negative active-material is by means of physical effects, for instance by obstructing the growth of lead sulfate crystals and/or by maintaining channels for irrigation of the electrode by the electrolyte. In both these cases, there is no need for the carbon to be in a conductive form. Each of the above three actions of carbon is considered in more detail, as follows.

Figure 7.6 Schematic of positive current fluxes during and after a short charge event on a negative plate that contains carbon. Reproduced from P. T. Moseley, D. A. J. Rand, K. Peters J. Power Sources 295 (2015) 268-274.

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Figure 7.7 Lead crystals electrodeposited on a carbon surface. Reproduced from D. Samuelis, Heraeus PorocarbÒ. A unique functional carbon additive for electrochemical energy storage devices, In: 16th Asian Battery Conference, Bangkok, Thailand, September 8‒11, 2015.

7.7.1 Capacitance: current flows during and after a charge event When a charge event applied to a leadeacid cell is discontinued, although the external current is zero, the double-layer remains charged, and this results in a local current between the component materials of the negative activemass [34]. The current is caused by the discharging of the double-layer via the Faradaic reaction, and thereby the electrode potential changes to its equilibrium value with a characteristic time constant. The amount of charge involved can be substantial if the surface-area of conducting material is augmented by the inclusion of an appropriate form of carbon. The time constant, s, for the equilibration process can be estimated by equating the double-layer discharge current to the charging current that is going into the Faradaic reaction and can be approximated [35] as: s ¼ RTC=Fio

(7.1)
1


1

where R is the universal gas constant (8.3145 J mol K ); T is the absolute temperature (K); C is the specific capacitance (F cm
2); F is the Faraday constant (96 458 As mole
1); io is the exchange-current density (A cm
2). Reactions that proceed relatively quickly have a small time constant, whereas those that are kinetically sluggish result in a large time constant. By way of example [35], the zinc electrode has rapid kinetics with an exchange-current of around 10
2 A cm
2 that gives rise to a time constant of the order of 50 ms. Alternatively, the nickel hydroxide electrode has a time constant of around 5 ms. Since both these time constants are so short, the

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involvement of the equilibration process outlined above has been largely ignored for both the zinc and the nickel electrode systems. By contrast, the kinetics of the electrode reactions in a lead‒acid cell are much slower, namely 4  10
7 A cm
2 at the positive plate and 4.96  10
6 A cm
2 at the negative. With an augmented carbon inventory, the negative plate can contribute a specific capacitance of up to 30 mF cm
2, whereby the time constant can be of the order of seconds and the charge equilibration (shown as function 3 in Fig. 7.6) can be regarded as a significant process that follows the removal of the external charging voltage. Under HRPSoC conditions, charge from external events, such as regenerative braking in vehicle applications, is taken up by the double-layer and thus boosts DCA efficiency. When the external input is discontinued, this charge is re-equilibrated between the double-layer and the Faradaic reaction. Recently, it has been shown [36] that cyclic voltammetry can be used to quantify the capacitive contribution to the charge-acceptance. The regions of a cyclic voltammogram that arise, respectively, from the Faradaic deposition of lead, the capacitive charging and the Faradaic evolution of hydrogen are illustrated in Fig. 7.8. The relative location of the three regions is significant. The area that represents hydrogen evolution has moved to the right (i.e., to a more positive potential) due to the addition of carbon. Simultaneously, the 0.010 Lead electrode 0.006

Carbon electrode Faradaic

Current / A

0.002

lead

Capacitive Faradaic

hydrogen

–0.002

–0.006

–0.010

–1.4

–1.2

–1.0

–0.8

–0.6

Cell potential / V vs. Hg|Hg SO 2

4

Figure 7.8 Identification of contributions from lead deposition (Faradaic), double-layer adsorption/desorption (capacitive) and hydrogen evolution (Faradaic) to cyclic voltammograms for lead and carbon electrodes (carbon black, acetylene black, graphite). Reproduced from A. Jaiswal, S.C. Chalasani, J. Energy Storage 1 (2015) 15e21.

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current flowing into the capacitance at all values of potential has expanded. To take advantage of this latter feature without invoking an increase in water electrolysis, it is necessary with a flooded cell to limit the potential or the duration of charge events, and with a valve-regulated cell to depend on the efficient operation of the oxygen cycle (even at low states-of-charge). Significantly, it has been observed [19] that the dominating process in the electrode reaction is the double-layer capacitance (non-Faradaic process) when the charge and the discharge cycles are limited to 5 s of duration. If the charge and the discharge durations are between 30 and 50 s, then the electrochemical reactions (Faradaic processes), related to lead sulfate dissolution or lead deposition, dominate. Although the introduction of extra carbon may reduce the hydrogen evolution overpotential [6], it is possible for cells thus treated to provide a long operational life without excessive loss of water provided that the charge events to which they are exposed are limited in duration or potential. The relative amounts of charge to the previously mentioned three reactions on three different forms of carbon are compared with those on a bare lead electrode in Fig. 7.9 [36]. All three carbons provide significantly higher charge capacity than the lead-only electrode. The charge capacity of lead electrodeposition for carbon black, acetylene black, graphite and lead during the reduction cycle is 8.9, 3.5, 6.9 and 2.3 mC, respectively. The charge capacity of the carbon black (44.9 mC) is about double that of the acetylene black (22.6 mC). By comparison, the graphite powder does not display any capacitive behaviour due to its much lower surface-area, but it does have notable gassing characteristics.

Faradaic

Pb

Capacitive H2

Charge / C

Faradaic

Type of carbon

Figure 7.9 Charge capacity (coulombs) for lead deposition, capacitance and hydrogen evolution for mixtures lead þ carbon black (CB-2), lead þ acetylene black (AB) and lead þ graphite (G) compared with that for a bare lead electrode (Pb). Reproduced from A. Jaiswal, S.C. Chalasani, J. Energy Storage 1 (2015) 15e21.

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7.7.2 Extending the conducting surface-area to assist electrochemistry Cycle tests under HRPSoC conditions of cells that contain extra carbon have provided strong evidence [37] that the electrochemical and chemical processes can take place not only on the surface of lead metal, but also on the surface of carbon; see Fig. 7.10. Subsequent research [19] has confirmed that two electrical systems are operating on carbon at the negative plate, namely:  a capacitive system, which involves high-rate charging and discharging of the electric double-layer; and  the conventional lead electrochemical system, which comprises the oxidation of lead to lead sulfate during discharge and the reverse process during charge. The capacitive process has been found to dominate during cycling with charge and discharge processes each of 5-s duration, whereas the electrochemical reactions dominate during cycling with longer pulses. These observations are consistent with the previous calculation of the time constant for the transfer of charge between the two component materials of the negative electrode.

7.7.3 Physical processes Studies have indicated [38,39] that the carbon added to the negative active-material acts as a steric hindrance to the crystallization of lead sulfate and thus helps to maintain a high surface-area for the discharge product. In support of this theory, it has been reported [40] that HRPSoC cycle-life is enhanced when titanium dioxide (a poorer electronic conductor), rather than graphite, is used as the additive, whereas cycle-life is improved by using either titanium dioxide or graphite (to different extents); the effect of the latter is not necessarily due to its conductivity alone.

Figure 7.10 Schematic illustration of how Faradaic reactions of the negative plate can take place on both the lead and carbon surfaces. Reproduced from D. Pavlov, T. Rogachev, P. Nikolov and G. Petkova, J. Power Sources 191 (2009) 58e75. EAC stands for electrochemically-active carbon.

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It has also been advocated [41] that activated carbon increases the porosity of the negative electrode by providing an additional structural skeleton that facilitates diffusion of the electrolyte solution from the surface to the interior of the plate. As a result, sufficient sulfuric acid is supplied to keep pace with the electrode reaction during HRPSoC operation. The same work has demonstrated [41] that longer cycle-lives under HRPSoC duty are achieved with carbon of larger particle size (i.e., micron size rather than nanometre). This information has given rise to the view [42] that small particles of carbon can become progressively buried within crystals of lead sulfate, and accordingly, their effectiveness is lost. Other investigations have revealed [19] that carbon additives can alter the pore structure of the negative electrode, and one consequence is that the access of SO2
4 ions to the innermost pores is impeded. On the other hand, Hþ ions may still diffuse out of the pores and allow the pH to rise locally to a value at which a-PbO is formed. This phase, which is clearly visible in X-ray diffraction records, is deleterious to the continued function of the negative active-material because the formation of the oxide is irreversible.

7.7.4 A possible pitfall: hydrogen evolution The addition of carbon to the negative active-mass can serve to augment the surface-area of the electrode substantially, and in such cases, it is common for there to be an increase in the hydrogen evolution rate (HER) during charging [43]. The HER may be increased by:    

a reduction in the hydrogen overpotential an increase in the applied potential an increase in the surface-area of the active-mass the presence of certain impurities in the carbon.

Reduction in surface-area to suppress the HER is likely to be counterproductive because both the capacitive effect and the rate of charge have a positive dependency on surface-area. Some control over hydrogen evolution might be exercised by limiting the potential that is applied to the cell via the battery management system. Nevertheless, efforts to limit the hydrogen evolution that results from the carbon additions have focused on the materials involved, their purity, surface functional groups and second-phase additives. There are marked variations in hydrogen gassing behaviour between the different classes of carbon (graphite, carbon black and activated carbon). A recent study [43] has, as expected, shown that the presence of a significant concentration of iron invests graphite with a high level of hydrogen evolution. The same study reported that graphites and carbon black materials both have notably higher specific currents (A g
1) than activated carbon

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materials. This latter observation is of particular importance, as it contradicts the commonly held belief that gassing is purely related to carbon surfacearea. Evidently, more research is required to complete the understanding of the range of factors that affect the HER on carbons. Clearly, the materials should be free of elements that reduce the hydrogen overpotential, and, building on this conclusion, it has been found [44] that certain other elements actually serve to suppress the evolution of hydrogen at the negative plate. For instance, it has been proposed that the carbons used in the UltraBatteryTM should be accompanied by compounds of elements such as zinc, cadmium, bismuth, lead or silver [45]. It is worth noting here that following the demonstration of long operational life in vehicles that are subjected to HRPSoC duty, UltraBatteryTM technology is being deployed in two new original equipment manufacturer vehicles (see Chapter 12).

7.8 Best choice of carbon Carbon can be found in various forms with a very wide range of physical properties, which depend very strongly on the respective electronic properties of the atoms. The principal allotropes of the element are:  diamond, in which the carbon atoms are sp3 hybridized so that the material is very hard and electronically resistive; and  graphite, in which the atoms are sp2 hybridized and invest the material with a softer, layered structure that exhibits significant conductivity along the planes of its hexagonal structure. The physical properties of diamond and graphite are listed in Table 7.1. Table 7.1 Physical properties of diamond and graphite Property

Diamond

Graphite

Cubic

Hexagonal

Orbital hybridization

sp3

sp2

Covalent radius, pma

77

73

3.515

2.267

10

w1

6.155

8.517

w 2200

w150

Crystal structure

Density, g cm
3 Mohs hardness Heat capacity, J mol


1

K


1

Thermal conductivity, W m
1 K
1 Resistivity, U m

a

Picometres; 100 pm ¼ 1 Å.

230

w10

12

w3  10
3 (c axis) w4  10
6 (a axis)

LeadeAcid Batteries for Future Automobiles

In the context of the materials present in the negative plate of a leadeacid cell, it is worth noting that the thermal conductivity of graphite is approximately four times that of lead (35.3 W m
1 K
1), and therefore, the presence of graphite will assist heat distribution within the negative active-material. The resistivity of graphite (both parallel to and perpendicular to the basal plane; see Table 7.1) is greater than that of lead (2.08  10
7 U m). Consequently, early theories that the benefits gained by the addition of carbon are due to an improvement in the conductivity of the negative active-material appear to be groundless, except when the electrode is discharged to such an extent that almost all of the sponge lead is replaced by lead sulfate. A wide range of amorphous, or poorly crystalline, substances can be prepared in which both sp2 and sp3 carbon atoms are present. Such materials exhibit physical properties that are intermediate between those of the diamond and graphite end-members, but also are strongly influenced by other parameters associated with materials. Particle size can be anywhere between a few nanometres and tens of microns, whereas surface-areas can vary from a few m2 g
1 (graphite) to over 2000 m2 g
1 (activated carbons and carbon blacks). Activated carbons are mainly amorphous with a fine pore-structure. Carbon blacks are composed of agglomerates of interconnected clusters within which there are regions that are ordered and have the graphite structure. In the absence of other factors, electrical conductivity is likely to follow the sequence graphite > carbon blacks > activated carbons. The surfaces of these materials, however, can accommodate a range of atoms or groups of atoms [46] that exercise considerable influence on properties such as wettability, double-layer formation and chemical reactivity. Impurity levels are also important. Depending on the production process, industrial carbons can contain up to 10,000 ppm of foreign elements that can include various amounts of iron, nickel, copper, zinc, silicon, potassium and sulfur. In view of the need to restrict the evolution of hydrogen during charging, it is, of course, particularly important to prevent or minimize the presence of those impurities that would promote such gassing. As discussed previously, there are at least three ways by which the presence of carbon can modify the performance of the negative plate of a lead‒acid battery, namely via: 1. a capacitive contribution 2. extension of the surface-area on which the electrochemical charge and discharge processes can take place 3. physical processes. The capacitive process (1) is favoured by carbon that has large surface-area, is conductive and is in contact with the current-collector (grid). It is not necessary, however, for the carbon to be mixed intimately with the sponge lead component of the negative electrode.

231

LeadeAcid Batteries for Future Automobiles

The surface-area effect (2) also requires the carbon to be conductive and in contact with the current-collector. On the other hand, given that carbon promotes a bulk rather than a surface process, the surface-area can be less than that required to instigate the capacitive process. Carbon deployed to take advantage of physical processes (3) does not have to be conducting, but it must be intimately mixed with the sponge lead and should not be very finely divided, or its efficacy will wane over time. In view of the conflicting requirements for the carbon to function in these several ways, it is not surprising that workers who are seeking to optimize the HRPSoC performance of lead‒acid batteries have resorted to evaluating combinations of different types of carbon.

Abbreviations, acronyms and initialisms AC Alternating current AGM Absorptive glass-mat BEV Battery electric vehicle CAN bus Controller area network bus CENELEC Comité Européen de Normalisation Électrotechnique (English: European Committee for Electrotechnical Standardization) CSIRO Commonwealth Scientific and Industrial Research Organisation DC Direct current DCA Dynamic charge-acceptance EAC Electrochemically active-material EAI Electric Applications Inc. EFB Enhanced flooded battery EFC Enhanced flooded cell EoD End-of-discharge EU European Union EUCAR European Council for Automotive R&D FCV Fuel cell vehicle GPS Global positioning system HER Hydrogen evolution reaction HEV Hybrid electric vehicle HRPSoC High-rate partial-state-of-charge ICE Internal combustion engine ICEV Internal combustion engine vehicle ISG Integrated starter generator ISS Idling start‒stop JIS Japanese Industrial Standard LDV Light-duty vehicle LH Left hand NEDC New European Driving Cycle Ni
MH Nickel
metal hydride OEM Original equipment manufacturer OVC Open-circuit voltage

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PSoC Partial state-of-charge rel. dens. Relative density RH Right hand RHOLAB Reliable, highly optimized, lead
acid battery SBA Standard of battery association of Japan SEM Scanning electron microscopy SoC State-of-charge SP Set point VDA Verband der Automobilindustrie (German Automobile Industry Association) VRLA Valve-regulated lead‒acid ZEV Zero emissions vehicle

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