*2.1. Flooded Batteries*

In flooded batteries positive grid corrosion is mitigated by using lead-antimony alloy grids [17,18], related to higher creep strength of this alloy [19]. Antimony also facilitates conversion of the highly resistive PbO corrosion product to the conductive PbO2, which has a conductivity 17 orders of magnitude higher [20]. However, antimony dissolves in the electrolyte, migrates to the negative electrode, and promotes hydrogen evolution during standby, charge and overcharge. Hence for maintenance-free batteries, low antimony positive grid alloys are used, while for VRLA, antimony free alloys of Pb-Ca-Sn are used [19,21], with calcium providing mechanical strength and tin mitigating grid corrosion, while Pb-Ca alloy is used for the negative gird. Positive active material softening and shedding, and negative active material sulfation, common to flooded and VRLA batteries, are covered in detail in the next section. Electrolyte stratification is mitigated by overcharging to promote gas evolution with associated electrolyte mixing. Electrolyte dry-out is prevented by periodic addition of deionized water as part of standard maintenance. For flooded batteries, grid corrosion mitigation by use of lead-antimony alloy is expected to reduce positive active material shedding due to better grid-active material bond, while mitigation of electrolyte stratification and starting with a lower negative to positive active material ratio relative to VRLA are expected to reduce negative electrode irreversible sulfation.

#### *2.2. VRLA Batteries*

For VRLA batteries, there are areas that need further study. Oxygen recombination depolarizes the negative electrode, thus raising its potential. For batteries charged at a constant potential, this raises the positive electrode potential, leading to greater positive grid corrosion, with associated water loss [19,21] and additional oxygen evolution, which corrodes the grid further [12]. The higher sulfuric acid concentration due to water loss accelerates grid corrosion [19]. This leads to a synergistic effect of water loss on grid corrosion. Hence it is important to avoid water loss, not just to ensure the cell capacity is not electrolyte-limited, but to limit positive grid corrosion. Control of overcharge current such that it does not exceed the recombination rate ensures complete recombination of oxygen evolved from the positive electrode. For fresh VRLA batteries, there may be some excess electrolyte, which slows down oxygen transport to the negative, resulting in water loss till the oxygen recombination current can be supported. During this period, the oxygen generation current at the positive is supported by charging of the negative electrode active material. Cell design should take this into account and add sufficient excess negative capacity to ensure the negative electrode does not get fully charged during this period to avoid hydrogen evolution. The unintended effect of excess negative is that some uncharged lead sulfate crystals may eventually grow in size and become difficult to charge, increasing electronic resistance and preventing electronic and ionic access to parts of the electrode. Hence it is important to ensure the positive and negative electrode active mass and electrolyte content are optimized to avoid (1) hydrogen generation at the negative, (2) irreversible sulfation of negative electrode (3) electrolyte dry-out.

Upon discharge, there is a near doubling of volume in the positive electrode and about a 2.5× increase in the negative electrode volume [19] since the lead sulfate discharge product has a lower density. The negative spongy lead is compressible; hence this volume growth is partially compensated by a reduction in the volume of the remaining lead during discharge. For VRLA batteries using glass mat separators, while volume change in the in-plane direction due to positive grid corrosion has been mitigated by the use of antimonyfree Pb-Ca-Sn alloy, the through plane increase in positive active material volume upon discharge is mitigated by applying a compressive force to the electrode assembly [19]. The glass mat is made of fibers with various diameters to optimize mechanical strength, pore size distribution, porosity, and wicking characteristics of the separator. Smaller fiber diameter lends itself to smaller pore size and greater wicking ability, while larger

fiber diameter improves mechanical strength and resistance to compression [12,22]. With proper glass mat separator design, electrolyte stratification is avoided or mitigated by wicking, and positive active material growth is mitigated by compression, with the glass mat pressed against the positive active material at pressures up to 138 kPA [22]. In a gel VRLA battery, where sulfuric acid is mixed with fumed silica, forming an immobile electrolyte, stratification is less of an issue [22]. Note that for flooded batteries, as discussed earlier, electrolyte stratification is mitigated during overcharge, where the gases evolved stir the electrolyte.

VRLA batteries have a higher specific gravity at full charge compared to flooded batteries to compensate for the lower electrolyte content. The positive electrode has two oxide types in the fully charged state–α-PbO2, which has lower activity, and β-PbO2 which has higher activity. As long as the acid concentration is maintained in the 0.9–5 M (1.05–1.28 specific gravity) range, the active material is in the more active state, with the less active α-PbO2 dominating outside this range [18]. Electrolyte stratification could result in greater α-PbO2 formation at the bottom of the electrode, while water loss can result in α-PbO2 dominating throughout the charged positive electrode. As discussed earlier, higher electrolyte specific gravity also accelerates positive grid corrosion, which manifests itself as a steady decrease in usable capacity at a fixed rate.

Electrolyte stratification also results in greater lead sulfate formation at the bottom. This uneven lead sulfate distribution across the electrode height results in lower electrode active mass utilization and associated capacity loss [12]. This is mitigated for tall cells and modules by placing the tall side horizontally [23].

Using the mitigation approaches described, the above failure modes have been reduced, resulting in the negative electrode failure being the main R&D topic for further work. The low surface area of the negative electrode and its low specific capacitance results in poor charge acceptance especially at high rates. The voltage range above which gassing occurs and below which charge is incomplete is quite narrow for VRLA batteries [23,24], which further contributes to poor negative electrode charge acceptance at high rates. The positive lead dioxide active material has an order of magnitude higher specific surface and three times higher specific capacitance relative to the negative electrode spongy lead [23,25]. This ensures uniform distribution of the charge transfer reaction across the bulk of the positive electrode, and formation of lead sulfate film at the negative electrode surface due to poor charge acceptance. This is especially the case for partial state of charge cycling of lead acid batteries [16,26], where the batteries are not fully charged at the end of each cycle (e.g., hybrid electric vehicle, frequency regulation). To overcome this, expanders are added to the negative electrode active mix during paste formulation. These consist of barium sulfate, which increases nucleation rate for the lead sulfate formation reaction [12,16,26], lignosulfonates with functionalities that promote the formation of lead sulfate uniformly across the electrode thickness by inhibiting lead sulfate crystal growth and formation of a passivating layer [12,24], and various forms of carbon such as graphite and carbon black that increase electronic conductivity, surface area and specific capacitance [24,27]. The higher electronic conductivity reduces isolation of negative active mass via electronic percolation, while the higher surface area allows uniform distribution of lead sulfate across the bulk of the electrode, with the higher capacitance improving charge acceptance at high rates [25]. The carbon is incorporated either in the paste mix or laminated onto the electrode to form a parallel lead-carbon hybrid, or simply replaces the negative spongy lead electrode or the grid [15,19]. For the batteries used in this work, high surface area carbon is introduced in the paste formulation step of the negative electrode [28]. Table 1 shows the summary of failure modes.


**Table 1.** Failure Modes of Absorbed Glass Mat Valve Regulated Lead Acid Batteries.
