Mitigating intergranular attack and growth in lead-acid battery electrodes for extended cycle and operating life
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INTRODUCTION
OPERATING and cycle lifetimes of conventional Pbacid batteries are limited by the resistance of the positive electrodes (grids) to intergranular corrosion, cracking, and creep.[1,2] Grid weight loss occurs when individual grains are removed from the electrode cross section as their surrounding boundaries are breached by intergranular corrosion/cracking. Alternatively, growth of the positive grid arises from the synergistic effect between creep and intergranular cracking. Dimensional changes occur in the electrode over time, causing adjacent plates to short, resulting in loss of capacity.[2] Creep, at the low stresses and relatively high temperatures to which the electrodes are exposed, originates primarily from grain boundary sliding.[4] Therefore, as ‘‘under-hood’’ temperatures in modern vehicles increase above 0.6 Tm, growth is becoming the predominant factor in determining operating life in automotive Starter Lights Ignition (SLI) applications.[5,6] Battery electrodes were originally cast from lead antimony (PbSb) alloys. While these alloys offer excellent cycling capacity, mechanical strength, uniform corrosion rates, and fluidity for casting, they typically suffer from ‘‘plate-back’’ effects where antimony from the positive electrode deposits onto the negative grid during successive charge-discharge cycles.[3] Antimony also reduces the hydrogen evolution voltage, promoting the formation of H2 gas and accompanying water losses during charging.[3] Hence, the prevalence of PbCaSn alloys, which eliminate plate-back and water losses, have increased in recent years with the popularity of valve-regulated (maintenance-free) battery designs in telecommunications, traction, and SLI applications. At the same time, however, PbCaSn grids are
more susceptible to weight loss, growth, and shedding of active material than their Sb-based counterpart.[3] Efforts to minimize the intergranular effects leading to weight loss and grid growth in PbCaSn alloys have traditionally focused on developing proprietary alloying recipes involving Ag, Se, As, etc., which can degrade internal resistivity performance.[7] Alloying additions have also been used as nucleants in order to refine grain size, producing some improvement in weight loss, although often at the expense of grid growth.[2] An attempt to alter grain boundary structure, which offers the potential for simultaneously improving weight loss and grid growth, has never before been pursued. Grain boundaries having misorientation relationships which are crystallographically described by low S values in the Coincident Site Lattice (CSL) model are recognized to exhibit ‘‘special’’ properties, including increased resistance to diffusion,[8] corrosion,[9] cracking,[10] and sliding and cavitation (creep),[11,12] relative to ‘‘general’’ or ‘‘random’’ boundaries. Special properties associated with CSL interfaces having S values of less than 29 have been ascribed to their ordered structure and lower free volume compared with their random counterparts.[13,14] A detailed review
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