Carbon Corrosion Mechanisms in Batteries during High-Rate, Partial State-of-Charge Cycling

Abstract

Battery cycling profiles in hybrid electric vehicles have high currents both during charging, when the vehicle brakes are applied, and during discharging, when the vehicle accelerates. Cycle life in these high-rate, partial-state-of-charge (HRPSoC) applications is improved by adding conductive carbons with high surface areas to the negative active material. During charge, however, carbon can be oxidized by water to form soluble carbon monoxides such as formic acid. Further oxidation produces gaseous carbon dioxide at the positive electrode. As the internal cell pressure increases, it may force the vents open to release the carbon dioxide and other gases in the battery. Fortunately carbon dioxide formation is slowed by limited transport of soluble carbons from the negative to the positive plates. The valve-regulated lead-acid (VRLA) battery life is extended by carbon additions to the negative plate, but the ultimate cause of failure may become cell dry-out.