Controlling Acid Diffusion – An Explanation for the ‘Extra Carbon’ Effect
The loss of charge-acceptance that severely limits the use of lead–acid batteries under high-rate partial state-of-charge (HRPSoC) duty is known to involve a complex interplay between: (i) negative active-material (NAM) composition; (ii) crystallization and morphology of lead sulfate; (iii) limitations on acid diffusion. This presentation focuses on the influence of acid diffusion and extra carbon in the NAM, and their respective effects on HRPSoC cycle-life. A unique electrochemical flow-cell has been constructed to control acid diffusion rates for a series of standard and carbon-enhanced negative electrodes. In preliminary characterization experiments, the formation of lead sulfate (during discharge) and hydrogen gas (at top-of-charge) were found to restrict flow rate reversibly. Under hybrid electric vehicle (HEV) duty, standard and carbon-enhanced electrodes achieved 600 and 7500 cycles, respectively. Electrolyte flow rates at failure were ~10% of the original values, with much of the decrease occurring early in service when the overall concentration of lead sulfate was relatively low. All electrodes ultimately failed due to bulk accumulation of lead sulfate (~80 wt.%). Early in service, elemental analysis of NAM cross-sections showed that all the electrodes developed a band of lead sulfate in their outer regions (nearest to bulk electrolyte). For carbon-enhanced electrodes, this feature was particularly dense and thin, thereby indicating that such a layer is necessary for sustained performance in HRPSoC duty. Combining compositional data with a study of flow-rates and charge return suggests that a dense, compact layer of lead sulfate restricts the supply of higher strength acid to the interior of the NAM, thereby preserving an ‘inner acid reservoir’ of lower concentration. The latter promotes greater charge return, which slows down the build-up of lead sulfate. These findings suggest several ways of designing batteries which are better suited to applications that involve HRPSoC duty.
Russel Newnham is the Chief Technical Officer at Electric Applications Incorporated in Phoenix, USA. For twenty year before this, he conducted the research and development of lead–acid and lithium batteries at CSIRO in Australia. Russell has also served as the Technical Manager at the NorthStar Battery Company in Springfield, USA.