Influence of charging protocols on the charging capability and aging of lithium-ion cells with silicon-containing anodes

Abstract

Silicon with a high gravimetric capacity of 3579 mAh g−1 of the pure material becomes increasingly common in the anode of lithium-ion batteries to increase energy density on the full cell level. However, silicon changes its volume excessively during (de-)lithiation making it prone to aging. In commercial cells, it is typically applied either as SiOx or nano-Si in small quantities in a composite anode together with graphite. A different operation voltage window of silicon, varying quantities of silicon in the anode but also different technologies of the applied silicon materials, however, result in different aging behavior of the full cell—even if silicon content and capacities are similar on the data sheet. In this study, two commercial cylindrical cells in the 18650 format with differing silicon technologies are thus compared with a focus on their charging behavior and degradation mechanisms. One way to increase cycling stability with respect to the different silicon cell chemistries is the adaption of the charging protocols. Voltage limits as well as current have therefore been varied in this study using five different charging protocols. Their influence on the charging times and aging behavior is analyzed in great depth: dilatometer investigations of the anodes and their lithiation behavior, differential voltage investigations as well as a detailed post-mortem analysis including ICP-OES, SEM and EDX have identified silicon expansion at low full cell voltages as root aging mechanism in both cells.