Effects of Succinonitrile (SN) as an Electrolyte Additive on the Impedance of LiCoO₂/Graphite Pouch Cells during Cycling

LiCoO 2 /graphite pouch cells with 1.0M LiPF 6 in EC:DEC (1:2 v/v) electrolyte were used to study the effect of succinonitrile (SN) as an electrolyte additive on the cell impedance during cycling. Pouch cells containing no additives, 2 wt% vinylene carbonate (VC), 2 wt% SN and 2 wt% VC + 2 wt% SN were studied for comparison. In order to investigate which electrode contributed to impedance changes during charge and discharge cycling, positive and negative electrode symmetric cells, fabricated using electrodes extracted from the parent pouch cells, were studied using electrochemical impedance spectroscopy (EIS). Two wt% VC added to the electrolyte suppressed impedance growth of the positive electrode during cycling, while the addition of 2 wt% SN greatly increased the impedance growth of the positive electrode during cycling. The addition of both 2 wt% VC and 2 wt% SN leads to intermediate behavior. In all cases, the negative electrode impedance decreased during cycling when VC, SN or VC+SN additives were present. The dominant contribution to the impedance growth of pouch cells containing SN during cycling comes from the positive electrode.