Preventing Gas Formation in Dual Carbon Battery By Using Mixed Supporting Salt of LiPF6-LiBF4

Abstract

Rechargeable battery with large energy capacity as well as superior rate property is strongly required for electric vehicle and hybrid vehicle. Dual carbon battery which is expected as a post Li ion battery with superior rate property, is based on intercalation of anion and cation into graphitic carbon and advantages on cycle stability and high discharge potential (>4V), however, low coulomb efficiency by gas formation is the most serious issue for this battery. In this study, gas formation mechanism was analyzed based on LiPF6/ethylene carbonate (EC) and diethylene carbonate (DMC). Mass analysis of the formed gas suggests that the main species in gas was C2H6, C2H4, CO, and CH4 and the formation of these gas may be assigned to the electrochemical decomposition of EC during charge step. In the initial charge step, gas formation was observed from anode side at initial period and from cathode side at final period. Therefore gas formation from anode side could be assigned to the formation of SEI and that from cathode side could be oxidation of EC in electrolyte. Therefore, by using simple DMC electrolyte, amount of gas formation can be decreased. Among the examined electrolyte, gas formation amount became smallest when DMC was used for electrolyte. In addition, it was found that amount of gas formation was significantly suppressed by using LiBF4-DMC electrolyte, however, discharge capacity became smaller (ca. 40mAh/g). Therefore, mixing LiPF6 with LiBF4 in DEC shows the reasonable large discharge capacity (ca. 87 mAh/g) and the amount of gas was also suppressed. The optimized ratio for mixing was LiPF6:LiBF4=3:1 and gas amount formed became smaller by an order of magnitude. Although low coulomb efficiency was observed at the fist cycle, high coulomb efficiency was sustained over 300 cycles examined.