A Comparative Study of Vinylene Carbonate and Fluoroethylene Carbonate Additives for LiCoO2/Graphite Pouch Cells

Abstract

Vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are compared as electrolyte additives for LiCoO2/graphite pouch cells using the ultra high precision charger (UHPC) at Dalhousie University, an automated storage system, electrochemical impedance spectroscopy (EIS) and long term cycling. Both VC and FEC are useful additives that improve couloumbic efficiency (CE), reduce charge end point capacity slippage, improve long-term cycling and reduce self-discharge during storage compared to cells with control electrolyte. Increasing the concentration of VC over 2% causes a dramatic increase in charge transfer resistance at the negative electrode surface, while the same effect is not observed for FEC. Therefore larger concentrations of FEC can be added to the electrolyte without this problem. However, when 4 or 6% FEC is used, greater gas generation during extended cycling at 40°C is detected. When only a single additive of VC or FEC is used in these LCO/graphite pouch cells tested at 40°C, a concentration of between 2% and 4% VC appears to be optimum as that provides high CE, low charge end point capacity slippage, a small increase in charge-discharge polarization with cycling and a small self-discharge during storage. The VC content would be optimized between these limits to trade off lifetime for rate capability.