A Systematic Study of the Concentration of Lithium Hexafluorophosphate (LiPF6) as a Salt for LiCoO2/Graphite Pouch Cells

Abstract

The effect of lithium hexafluorophosphate (LiPF6) concentration in the electrolyte of LiCoO2/graphite pouch cells was studied using the ultra high precision charger (UHPC) at Dalhousie University, an automated storage system, electrochemical impedance spectroscopy (EIS), gas evolution measurements, and high rate cycling. LiCoO2/graphite pouch cells with higher LiPF6 concentrations maintained their discharge capacity well during high C-rate discharge at all temperatures tested while those with 0.5 M LiPF6 did not. The UPHC cycling data shows that coulombic efficiency (CE) and charge end point capacity slippage (charge slippage) both became worse as the LiPF6 concentration increased for cells without 2% VC. When 2% VC was added, the CE and charge end point capacity slippage improved markedly for all LiPF6 concentrations in this study. In addition, the CE and charge slippage were virtually independent of LiPF6 concentration when 2% VC was added to the electrolyte. Electrochemical impedance spectroscopy experiments were used to probe how LiPF6 concentration affected cell impedance after storage experiments and after UHPC experiments. In the presence of 2% VC, there appear to be no issues associated with using large concentrations of LiPF6, at least at temperatures above 10°C. Researchers engaged in studies of the “solvent in salt” concept should therefore consider the inclusion of useful electrolyte additives like VC in their solutions,