Coulombic inefficiency of graphite anode at high temperature

Temperature imposes a diversified set of problems to the successful operation of Li-ion batteries. Graphite anodes, in particular, are especially fragile at elevated temperatures, as continuous electro-reduction processes cause detrimental transformations in the solid electrolyte interphase (SEI). In the present work, we investigate the factors that contribute to the performance decay of graphite in cells containing a pyrrolidinium-based ionic liquid, exploring the effect of different lithium salts, electrode formulations and test conditions on the thermal resilience of the system. We show that the contribution of FSI anions to the SEI provides remarkable protection to graphite particles, preventing the electronic isolation of electrode domains. We further propose that the positive activity exhibited by these species is associated with their specific decomposition pathway. A coulombic efficiency of 99.4% could be achieved in Li/graphite half-cells at 90 °C, demonstrating that compositional control of the solid electrolyte interphase is essential to extend the environmental range of Li-ion batteries.