Understanding the thermal instability of fluoroethylene carbonate in LiPF6-based electrolytes for lithium ion batteries

Lithium ion batteries (LIBs) have been successfully commercialized in a diverse mobile electronic device and perceived as a solution to the demand for ecofriendly electric vehicles and grid-scale energy storage systems owing to their high-energy density and long cycle life. To improve the energy density of LIBs, Si is considered as a promising anode material due to its high theoretical capacity of 4212 mAh g-1 for Li22Si5. However, the Si particles suffers from the severe volume changes up to 400%, which leads deterioration of the Si-electrolyte interfaces during cycling. Fluoroethylene carbonate (FEC) has been highly regarded as effective reducible additive for stabilizing the Si-electrolyte interfaces due to the formation of stable solid electrolyte interphase (SEI) on Si anode, which improves the electrochemical performance of Si anode. However, there have been some reports of detrimental effects of the FEC-derived SEI on the electrochemical properties of Li/graphite half cells and spinel-type Li1.1Mn1.9O4/graphite full cells. Our investigation revealed that Lewis acids, such as PF5, in LiPF6-based electrolytes caused the defluorination of FEC, which was used as a co-solvent, and consequently, excess HF formed in the LiCoO2 (LCO)-LiNi0.5Co0.2Mn0.3O2 (NCM)/pitch-coated silicon alloy-graphite (Si-C) full cell at elevated temperature. The undesirable formation of HF and acid compounds that occurs upon defluorination of FEC promoted transition metal ion dissolution from the LCO-NCM cathode at elevated temperatures; this transition metal dissolution results in a self-discharge of the Si-C anode in a full cell coupled with an LCO-NCM cathode. Thus, we have observed inferior cycling properties and storage performance of LCO-NCM/Si-C full cells with the FEC-based electrolyte at elevated temperatures