LiPF6/Methyl difluoroacetate (MFA) shows a very good thermal stability with Li metal. However, LiPF6/Ethyl difluoroacetate (EFA) did not show any improvement of the thermal stability. Using FTIR and XPS, the main component of SEI in the MFA electrolyte was found to be CHF 2COOLi. However, CHF2COOLi does not exist in the SEI of the EFA electrolyte, because CHF2COOLi dissolves in EFA electrolyte. CHF2COOLi was found to dissolve in EC, DMC, and DEC. Therefore, the addition of a small amount of MFA in 1 M LiPF6/EC + DMC, DMC, and DEC electrolytes was not effective to improve thermal stability.
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