Properties of carbon anodes and thermal stability in LiPF₆/methyl difluoroacetate electrolyte

The thermal stability of 1 M LiPF₆/CHF₂COOCH₃ (methyl difluoroacetate, MFA) with lithiated carbon anodes and the electrochemical characteristics of carbon anodes in this electrolyte have been investigated in terms of the use of this electrolyte in lithium-ion batteries. Differential scanning calorimeter measurement of LiPF₆/MFA with lithiated carbon anodes indicated that the main exothermic peak is around 400°C. The peak temperature was 110°C higher than the peak of LiPF₆/ethylene carbonate-dimethyl carbonate (EC-DMC) (1:1 in vol) with lithiated carbon anodes. The chemical compositions of the solid electrolyte interphase (SEI) on the lithium metal anode in LiPF₆/MFA electrolyte were characterized by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. These spectroscopic measurements revealed that CHF₂COOLi existed as a major component of SEI. It was expected that a reaction product of CHF₂COOLi and lithiated carbon increased the thermal stability. Compared with the case in 1 M LiPF₆/EC-DMC [1:1 in vol], the discharge capacity of carbon anodes was slightly smaller in 1 M LiPF₆/MFA, while the cycling performance was similar in both electrolytes. From the result of the impedance measurement, the reason for the small capacity is the large resistance of SEI. Moreover, good cycle performance was obtained for the lithium ion cell used in 1 M LiPF₆/MFA, while the cells discharge capacity was slightly lower than of the lithium-ion cell in 1 M LiPF₆/EC-DMC (1:1 in vol). It is noted that 1 M LiPF₆/MFA electrolyte is a good candidate to improve the thermal stability of the lithium ion and lithium metal anode battery.