Methyl difluoroacetate (MFA) was used as an electrolyte solvent in Li/CuCl2 batteries, and the LiPF6/MFA electrolyte was effective for the suppression of the self-discharge of Li/CuCl2 batteries. The dependence of self-discharge behavior on LiPF6 concentration (1.0, 1.4, 1.8, 2.2, and 2.7 mol dm−3) in MFA-based electrolytes was investigated. The lowest self-discharge was observed for a 2.2 mol dm−3 LiPF6/MFA electrolyte under the described conditions, and an initial discharge capacity of 395 mAh g−1 was maintained after 678 h (4 weeks) of rest. The cycling performance of the LiPF6/MFA electrolytes was evaluated, and the discharge capacity for the 2.2 mol dm−3 LiPF6/MFA electrolyte was found to be maintained at 183 mAh g−1 (10th cycle). Therefore, 2.2 mol dm−3 LiPF6/MFA electrolyte is confirmed to be effective for the suppression of the self-discharge of Li/CuCl2 batteries.
As the cathode materials in conversion-type lithium ion batteries, transition-metal chlorides are well-known to easily dissolve in many organic solvents. However, the LiPF 6 /methyl difluoroacetate (MFA; CHF 2 COOCH 3 ) electrolyte is effective in suppressing the dissolution of the CuCl 2 electrode, as reported for a Li/CuCl 2 battery. This effect did not arise in the MFA solvent without LiPF 6 , but did on slightly dissolving LiPF 6 in the MFA solvent. This was because a very small amount of film was formed on the surface of the CuCl 2 electrode due to the addition of the small amount of LiPF 6 , and the film was composed of copper fluoride hydroxide, partial release of Cl from CuCl 2 , and CuCl.
As conversion-type cathode materials, transition-metal chlorides are known to suffer from dissolution in organic solvents. However, our previous investigation revealed that in the Li/CuCl 2 battery, the dissolution of CuCl 2 /C cathode materials could be suppressed by using LiPF 6 /methyl difluoroacetate (MFA; CHF 2 COOCH 3) electrolyte. Consequently, its capacity decline was lessened by raising the charged voltage. Herein we examine the reconversion reaction cathode of Cu/LiCl instead of the conversion reaction cathode of CuCl 2 /C. The charge-discharge characteristics of both electrodes are reported. The Cu/LiCl electrode with LiPF 6 /MFA could charge and discharge without carbon additives.
X-ray absorption fine structure (XAFS) analysis was performed on the CuCl2 cathode in Li/CuCl2 battery with LiPF6/methyl difluoroacetate (MFA) electrolyte to investigate the valence states of Cu during discharge/charge. X-ray absorption near edge structure (XANES) spectrum indicated that the Li/CuCl2 battery can discharge and charge reversibly. The disproportionation side reaction is confirmed to occur during charging. The side reaction, known to induce battery self-discharge, can be suppressed if reaction Cu + LiCl → CuCl + Li+ + e− proceeds completely, which may be accomplished by using a more homogeneous composition in the cathode.
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