Suppression Mechanism for Dissolution of Conversion-Type CuCl₂ Electrode in LiPF₆/methyl Difluoroacetate Electrolyte

Abstract

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.

Metal chloride cathodes for next-generation rechargeable lithium batteries

“…More interestingly, in the case of metal chlorides, both chloride ions and lithium ions can serve as charge carriers during conversion reaction. 19 , 21 , 52 , 53 , 68 However, the distinction between the two lacks sufficient research. Additionally, the aggressive conversion between halogens and lithium halides holds the promise of higher energy densities, 17 , 39 but unfortunately also presents greater challenges in terms of safety and reversibility ( Figure 7 D).…”

“…Reproduced with permission from a study by Hashizaki et al. 68 Copyright 2019 IOP Publishing. (C) Discharge characteristics of CuCl 2 electrode in different electrolytes.…”

“…Reproduced with permission from a study by Hashizaki et al. 68 Copyright 2019 IOP Publishing. (D) Variation of VCl 3 solubility with LiFSI/DMC electrolytes as a function of the LiFSI concentration (c LiFSI ).…”

“…Ogumi’s group reported the application of CuCl 2 as a conversion-type cathode by utilizing fluorinated electrolytes. 21 , 68 , 69 Their particular electrolyte formulation of methyl difluoroacetate (MFA) as the solvent and LiPF 6 as the lithium salt successfully mitigated the dissolution of the CuCl 2 cathode. This improvement could be attributed to two factors.…”

Non‐Flammable Electrolyte Enables High‐Voltage and Wide‐Temperature Lithium‐Ion Batteries with Fast Charging

“…In this study, a fluorinated carboxylic acid ester in which partial (two) H atoms of the methyl in the methyl acetate (MA) molecule are substituted by F atoms, that is methyl difluoroacetate (MDFA) (inset in Figure 1a), is chosen as the main solvent, due to its wide-temperature operation capability [19] and good oxidative stability (Figure S1). Unlike traditional fluorinated linear carbonate solvents (e.g., methyl 2,2,2-trifluoroethyl ester (FEMC) [20] and bis(2,2,2-trifluoroethyl) carbonate (BTFEC) [21] ), in which the solubility of LiPF 6 salt is limited owing to the effect of trifluoromethyl group, the chosen fluorinated carboxylic acid ester has a higher dis-solvation capability for the LiPF 6 salt (e.g., 2.7 M LiPF 6 in MDFA [22] ). Moreover, the low coordination ability of Li + with MDFA that results from the influence of fluorine atoms with an electron-withdrawing effect is beneficial for reducing the desolvation barrier.…”