Thermal studies of fluorinated ester as a novel candidate for electrolyte solvent of lithium metal anode rechargeable cells

The thermal stability of fluorinated ester electrolytes with and without lithium metal and the positive electrode material at the charged state were investigated, in terms of application for electrolytes in lithium metal anode cells. The fluorinated ester electrolytes are solutions dissolving LiPF₆ in carboxylic acid esters whose original carboxylic acids are partially fluorinated. The corresponding non-fluorinated ester electrolytes were also studied for comparison. According to differential scanning calorimetry (DSC) measurement, fluorinated ester electrolytes exhibited significant thermal stability when coexisting with lithium metal or Li_0.5CoO₂. LiPF₆/methyl difluoroacetate showed the best stabilization effect, which shifted the exothermic peak of the electrolyte with lithium metal or Li_0.5CoO₂ to about 300°C. In addition, LiPF₆/methyl difluoroacetate exhibited a good lithium anode cycling efficiency. We believe that LiPF₆/methyl difluoroacetate is a very promising electrolyte for use in realizing lithium metal anode secondary cells.