Dilution of Highly Concentrated LiBF₄/Propylene Carbonate Electrolyte Solution with Fluoroalkyl Ethers for 5-V LiNi0.5Mn1.5O₄Positive Electrodes

Various kinds of fluoroalkyl ethers were investigated as diluents to reduce the high viscosity of highly concentrated LiBF4/PC electrolyte solutions. 1,1,2,2–tetrafluoroethyl 2,2,3,3–tetrafluoropropyl ether (HFE) was the most suitable diluent because of a high solubility of LiBF4. 2.50 mol kg−1 LiBF4/PC+HFE (2:1 by volume) was a reasonable compromise to attain a low viscosity (51.7 mPa s) and a low PC/Li molar ratio (2.39). Raman spectroscopy revealed that the fraction of free PC molecules in 2.50 mol kg−1 LiBF4/PC+HFE (2:1) was much smaller than 2.50 mol kg−1 LiBF4/PC (PC/Li molar ratio = 4.00), and that the interactions of HFE with Li+ cations and BF4− anions were very weak. LiNi0.5Mn1.5O4 positive electrodes showed high charge/discharge performance with low irreversible capacities in 2.50 mol kg−1 LiBF4/PC+HFE (2:1), which showed that the highly concentrated LiBF4/PC system can be diluted with HFE without losing the high stability against oxidation.