Fluoroalkyl ether-diluted dimethyl carbonate-based electrolyte solutions for high-voltage operation of LiNi0.5Co0.2Mn0.3O2 electrodes in lithium ion batteries

Abstract

The energy density of lithium-ion batteries can be increased by improving the battery voltage and/or specific capacity. However, conventional electrolyte solutions decompose oxidatively at high voltages. The stability against oxidation of electrolyte solutions could be enhanced by increasing the concentration of lithium salts. The nearly saturated 8.67 mol kg⁻¹ LiBF₄/dimethyl carbonate (DMC) electrolyte solution enabled high-voltage operation of LiNi_0.5Co_0.2Mn_0.3O₂ positive-electrodes at 4.6 V, and extended the discharge capacity to ca. 200 mA h g⁻¹. BF₄⁻ anions, as well as DMC solvents, were stabilized at the high concentration. To reduce the viscosity and Li concentration without losing the high stability against oxidation toward practical use, the highly concentrated electrolyte solution was diluted with fluorinated co-solvents having a low donor ability. Raman spectroscopy revealed that the solvation structure of LiBF₄/DMC was maintained after dilution with a specific fluoroalkyl ether. The resultant low-viscosity and -concentration electrolyte solution was highly stable against oxidation at the LiNi_0.5Co_0.2Mn_0.3O₂ electrodes as long as the DMC/LiBF₄ molar ratio was kept low. The charge/discharge performance was much better than that for a conventional 1 M LiPF₆/ethylene carbonate-based electrolyte solution and the energy density far exceeded that of 5 V class LiNi_0.5Mn_1.5O₄ electrodes.