Non-Flammable and Highly Concentrated Carbonate Ester-Free Electrolyte Solutions for 5 V-Class Positive Electrodes in...

Non-flammable and highly concentrated electrolyte solutions were designed using tris(2,2,2-trifluoroethyl) phosphate (TFEP) as a main solvent toward a radical improvement in the safety and energy density of lithium-ion batteries. Unlike conventional carbonate ester-based solutions, simple TFEP-based electrolyte solutions were not intrinsically compatible with 5 V-class LiNi_0.5 Mn_1.5 O₄ positive electrodes, even at high concentrations. Based on the degradation mechanism that was analyzed by Raman spectroscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, a fluorinated diluent of methyl 3,3,3-trifluoropropionate (FMP) was introduced to suppress the decomposition of LiBF₄ and TFEP at high potentials. A nearly saturated LiBF₄ /TFEP+FMP electrolyte solution with a specific composition improved the charge and discharge performance of a LiNi_0.5 Mn_1.5 O₄ electrode, and the solution structure was studied by pulsed-field-gradient NMR spectroscopy.