Non‐Flammable and Highly Concentrated Carbonate Ester‐Free Electrolyte Solutions for 5 V‐Class Positive Electrodes in Lithium‐Ion Batteries

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 O 4 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 LiBF 4 and TFEP at high potentials. A nearly saturated LiBF 4 /TFEP+FMP electrolyte solution with a specific composition improved the charge and discharge performance of a LiNi 0.5 Mn 1.5 O 4 electrode, and the solution structure was studied by pulsed‐field‐gradient NMR spectroscopy.