Effects of a High-Concentration LiPF₆-Based Carbonate Ester Electrolyte for the Electrochemical Performance of a High-Voltage Layered LiN_i0.6Co_0.2Mn_0.2O₂ Cathode

Lithium-ion battery electrolytes are key components contributing to the Li⁺ transference and the formation of solid electrolyte interface (SEI) film. A high-concentration LiPF₆-based EC/DMC electrolyte is employed to investigate the electrochemical performance of layered LiNi_0.6Co_0.2Mn_0.2O₂ cathode material at the high voltage of 4.6 V. Cyclic voltammetry tests indicate that the 6.5 M EC/DMC-LiPF₆ high-concentration electrolyte enjoys a stronger oxidation resistance compared with 1 M diluted electrolyte. It is indicated that the highest occupied molecular orbital (HOMO) energy of nEC-Li⁺ and nDMC-Li⁺ solvation complexes is reduced with the Li⁺ concentration increase, suggesting that the high Li⁺ concentration enhances the oxidation resistance of the solvent molecules by the density functional theory (DFT) method. X-ray photoelectron spectroscopy (XPS) tests demonstrate that the SEI film is different from that of the diluted electrolyte on the cathode surface by using the high-concentration electrolyte. The composite cathode of LiNi_0.6Co_0.2Mn_0.2O₂ exhibits a 161.3 mAh/g reversible capacity at the rate of 0.2C (1C = 180 mA g^-1) after 100 cycles by using the high-concentration electrolyte, while a 138.6 mAh/g lower capacity is exhibited in the diluted electrolyte. The improvement of cycling performance should be attributed to the prevention of the interface side reactions on the cathode.