An electrolyte to improve the deep charge–discharge performance of LiNi_0.8Co_0.15Al_0.05O₂ cathode

The electrochemical behaviors of electrolytes, 1 M lithium difluoro(oxalate)borate (LiODFB) in sulfone and dimethyl carbonate blends, have been investigated for LiNi_0.8Co_0.15Al_0.05O₂ (NCA) cathode. The viscosity and conductivity tests have been used to investigate the relationship among the solution viscosity, conductivity and electrolyte composition. Through linear sweep voltammetry, cyclic voltammetry, AC impedance and charge–discharge test, we have compared the electrochemical stability of different electrolytes, the reversibility, capacity, cycling performance, rate capability and interfacial impedance of NCA cathode in different electrolytes. SEM, XPS, TEM and FTIR analyses have utilized to discuss the surface nature of NCA cathode after cycles at deep charge–discharge voltage. These results demonstrate that 1 M LiODFB/(SL–DMC, 4:6) electrolyte shows the maximum conductivity value (κ_max = 4.25 S cm⁻¹), proper viscosity (η = 3.43 mPa s) and high oxidative decomposition potential (5.66 V) and NCA cathode can present superior reversibility, higher columbic efficiency during cycles in this electrolyte than in 1 M LiPF₆/(EC–DMC, 3:7) electrolyte. Besides, a dense deposition layer is formed on the NCA cathode when NCA/Li cells cycle in 1 M LiODFB/(SL–DMC, 4:6) electrolyte with deep charging and discharging process, which can avoid the erosion of electrolyte to transition-metal elements, restrain the damage of NCA structure, and then improves the cycle performance of NCA/Li cells.