Study of the LiMn_1.5Ni_0.5O₄/Electrolyte Interface at Room Temperature and 60°C

The surface layer (Cathode-Electrolyte Interface; CEI) on LiMn_1.5Ni_0.5O₄, a promising, high voltage positive electrode for Li-ion batteries, was studied by XPS, AC impedance spectroscopy and FTIR spectroscopy. Half cells and full cells with LiMn_1.5Ni_0.5O₄as positive electrode material and Li₄Ti₅O₁₂as a negative electrode material were assembled in conventional carbonate-based electrolytes with LiPF₆or LiBF₄as the salt, and the effect of cycling at different operating conditions (short and long storage time, state of charge and temperature) on the surface layer composition was assessed. Capacities reaching near the theoretical value of 140 mAh g₋₁were obtained in half cells cycled at C/2 and room temperature, with 85% of the capacity being retained after 100 cycles. Cycling at 60°C leads to a decrease in capacity and coulombic efficiency. The surface analysis by XPS revealed that the CEI is composed of inorganic species such as LiF and Li_xPF_yO_zor Li_xBF_yO_zas well as organic species such as polyethers and carbonates. Generally, it was found that cycling or storing the material at 60°C with an electrolyte using LiPF₆as a salt yield more organic species and less LiF at the surface than the one with LiBF₄.