Study of the LiMn1.5Ni0.5O4/Electrolyte Interface at Room Temperature and 60°C

Abstract

The surface layer (Cathode-Electrolyte Interface; CEI) on LiMn1.5Ni0.5O4, 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 LiMn1.5Ni0.5O4 as positive electrode material and Li4Ti5O12 as a negative electrode material were assembled in conventional carbonate-based electrolytes with LiPF6 or LiBF4 as 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−1 were 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 LixPFyOz or LixBFyOz as 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 LiPF6 as a salt yield more organic species and less LiF at the surface than the one with LiBF4.