Computational and Experimental Investigation of Ti Substitution in Li₁(Ni_<i>x</i>Mn_<i>x</i>Co_{1–2<i>x</i>–<i>y</i>}Ti_<i>y</i>)O₂ for Lithium Ion Batteries

Abstract

Aliovalent substitutions in layered transition-metal cathode materials has been demonstrated to improve the energy densities of lithium ion batteries, with the mechanisms underlying such effects incompletely understood. Performance enhancement associated with Ti substitution of Co in the cathode material Li₁(Ni_<i>x</i>Mn_<i>x</i>Co_{1–2<i>x</i>})­O₂ were investigated using density functional theory calculations, including Hubbard-U corrections. An examination of the structural and electronic modifications revealed that Ti substitution reduces the structural distortions occurring during delithiation due to the larger cation radius of Ti⁴⁺ relative to Co³⁺ and the presence of an electron polaron on Mn cations induced by aliovalent Ti substitution. The structural differences were found to correlate with a decrease in the lithium intercalation voltage at lower lithium concentrations, which is consistent with quasi-equilibrium voltages obtained by integrating data from stepped potential experiments. Further, Ti is found to suppress the formation of a secondary rock salt phase at high voltage. Our results provide insights into how selective substitutions can enhance the performance of cathodes, maximizing the energy density and lifetime of current Li ion batteries