Stabilization of Lithium Transition Metal Silicates in the Olivine Structure

Abstract

While olivine LiFePO₄ shows amongst the best electrochemical properties of Li-ion positive electrodes with respect to rate behavior owing to facile Li⁺ migration pathways in the framework, replacing the [PO₄]^3- polyanion with a silicate [SiO₄]^4- moiety in olivine is desirable. This could allow additional alkali content and hence electron transfer, and increase the capacity. Herein we explore the possibility of a strategy toward new cathode materials and demonstrate the first stabilization of a lithium transition metal silicate (as a pure silicate) in the olivine structure type. Using LiInSiO₄ and LiScSiO₄ as the parent materials, transition metal (Mn, Fe, Co) substitutions on the In/Sc site were investigated by computational modeling via atomic scale simulation. Transition metal substitution was found to be only favorable for Co, a finding confirmed by the successful solid state synthesis of olivine Li_xIn_yCo_2-x-ySiO₄. Stabilization of the structure was achieved by entropy provided by cation disorder.