On Vanadium Substitution in Li2MnSiO4/C as Positive Electrode for Li-ion Batteries

Abstract

Vanadium substitution is an interesting approach to manipulate the properties of the poor electronic and ionic conducting lithium transition metal orthosilicates. Especially, if incorporated on the Si site, it could alter the highly insulating character of the SiO4 framework. This study addresses the feasibility and limitations of V substitution in Li2MnSiO4. Nominal compositions of Li2Mn1–xVxSiO4 (0 ≤ x ≤ 0.2) and Li2MnSi1–xVxO4 (0 ≤ x ≤ 0.3) were synthesized by a sol–gel method, and the structural evolution was analyzed by X-ray diffraction and transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). While the solid solubility of V on tetrahedral Mn sites was shown to be limited, substantial amounts of V entered the structure when intended to substitute Si. Elemental mapping by TEM showed that V was highly inhomogeneously distributed, and high energy resolution EELS demonstrated that the majority of V was present in a tetravalent state. The nominal compositions Li2MnSi1–xVxO4 (0 ≤ x ≤ 0.3) showed superior electrochemical performance, with reduced charge transfer resistance and an increased Li ion diffusion coefficient. Furthermore, cyclic voltammetry revealed increased redox activity which can be attributed to V within the concentration series. The best performance was achieved with 25 mol % V substitution. V substitution beyond 25 mol % caused deterioration of the properties.