A thermodynamic investigation on the substitution mechanism of Mg-doped lithium vanadium phosphate

Abstract

Monoclininc lithium vanadium phosphate, Li3V2(PO4)3, is regarded as a potential cathode material for the next generation of high-performance lithium ion batteries, since it exhibits a high discharge voltage (up to ∼4.1 V) and a large theoretical specific capacity (197 mAhg−1). However, the low intrinsic electronic conductivity of Li3V2(PO4)3, which is a prevailing challenge for olivine type compounds, inhibits its use in commercial applications. Although the substitution of V3+ by other cation species is a common procedure to increase the conductivity and electrochemical performance of Li3V2(PO4)3, the underlying mechanisms for the improved properties are not yet well understood. Therefore, a thermodynamic approach is used in this work to investigate the influence of dopant, i.e. Mg2+ as well as vacancies on the V3+ site on the stability of the resulting materials. On the basis of the measured partial molar Gibbs energies, entropies and enthalpies of the electrochemical reaction, a detailed discussion of the substitution mechanisms and their influence on the electrochemical performance is presented.