Approaching better cycleability of LiCoPO4 by vanadium modification

A series of (1 − y)LiCoPO4·yLi3V2(PO4)3 with 0 ≤ y ≤ 1 was prepared via mechanochemically assisted carbothermal reduction at 750 °C. Structure and electrochemistry were studied by a complex of physico-chemical methods including XRD, NPD, FTIR, NEXAFS, TEM, EDX STEM, and galvanostatic cycling. It has been shown that only a small amount of V ions incorporates into the LiCoPO4 structure. The other V ions form monoclinic Li3V2(PO4)3. Incorporated V ions are in a mixed oxidation state and predominantly substitute for Co2+. The compensation mechanism for supervalent V ions substitution in LiCoPO4 involves the lithium vacancies formation. The as-prepared (1 − y)LiCoPO4·yLi3V2(PO4)3 with low y show the enhanced electrochemical behavior compared to pure LiCoPO4, especially in the 4.0-5.0 V range, due to a suitable amount of structural defects, which improve ionic and electronic conductivity, and to the presence of the Li-conductive Li3V2(PO4)3 on the surface of the LiCoPO4 particles promoting the deliverance of the Li ions from the electrolyte to the (0 1 0) facet.