Mixed sodium-lithium vanadium fluorophosphates Na3-xLixV2(PO4)2F3: The origin of the excellent high-rate performance

The Na+/Li+ substitution in Na3V2(PO4)2F3 is studied using different synthesis approaches: equilibrium and non-equilibrium. In the equilibrium solid-state process, the reagent mixtures with various Na/Li ratios are preliminary mechanically activated using a high-energy AGO-2 planetary mill and then annealed in an Ar flow. Non-equilibrium chemical Na/Li ion exchange is performed in the organic solution of LiBr as a Li source. Phase composition, crystal structure, morphology and electrochemical properties of the as-prepared materials are analysed by XRD, SEM, TEM, EIS, galvanostatic cycling, and cyclic voltammetry. It is shown that the solid-state synthesis leads to the lower substitution degree and is accompanied by the formation of the V2O3 impurity. These samples are characterized by enhanced electrical conductivity (by about 4 orders of magnitude) compared to that of the pristine Na3V2(PO4)2F3, and improved high-rate performance both in Na and Li-cells. The origin of the excellent high-rate capability of the as-prepared Na3-xLixV2(PO4)2F3 is partially attributed to the pseudocapacitive effect initiated from the rational design of the materials, including submicron particles, mixed in-situ coating with electron-conductive carbon and V2O3.