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

Abstract The Na+/Li+ substitution in Na 3 V 2 (PO 4) 2 F 3 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 V 2 O 3 impurity. These samples are characterized by enhanced electrical conductivity (by about 4 orders of magnitude) compared to that of the pristine Na 3 V 2 (PO 4) 2 F 3 , and improved high-rate performance both in Na and Li-cells. The origin of the excellent high-rate capability of the as-prepared Na 3-x Li x V 2 (PO 4) 2 F 3 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 V 2 O 3. Graphical abstract Image 1 Highlights • Na 3-x Li x V 2 (PO 4) 2 F 3 were prepared by the solid-state and ion exchange approaches. • Na 3-x Li x V 2 (PO 4) 2 F 3 solid solutions preserve the crystal structure of Na 3 V 2 (PO 4) 2 F 3. • Solid-state synthesis of Na/Li compositions is accompanied by in situ V 2 O 3 formation. • V 2 O 3 significantly enhances electronic conductivity and high-rate performance. • High-rate capability was partially attributed to the pseudocapacitive effect. [ABSTRACT FROM AUTHOR]