Structure and electrochemical properties of Na2±xV3P2O13 (x = 0 and 1): a promising cathode material for sodium-ion batteries

Owing to the large abundance of sodium resources and its low cost, sodium-ion batteries (NIBs) are being considered as a promising, feasible alternative to lithium-ion batteries (LIBs), notably for stationary applications. Research activities on sodium-ion batteries are growing worldwide but do still require a great deal of basic and applied research. The design and synthesis of new cathode materials are of great interest to realize the structural requirements to build sustainable and safe NIBs. Herein, we report the synthesis, structure and electrochemical properties of sodium vanadium oxy-phosphate (NVOP), Na2±xV3P2O13 (x = 0 and 1), a stable host for the reversible insertion of sodium. Na3V3P2O13 delivers a reversible capacity of 132 mA h g−1 at an average potential of 2.7 V vs. Na/Na+, which amounts to a specific energy of 356 W h kg−1. Furthermore, NVOP compounds exhibit excellent cycling stability. Besides, NVOP shows a rich structural chemistry during the sodium insertion and deinsertion process. A reversible switching of V5+ and V4+ between two crystallographic sites during sodiation and desodiation reactions was observed, hitherto unknown in battery materials. Na2±xV3P2O13 (x = 0 and 1) compounds were characterized by various experimental tools to understand the structure and related properties. In addition, density functional theory (DFT) calculations were performed to complement experimental observations and to understand sodium diffusion behavior in Na2±xV3P2O13 (x = 0 and 1)