Understanding the electrochemical mechanism of high sodium selective material Na3V2(PO4)2F3 in Li+/Na+ dual-ion batteries

Na3V2(PO4)2F3 (NVPF) has been attracting great interests as a potential positive electrode for lithium/sodium (Li+/Na+) hybrid batteries. However, it is still not clarified as what type of alkali metal ion is intercalated at different stage in the Na-based cathode material during discharge process. Herein, the intercalation sites and ratios of Li+ and Na+ ions are firstly studied systematically. The results show that the intercalation of Li+ ions is mainly at the original Na1 and Na2 sites. However Li+ ions are metastable in NVPF, and are easy to be replaced by Na+ ions through ion exchange in Na+ ion containing hybrid electrolyte. There exists a competitive insertion between Li+ and Na+ ions caused by the impact of ion exchange rate, and the difference of the ion diffusion speed of Li+ and Na+ ions in the hybrid electrolyte with certain concentration of these two ions. Thus a competitive insertion mechanism of Li+ and Na+ ions in NVPF is proposed. Furthermore, the intercalating ratio of Li+ and Na+ ions can be tuned by controlling the external current density, which can effectively change the ion diffusion speed of Li+ and Na+. Conclusively NVPF with double coating of polytetrahydrofuran and carbon has better rate capability and cycle stability in hybrid electrolyte with Na+ concentration higher than 0.2 mol L−1, which is of potential to be used as cathode material for hybrid batteries with high specific energy.