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

Abstract Na 3 V 2 (PO 4 ) 2 F 3 (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.