Superior Na-storage performance of molten-state-blending-synthesized monoclinic NaVPO4F nanoplates for Na-ion batteries

Abstract

A superior monoclinic NaVPO₄F nanoplate for Na-ion batteries has been prepared by a molten-state-blending technique. By this molecular level blending method, a nanoscale-laminated NaVPO₄F@C sample with high crystallinity can be obtained. High thermal stability, stable Na⁺ insertion/extraction and superior electron/Na⁺ transport of NaVPO₄F have been elucidated by temperature-dependent IR, in situ XRD and kinetic investigations. Accordingly, the as-prepared NaVPO₄F with moderate carbon coating exhibits superior Na-storage performance. It can deliver a high initial specific capacity of 135.0 mA h g⁻¹ at 0.2C, ultra-high rate performance (up to 112.1 mA h g⁻¹ at 30C) and super-stable cycling performance (capacity fading rate of 0.0064% per cycle during 1500 cycles at 20C). The potential application of NaVPO₄F as the anode has been explored, and a symmetrical battery with NaVPO₄F as both the anode and cathode has been successfully assembled for the first time. More significantly, in situ XRD and ex situ NMR have been employed to explore the charge–discharge behavior in a Na-ion battery, and the result clearly demonstrates that less than one Na has been intercalated/extracted from NaVPO₄F during the charging/discharging process.