Sequentially-processed Na3V2(PO4)3 for cathode material of aprotic sodium ion battery

Abstract

Na3V2(PO4)3 is considered as one of the most promising cathode materials for SIB, electrodes through exploiting unconventional and relatively expensive vanadium salts or carbon sources. For the latter, the obtained superior rate performance is generally obtained at relatively high content of the carbon additives. A carbon-coated sodium ion containing vanadium phosphate, Na3V2(PO4)3, is prepared by a simple sequentially-processed method. The sequentially-processed method is potential for mass-production and preparation of other high-quality vanadium-based electrode materials. A special dissolution sequence leads to an un-precipitated solution, which is used to produce an amorphous-state precursor. The carbon mass fraction of sequentially-processed Na3V2(PO4)3 is estimated to be less than 2% measured by thermo-gravimetric analyzer. Electrochemical performance of sequentially-processed Na3V2(PO4)3 is examined as cathode material for aprotic sodium ion battery. The sequentially-processed Na3V2(PO4)3 delivers 113 mA h g−1 with a flat voltage plateau at 3.4 V vs sodium metal. The simple sequentially-processed method also endows excellent rate capacity and cyclability to sequentially-processed Na3V2(PO4)3 at the voltage window of 2.8–3.8 V. We further investigate cyclic and rate performance of sequentially-processed Na3V2(PO4)3 as a high-capacity cathode material with a widened voltage window of 1–4 V vs Na+/Na. A capacity of more than 210 mA h g−1 can be delivered with appearance of another additional voltage plateau at 1.6 V but the capacity quickly fades away after dozens of cycles. The result validates the potential of Na3V2(PO4)3 in reaching high cathode capacity (> 200 mA h g−1) and also calls for new structure design to better accommodate the intercalation of sodium ions and new binder materials that can tolerate a larger degree of electrode expansion.