Na-Rich Na_3V_2(PO_4)_3 Cathodes for Long Cycling Rechargeable Sodium Full Cells

摘要

NASICON-type sodium vanadium phosphate (Na_3V_2(PO_4)_3, or NVP) cathode materials have great potential for fast charging and long cycling sodium-ion batteries (SIBs) similar to lithium iron phosphate (LiFePO_4, or LFP) cathode materials used in lithium-ion batteries (LIBs). However, the cycle life and energy density in the full cell using NVP materials need to be significantly improved. This paper investigates the degradation mechanisms of NVPbased SIBs and identifies the Na loss from the cathode to the anode solid electrolyte interphase (SEI) reactions as the main cause of capacity degradation. A new Na-rich NVP cathode (e.g., Na_4V_2(PO_4)_3, or Na_4VP) is developed to address the Na loss problem. Conventional NVP can be easily transformed into the Na_4VP by a facile and fast chemical solution treatment (30 s). Na-free-anode Na_4VP and hard carbon-Na_4VP full cells are assembled to evaluate the electrochemical properties of the Na-rich NVP cathode. The Na_4VP cathode provides excess Na to compensate for the Na loss, resulting much longer cycle life in the full cells (>400 cycles) and a high specific energy and power density. Good low-temperature performance is also observed.