A sodium-ion cell based on the fluorophosphate compound NaVPO4F

Preliminary performance data is presented for a novel sodium-ion cell based on a hard carbon/NaVPO4F chemistry. To our knowledge this is the first time that a viable sodium-ion system based on a polyanion cathode material has been demonstrated. The NaVPO4F was prepared by a simple incorporation reaction involving vanadium(III) phosphate, VPO4 and NaF. The hard carbon was from a commercial source. Constant current cycling indicated reversible alkali-ion insertion behavior for the fluorophosphate material, while the generally symmetrical nature of the charge/discharge curves indicated the energetic reversibility of the insertion system. The average discharge voltage for the sodium-ion cells was demonstrated to around 3.7 V, a figure comparable with commercially available lithium-ion cells. The reversible specific capacities for the cathode and anode active materials were determined to be 82 and 202 mAh/g, respectively. Representative test cells cycled more than 30 times before their discharge capacity had declined to less then 50% of the original. The relatively high first cycle charge inefficiency is thought to be due to irreversible passivation reactions occurring at the carbon surface. Significant performance enhancements may be made by optimization of the electrolyte system and improving the active material mass balance. In summary, low overvoltage, good insertion reversibility, and reasonable capacity retention characterize this fluorophosphate-based sodium-ion cell. (C) 2002 The Electrochemical Society.