A high-energy-density NASICON-type Na 3 V 1.25 Ga 0.75 (PO 4 ) 3 cathode with reversible V 4+ /V 5+ redox couple for sodium ion batteries.

The stable three-dimensional framework and high operating voltage of sodium superionic conductor (NASICON)-type NaV(PO) has the potential to work with long cycle life and high-rate performance; however, it suffers from the poor intrinsic electronic conductivity and low energy density. Herein, Ga is introduced into NaV(PO) to activate the V/V redox couple at a high potential of 4.0 V for enhancing energy density of the materials (NaVGa(PO)). After the partial substitution of Ga for V, three redox couples (V/V, V/V and V/V) of V are reversibly converted in the voltage range of 1.4-4.2 V, suggesting multi-electrons (>2e) involved in the reversible reaction, and simultaneously the electronic conductivity of the materials is effectively enhanced. As a result, the cathode with x = 0.75 exhibits excellent electrochemical properties: in the voltage range of 2.2-4.2 V, delivering an initial capacity of 105 mAh/g at 1C with a capacity retention rate of 92.3% after 400 cycles, and providing a stable reversible capacity of 88.3 mAh/g at 40C; and in the voltage range of 1.4-4.2 V, presenting the reversible capacity 152.3 mAh/g at 1C (497.6 Wh kg), and cycling stably for 1000 cycles at 20C with a capacity decay of 0.02375% per cycle. It is found that the NaVGa(PO) cathodes possess the sodium storage mechanism of single-phase and bi-phase reactions. This investigation presents a useful strategy to enhance the energy density and cycling life of NASICON-structured polyanionic phosphates by activating high-potential V/V redox couple.