Achieving reversible multi-redox activity in NASICON-type sodium vanadium phosphate towards ultrafast and high-energy sodium storage

NASICON-type Na 3 V 2 (PO 4 ) 3 (NVP) has been identified as an ideal cathode for sodium-ion batteries (SIBs) because of its ultra-fast Na + diffusion and very stable structure. However, it still suffers from significant problems such as a low operating voltage plateau (3.4 V, vs. Na + /Na), inferior electronic conductivity and unsatisfactory energy density (370 Wh kg −1 ), thereby hampering the commercialization of NVP. Herein, a series of Al 3+ /Ga 3+ co-doping Na 3 V 2 (PO 4 ) 3 cathodes were successfully prepared through a facile solid-state reaction process. The substitution of Al 3+ /Ga 3+ for V 3+ in Na 3 V 2–2x Al x Ga x (PO 4 ) 3 ( x = 0, 0.1 and 0.2) effectively activates the electrons outside of V and achieves an extra redox couple V 4+ /V 5+ for high-voltage (3.9 V, vs. Na + /Na) reactions. Moreover, the presence of Ga in this series of electrode materials increases the proportion of sp 2 -type carbon. The optimized Na 3 V 1.8 Al 0.1 Ga 0.1 (PO 4 ) 3 material possesses high ionic/electronic conductivity and superior electrochemical performance: the energy density is enhanced by approximately 13.1 % at 0.1 C (from 345.4 Wh kg −1 for NVP to 390.7 Wh kg −1 for Na 3 V 1.8 Al 0.1 Ga 0.1 (PO 4 ) 3 ), and the power density is increased by approximately 135 % at 30 C (from 2574 W kg −1 for NVP to 6039 W kg −1 for Na 3 V 1.8 Al 0.1 Ga 0.1 (PO 4 ) 3 ). Furthermore, Na 3 V 1.8 Al 0.1 Ga 0.1 (PO 4 ) 3 exhibits excellent cycling stability with 93.4 % capacity retention after 350 cycles at 1 C. Ex-situ XPS and electrochemistry investigations imply the high reversibility of the V 3+ /V 4+ /V 5+ redox reaction in Na 3 V 1.8 Al 0.1 Ga 0.1 (PO 4 ) 3 . This work proposes a novel strategy to modulate the electronic structure of NVP by co-doping with Al 3+ and Ga 3+ to achieve reversible multi-redox activity (V 3+ /V 4+ /V 5+ ) in NASICON-type NVP, thus providing a viable approach for advancing ultrafast high-energy sodium storage.