Unlocking highly reversible V5+/V4+redox reaction and fast-stable Na storage in NASICON cathodes by electronic structure optimization and solid-solution behavior regulation

It is formidable to fully exert NASICON-structured Na 3 V 2 (PO 4 ) 3 's (NVP) energy density due to the serious structural degradation aroused from the undesirable phase evolution and transition metal ion migration upon high potential versus Na + /Na. To break through the energy limitation of NVP, we meticulously selected three classes of metal elements to employ a novel quaternary substitution in NVP. Theoretical calculation expresses that their 3p, 3d, 4 f orbits hybrid synergy can favor the electron rearrangement signally, increasing the accessibility of the V 5+ /V 4+ redox reaction. Therefore, Na 3 V 1.6 (CrAlFeCe) 0.1 (PO 4 ) 3 (NVMP) cathode delivers a groundbreaking energy density of 420 Wh kg -1 . Besides, the solid-solution mechanism of Na storage during the V 5+ /V 4+ reaction process was validated by ex situ XRD, demonstrating the significant effect of quaternary substitution on inhibiting irreversible phase evolution under high charge states. Hence, NVMP can maintain the activated V 5+ /V 4+ after 10000 cycles. The fast Na-storing kinetics of NVMP were analyzed by in situ EIS, in situ DRT, GITT, and various scan rate CV. This work provides new insights for adjusting electronic structure and Na storing behavior in NASICON cathodes.