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 exert NASICON-structured Na3V2(PO4)3's (NVP) energy density fully 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 select three classes of metal to employ a novel quaternary substitution in NVP. Theoretical calculation expresses that their 3p, 3d, 4 f orbits hybrid synergy favors the electron rearrangement signally, increasing the accessibility of the V5+/V4+ redox reaction. Therefore, Na3V1.6[CrAlFeCe]0.1(PO4)3 (NVMP) cathode delivers a groundbreaking energy density of 420 Wh kg−1. Besides, the solid-solution mechanism of Na storage during the V5+/V4+ 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 V5+/V4+ after 10000 cycles. The fast sodium-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.