Localized electronic interactions in phosphate cathode: Breaking the V4+/V5+ high-voltage barrier for high-energy and long-life sodium-ion batteries

Achieving multi-electron reaction at high operation voltage is the key to increase the energy density of Na3V2(PO4)3 (NVP) cathode. However, the motivated V4+/V5+ redox usually shows inferior reversibility and causes serious volume changes. Herein, this article proposes a local electronic interaction mechanism which achieves highly reversible multi-electron reaction of NVP. Particularly, Al-Sn co-doped and carbon coated NVP (Na3Al0.1Sn0.1V1.8(PO4)3@C, abbreviated as NASVP@C-2) was prepared by sol-gel method. The doped-Al can activate the redox of V4+/V5+ and generate the “pinning effect” to stabilize the crystal structure, and the Sn acts as localized electronic reservoir for charge compensation of V redox. The localized electronic interaction mechanism between Sn and V is revealed by multi ex-situ characterizations. Kinetics tests and density functional theory (DFT) calculations suggest that the Al-Sn co-doping enhances the electronic conductivity and reduces the Na+ diffusion barrier in NVP. An extremely low volumetric variation (1.07 %) is detected in NASVP@C-2 during cycling. As a result, the highly reversible multi-electron (2.53) reaction is achieved in NASVP@C-2, which releases a high capacity of 147.6 mAh g−1 at 1 C and exhibits exceptional cycle stability and rate capability. This work provides a new strategy to design high energy density and durable NASICON cathode.