Unraveling the Modified Regulation of Ternary Substitution on Na <sub>3</sub>V <sub>2</sub>(PO <sub>4</sub>) <sub>3</sub> for Sodium Ion Battery

The opening three-dimensional Na3V2(PO4)3 (NVP) brings much attention for its high reversible capacity and long voltage platform. Unfortunately, its popularization and application are limited by the poor instinct conductivity property. Herein, a convenient sol-gel method is proposed to synthesize the potassium, lanthanum and silicon co-substituted NVP. The potassium ions are responsible for the extending along c axis while other two ions with larger ionic radius facilitate the extension of crystal structure along other directions. Accordingly, it can reduce the resistance existing in the neighboring coordination environment and improve the stability of crystal framework by enlarging the migration channels for sodium ions. Significantly, with the simultaneous function of ternary substitutions, slight lattice distortions occur in the crystal bulk and more active sites for reversible de-intercalation of sodium ions emerge for the first time, favorably supplementing the discharge capacity during the period of structural phase transition. Moreover, first-principle calculations demonstrate the introduction of three heteroatoms could decline the gap between conductive band and valence band and reduce the energy barrier of sodium migration. The optimized KLS0.07 (Na3.03V1.93La0.07(PO4)2.9(SiO4)0.1) cathode possesses superior sodium storage property in both half cell and full cell. This work reveals the distinctive advantages of ternary substitution on NVP system through the theoretical and experimental methods, contributing to the further exploration in other cathode materials in sodium ion battery.