Unraveling the modified regulation of ternary substitution on Na3V2(PO4)3 for sodium ion batteries

Abstract

The emergence of three-dimensional Na₃V₂(PO₄)₃ (NVP) has attracted a lot of attention due to its high reversible capacity and long voltage platform. Unfortunately, its popularization and application are limited by its poor intrinsic conductivity. Herein, a convenient sol–gel method is proposed to synthesize potassium/lanthanum/silicon co-substituted NVP. The potassium ions are responsible for extension along the c axis while the other two ions facilitate the expansion of the crystal structure along the other directions. Accordingly, the ternary substitution reduces the resistance existing in the neighboring coordination environment and improves the stability of the crystal framework by enlarging the migration channels for Na⁺. Significantly, with the simultaneous function of ternary substitution, slight lattice distortions occur in the bulk and more active sites for reversible de-intercalation of Na⁺ emerge, favorably supplementing the discharge capacity during the period of structural phase transition. Moreover, first-principles calculations demonstrate that the introduction of three heteroatoms could decrease the gap between the conductive and valence bands and reduce the energy barrier for sodium migration. The optimized Na_3.03V_1.93La_0.07(PO₄)_2.9(SiO₄)_0.1 possesses a superior sodium storage property in both half- and full-cells. This work reveals the distinctive advantages of ternary substitution on the NVP system through theoretical and experimental methods, contributing to further exploration of other cathode materials.