Abstract

Multi-electron NAtrium SuperIonic CONductor (NASICON)-Nb₂(PO₄)₃ (N0NbP) is an attractive Na-ion battery anode, owing to its low intercalation voltage (1.4 V vs Na⁺/Na⁰) and high capacity (≈150 mAh g⁻¹). However, it suffers from poor capacity retention due to structural degradation. To overcome this issue, extra Na+ ions are introduced at the Na(1) sites, via V³⁺ substitution, which can act as stabilizing agents to hold lantern units together during cycling, producing NASICON-Na_1.5V_0.5Nb_1.5(PO₄)₃ (N_1.5VNbP). The N_1.5VNbP anode exhibits reversible capacities of ≈140 mAh g⁻¹ at 1.4 V versus Na⁺/Na⁰ through Nb⁵⁺/Nb⁴⁺/Nb³⁺ and V³⁺/V²⁺ redox activities. The extra Na⁺ ions in the framework forms a complete solid-solution during Na (de)intercalation and enhances sodium diffusivity, in agreement with first-principles calculations. Further, N_1.5VNbP demonstrates extraordinary cycling (89% capacity retention at 5C after 500 cycles) and rate performances (105 mAh g⁻¹ at 5C). Upon pairing the N_1.5VNbP anode with the NASICON-Na₃V₂(PO₄)₃ cathode, the full Na-ion cell delivers a remarkable energy density of 98 Wh kg⁻¹ (based on the mass of anode and cathode) and retains 80% of its capacity at 5C rate over 1000 cycles. The study opens new possibilities for enhancing the electrochemical performance of NASICON anodes via chemical and structural modulations.