Cation-Anion Co-doped Na₃V₂(PO₄)₃ Cathode for Robust and High-Performance Sodium-Ion Storage

Sodium ion superconductors (NASICON) are widely perceived as potential cathodes for sodium-ion batteries (SIBs) because of their good structural stability and high operation potential for Na⁺ de/intercalation. Nevertheless, the limited sodium ion storage capacity, rate capability, and stability due to the poor electronic conductivity hinder their widespread application. In this work, cation (Fe³⁺) and multivalent anion group (MoO₄ ^2-) are co-doped into Na₃V₂(PO₄)₃ (NVP) by replacing V³⁺ and PO₄ ^3-, producing a Fe³⁺/MoO₄ ^2-co-doped NVP, i.e., Na₃V_2-2xFe_2x(PO₄)_3-3x(MoO₄)_3x (0 ≤ x ≤ 0.06) compound. In comparison with the pristine NVP, this co-doped NVP delivers much enhanced rate performance, high specific capacity, and cyclic stability. The stabilized V⁴⁺/V⁵⁺ redox reaction at 4.0 V (vs Na/Na⁺), enabled by cation-anion co-doping, can remarkably promote the sodium-ion de/intercalation potential and specific capacity compared to pristine NVP. Additionally, density functional theory (DFT) simulation confirms the enhanced electronic conductivity and sodium ion diffusion kinetics, which can further boost the rate capability and cycling stability. The proposed cation-anion co-doping strategy offers a promising pathway for scaling up the manufacturing of NVP-based cathodes for SIBs.