Activating Reversible Multielectron Redox in Mg/Al Co-doped Na3V2(PO4)3 toward High-Performance Sodium-Ion Batteries

Abstract

Sodium superionic conductor (NASICON) type Na3V2(PO4)3 (NVP) is a promising cathode material for sodium ion batteries due to its high structural stability and excellent safety. However, its low intrinsic electronic conductivity during sodium ion extraction/insertion restricts its further practical application. Herein, a bimetallic co-doping strategy is employed to partially substitute vanadium, forming Na₃V₁.₉Al₀.₀₅Mg₀.₀₅(PO₄)₃. The synergistic effect of Mg and Al enhances structural stability and Na⁺ diffusion kinetics, thereby improving rate capability and cycling performance. As a cathode material for sodium-ion batteries, it delivers a high reversible specific capacity of 125.5 mAh g⁻¹ at 0.2 C and excellent cycle stability (79.5% capacity retention after 2,500 cycles at 20 C). Furthermore, upon metal doping, the reversible V⁴⁺/V⁵⁺ redox couple is activated at a high voltage of ~4.0 V. This activation arises from charge compensation when divalent Mg²⁺ replaces trivalent V³⁺, which is accompanied by additional Na⁺ insertion that induces an Na-rich phase.This activation, together with the improved discharge capacity, contributes to a substantial enhancement in both the energy density and operating voltage of the material. Moreover, density functional theory (DFT) calculations clarified that doping ions can effectively promote the migration dynamics of sodium ions. Collectively, this study demonstrates that a broadly viable doping strategy plays a pivotal role in attaining multielectron reactions and higher energy densities in NASICON cathodes, paving the way for advanced sodium-ion batteries.