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Activating Reversible Multielectron Redox in Mg/Al Co-doped Na3V2(PO4)3 toward High-Performance Sodium-Ion Batteries
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DOI：10.1039/D6TA03429B.png)
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Abstract
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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.
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