Ti/Cr dual-doped NASICON-type Na3V2(PO4)3 cathodes for multi-electron high-energy sodium storage

Abstract

Sodium-ion batteries (SIBs) have attracted considerable attention as a cost-effective and sustainable alternative to lithium-ion batteries, owing to the earth-abundant and low-cost nature of sodium resources. NASICON-type compounds, such as Na₃V₂(PO₄)₃, possess a stable three-dimensional framework and open Na⁺ diffusion channels; however, fewer than two electrons per formula unit participate within its operating voltage window, restricting capacity improvement. Herein, a Ti/Cr dual-doping strategy is employed to partially substitute V sites, simultaneously introducing the Ti³⁺/Ti⁴⁺ redox couple while enabling the V⁴⁺/V⁵⁺ pair. The resulting Na₃V_0.7Ti_0.8Cr_0.5(PO₄)₃ cathode exhibits a remarkable discharge capacity of 165.81 mAh g⁻¹ at 0.1 A g⁻¹, along with outstanding cycling stability, maintaining 92.8% of its initial capacity after 200 cycles. Moreover, it retains 88.9% of its original capacity (79.16 mAh g⁻¹) after 5 000 cycles at a high current density of 5 A g⁻¹. In situ XRD elucidates the Na⁺ storage mechanism, while DFT calculations reveal that dual-doping modulates local electronic states, enhances structural stability, and facilitates electronic conductivity. This work offers an effective design strategy for high-performance NASICON-type cathodes for sodium-ion storage.