Anti-site defect regulation promoting V activity to induce brand new sodium storage sites for Na-rich type Na3+2xV2-xNax(PO4)3 with advanced performance

Abstract

The electrochemical activity of V in Na₃V₂(PO₄)₃ (NVP) is the key for elevating reversible capacity. Herein, the V-site is modulated through an anti-site doping strategy by replacing it with Na⁺. Initially, a typical P-type doping effect is generated, increasing the concentration of hole carriers. XAFS confirms an increase in the length of V-O bond, making it easier to break, and consequently, enhances the activity of V. To maintain charge balance, V-site is supplemented with more Na⁺, resulting in a strongly structured sodium-rich material NVP/C-Na0.07. Taking advantage of the significant difference in ionic radii and electronic structures of Na⁺ and V³⁺, a brand newly sodium storage site (24f) is activated, which is further verified by Na²³NMR. Correspondingly, more active Na⁺ can be effectively utilized, contributing to the increased capacity. Contact angle tests and AFM certify NVP/C-Na0.07 exhibits excellent wettability with electrolyte and highly smoothness of electrode surface. Ex-situ XPS and XRD reveal the charge compensation mechanism, indicating high reversibility of V valence state and low strain properties of crystal structure. DFT calculations demonstrate the optimized electronic construction, both in initial and desodiumised state. Meanwhile, full battery tests paired with various anodes and ARC suggest NVP/C-Na0.07 reveals remarkable practicality and excellent thermal safety.