An Effective Cation-Anion Codoping Strategy for Superior Stability and High-Performance of Na4Fe0.5Mn0.5V(PO4)3 Cathode.

The Na+ superionic conductor (NASICON)-type Na4Fe0.5Mn0.5V(PO4)3 cathode possesses high application potential due to its high structural stability, fast Na+ mobility, and low vanadium content. However, the Jahn-Teller effect results in unfavorable structural distortion and capacity fade. Herein, a Ti4+ and Br- codoping strategy to enhance structural stability and electronic/ionic conductivity is proposed. Ti4+ substitutes for Fe/Mn sites, acting as a donor dopant with strong Ti-O bond. Br- widens the ion channel with the larger ionic radius. The results show that it leads to a 60-fold surge in carrier density, a 9-fold improvement in electronic conductivity, and increase the ionic mobility to 2 × 10-11 cm2 s-1 (nearly 20-fold). Besides, ex-situ XRD analysis reveals a nearly zero structural strain (0.2%) during cycling. Ex-situ XPS analysis reveals the proportions of V5+ and Mn4+ increase significantly after constant-voltage charging at 4.4 V. Meanwhile, the V2+/V3+ redox couple is activated within 1.0-1.8 V. The optimized Na4Fe0.4Mn0.4Ti0.2V(PO4)2.9Br0.3 cathode demonstrates a remarkable discharge capacity of 183.8 mAh g-1 at 0.1C within 1.0-4.4 V and even 115.5 mAh g-1 at 5C with a retention of 82.4% after 5000 cycles. Importantly, its sodium-ion storage properties have been successfully validated under high-areal-loading conditions, which indicates it could be a promising cathode.