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An Effective Cation-Anion Codoping Strategy for Superior Stability and High-Performance of Na4Fe0.5Mn0.5V(PO4)3
Zhuohui Sun1,2, Hongwei Zhang2, Yilong Jia2
1Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education, School of Future Technology, National Innovation Platform (Center) for Industry Education Integration of Energy Storage Technology, Xi'an Jiaotong University, Xi'an710049, China.
Abstract:
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.
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