In-situ structural evolution of Zr-doped Na₃V₂(PO₄)₂F₃ coated by N-doped carbon for SIB

Na₃V₂(PO₄)₂F₃(NVPF), a cathode material used in sodium-ion batteries (SIB), features ultrafast Na⁺ migration and high structural stability because of its three-dimensional open framework. However, the poor intrinsic electronic conductivity of NVPF often leads to high polarization, low Coulombic efficiency, and unsatisfactory rate performance, which hinder its commercial application.

Recently, a group led by Prof. Shuangqiang Chen and Prof. Yong Wang from Shanghai University synthesized zirconium-doped NVPF nanoparticles coated with a nitrogen-doped carbon layer and demonstrated a synergistic effect on the overall electrochemical performance. Specifically, the optimized NVPF-Zr-0.02/NC electrode led to high reversible capacity (119.2 mA h g-1 at 0.5 C), superior rate capacity (98.1 mA h g^-1 at 20 C), and excellent cycling performance (capacity retention of 90.2% in 1000 cycles at 20 C). In situ XRD characterization of the NVPF-Zr-0.02/NC electrode was performed to monitor the real-time structural evolution in different charge/discharge states. The results confirmed the presence of several intermediates with new phases, following a step-wise Na-extraction/intercalation mechanism with reversible multiphase changes. In addition, NVPF-Zr-0.02/NC//hard carbon full cells demonstrated a high reversible capacity of 99.8 mA h g-1 at 0.5C, with an average output voltage of 3.5 V, high energy density of ~194 Wh kg^-1, and good cycling stability, thus indicating excellent potential for practical application.

"Such attempts provide meaningful guidance and reference for practical SIBs with high capacity, long cycle life, and good structural stability," said Prof. Chen.

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The authors gratefully acknowledge the National Natural Science Foundation of China (21975154), the Shanghai Municipal Education Commission (Innovation Program (2019-01-07-00-09-E00021) and Innovative Research Team of High-level Local Universities in Shanghai. Research is also supported by The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power. The authors thank Laboratory for Microstructures, Instrumental Analysis, and Research Center of Shanghai University for offering access to material characterizations.