Recently, the research team led by Assistant Professor Gang Sun from Shenzhen University and Professor Zhenbo Wang from Harbin Institute of Technology has achieved a major breakthrough in kinetic modulation and extreme-environment application of cathode materials for sodium-ion batteries. The research findings have been published in the top international journal Advanced Functional Materials, with the article entitled “Defying the Kinetics Limitations of Na₃V₂(PO₄)₂F₃ Cathode by a Tailored Sol-Gel Thermal Protocol: Toward Ultra-Stable Sodium Storage at High Rates and Subzero Temperatures”. Jianhong Guo, a master’s student at Shenzhen University, is the first author. Assistant Professor Gang Sun (Shenzhen University) and Professor Zhenbo Wang (Harbin Institute of Technology) are the corresponding authors, and Shenzhen University serves as the primary corresponding institution for this work. Meanwhile, key outcomes of this research have been granted a Chinese national invention patent: A Preparation Method of Carbon-Coated Na₃V₂(PO₄)₂F₃ Cathode Material for Sodium-Ion Batteries (Patent No. CN 119725474 B), laying a solid foundation for industrial transformation and intellectual-property protection.
Benefiting from abundant sodium reserves and low cost, sodium-ion batteries have attracted wide attention in energy-storage fields. Sodium vanadium fluorophosphate (Na₃V₂(PO₄)₂F₃, abbreviated as NVPF) stands out as a promising cathode candidate, owing to its high operating voltage of ~3.9 V and robust three-dimensional framework. Nevertheless, its polyanionic skeleton brings about low intrinsic electronic conductivity and sluggish Na⁺ diffusion kinetics, severely limiting its rate capability and practical deployment under low-temperature conditions. Conventional isothermal sol-gel routes cannot simultaneously realize fine primary particles, uniform carbon coating and optimized lattice active sites.
Based on systematic investigation of sol-gel reaction temperatures, this work reports a two-step tunable-temperature sol-gel strategy with “high-temperature first followed by low-temperature” treatment. A relatively high temperature (90 ℃) in the early stage promotes full precursor dissolution and initial chelation to form homogeneous precursor particles. Subsequent lower temperature (80 ℃) precisely tunes polycondensation kinetics, guides ordered assembly of gel networks and suppresses vanadium-ion hydrolysis. This strategy refines primary particles and drives them to self-assemble into uniform secondary spherical morphology, greatly shortening Na⁺ transport paths. Moreover, an approximately 5 nm-thick, dense and continuous amorphous carbon-coating layer is constructed, while the Na-ion occupancy at lattice Na(2) sites is improved, intrinsically activating electrochemical reaction kinetics.
The optimized cathode delivers outstanding rate performance and wide-temperature adaptability. It achieves a reversible specific capacity of 124.5 mAh g⁻¹ at 1 C and retains 103.6 mAh g⁻¹ even at an ultra-high rate of 30 C. Under harsh subzero condition of −30 ℃, an initial specific capacity of 100.1 mAh g⁻¹ is obtained at 0.1 C, and 93.0 % capacity retention is maintained after 780 cycles at 0.2 C. When paired with commercial hard-carbon anode for full-cell assembly, the full cell still outputs 103.0 mAh g⁻¹ at 10 C, demonstrating great commercial application potential.
From the perspective of balancing synthetic thermodynamics and kinetics, precise temperature control realizes triple synergistic optimization of NVPF: micro-morphology, continuity of carbon coating and highly-active Na(2) lattice sites. The work not only effectively addresses the common challenge of reconciling high activity with long-term stability in polyanionic materials, but also offers a new design paradigm and theoretical guidance for developing advanced sodium-ion battery cathode materials suited to all-climate, high-power applications.
Figure 1. (a) Schematic illustration of the tunable-temperature sol-gel synthetic route; (b) ²³Na solid-state NMR spectra; (c) SEM image of NVPF-75; (d) SEM image of NVPF-9080; (e) Comparison of rate capability from 1 C to 30 C at 25 ℃; (f) Cycling performance of NVPF-9080 cathode at 50 ℃.
Acknowledgements: This work is financially supported by the National Natural Science Foundation of China, Shenzhen High-Level Team Project, Shenzhen Science and Technology Program, Guangdong Basic and Applied Basic Research Foundation, Shandong Provincial Key Research and Development Program, Heilongjiang Head-Goose Team Program, and Fundamental Research Funds for the Central Universities.
Article Link: https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.76147
Proofread by: Xiaoying Zhang, Dong Wang, Lei Wang
