Enhanced electrochemical performance of N-doped carbon coated Na2FePO4F cathode materials for sodium-ion batteries: achieving high capacity and cycle stability†

Iron-based phosphates, known for their abundance, cost-effectiveness, and eco-friendliness, have garnered significant attention as viable cathode materials for sodium-ion batteries. However, synthesizing pure phase, high-capacity cathodes remains a substantial challenge. This research focuses on Na₂FePO₄F and achieves a significant enhancement in its electrochemical properties through a refined preparation process, leading to the development of Na₂FePO₄F@C. Initially synthesized via a sol–gel method and subsequently coated with carbon through wet grinding, Na₂FePO₄F@C exhibits a commendable initial discharge capacity of 121 mA h g⁻¹ at 0.1C. Yet, it is plagued by inadequate cycle stability. To address this issue, an N-doped carbon coated Na₂FePO₄F@C@NC composite is developed, primarily based on Na₂FePO₄F@C. The successful incorporation of N atoms into the carbon layer introduces surface defects and active sites, thereby enhancing electron conductivity and bolstering the electrochemical performance. Notably, Na₂FePO₄F@C@10NC demonstrates a specific discharge capacity of 88 mA h g⁻¹ at 1C and maintains an impressive capacity retention rate of 98% after 5000 cycles at 40C. Additionally, the material shows robust long-term cycle performance under 60 °C and 1C, with an initial cycle capacity of 102.1 mA h g⁻¹. Na₂FePO₄F@C@NC also exhibits excellent compatibility with hard carbon in a full battery configuration, achieving a 0.1C specific discharge capacity of 80.2 mA h g⁻¹. These findings provide valuable insights and guidance for the practical deployment of Na₂FePO₄F.