Tailored voltage plateau enabling superior sodium storage for Fe-based fluorophosphate cathode

Iron-based fluorophosphate (NFPF) is the most promising cathode materials integrating cost efficiency and voltage advantages. However, undesirable electrochemical performance is limited by poor electron conductivity and diffusion kinetics as well as ambiguous mechanism of plateau behavior. The article investigates the optimization of the voltage plateau mediated by local structure through Zr modification. In-depth experiment and theoretical calculations reveal that the heteroatom excites electrochemical Na activity and induces broadened Na+ pathway with the embellishment of intrinsic conductivity and highly stable framework. On the grounds of co-facilitation on electrons and ions, the first plateau is significantly extended and its proportion is increased, which liberates the confinement on electrochemical performance. The NFPF-0.07Zr sample maintains capacity of 73.78 mAh g -1 at 5 C with capacity retention of 68.67% and only 0.02% degradation per cycle over 2000 cycles. The sodium storage mechanism of the dual-biphase reaction is revealed by in situ XRD with slight volume change (3.48%). The assembled full cell outputs initial energy density of 222.3 Wh kg-1 based on the total electrode mass, enabling 150 stable cycles at 1 C. Such a fundamental understanding of the intrinsic mechanism of voltage plateaus offers a rational perspective for designing Fe-based fluorophosphate cathode.