Identifying the β-to-α phase transition during the long cycling process in Na2FePO4F cathode

Abstract

Na₂FePO₄F has emerged as a promising cathode for large-scale electrochemical energy storage, primarily due to its abundance of raw materials and distinctive two-dimensional ion channels. Counterintuitively, pristine Na₂FePO₄F lacks the long-term stability typically seen in polyanionic cathodes, which severely impedes its practical application. Traditionally, the origin of this problem has been only phenomenologically attributed to the poor intrinsic electronic conductivity and structural instability of Na₂FePO₄F. Here, we identify that rapid capacity fading of Na₂FePO₄F is closely related to the β-to-α phase transition during the long cycling process. Furthermore, we develop a practical high-entropy doping strategy and the corresponding microstructure engineering to mitigate the impact of the phase transition on the crystal structure, thereby increasing the capacity retention from 56 % to 94 % over 400 cycles at 0.5C in coin cell, and attain almost 100 % capacity retention after 300 cycles at 0.1C in pouch cell. Overall, this work unveils the mechanism of rapid capacity decay in Na₂FePO₄F and lays the groundwork for the rational design of durable polyanionic electrodes.