Fluorine Dissolution-Induced Capacity Degradation for Fluorophosphate-Based Cathode Materials.

NaV(PO)F has been considered as a promising cathode material for sodium-ion batteries due to its high operating voltage and structural stability. However, the issues about poor cycling performance and lack of understanding for the capacity degradation mechanism are the major hurdle for practical application. Herein, we meticulously analyzed the evolution of the morphology, crystal structure, and bonding states of the cathode material during the cycling process. We observed that capacity degradation is closely related to the shedding of the active material from the collector caused by HF corrosion. Meanwhile, HF is produced through F anion dissolution from NaV(PO)F induced by trace HO during the cycling process. The F dissolution-induced degradation mechanism based on fluorine-containing cathode materials is proposed for the first time, providing a new insight for the understanding, modification, and performance improvement for fluorophosphate-based cathode materials.