Cu nanoparticles induced-reconstruction of NaF enabling the reversibility of Na/CF_x batteries

Making CF_x-based batteries rechargeable represents a superior promise route to dramatically boost of secondary battery systems due to the unprecedented theoretical capacity and specific energy density of CF_x. However, this goal has been plagued by a fundamental challenge of the high dissociation energy and the non-uniform deposition of the discharge products, alkaline metal fluoride (e.g., NaF). Herein, a novel strategy of engineering the discharge product NaF is developed. The hybrid Cu@CF_0.88 cathode material delivers a reversible discharge capacity of 700.6 mAh g^-1 at 20 mAh g^-1 and an energy density of 1541.1 Wh kg^-1, corresponding to 81.4% of the theoretical value. At 200 mAh g^-1, it gives a reversible discharge capacity of 250.1 mAh g^-1 after 10 cycles. The Cu nanoparticles serve as preferential nucleation sites, effectively regulating the nucleation, crystallization, and spatial distribution of NaF in the initial discharge process. More importantly, an in situ generated intermediate NaCuF₃ induces the reconstruction of NaF into a more readily decomposable and electrochemically active amorphous phase. This strategy remarkably improves the reaction reversibility of the Na/CFₓ batteries, while additional efforts are required to optimize the long-term cycling stability for practical applications.