Atomic-Scale Dynamics and Storage Performance of Na/K on FeF Nanosheet.

Developing highly efficient FeF-based cathode materials for Na/K-ion batteries is greatly needed, which needs long cycling life and rate performance besides large voltage and capacity. Accordingly, we designed a two-dimensional (2D) FeF nanosheet to obtain highly efficient Na/K-ion batteries. Moreover, first-principles calculations were implemented to discuss systematically the Na and K storage mechanism on the FeF(012) nanosheet. The adsorption energies of Na and K are -3.55 and -3.98 eV, respectively, which can guarantee the Na/K loading process. Interestingly, Na and K adatoms on FeF(012) prefer to get together in the form of the Na dimer and K tetramer, respectively. Energy barrier of the K tetramer is lower than that of the Na dimer (0.43 eV vs 0.45 eV). As a result, the K tetramer possesses a larger diffusion coefficient than the Na dimer (4.22 × 10 cm·s vs 3.32 × 10 cm·s). That is to say, good Na/K-ion mobility can be achieved. Also, the FeF(012) nanosheet exhibits high initial discharge voltage (4.10 V for K and 3.74 V for Na). Moreover, it has a stable discharge voltage curve in Na/K-ion batteries. Besides, the FeF(012) nanosheet is more favorable to be fabricated as a flexible cathode material for potassium batteries. Therefore, the 2D FeF nanosheet belongs to a promising cathode material in Na/K-ion batteries.