Constructing a charged-state Na-NiCl2 battery with NiCl2/graphene aerogel composite as cathode

The hierarchical nickel chloride/reduced graphene oxide (NiCl 2 -rGO) aerogels are synthesized via a facile hydrothermal and freeze-drying methods using low-cost nickel chloride hexahydrate (NiCl 2 ·6H 2 O) as raw material. The NiCl 2 -rGO hybrid aerogel structure enables the uniform distribution of lamellar NiCl 2 on the three-dimensional (3D) cross-linked conductive graphene sheets. When employed as cathode for sodium-nickel chloride (Na-NiCl 2 ) batteries, the as-obtained NiCl 2 -rGO aerogels exhibit excellent electrochemical performance with high reversible capacities (116 mAh g −1 after 50 cycles, 128 mAh g −1 in a fixed capacity window (20%–80% state of charge (SOC)) after 50 cycles), and a ultra-low polarization (0.05 V in the fixed capacity window). The outstanding performance of NiCl 2 -rGO aerogels can be attributed to the excellent conductivity of graphene. Moreover, the strong contact between NiCl 2 and graphene sheets can effectively promote the transfer of electrons. Compared with traditional tubular Na-NiCl 2 batteries, the molar ratio of Ni/NaCl of NiCl 2 -rGO aerogels can be decreased from 1.8 to the stoichiometric ratio (0.5). To the best of our knowledge, this work paves a perspective way for the first time to develop charged-state NiCl 2 -based cathodes with splendid performance for Na-NiCl 2 batteries.