Ni S -Ni Hybrid Nanospheres with Intra-Core Void Structure Encapsulated in N-Doped Carbon Shells for Efficient and Stable K-ion Storage.

Iron group metals chalcogenides, especially NiS, are promising candidates for K-ion battery anodes due to their high theoretical specific capacity and abundant reserves. However, the practical application of NiS-based anodes is hindered by slow electrochemical kinetics and unstable structure. Herein, a novel structure of Ni S -Ni hybrid nanosphere with intra-core voids encapsulated by N-doped carbon shells (Ni S -Ni@NC-AE) is constructed, based on the first electrodeposited NiS nanosphere particles, dopamine coating outer layer, oxygen-free annealing treatment to form Ni S -Ni core and N-doped carbon shell, and selective etching of the Ni phase to form intra-core void. The electron/K transport and K storage reaction kinetics are enhanced due to shortened diffusion pathways, increased active sites, generation of built-in electric field, high K adsorption energies, and large electronic density of states at Fermi energy level, resulting from the multi-structures synergistic effect of Ni S -Ni@NC-AE. Simultaneously, the volume expansion is alleviated due to the sufficient buffer space and strong chemical bonding provided by intra-core void and yolk-shell structure. Consequently, the Ni S -Ni@NC-AE exhibits excellent specific capacity (438 mAh g at 0.1 A g up to 150 cycles), outstanding rate performances, and ultra-stable long-cycle performance (176.4 mAh g at 1 A g up to 5000 cycles) for K-ion storage.