Hollow shell-in-shell Ni₃S₄@Co₉S₈ tubes derived from core-shell Ni-MOF-74@Co-MOF-74 as efficient faradaic electrodes

This paper reports the novel design of shell-in-shell Ni₃S₄@Co₉S₈ double-shelled tube (DST) hierarchical structures with different shell compositions, and the subsequent attempt to fabricate these structures. We fabricated such unique shell-in-shell hierarchical structures through transition from isomorphic metal-organic framework core-shell structures of Ni-MOF-74@Co-MOF-74 in the presence of Na₂S under solvothermal conditions. The as-prepared Ni₃S₄@Co₉S₈ DSTs demonstrated high specific capacity (up to 1257.4 F g^-1 at a current density of 4 A g^-1) with excellent charge-discharge reversibility (with 81.5% capacity retention after 1000 cycles), which is superior to that of the single-shelled Co₉S₈ tubes (337.8 F g^-1 at 1 A g^-1). The specific capacity of the prepared DST electrode also showed exceptional performance at a current density of 20 A g^-1. Approximately 74.0% of the obtained value of the specific capacity was retained at 1 A g^-1. This approach is a facile method to produce novel electrode materials for energy storage devices and a useful selflated strategy for the preparation of multi-shelled hollow materials with different shell compositions for different functions.