Improved cycling stability of NiS2 cathodes through designing a kiwano hollow structure

As one of the promising cathode materials, NiS2 delivers an ultrahigh theoretical capacity of 870 mA h g(-1). However, it suffers from huge capacity fading during multiple cycles because of the close formation enthalpy between NiS2 and other nickel sulphides (Ni3S4, Ni3S2 and NiS) and the large volumetric expansion during charging. To overcome these drawbacks, and being inspired by the morphology of kiwano, i.e. the African horned melon, with many protrusions, we design and synthesize a unique kiwano-like hollow structure via a facile approach, which is beneficial to shorten the diffusion lengths, buffer the volume expansion and especially control the evolution of intermediate phases. The fabricated electrodes present significantly improved capacity (681 mA h g(-1) after 100 cycles at 50 mA g(-1)), superior cycling stability (580.6 mA h g(-1) even after 400 cycles at 0.2C) and fast Li+ storage properties (264 mA h g(-1) at 2C). The single intermediate phase Ni3S4, rather than other nickel sulphides, was observed in real time by using in situ transmission electron microscopy (TEM) upon direct lithiation. This was one of the key factors for the outstanding cycling stability. As visualized by in situ TEM, the pores in the kiwano structure can effectively buffer the volume expansion. In addition, Li+ ions prefer to insert into NiS2 through the (111) facet owing to their low activation energy. Density of states (DOS) calculations reveal that NiS2 and the intermediate phase Ni3S4 present extremely high electronic conductivity, thus delivering high rate capacity. These promising findings can provide a new perspective in high-performance lithium-ion batteries.