Unraveling the reaction reversibility and structure stability of nickel sulfide anodes for lithium ion batteries

The electrochemical performance of lithium-ion batteries, i.e. specific capacity and cyclability, is primarily determined by chemical reversibility and structural stability of the electrodes in cycling. Here we have investigated the fundamental reaction behaviors of nickel sulfide (NixSy) as lithium-ion battery anodes by in-situ TEM. We find that Ni3S2 is the electrochemically stable phase, which appears in the first cycle of the NixSy anode. From the second cycle, conversion between Ni3S2 and Li2S/Ni is the dominant electrochemical reaction. In lithiation, the NixSy nanoparticles evolve into a mixture of Ni nanocrystals embedded in Li2S matrix, which form a porous structure upon full lithiation, and with the recrystallization of the Ni3S2 phase in delithiation, a compact and interconnected network is built. Structural stability in cycles is susceptible to particle size and substrate restraint. Carbon substrate can certainly improve the tolerance for size-dependent pulverization of NixSy nanoparticles. When NixSy nanoparticle exceeds the critical size value, the morphology of the particle is no longer well maintained even under the constraints of the carbon substrate. This work deepens the understanding of electrochemical reaction behavior of conversion-type materials and helps to rational design of high-energy density battery anodes.