Failure mechanism of Au@Co9S8 yolk-shell anode in Li-ion batteries unveiled by in-situ transmission electron microscopy

Co9S8 has been regarded as a desirable anode material for lithium-ion batteries (LIBs) due to its high theoretical capacity. However, lithium (Li) insertion caused large volume expansion, and subsequent pulverization and capacity decay are the major challenges for its practical applications in commercial LIBs. In this work, we design an Au@Co9S8 yolk-shell nanoparticle (NP) anode and use in-situ transmission electron microscopy to study the structural, morphological, and chemical evolutions in the NPs during lithiation. Our experimental results reveal that Li insertion leads to the volume expansion of the lithiated Co9S8 shell dominantly in the outward direction, and the conversion reaction between Li and Co9S8 induces the formation of a two-phase shell with metallic Co clusters dispersed in the as-formed Li2S matrix. With corroboration of chemo-mechanical simulations, we elucidate that the lithiation-generated comparative softer phase together with the inner stiffer unlithiated Co9S8 phase contributes to the outward only deformation pattern. In addition, the insulating nature of the as-formed Li2S results in an unexpected non-reactive Au yolk during lithiation. Our findings elucidate the significance of considering the dynamical phase transformations of active materials (i.e., Co9S8) for LIBs and provide valuable insights into the rational design of new generation high-performance LIBs.