Morphology-Controlled Discharge Profile and Reversible Cu Extrusion and Dissolution in Biomimetic CuS

Metal sulfide materials such as CuS, SnS2, Co9S8, and MoS2 are high-capacity anode materials for Li-ion batteries with high capacity. However, these materials go through a conversion reaction with Li+, which is accompanied by inevitably huge volume expansions, thereby causing performance degradation. Here, we report a nanoscale engineering route to efficiently control the overall volume expansion for enhanced performance. We engineered CuS with nanoplate assembly on a nanostring, leading to a nanostructure mimicking the crassula baby necklace (CBN) in the natural plant. Using in situ transmission electron microscopy, we probed the lithiation kinetics and dynamic structural transformations. Due to the linkage of the central nanostring, the CuS CBN exhibited a fast Li+ diffusion along the axial direction and high mechanical stability during lithiation. The volume expansion was minimal for our CuS CBN due to the pre-engineered gap and pores between these plates. The CuS followed a two-step lithiation process, with Cu2S and Li2S formation as the first step and Cu extrusion in the later stage. Interestingly, during the Cu2S-to-Cu conversion, we observed an incubation period before the metallic Cu extrusion, which is featured by the formation of an amorphous structure due to the large lattice strain and distortion associated with the displacement of Cu by Li ions. In the final stage, the lithiated amorphous phase recrystallized to a composite of Cu nanocrystals in a polycrystalline Li2S matrix. Associated with the nanoscale size, the Cu nanocrystals can reversibly dissolve into the matrix upon delithiation. The present work demonstrates tailoring of desired functionality in electrodes using bionic engineering methods.