PNNL

Abstract

Metal sulfides, such as CuS, SnS2, Co9S8, MoS2, are high capacity anode materials for Li-ion batteries. However, these materials go through a conversion reaction with Li+, which is accompanied by the phase transformation and inevitably results in a huge volume expansions, therefore causing performance degradation. Here, we report 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 of mimicking the crassula baby necklace (CBN) in the natural plant. Using in-situ transmission electron microscopy (in-situ TEM), we probed the lithiation kinetics and dynamic structural transformations. Due to the linkage of the central nanostring, the CuS CBN exhibited fast Li+ diffusion along the axial direction and high mechanical stability during lithiation. The volume expansion was minimal for the CuS CBN due to the pre-engineered gap between these plates. The CuS followed a two-step lithiation process with Cu2S and Li2S formation as the first step and Cu extrusion from Cu2S in the later stage. Interestingly, during the Cu2S to Cu conversion, there was a 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 dispersed in a polycrystalline Li2S matrix. Associate with the nanoscale size, the Cu nanocrystals can reversibly dissolve into the matrix upon delithiation. The present work demonstrates tailoring of desired functionality through nanoscale engineering of using bionic methods.