Disclosing the Origin of Irreversible Sodiation-Desodiation of a Cu₄SnP₁₀ Nanowire Anode by Ex Situ Transmission Electron Microscopy

Cu₄SnP₁₀, a promising phosphide material for sodium-ion battery anode applications, suffers from poor cycling stability, and its mechanism remains unclear. This is largely due to the amorphous nature of the active materials upon cycling and its possible structural change at a small length scale (e.g., nanometers), making it difficult to access the phase/structural evolution of the electrode. In the present work, we show that the phase/structural change of the Cu₄SnP₁₀ nanowire electrode can be systematically investigated using a comprehensive set of ex situ transmission electron microscopy-based techniques, which are ideal for decay mechanism analysis of electrode materials of amorphous nature and with nanoscale structural evolution. The compositional elements of Cu₄SnP₁₀ nanowires are found to be spatially redistributed at a nanometer scale upon the initial sodiation, and this is partially reversible in the following desodiation process. Damage accumulates until a critical size of phase separation/segregation is reached, when the active material loss takes place, leading to fast deterioration of the entire Cu₄SnP₁₀ nanowire structure and thus its electrochemical performance. The phase segregation driven-active material loss is found to dominate the cycle-dependent capacity decay of the Cu₄SnP₁₀ nanowire electrode.