A number of research for development of novel electrode with high capacity, stable cyclability and low cost for rechargeable battery have been carried out to various portable electronic device and electric vehicles and hybrid electric vehicles. To develop more enhanced battery, various in-situ studies have been carried out to understand reaction mechanisms.
A number of studies about CuS as an anode material of rechargeable batteries because of its high capacity (~ 560mah/g) and high electronic conductivity $(~ 10^3 S/cm)$. Understanding clear mechanism of reaction mechanism is necessary for development durable anode materials for batteries. In this study, transmission electron microscopy (TEM) is used for observation of lithiation and sodiation reaction of hexagonal CuS nanoparticles. LiF and NaF were employed for providing lithium and sodium metal to CuS. LiF and NaF were dissociated by electron beam, generating Li and Na sources. Generated Li and Na sources reacted with CuS nanoparticles, showing different reaction mechanisms. Displacement reaction was observed in lithiation reaction where sulfur position was retained while copper was completely replaced by Li. In terms of sodiation, displacement-conversion combined reaction was observed in sodiation reaction in which sodium sulfide micro-grains were formed and copper spheres were embedded in the grains. This different aspect is very unique because lithium and sodium both are monovalent and lithium sulfide and sodium sulfide both also have same cubic structure. This unique phenomenon might be happened by difference of diffusion barrier of copper in $Li_2S$ and $Na_2S$.