Reactivity of transition metal (Co, Ni, Cu) sulphides versus lithium: The intriguing case of the copper sulphide

Transmission electron microscopy (TEM), in situ X-ray diffraction (XRD) and electrochemical techniques have been used to study the electrochemical reactivity of transition metal (cobalt, nickel, and copper) sulphides. We show that cobalt and nickel sulphides react versus lithium through conversion reactions similarly to their homologous oxides with during the discharge step the formation of metallic nano-particles embedded in Li2S that on the following charge convert back into sulphides. In contrast, the electrochemical reactivity of CuS towards Li was shown to follow a displacement reaction leading to the growth and disappearance of large copper dendrites with a concomitant reversible decomposition/re-crystallization of the initial electrode material. We show from structural considerations that this mechanism is nested in the creation of Cu2−xS phases, since intermediary phases during cycling have both high copper mobility and a sulphur network close to that of Li2S. In spite of their attractive capacity, none of these compounds show good capacity retention over the studied voltage range (0 to 2.5 V); the main reason being rooted in the partial solubility of Li2S into the electrolyte.