Copper Extrusion/Reinjection in Cu-Based Thiospinels by Electrochemical and Chemical Routes

The electrochemical reactivity of CuTi 2 S 4 and CuCr 2 S 4 spinel samples toward Li was studied over a wide range of voltages. Their electrochemical behavior can be decomposed into two steps. For less than two inserted lithium, we emphasized a Li-driven electrochemical displacement reaction that led to the extrusion of copper from lithium, leading to the rocksalt-type structure phases Li 2 Ti 2 S 4 and Li 2-x Cu 0.3 Cr 2 S 4 , whereas conversion reactions were observed beyond x = 2. These conversion reactions were shown to have a large rechargeable uptake and release of Li + ions (7 per formula unit) that rapidly fade on cycling. Regarding the Cu extrusion, we demonstrated that this process is reversible for the Ti-based spinel upon oxidation through a chemically assisted electrochemical process. For the Cr-based system, a new phase LiCu 0.7 Cr 2 S 4 was evidenced to form on oxidation. This difference is explained in terms of band structure considerations and d-sp redox chemistry. Chemical reinjection of Cu into Ti 2 S 4 forming the single phase Cu x Ti 2 S 4 (0 < x < 1) from the reaction of Ti 2 S 4 powders with an aqueous CuS O ·nH 2 O solution is also reported. It is shown that traces of H 2 S coming from the slight chemical instability of Ti 2 S 4 in water are necessary for initiating the chemical reinjection reaction to form Cu x Ti 2 S 4 .