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

Abstract

The electrochemical reactivity of CuTi2S4 and CuCr2S4 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 Li2Ti2S4 and Li2-xCu0.3Cr2S4, 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 LiCu0.7Cr2S4 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 Ti2S4 forming the single phase CuxTi2S4 (0 < x < 1) from the reaction of Ti2S4 powders with an aqueous CuSO4·nH2O solution is also reported. It is shown that traces of H2S coming from the slight chemical instability of Ti2S4 in water are necessary for initiating the chemical reinjection reaction to form CuxTi2S4.