Cation deficient Cu4–xGeCo4Sn12S32 thiospinels: electrochemical behaviour and induced structural modifications

Abstract

Characterization of the structure and electrochemical behaviour of two deficient thiospinels, Cu ₄ GeCo ₄ Sn ₁₂ S ₃₂ and Cu _2.6 GeCo ₄ Sn ₁₂ S ₃₂ , where the latter was obtained by chemical deintercalation of Cu from the former, has been performed. The structural study shows that tetragonal distortion is present in Cu ₄ GeCo ₄ Sn ₁₂ S ₃₂ , due to the Jahn–Teller effect for low-spin Co ^II , while this distortion is negligible for Cu _2.6 GeCo ₄ Sn ₁₂ S ₃₂ with partial oxidation of Co ^II to Co ^III .The electrochemical study of lithium insertion into Cu ₄ GeCo ₄ Sn ₁₂ S ₃₂ reveals the presence of three well defined voltage steps located at ca. 2.15, 1.67 and <1.65 V. The spinel structure is preserved during the first steps of lithium insertion, while a two-phase region is observed for x ≥ 6. The step at 2.15 V can be attributed to the filling of vacancies in tetrahedral sites. When Cu _2.6 GeCo ₄ Sn ₁₂ S ₃₂ is used as an active cathode material, a new lithium insertion process appears in the first step of the discharge curve, centered at ca. 2.24 V, followed by others at 2.13 and <1.8 V, the host framework is also retained during the Li insertion. The new first step may be attributed to Co ^III –Co ^II reduction while the second step at 2.13 V is assigned to the filling of tetrahedral vacancies, accompanied by copper extraction. Finally, ¹¹⁹ Sn Mössbauer spectroscopy shows that the Sn ^IV –Sn ^II reduction takes place at a large depth of discharge.