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

Characterization of the structure and electrochemical behaviour of two defi cient thiospinels, Cu4GeCo4Sn12S32 and Cu2.6GeCo4Sn12S32, where the latter was obtained by chemical deintercalation of Cu from the former, has been pe rformed. The structural study shows that tetragonal distortion is present i n Cu4GeCo4Sn12S32, due to the Jahn-Teller effect for low-spin Co-II, while this distortion is negligible for Cu2.6GeCo4Sn12S32 with partial oxidation of Co-II to Co-III. The electrochemical study of lithium insertion into Cu4GeCo4Sn12S32 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 l ithium insertion, while a two-phase region is observed for x greater than o r equal to 6. The step at 2.15 V can be attributed to the filling of vacanc ies in tetrahedral sites. When Cu2.6GeCo4Sn12S32 is used as an active catho de material, a new lithium insertion process appears in the first step of t he 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 ne w first step may be attributed to Co-III-Co-II reduction while the second s tep at 2.13 V is assigned to the filling of tetrahedral vacancies, accompan ied by copper extraction. Finally, Sn-119 Mossbauer spectroscopy shows that the Sn-IV-Sn-II reduction takes place at a large depth of discharge.