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The electrochemical reactivity of layered oxysulfide Sr₂MnO₂Cu_4-δS₃ towards Li and its structural changes during the (de)lithiation processes have been investigated intensively to determine the mechanisms by which this material reversibly cycles lithium. This material consists of alternating Perovskite-type [Sr₂MnO₂] layer and antifluorite-type [Cu₂S] layer. It displays a so-called combined displacement /intercalation (CDI) mechanism for its electrochemical reactions towards Li, where most of the inserted Li replaces Cu and forms Li₂S-like environment and extrudes Cu out of the framework as metallic particles. Constant current and galvanostatic intermittent titration (GITT) measurements and a combination of characterization techniques, including XRD, Cu, Mn, and S K-edge XANES, and Li NMR have been used to investigate these processes. In order to interpret the S XANES results, a number of model compounds were prepared and investigated.  Cu and Mn XANES show the full reduction of Cu⁺ to Cu⁰, Mn being reduced from 2.5+ to 2+ at the initial stage of the 1^st discharge.  This initial Mn reduction was rationalized by Li insertion into the vacant tetrahedral sites in the sulfide layers prior to the Li-Cu exchange process. The following charge shows several processes at 1.8, 2.5 and 3.3 V.  The 1.8 and 2.5 V processes both involve the removal of Li and reduction of concentration of Li₂S-like structures (as seen by NMR and S XANES), and Cu is oxidized and re-inserted into the framework. Oxidation of Mn from 2+ to 2.5+, however, only occurs during the second (2.5 V) process. If the applied charge cutoff voltage is limited to 2.75 V. the 1.8 and 2.5 V processes show good reversibility, and similar processes are seen on both charge and discharge. The 3.3 V process, seen on charging to higher voltages, is ascribed to the fully oxidation of residual Cu metal, as there still remains small amount of un-reduced Cu metal after the 2.5 V process. When the system is charged to above the 3.3 V process, the sulfur framework undergoes drastic changes, possibly involving redox reactions involving sulfur. The electrochemistry on discharge resembles that on the 1^st discharge, but the capacity retention is much worse.   Thus, by controlling the voltage window used in the cycling, good cycling behavior can be obtained.

In recent experiments XANES and X-ray diffraction 2 dimensional mapping and tomography (imaging) were used to follow the Cu extrusion from the particles, and follow the redox processes of the individual particles.

The work was performed in collaboration with Dr. Simon Clarke at the University of Oxford.