Structural Evolution of Layered Manganese Oxysulfides during Reversible Electrochemical Lithium Insertion and Copper Extrusion

The electrochemical lithiation and delithiation of the layered oxysulfide Sr2MnO2Cu4-delta S3 has been investigated by using a combination of in situ powder X-ray diffraction and ex situ neutron powder diffraction, X-ray absorption and Li-7 NMR spectroscopy, together with a range of electrochemical experiments. Sr2MnO2Cu4-delta S3 consists of [Sr2MnO2] perovskite-type cationic layers alternating with highly defective antifluorite-type [Cu4-delta S3] (delta approximate to 0.5) anionic layers. It undergoes a combined displacement/intercalation (CDI) mechanism on reaction with Li, where the inserted Li replaces Cu, forming Li4S3 slabs and Cu+ is reduced and extruded as metallic particles. For the initial 2 3% of the first discharge process, the vacant sites in the sulfide layer are filled by Li; Cu extrusion then accompanies further insertion of Li. Mn-2.(5+) is reduced to Mn2+ during the first half of the discharge. The overall charging process involves the removal of Li and re-insertion of Cu into the sulfide layers with re-oxidation of Mn(2+ )to Mn-2.(5+). However, due to the different diffusivities of Li and Cu, the processes operating on charge are quite different from those operating during the first discharge: charging to 2.75 V results in the removal of most of the Li, little reinsertion of Cu, and good capacity retention. A charge to 3.75 V is required to fully reinsert Cu, which results in significant changes to the sulfide sublattice during the following discharge and poor capacity retention. This detailed structure-property investigation will promote the design of new functional electrodes with improved device performance.

Automated summary

The electrochemical lithiation and delithiation of the layered oxysulfide Sr2MnO2Cu4-delta S3 has been investigated by a combination of techniques, revealing a complex reaction mechanism involving the insertion/extrusion of Li and Cu, which significantly impact the material's performance.