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

[Image: see text] The electrochemical lithiation and delithiation of the layered oxysulfide Sr(2)MnO(2)Cu(4−δ)S(3) has been investigated by using a combination of in situ powder X-ray diffraction and ex situ neutron powder diffraction, X-ray absorption and (7)Li NMR spectroscopy, together with a range of electrochemical experiments. Sr(2)MnO(2)Cu(4−δ)S(3) consists of [Sr(2)MnO(2)] perovskite-type cationic layers alternating with highly defective antifluorite-type [Cu(4−δ)S(3)] (δ ≈ 0.5) anionic layers. It undergoes a combined displacement/intercalation (CDI) mechanism on reaction with Li, where the inserted Li replaces Cu, forming Li(4)S(3) 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 Mn(2+) 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.