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

Abstract

<p>The electrochemical lithiation and delithiation of the layered oxysulfide Sr<sub>2</sub>MnO<sub>2</sub>Cu<sub>4−<i>δ</i></sub>S<sub>3</sub> has been investigated by using a combination of <i>in situ </i>powder X-ray diffraction and <i>ex situ</i> neutron powder diffraction, X ray absorption and Li NMR spectroscopy, together with a range of electrochemical experiments. Sr<sub>2</sub>MnO<sub>2</sub>Cu<sub>4−<i>δ</i></sub>S<sub>3</sub> consists of [Sr<sub>2</sub>MnO<sub>2</sub>] perovskite-type cationic layers alternating with highly defective antifluorite-type [Cu<sub>4−</sub><sub>d</sub>S<sub>3</sub>] (d ~ 0.5) anionic layers. It undergoes a combined displacement/intercalation (CDI) mechanism on reaction with Li, where the inserted Li replaces Cu, forming Li<sub>4</sub>S<sub>3</sub> slabs and Cu<sup>+</sup> is reduced and extruded as metallic particles. For the initial 2-3% of the 1<sup>st</sup> discharge process, the vacant sites in the sulfide layer are filled by Li; Cu extrusion then accompanies further insertion of Li. Mn<sup>2.5+</sup> is reduced to Mn<sup>2+</sup> 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<sup>2+</sup> to Mn<sup>2.5+</sup>. 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 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. </p>

Anion Redox as a Means to Derive Layered Manganese Oxychalcogenides with Exotic Intergrowth Structures

“…This is a chemical complement to electrochemical cycling previously carried out for the related oxysulphide Sr2MnO2Cu3.5S3. 38 This chemistry highlights redox competition between cations and anions. Mn 4+ is common in oxides such as MnO2, while MnS2 contains oxidised (S2) 2species, due to Mn 2+/3+ bands lying below the top of the S 3s/3p bands.…”

“…[36][37] Our analysis of 6,7 Li spectra of Sr2MnO2LixS2 and structural homologues estimated that each Mn 2+ -S-Li interaction resulted in a shift of 101 ppm 32 while a Mn 3+ -S-Li interaction gave a larger shift of 165 ppm due to greater Mn 3+ -S covalency. 38 Since each Li is surrounded by four neighbouring Mn in the parent-type structure (Fig. 5d), and hyperfine shifts are additive, four Mn 2+ -S-Li interactions should result in a hyperfine shift of ~404 ppm, increasing to 468 or 532 ppm when one or two of the Mn ions are oxidised the 3+ state.…”