Anion redox as a means to derive layered manganese oxychalcogenides with exotic intergrowth structures

Abstract

Topochemistry enables step-by-step conversions of solid-state materials often leading to metastable structures that retain initial structural motifs. Recent advances in this field revealed many examples where relatively bulky anionic constituents were actively involved in redox reactions during (de)intercalation processes. Such reactions are often accompanied by anion-anion bond formation, which heralds possibilities to design novel structure types disparate from known precursors, in a controlled manner. Here we present the multistep conversion of layered oxychalcogenides Sr₂MnO₂Cu_1.5Ch₂ (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu_1.5Ch₂]^2.5- slabs collapsed into two-dimensional arrays of chalcogen dimers. The collapse of the chalcogenide layers on deintercalation led to various stacking types of Sr₂MnO₂Ch₂ slabs, which formed polychalcogenide structures unattainable by conventional high-temperature syntheses. Anion-redox topochemistry is demonstrated to be of interest not only for electrochemical applications but also as a means to design complex layered architectures.

Fig. 1. Topochemistry leading to collapse of interlayer spacing between Sr₂MnO₂S₂ slabs.

a Synthetic scheme displaying the main reaction at each step and required reagents. Elemental Cu content y in the mixture Sr₂MnO₂Li_xS₂ + y Cu was quantified by Rietveld refinement. The SEM image of the Step1 product is overlaid with the EDX mapping of Cu (cyan) and S (yellow). b Laboratory powder XRD patterns of Sr₂MnO₂Cu_1.5S₂ and its products after first (Step 1) and second (Step 3) treatments with n-BuLi at 50 °C. The molar ratio between the oxysulfide and copper was estimated by Rietveld refinements using the Sr₂MnO₂Li_1.9S₂ model reported by Rutt et al., whose theoretical pattern is also displayed for comparison. c Laboratory powder XRD patterns after first (Step 2) and second (Step 4) treatments with disulfiram as well as after the final treatment at 80 °C (Step 5). Rietveld refinements were performed to account for the sharp peaks in Step 2 and 4 products, fixing the atomic parameters to the Sr₂MnO₂Li_1.9S₂ structure model. They gave a = b = 4.00 Å, c = 17.62 Å, which was slightly smaller than a = b = 4.03 Å, c = 17.83 Å refined for the Step 1 product Sr₂MnO₂Li_1.9S₂ + 1.5 Cu. * = Si 111 reflection.

Fig. 2. Rietveld refinement and structure modelling of the collapsed phase.

a Rietveld fit to powder PXRD data (I11, Diamond) of the final product of Fig. 1a. Data (black), fit (red); R_wp = 5.74%, χ² = 17.1. See Methods section for details. b The structural model used for the refinement. Its unit cell (a = b = 4.030 Å, c = 792.3 Å) was defined by one hundred Sr₂MnO₂S₂ slabs stacked in a body-centred manner. Sliding of each layer in xyz directions (S_x, S_y, S_z) was parametrised and refined as fractional values of unit cell parameters. The shift (S_x, S_y) = (0.32, 0.32) suggested by the refinement redefined the original I4/mmm cell of the parent phase (middle) into the new C2/m cell (right, see Supplementary Table 1 for its structure parameters), c 2D kernel plot representing occurrence of (S_x, S_y) values refined for sliding of each Sr₂MnO₂S₂ layer (See also Supplementary Fig. 8).

Fig. 3. Experimental evidence of sulphur oxidation and disulphide bond formation.

a, b XANES spectra at the Mn K-edge (a) and S K-edge (b) measured for pristine Sr₂MnO₂Cu_1.5S₂ (black), Sr₂MnO₂Cu_1.33S₂ (blue, reproduced from the data published in ref. ), and the collapsed phase (red). Dotted lines represent the absorption edges of reference standards MnO and Mn₂O₃, which correspond to the lowest-energy maxima on the d(χμ(E))/dE plot (See ref. for their spectra). c Inverse molar susceptibility 1/χ_mol of the collapsed phase plotted against temperature (2 K ≤ T ≤ 300 K). Magnetisation (emu) was measured at the applied field H = 100 Oe after zero field cooling (ZFC) and converted into 1/χ_mol (emu^-1 mol Oe) supposing the tentative formula Sr₂MnO₂S₂ (molar mass = 326.31). The linear region (120 K ≤ T ≤ 280 K) of the obtained 1/χ_mol – T plot was fitted by Curie–Weiss law 1/χ_mol = (T/C) – (θ/C), where C and θ represented the Curie and Weiss constants, respectively. See Supplementary Fig. 17 for other magnetic data. d Raman spectra of Sr₂MnO₂Cu_1.5S₂ (black) and the collapsed phase (red). The measurement was performed under air with excitation wavelength λ_ex = 532 nm. See Supplementary Fig. 19 for the spectra measured at λ_ex = 785 nm.

Fig. 4. Characterisation of residual Cu and Li in the collapsed phase.

a Secondary electron (SE) image of Sample 2 (bulk composition estimated by ICP-MS: Sr_2.00(2)Mn_1.07(2)O₂Cu_0.24(0)Li_0.23(1)S₂) and b its EDX mapping overlaying signals from Mn K_α1 and Cu K_α1 lines. EDX mappings of other elements are found in Supplementary Fig. 21. c Sr, Mn, O, Cu and S contents plotted over the 23 μm line section shown in Fig. 4a. To obtain better precision than these line-scan data, single-point EDX spectra were acquired separately at the positions corresponding to x = 8.9 (Point 1) and 21.5 μm (Point 2) of the line section. d Voltage vs specific capacity plot for a half cell constructed with Sample 1 (bulk composition estimated by ICP-MS: Sr_2.00(2)Mn_1.07(2)O₂Cu_0.35(0)Li_0.13(0)S₂) as the positive electrode and Li metal as the anode, cycled at a C/10 rate.

Fig. 5. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images of Sample 1 (Sr₂MnO₂Cu_0.35Li_0.13S₂).

a Close-up view displaying intergrowth of [100] and [010] zones of the collapsed phase (See Supplementary Fig. 23 for the whole image). The structure of the C2/m model (Fig. 2b) was overlaid for comparison. b Close-up view of the domain where the collapsed-type layers intergrow with the parent-type layers. White arrows highlight darker and thinner strips that represent the metal-free layers of disulphide ions. Between the collapsed-type layers, Sr₂MnO₂ slabs shift either left or right (highlighted by red or blue lines, respectively). The overlaid image depicts the tentative structure model of the intergrowth domain, where structural parameters within the respective intergrowing slabs are fixed to those of the C2/m model and Sr₂MnO₂Cu_1.5S₂. See Supplementary Fig. 24 for details. c, d Structural parameters obtained from the convoluted intensity profile (Supplementary Fig. 25) of Fig. 5a, b. The distances between planes of Sr²⁺ ions are shown.

Fig. 6. ⁷Li NMR spectra before and after lithiation.

a ⁷Li NMR spectrum recorded for the collapsed oxysulfide phase (Sample 1) and b after its electrochemical lithiation (i.e. after discharge down to 0.5 V (vs. Li/Li⁺) corresponding to the intercalation of 1.75 equivalents of Li; see Fig. 4d) and c after chemical lithiation (i.e. after addition of 1.5 equivalents of n-BuLi; see Supplementary Fig. 22). Each spectrum was deconvolved (black line: observed, red line: fit) into Gaussian or Lorentzian peaks and their hyperfine shifts were interpreted in terms of the Li local structure as well as the Mn²⁺/Mn³⁺ ratio. Sidebands of the peak at 0 ppm (due to diamagnetic impurities) are labelled with a asterisk (*). See Supplementary Fig. 26 for the whole spectra. d Three possible local environments around Li. Black thick lines highlight connectivity between Li and four nearby Mn^2+/3+ cations. Mn^2+/3+ cations are either sandwiched symmetrically by S²⁻ anions at apexes of tetrahedra, or asymmetrically by S²⁻ apexes and S₂ dimers. In the latter case, charge transfer from S₂ dimers to Mn leads to stronger Mn-S-⁷Li interactions.

Fig. 7. Collapsed manganese oxyselenide and oxidation of its selenide anions.

a Rietveld fit to powder synchrotron PXRD data (I11, Diamond) of the collapsed oxyselenide derived from Sr₂MnO₂Cu_1.5Se₂. R_wp = 1.40%, χ² = 4.7. The same method described in Fig. 2b was used for the refinement. b 2D kernel plot representing the occurrence of (S_x, S_y) values refined for sliding of each Sr₂MnO₂Se₂ layer. See Supplementary Fig. 29 for details. c The collapsed Sr₂MnO₂Se₂ model derived by the most frequent shift (S_x, S_y) = (0.3, 0.3). d, e XANES spectra at Mn K-edge (d) and Se K-edge (e) were measured for the pristine Sr₂MnO₂Cu_1.5Se₂ (black) and its collapsed phase (red). Dotted lines represent the absorption edges of reference standards MnO and Mn₂O₃. f Raman spectra of Sr₂MnO₂Cu_1.5Se₂ (black) and its collapsed phase (red). The measurement was performed under air with excitation wavelength λ_ex = 532 nm.