Research ArticleELECTROCHEMISTRY

A reversible oxygen redox reaction in bulk-type all-solid-state batteries

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Science Advances  19 Jun 2020:
Vol. 6, no. 25, eaax7236
DOI: 10.1126/sciadv.aax7236
  • Fig. 1 Synthesis of the Li2Ru0.8S0.2O3.2 positive electrode active material.

    (A) Schematic image for the synthesis strategy of Li2RuO3-Li2SO4 positive electrode active materials via mechanochemistry from Li2RuO3 [space group (SG): C2/c] and Li2SO4 (SG: P21/c) crystals. (B) XRD patterns of Li2Ru0.8S0.2O3.2 after the mechanochemical synthesis. Cation-disordered Li2RuO3 crystal (SG: Fm3¯m) has not been reported to date; therefore, the XRD pattern of the cubic structure was simulated with RIETAN-FP (36) and VESTA (37). (C) SEM image and corresponding EDX mappings for Ru, S, and O elements in Li2Ru0.8S0.2O3.2 powders. (D) DF-TEM image for Li2Ru0.8S0.2O3.2 particles and the corresponding ED pattern (inset). The bright spots in the DF-TEM image show the cation-disordered Li2RuO3 nanocrystalline region. Nanosized crystalline particles were dispersed in the amorphous Li2RuO3-Li2SO4 matrix. The ED pattern also indicates that these crystals have a cation-disordered rocksalt structure with the space group of Fm3¯m. (E) Temperature dependence of the electronic (blue circles) and lithium ionic conductivity (red circles) for the Li2Ru0.8S0.2O3.2 electrode material. (F) Cross-sectional SEM image of the Li2Ru0.8S0.2O3.2 pellet pressed under 540 MPa at room temperature. Cross section of the pellet was polished by an argon ion milling system.

  • Fig. 2 Charge-discharge profile for bulk-type all-solid-state cell with the monolithic Li2Ru0.8S0.2O3.2 positive electrode.

    (A) Schematic for high energy density of bulk-type all-solid-state batteries. Typical bulk-type all-solid-state batteries are composed of the composite positive electrode, solid electrolyte, and lithium-indium alloy negative electrode layers. Especially in the composite positive electrode layer, active material contents should be increased to improve the energy density of battery. The positive electrode layer composed only of active material is the most ideal battery configuration for high energy density. (B) Charge-discharge curves of the all-solid-state cell operated under the constant current density of 0.25 mA cm−2 (0.028 C rate) at 100°C. (C) Charge-discharge performance under the operation of various current densities. (D) Cycle performance of the bulk-type all-solid-state cell. (E) Relationship between current density and discharge capacity. C rate (C = 354 mA g−1) is defined by the theoretical capacity when all the lithium ions are extracted from the Li2Ru0.8S0.2O3.2 positive electrode active material. (F) Relationship between energy density and average power density based on the weight of the positive electrode. (G) Relationship between coulombic/energetic efficiencies, average discharge voltage, and cycle number for an all-solid-state cell operated at 0.07 C (0.64 mA cm−2) at 100°C. (H) Charge-discharge curves at the 25th, 40th, 50th, and 60th cycles.

  • Fig. 3 Electrochemical analysis of the Li2Ru0.8S0.2O3.2 positive electrode active material during the initial charge-discharge process.

    GITT profiles (constant current charge in 0.25 mA cm−2 for 2 hours and OCV rest for 10 hours) (A) in charging state and (B) in discharging state. Resistance of solid electrolyte layer (RSE) was determined by the real axis intercept in the high-frequency region in AC impedance plots. Charge transfer resistance (Rct) is defined by the value of the semicircle observed in the middle frequency region with a capacitance of about 10−6 F. Mean diffusion coefficient (D · R−2) is calculated from the GITT profiles.

  • Fig. 4 Structural change of the Li2Ru0.8S0.2O3.2 positive electrode active material during the charge-discharge process.

    (A) Ex situ XRD patterns before and after the initial charge-discharge processes. (B) DF-TEM images and the corresponding ED pattern after the initial charge and discharge processes. (C) HR-TEM image for the Li2Ru0.8S0.2O3.2 positive electrode active material after the initial discharging. Nanosized cation-disordered rocksalt-type crystals are reprecipitated by the insertion of lithium ions, which are dispersed in the amorphous Li2RuO3-Li2SO4 matrix. The inserted figure shows the fast Fourier transform (FFT) images for the crystalline (yellow circle) and amorphous (red circle) regions in the Li2Ru0.8S0.2O3.2 active material after the initial discharge process.

  • Fig. 5 Charge compensation mechanism for the Li2Ru0.8S0.2O3.2 positive electrode active material by XAFS measurements.

    (A) Ru K-edge XANES spectra. (B) EXAFS oscillations for Li2Ru0.8S0.2O3.2 in the charging state. (C) Ru LIII-edge XANES spectra. (D) O K-edge XANES spectra. (E) S K-edge XANES spectra for the Li2Ru0.8S0.2O3.2 active material after the charge-discharge measurement.

  • Fig. 6 Electronic structural analysis for the Li2Ru0.8S0.2O3.2 positive electrode active material during the charge-discharge process by XPS.

    All peaks were calibrated with the peak position of Au4f7/2 to 84.0 eV. (A) O1s XPS spectra. (B) S2p XPS spectra for the Li2Ru0.8S0.2O3.2 positive electrode active material. As a reference, the spectra for the crystalline Li2RuO3 and Li2SO4 were also recorded. In the O1s spectra, there are three characteristic peaks. The peak at the lowest energy side (dark blue) is assigned to lattice oxygen in the Li2RuO3 structure. The peak at the higher energies (red) denotes the oxygen in the sulfate anion. The peak in the middle is presumably due to the surface-adsorbed oxide species such as carbonate. In particular, the peak attributed to the peroxo-like (O─O)n− species has not been confirmed in the charged state; its position overlaps with the peak of the sulfate. (C) Summary of the peak positions in O1s and S2p XPS spectra for the sulfate anion during the charge-discharge process.

Supplementary Materials

  • Supplementary Materials

    A reversible oxygen redox reaction in bulk-type all-solid-state batteries

    Kenji Nagao, Yuka Nagata, Atsushi Sakuda, Akitoshi Hayashi, Minako Deguchi, Chie Hotehama, Hirofumi Tsukasaki, Shigeo Mori, Yuki Orikasa, Kentaro Yamamoto, Yoshiharu Uchimoto, Masahiro Tatsumisago

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