We report the conduction mechanism in oxygen-substituted lithium conductors composed of the Li 6.15 Al 0.15 Si 1.35 S 6Àx O x (LASSO) system, which is a novel member of the argyrodite-type family and has superionic conductivities, making it suitable for all-solid-state batteries. The crystal structures, ionic conductivities, and electrochemical properties of these systems were examined using powder X-ray and neutron diffractometry combined with impedance spectroscopy and cyclic voltammetry measurements.The optimal Li 6.15 Al 0.15 Si 1.35 S 5.4 O 0.6 (x = 0.6) material exhibited a high ionic conductivity of 1.24 mS cm À1 at 25 1C with a low activation energy of 36.6 kJ mol À1 . Rietveld refinement and maximum-entropymethod analysis using neutron diffraction data revealed unique interstitial Li + and O 2À /S 2À site disorder, which led to a flatter energy landscape for migrating Li + ions and, thus, a low percolation threshold for three dimensional (3D) Li-ion diffusion. Oxygen substitution also stabilized the structure, and a wide electrochemical window from À0.1 V to 5 V vs. Li/Li + was achieved. The significant improvements in the ionic conductivity and stability owing to structural changes after cation and anion substitutions reveal an important strategy toward the development of argyrodite-type superionic conductors.

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is a promising cathode candidate for Li-ion batteries because of its high discharge capacity; however, its reaction mechanism during cycling has not been sufficiently explicated. Observations of Mn and O binding energy shifts in operando hard X-ray photoelectron spectroscopy measurements enabled us to determine the charge-compensation mechanism of Li 2 MnO 3 . The O 1s peak splits at an early stage during the first charge, and the concentration of lower-valence O changes reversibly with cycling, indicating the formation of a low-valence O species that intrinsically participates in the redox reaction. The O 1s peak-splitting behavior, which indicates the number of valences of O in Li 2 MnO 3 , is supported by the computational results for an O3 to O1 structural transition. This is in agreement with the results of our previous study, wherein we confirmed this O3 to O1 transition based on in situ surface X-ray diffraction analysis, X-ray photoelectron spectroscopy, and firstprinciples formation energy calculations.

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