All-solid-state batteries with sulfide solid electrolytes (SEs) are promising next-generation energy storage devices that are safe and have a cycle life and energy densities. To improve their electrochemical performances, we investigated the electrochemical reactions of the SEs and their structural changes. The detailed changes in the structure and electronic states have not yet been measured experimentally because of the difficulty in observing the microscopic area formed at the interface between the electrode materials and the SEs. Thus, we prepared composite electrodes composed of Li3PS4 SE and carbon to increase the electrochemical reaction area. The structural and electronic-state changes in Li3PS4 during the Li deinsertion–insertion processes were revealed using X-ray diffraction and Raman, X-ray photoelectron, and X-ray absorption spectroscopies. We found that the sulfide ions in Li3PS4 contribute to charge compensation during the charge–discharge processes. The S–S bonds between PS4 units associate and dissociate via the retention of the covalent bonds between the P atoms and S–S bonds. Although the anion redox behavior is generally discussed with regard to transition metal chalcogenides, here we first report the reversible association and dissociation of S–S bonds in typical element compounds. This knowledge contributes to an improved understanding of the anion redox reactions and the interfacial resistances between high-voltage positive electrodes and SEs.

stability for evaluating the sulfur electrode although Li-In alloy is heavy weight and high voltage operation. On the other hand, the all-solid-state Li metal cells have showed extremely low areal capacity of 0.1 to 0.2 mAh cm −2 . [2] It is thus difficult to construct all-solid-state full cells with Li metal at the present stage.Li 2 S, which is a discharge reaction product of Li/S batteries, is a promising positive electrode because it has a high theoretical specific capacity (1167 mAh g −1 ) and can be coupled with Li-free high capacity negative electrode materials [3] such as Sn, [3a] Si-Sn alloy, [3b] black P, [3c] and Sn 4 P 3 . [3d] To achieve the Li/S cells with high energy density, not only increase of the Li 2 S active material content in a positive electrode but also improvement of the Li 2 S utilization (the ratio of the obtained capacity to the theoretical capacity) are required. [4] However, the increase of the Li 2 S content in a composite electrode leads to a decrease of the Li 2 S utilization because of less electrical conduction paths in the electrode. The improvement of Li 2 S utilization is an important issue for achieving cells with high energy density. The utilizations of Li 2 S have remained only 70% even though many efforts [5] have been paid such as the use of several mixing techniques, [5a,b] the preparation of nanoparticles of Li 2 S, [5c,d] and a balanced ionic/ electronic conducting matrix. [5d] Moreover, in batteries with organic liquid electrolytes, the highest reversible utilization is 75% although extensive studies such as mixing techniques with carbon and development of carbon matrix have been done. [6] Therefore, effective approach which is entirely different from previous conventional techniques should be investigated for the full utilization of Li 2 S. Recently, reaction mechanism of lithium and sulfur has been investigated by using in-situ transmission electron microscope. [7] These researches have suggested a new lithiation process, where electronic conduction paths are not only in carbon but also at the interface between Li 2 S and S. On the other hand, an ionic conduction path is only in Li 2 S, but it has a quite low ionic conductivity of 10 −13 S cm −1 . An ionic conduction path in a bulk Li 2 S is important for increasing the utilization of Li 2 S and improving the rate capability. In our previous work, an increase of the ionic conductivity of Li 2 S itself has been investigated by preparing solid solutions composed of Li 2 S and LiI. [8] The all-solid-state cells with Li 2 S-LiI solid solution have showed the highest reversible capacity of 920 mAh g −1 All-solid-state Li/S batteries have received a lot of attention in the view point of high safety and long cycle life. However, the all-solid-state Li/S cells have suffered from the low utilization of Li 2 S. In this study, for improving the utilization and understanding the conversion reaction of Li 2 S/S in the all-solid-state cells, Li 2 S-based solid solutions composed of Li 2 S and one of lithium halides of...

h i g h l i g h t sUse of the Li 3 PS 4 glass electrolyte as an active material was examined. Composite Li 3 PS 4 -AB electrodes were prepared by mechanical milling. All-solid-state cells with the Li 3 PS 4 -AB electrode successfully operated. a b s t r a c tFor increasing energy densities of all-solid-state cells, utilizing solid electrolytes in the electrode layer as an active material is useful. Favorable electron conduction paths to the Li 3 PS 4 glass electrolyte were formed by mechanical milling with AB as the conductive additives. The all-solid-state cells with Li 3 PS 4 glass only or mixture prepared by grinding the Li 3 PS 4 glass and AB as a positive electrode did not work. On the other hand, the cell with Li 3 PS 4 -AB composite electrodes prepared by milling was successfully charged and then discharged. Initial charge capacity of the cell with the Li 3 PS 4 -AB composite electrode was 220 mAh g À1 at 0.064 mA cm À2 . Open circuit voltage of the cell was 2.6 V vs. Li, which was higher than that with Li 2 S active material. In the cell with the Li 2 SeLi 3 PS 4 -AB composite electrode, the Li 3 PS 4 glass functioned as not only Li þ ion conductive paths to Li 2 S but also active material itself, suggesting that the use of the solid electrolytes as active materials in electrode layers is useful for increasing reversible capacity per gram of a positive electrode.

The electrochemical window of solid electrolytes (SEs) plays a crucial role in designing active material-SE interfaces in high-energy-density all-solidstate batteries (ASSBs). However, the suitable electrochemical window for individual active materials is not yet investigated, as the electrochemical window of SEs is overestimated. In this study, the oxidation onset voltages (OOVs) of several SEs, namely those compatible with Li 2 S as a high-capacity positive electrode material are determined. Results reveal that SEs with low OOVs decrease the capacity and increase the interfacial resistance of the corresponding ASSBs. The OOVs of SEs must exceed that of Li 2 S by more than 0.2 V to achieve high capacity, which in turn depends on SE ionic conductivity. Therefore, an Li 2 S positive electrode is combined with pseudobinary Li-oxyacid salts as SEs, exhibiting high OOVs and ionic conductivities, to afford a high-capacity (500 Wh kg −1 ) ASSB with high Li 2 S content.

To improve the utilization of Li 2 S active material, an essential increase of Li + ion conductivity for Li 2 S itself was investigated by modifying the crystal structure of Li 2 S. Li 2 SLiI solid solutions were prepared by mechanical milling and their ionic conductivities were remarkably increased. All-solid-state cells with Li 2 SLiI composite electrodes exhibited the highest utilization of Li 2 S with high reversibility and cyclability in allsolid-state cells with Li 2 S reported so far.Development of rechargeable batteries with high energy density and high safety is desired. Elemental sulfur with a high theoretical capacity (1672 mA h g ¹1 ) attracts much attention as a positive electrode for a Li/S battery with high energy density. However, Li/S batteries with an organic liquid electrolyte suffered from rapid capacity fading, mainly due to the dissolution of polysulfides, which are formed during charge discharge processes in the sulfur electrode. In order to suppress the dissolution of polysulfides into the liquid electrolyte, various approaches such as trapping in pore in carbon, 1 the use of solid polymers, 2,3 ionic liquid-based electrolytes, 4,5 and protection of lithium anodes 6 have been examined. We reported that allsolid-state Li/S cells with composite electrodes prepared by the mechanical milling (MM) of a mixture of sulfur active material, acetylene black, and Li 2 SP 2 S 5 solid electrolyte (SE) showed a high reversible capacity with good cyclability.7 However, the sulfur positive electrode requires the use of a lithium metal negative electrode because the sulfur positive electrode has no lithium, which poses serious safety hazards and may not be practical. Li 2 S active material, with a theoretical capacity of 1167 mA h g ¹1 , has received much attention because of its potential to use a non-lithium negative electrode such as highcapacity negative electrodes (e.g., silicon-or tin-based compounds, which can form alloys with lithium).8 We reported that all-solid-state cells with Li 2 S nanocomposite electrodes showed high capacity of 600 mA h g ¹1 at 0.13 mA cm ¹2 (0.07 C). However, the utilization of Li 2 S was still 50%, and thus, further improvement of the utilization is important for increasing the energy density of all-solid-state cells. To resolve the insulating nature of Li 2 S that prevents the achievement of high utilization, various efforts have been made to improve the contact between Li 2 S and electrical conductive materials such as carbon, metal, and solid electrolyte for all-solid-state batteries with Li 2 S as the active material. 911 On the other hand, shortening the Li + ion diffusion distance in Li 2 S by decreasing the Li 2 S particle size was also investigated for improving the utilization.9,12 The utilization of Li 2 S was increased from 43% to 50% by using pulverized Li 2 S as an active material for all-solid-state cells. 12 The all-solid-state Li/S batteries with the Li 2 S nanoparticle composite exhibited an initial discharge capacity of 848 mA h g ¹1 at a rate of 0.1 C ...