X-Ray Absorption Spectroscopic Analysis of Li3PS4 Glass-Carbon Composite Cathodes for All-Solid-State Lithium Secondary Battery
Authors: Saito Kohei , Mori Takuya , Orikasa Yuki , Hakari Takashi , Deguchi Minako , Hayashi Akitoshi , Tatsumisago Masahiro , Mitsuhara Kei , Ohta Toshiaki , Kowada Yoshiyuki , Uchimoto Yoshiharu 
Published: February 2020

All-solid-state lithium secondary batteries with sulfide inorganic solid electrolytes have attracted attention because of their outstanding safety performance. However, the practical capacity of cathodes for all-solid-state batteries is smaller than lithium-ion battery cathodes because the composite electrodes are contained by solid electrolyte powder. To increase the practical capacity of composite cathodes, the composite material of milling Li3PS4 solid electrolyte with acetylene black was examined as an active material.1 While the improved capacity of cathodes was reported, the reaction mechanism of this active material has not been clarified yet. In this study, we investigated the electronic structure change of Li3PS4-acetylene black composite electrodes during charge-discharge for sulfide all-solid-state lithium secondary battery.
The 75Li2S-25P2S5 (mol %) glass used as solid electrolyte for all-solid-state cells was prepared by mechanical milling.　The Li3PS4-acetylene black composite electrode was obtained by ball milling of the mixture of the Li3PS4 and acetylene black with the weight ratio of 70:30 at 370 rpm for 1 h. All-solid-state battery cells were fabricated as follows. The Li3PS4-AB composite electrode (5–6 mg) and the 75Li2S-25P2S5 glass (80 mg) were set in a polycarbonate tube (φ10 mm) and pressed together under 360 MPa. An indium foil with a thickness of 100 μm was then placed on the surface of the solid electrolyte side of the bilayer pellet as a counter-reference electrode. Electrochemical tests of the cells were conducted at a constant current density of 0.064 mA cm−2 at 25 °C in an Ar atmosphere. X-ray absorption spectroscopy (XAS) was carried out at the beamline BL-13 in the SR center, Ritsumeikan University. Charged or discharged Li3PS4-AB composite electrodes were transferred into the chamber for XAS measurements.
XRD patterns of charged and discharged sample showed no new peaks and Raman spectra showed remained PS4
3- anions in charge-discharge process. XPS analysis showed that new S-S covalent bonds were detected after charge. From S K-edge　XAS　spectra,　the peak top was shifted from 2471 eV to 2472 eV and the intensity of the peak increased during charging. The change of S K-edge XAS clearly indicates the electronic structure of sulfur largely contribute the redox reaction in Li3PS4-AB composite electrode. Moreover, the extended X-ray absorption fine structure analysis showed the formation of S-S bonds in the charge reaction. 2

Reference
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2. Cuisinier, M., Cabelguen, P.-E., Evers, S., He, G., Kolbeck, M., Garsuch, A., Bolin, T., Balasubramanian, M., and Nazar, L. F.,The Journal of Physical Chemistry Letters,
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4(19), 3227-3232.
Acknowledgement
This research was financially supported by Advanced Low Carbon Technology Research and Development Program (ALCA).