Depth-Resolved X-Ray Absorption Spectroscopic Studies on Reaction Mechanism at the Cathode/Electrolyte Interface in All-Solid-State Battery
Authors: Chen Ke Zheng , Mori Takuya , Orikasa Yuki , Ito Yusuke , Yubuchi So , Matsuyama Takuya , Hayashi Akitoshi , Tatsumisago Masahiro , Nitta Kiyofumi , Uruga Tomoya , Uchimoto Yoshiharu 
Published: February 2020

As one of the next-generation batteries, all-solid-state batteries have attracted attention due to their outstanding safety performance and energy density. However, their power performance are still far from the increasing demand of electric vehicles, which seems to be caused by the large interfacial resistance between cathode and solid electrolyte. To overcome the inherent interfacial problem of all-solid-state batteries, there is an urgent need to understand the reaction mechanism at the cathode/electrolyte interface. As the origin of the interfacial resistance, three hypotheses have been proposed: the space-charge layer [1], the reaction product [2] and the mechanical structure change [3]. In this study, we applied thin-film model electrode for electrochemical measurement and depth resolved X-ray absorption spectroscopy (DR-XAS) measurement to investigate the reaction mechanism of interface between LiCoO2 cathode and Li2S-P2S5 solid electrolyte with interface modification by using Li3PO4 interlayer.

In order to analyze the interfacial reaction between cathode and electrolyte, thin-film model electrode with LiCoO2 cathode and Li2S-P2S5 solid electrolyte was deposited on mirror-polished platinum substrate by pulsed laser deposition (PLD). A Li3PO4 interlayer was also deposited between cathode and solid electrolyte by PLD. Electrochemical measurements were performed by using liquid electrolyte (1M LiClO4 in propylene carbonate) and lithium metal anode. Cyclic voltammetry measurement (CV) was carried out from 3.2 V to 4.4 V at 0.1 mV/s. EIS measurement was performed at 3.9 V, 4.0 V, 4.1 V, 4.2 V, 4.3 V and 4.4 V with a 30 mV amplitude. The frequency range of EIS was limited from 10-2 to 106 Hz. Depth-resolved XAS measurements were conducted using the BL37XU beamline, SPring-8, Japan, with a two-dimensional pixel array detector, PILATUS (Dectris, Switzerland).

The first cycle of CV curves of the film without Li3PO4 interlayer shows a current peak around 3.2 V, which is considered to come from the reaction between cathode and electrolyte. EIS measurements show that the interfacial resistance of the films with Li3PO4 interlayer does not increase compared with the film without Li3PO4 interlayer. In order to discuss the interfacial structure, depth resolved X-ray near edge structure at Co K-edge is analyzed. The energy shift around the interface in the film with Li3PO4 interlayer is smaller than that of sample without Li3PO4 interlayer. These results indicate that valence changes of cobalt ions near cathode/solid electrolyte interface are suppressed by using Li3PO4 interlayer. The introducing of Li3PO4 interlayer reduces the inter-diffusion between cathode and solid electrolyte.

Reference

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[2] A. Sakuda, A. Hayashi, M. Tatsumisago., Chem Mater,
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[3] K. Kishida, N. Wada, H. Adachi, K. Tanaka, H. Inui, C. Yada, Y. Iriyama, Z. Ogumi., Acta Materialia,
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 Acknowledgement

This research was financially supported by the Japan Science and Technology Agency (JST), Advanced Low Carbon Technology Research and Development Program (ALCA), Specially Promoted Research for Innovative Next Generation Batteries (SPRING) Project.