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

[1] K. Takada, N. Ohta, L. Zhang, K. Fukuda, I. Sakaguchi, R. Ma, M. Osada, T. Sasaki., Solid State Ionics, 179 (2008) 1333.

[2] A. Sakuda, A. Hayashi, M. Tatsumisago., Chem Mater, 22 (2010) 949.

[3] K. Kishida, N. Wada, H. Adachi, K. Tanaka, H. Inui, C. Yada, Y. Iriyama, Z. Ogumi., Acta Materialia, 55 (2007) 4713.

 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.