Elucidation of Reaction Mechanism at the Cathode/Electrolyte Interface in All-Solid-State Battery By Using Depth-Resolved X-Ray Absorption Spectroscopy

Abstract

As one of the next-generation batteries, all-solid-state batteries have attracted a lot of attention due to their outstanding safety performance and energy density. However, their power performance are still far from traditional lithium ion batteries using liquid electrolyte, which is considered as a result of interfacial resistance existing between cathode and solid electrolyte. To overcome this 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, so far there are three hypotheses have been proposed: the space-charge layer [1], the reaction product layer [2] and the mechanical structure change [3]. In this study, two types of LiCoO2 thin-film model electrodes with and without interface modification by using Li3PO4 interlayer were fabricated by pulsed laser deposition (PLD). The electrodes were applied 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. In order to analyze the interfacial reaction, thin-film model electrode with LiCoO2 cathode and Li2S-P2S5 solid electrolyte was deposited on mirror-polished platinum substrate by 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 (CV) measurement was carried out from 3.2 V to 4.4 V at 0.1 mV/s. Electrochemical impedance spectroscopy (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 during first charge and discharge cycle. The frequency range of EIS was limited from 10-2 to 106 Hz. DR-XAS measurements were conducted using the BL37XU beamline, SPring-8, Japan, with a two-dimensional pixel array detector, PILATUS (Dectris, Switzerland). Comparing to the thin-film electrode with Li3PO4 interlayer, the first cycle of CV curves of the electrode 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 Li3PO4 interlayer can suppress the increase of interface resistant during charge and discharge, which means a more stable interface can be kept by interface modification. In order to make a more detailed observation of the interfacial structure, DR-XAS near edge structure of the electrodes are analyzed at Co K-edge. The energy shift around the interface in the electrodes with Li3PO4 interlayer is smaller than that of samples without Li3PO4 interlayer. These results indicate that valence changes of cobalt ions near cathode/solid electrolyte interface are suppressed by interface modification. The introducing of Li3PO4 interlayer reduces the inter-diffusion between cathode and solid electrolyte. The results of both electrochemical and DR-XAS measurements present that the reaction mechanism is the reaction product layer at cathode/electrolyte interface in all-solid-state battery using sulfide 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.