Operando Hard X-Ray Photoelectron Spectroscopy for Observing Interfacial Electrochemical Reactions in All-Solid-State Batteries
Authors: Iwama Tsukasa , Ohnishi Tsuyoshi , Masuda Takuya 
Published: September 2023

All-solid-state lithium-ion batteries (ASS-LIBs) have been attracting wide attention for their potential superior performance, such as high safety and long cycle life compared with the conventional lithium-ion batteries with liquid organic electrolytes. However, one of the critical problems of the ASS-LIBs for wide spread applications, especially for electric vehicles, is the large interfacial resistance across the electrode/solid electrolyte (SE)1. When an electrode and SE come into contact, some mobile ions are redistributed at the interface according to their potential difference2,3. In addition, the distribution of the mobile ions and electric potential are also changed during the battery operations. Therefore, dynamic observation of their distribution across the electrode/SE interface is essential to identify and control the origin of the interfacial resistance.

Hard x-ray photoelectron spectroscopy (HAXPES) is capable of nondestructive analysis of the elemental composition, oxidation states and electronic structure of sample surfaces and interfaces including those buried with over layers at depth > 10 nm[4].

In this study, we developed a laboratory-based
operando HAXPES system equipped with a Cr-Kα source (5414.9 eV), which enables angle-resolved measurements of and bias applications to the sample transferred without air-exposure, and applied it to the charging/discharging reactions of a LiCoO2 (LCO) positive electrode deposited on a Li6.6La3Zr1.6Ta0.4O12 (LLZT) solid electrolyte in a half cell configuration. HAXPES measurements were performed while the LCO electrode was electrically grounded to a hemispherical analyzer.

The galvanostatic charge/discharge potential profiles exhibited a characteristic plateau due to delithiation of a O3-LiCoO2 at ~3.9 V vs. Li+/Li5. In the Co 2p
3/2 region of the pristine cell, a sharp intense main peak and a broad satellite peak typical to a LCO were observed. At the charged states, however, the main peak was asymmetrically broadened to the higher binding energy. The main peak was deconvoluted into two peaks at 780 eV and ~781 eV, corresponding to Co3+ ions and Co4+ ions, respectively. The full width half maximums (FWHMs) of the Co4+ ions peak increased at charged states, while that of the Co3+ ions peak was remained unchanged. Furthermore, the relative areas of satellite peak decreased at the charged states. These results suggest that Co3+ ions were oxidized to the Co4+ ions by delithiation of the LCO electrode6,7. After the subsequent discharging, the component of higher binding energy of the main peaks decreased, and the satellite peak increased. In addition, the FWHMs of the Co4+ ions peak decreased, suggesting that the Co4+ ions were reduced by lithiation into the LCO electrode during discharging. Thus, the lithiation/delithiation of LCO was successfully observed by operando HAXPES.

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