Operando Observation and Analysis of Reaction Distribution in Composite Electrodes for All-Solid-State Lithium Ion Batteries

All-solid-state lithium ion secondary battery is a promising next generation one in terms of safety, energy density and flexibility in shape. In the all-solid-state lithium ion secondary battery, composite electrodes are organized by particles of active material, solid electrolyte, conductive additive material and binder forming three-dimensional ionic and electronic conduction pass. It is recognized the suitable ratio of active material is an important factor against high volumetric energy density and rate performance. Inhomogeneity of reaction in composite electrodes derived from their morphology affects the battery performance. Therefore, it is necessary to understand the relationship between morphology of composite electrode and reaction distribution phenomenon. Nevertheless, there are few studies from such a point of view both for the liquid and solid electrolyte system, 1-4 and it has never been reported observing the phenomenon experimentally in solid-state battery. In this study, we developed operando 2D-imaging X-ray Absorption Spectroscopic (2D-XAS) technique to investigate the reaction distribution within composite electrodes of in-plane and simulated cross sectional direction. We discuss here the correlation between electrochemical behavior and electrode morphology along two kinds of electrodes composed by different binder material. Composite electrodes were prepared by mixing active material (LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM)), solid electrolyte (75Li 2 S-25P 2 S 5 glass (SE)), conductive additive (Acetylene Black (AB)) and binder with some solvent in the ratio of NCM:SE:AB:binder = 70:30:3:3 [wt%]. Styrene butadiene copolymer rubber (SBR) or styrene ethylene butylene styrene copolymer (SEBS) was used as a binder material in each electrochemical cell. The composite electrodes and SE sheets were pressed together under 300 MPa uniaxially, and then placed in laminate-type layered cells with counter electrodes (Li metal foil). All the process was done in Ar-filled grove-box. These cells were designed to observe the reaction distribution in not only the in-plane but also the cross sectional direction by the operando 2D-XAS measurements. The operando 2D-XAS measurements were performed at BL37XU, SPring-8, Japan. Ni K -edge XAS spectra of the NCM electrodes were collected every 20 minutes using CMOS 2D detector with a spatial resolution of 1.3 square micrometer, under running 1/40 C rate charging program. Hereafter, we refer to the two kinds of el