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

Abstract

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 (LiNi1/3Co1/3Mn1/3O2 (NCM)), solid electrolyte (75Li2S-25P2S5 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 electrochemical cells with SBR binder and with SEBS as SBR-cell and SEBS-cell, respectively. In the same electrochemical measurement condition, the SBR-cell showed higher capacity at 10th cycle than the SEBS-cell. Also, it was observed that the SEBS-cell had low density, and that active material and SE distributed not homogeneously over the whole electrode from a cross section image through Scanning Electron Microscope.During the 2D-XAS measurements, galvanostatic charge tests were performed for both of the SBR- and the SEBS-cell with the State Of Charge (SOC) range of 0-22% under stable voltage. The x-ray absorption image obtained from the 2D-XAS measurements, means the existence ratio of the active material in the composite electrode (containing information along thickness dimension), reflecting the dispersiveness of the active material. The active material in the SBR-cell dispersed homogeneously, while that in the SEBS-cell localized. This suggests that dispersiveness of the active material in the composite electrode was influenced by characteristics of the binder material. Concerning the reaction distribution behavior in in-plane, the SBR-cell almost homogeneously reacted across the electrode from the initial of the charging process. In contrast, some specific regions reacted preferentially in the SEBS-cell even at the early stage of the charging process, which generated the distribution clearly through the measurement. Comparing existence ratio of Ni with reaction mapping in the both composite electrodes, it suggests that the reaction occurs preferentially in specific region containing a lot of active material particles along thickness dimension when they are greatly localized. In terms of cross sectional study of the composite electrodes, we observed that the electrode reaction proceeded isotropically from near the electrolyte to the current collector side in the case of charging. This result indicates that the limiting factor here is the ionic conductivity.