Three-Dimensional Visualization of Inhomogeneous Reaction within Individual Active Material Particles in Composite Solid-State Battery Electrodes Using Nano CT-XANES

Introduction Solid-state batteries (SSBs) are next-generation batteries that are expected to exhibit higher energy density, power density, and safety than current lithium-ion batteries. In composite SSB electrodes, randomly distributed particles of active material (AM) and solid electrolyte (SE), and voids form three-dimensionally complicated ion/electron conduction paths and AM/SE interfaces. Such complex mass transport paths and limited AM/SE interfaces can locally impede the ion/electron supply to AM particles particularly under the rapid (dis)charging, potentially leading to a three-dimensional (3D) inhomogeneous reaction within each AM particle, as well as between particles. The occurrence of such inhomogeneous reaction can severely deteriorate the capacity, power output, and cycle life of SSBs. Therefore, it is essential to understand the mechanism of the occurrence of the inhomogeneous reaction and to design electrodes that enable uniform reaction progression. The most effective approach for this is the direct observation of the 3D inhomogeneous reaction within individual AM particles in actual composite SSB electrodes. However, most of existing methods enables only one- or two-dimensional observation of the inhomogeneous reaction 1, 2 , offering limited insight into the 3D reaction inhomogeneity in AM particles in SSB electrodes. With this background, in this work, we developed a 3D visualization technique of inhomogeneous reaction within individual AM particles in a composite SSB electrode using X-ray nano computed tomography technique combined with X-ray absorption near-edge structure spectroscopy (nano CT-XANES). Using this technique, we three-dimensionally observed the inhomogeneous reaction in each of thousands of AM particles in composite SSB electrodes. Furthermore, based on the obtained information, we discussed the optimal AM particle parameters (e.g., size, shape, particle distribution) that can alleviate the inhomogeneous reaction within and between AM particles, thereby maximizing electrode performance. Experimental The composite cathodes to be observed were prepared by mixing LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) primary particle powders and Li 2.2 C 0.8 B 0.2 O 3 (LCBO) solid electrolyte powders in a 1:1 weight ratio. The model SSBs were fabricated with the composite cathode (~50 μm thickness), LCBO solid electrolyte, poly (ethylene oxide) (PEO)-based polymer electrolyte, and Li metal anode. The model SSBs were charged to 100 mAh/g at 0.1~0.2 C, and the inhomogeneous reactions within AM particles in the composite cathodes after charging were visualized using nano CT-XANES. In the nano CT-XANES measurements, CT measurements were conducted near the Ni K -edge (8345.9-8352.4 eV) by incrementing the X-ray energy in 0.2 eV steps, with the entire set of measurements requiring approximately 25 minutes. The state-of-charge (SOC) of NMC ( x in Li x Ni 1/3 Mn 1/3 Co 1/3 O 2 ) in each voxel in the 3D CT images was evaluated based on the peak top energy shift of the Ni K -edge XANES spectrum in the respective voxels. Results and discussion Figures 1(a) and (b) show 3D SOC maps of the composite cathode after charging to 100 mAh/g at 0.2 C viewed from two different angles, and the corresponding charging curve, respectively. The observation area was approximately 34 × 47 μm in the in-plane direction of the electrode and approximately 34 μm thick with the voxel size of approximately 0.3 μm. The red/blue colored regions represent the charged ( x = 0.45)/uncharged ( x = 1.0) AM particles, respectively, while the transparent regions correspond to the SE or voids. As shown in this figure, the SOC of individual AM particles after charging varied significantly among the particles. While some particles, such as particle A, were charged to a Li content corresponding to the charging capacity of the entire electrode of 100 mAh/g ( x = 0.64), there were also insufficiently charged particles (particle B) and excessively charged ones (particle C) relative to the average capacity of the entire electrode. These results indicates that the charge reactions proceeded inhomogeneously between AM particles. Furthermore, as shown in the 3D SOC maps of representative AM particles in Fig. 1(c), the reaction proceeded inhomogeneously within a particle. While the reaction progressed relatively uniformly within particles 1 and 2, the reaction proceeded quite inhomogeneously within particles 3 and 4. As described above, the charging state and its intra-particle inhomogeneity varied significantly among AM particles. To identify the characteristics of AM particles that determine the charging state and its inhomogeneity in each particle, in the presentation, we will statistically investigate the correlation between reaction states of each particle and morphological characteristics such as size and shape, for thousands of AM particles in the observation area. References [1] C. Xu, et al. , Joule , 6 , 2535 (2022). [2] D. A. Shapiro, et al. , Nat. Photonics , 8 , 765 (2014). Acknowledgements This work was supported by JST PRESTO Grant Number JPMJPR23J3, JST Mirai Program Grant Number JPMJMI21G3, and JST GteX Grant Number JPMJGX23S2, Japan. Figure 1