Solid state batteries (SSBs) are one of the emerging options for next-generation energy storage systems due to their improved safety and high power/energy densities. In composite SSB electrodes, many active material (AM) particles are randomly and three-dimensionally distributed together with solid electrolyte (SE) particles, forming complex ion and electron conduction pathways. Such complicated mass transport pathways lead to a local depletion of ion and/or electron suppy in the electrode, especially at high current densities, resulting in a three-dimensional (3D) heterogeneous electrochemical reaction within the AM particle ensemble and within each individual AM particle. Such heterogeneous reactions can degrade the capacity/power output and lifetime of SSBs. Therefore, an understanding of the influence of heterogeneous reactions on battery performance is important for the development of high-performance SSBs. However, there are few techniques that allow tracking of 3D heterogeneous reactions within an AM particle ensemble in a composite SSB electrode with sufficiently high spatial and temporal resolution.

          Here, we developed an imaging technique that allows 3D operando tracking of the evolution of the heterogeneous reaction within the AM particle ensemble in the electrode and each individual particle in the ensemble with sufficiently high spatial (~200 nm) and temporal (40 min.) resolution, based on our previously reported technique using computed-tomography with X-ray absorption fine structure spectroscopy (CT-XAFS) [1], and investigated the heterogeneous reaction in composite SSB electrodes. Furthermore, we quantitatively analyzed the optimal particle parameters (e.g., particle size, shape) based on the data set of both chemical and geometric information of over 10⁷ voxels and 10²~10³ of AM particles obtained by the 3D imaging.

          Composite SSB cathodes were prepared by mixing octahedral single-crystal Li(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC) particles with an average size of ~ 3 or 10 µm and Li_2.2C_0.8B_0.2O₃ (LCBO) solid electrolyte in a weight ratio of 5:5. A model SSB was then fabricated using this composite cathode, Li foil as the anode, and LCBO as the solid electrolyte. We performed the imaging nano CT-XAFS measurements near the Ni-K edge, employing an imaging optics consisting of a condenser zone plate and an objective Fresnel zone plate. The charge state distribution in the electrode was evaluated from the peak top energy of the Ni-K edge at each voxel in the obtained 3D image.

          We three-dimensionally observed the evolution of the electrochemical reaction in 10²~10³ NMC particles in the region of 50×50×50 µm³ in the composite SSB electrode during constant-current electrochemical of 0.05~0.3C. The electrochemical reaction proceeded heterogeneously both in the whole AM particle ensemble and in each single particle. We statistically analyzed the relationship between the features of the electrochemical reaction in each particle such as average state-of-charge (SOC), intra-particle reaction inhomogeneity, and inter-particle reaction inhomogeneity, and particle parameters (e.g. spatial coordinates, volume, and shape). In the investigated electrode, larger particles tended to react heterogeneously within each particle while smaller particles tended to react heterogeneously between each particle, thus there was an optimal AM particle size that allowed for a uniform reaction. Our technique allows for the quantitative determination of the optimal active material parameters through the data-driven analysis of the experimentally obtained information of the heterogeneous reaction in the AM particle ensemble in a composite SSB electrode. Therefore, our technique provides useful information for SSB electrode design that cannot be obtained by other existing techniques.