(Invited) Development of a 5D Analytical Tool for Probing Electrochemical Reactions in Composite Solid-State Battery Electrodes: Toward Designing Better Electrodes
Authors: Kimura Yuta , Tanaka Mai , Ishiguro Nozomu , Nakamura Takashi , Sekizawa Oki , Nitta Kiyofumi , Uruga Tomoya , Ariyoshi Kingo , Takeuchi Tomonari , Okumura Toyoki , Tada Mizuki , Uchimoto Yoshiharu , Amezawa Koji 
Published: December 2024

The emergence of solid lithium superionic conductors exhibiting conductivity surpassing that of liquid electrolytes has positioned solid-state batteries (SSBs) as one of the most promising and realistic next-generation energy storage devices. SSBs are expected to demonstrate superior safety, as well as higher capacity and power density compared to conventional lithium-ion batteries. In practice, however, SSBs often fail to achieve the high performance expected from the superior properties of their individual constituent materials. This is partly because the performance of SSBs is not solely dependent on the constituent materials but also heavily influenced by the design of each battery component. For example, for the electrode of SSBs, composite electrodes consisting of active materials and solid electrolytes (and conductive additives, if necessary) are often employed. Within these composite electrodes, particles of active materials and solid electrolytes, along with voids, are randomly interspersed, forming a complex microstructure. This intricate microstructure creates highly tortuous electron and ion conduction pathways and complex heterogeneous material interfaces within the electrode. Consequently, particularly under high current charging/discharging, local stagnation of ion/electron transport within the electrode can arise, leading to spatially inhomogeneous electrochemical reactions throughout the entire electrode or within individual active material particles. Such inhomogeneous electrochemical reactions hinder the materials from fully exhibiting their inherent performance, potentially leading to severe degradation of battery capacity, power output, as well as the impairment of cycle life due to localized overcharging/overdischarging. To suppress the occurrence of such inhomogeneous reactions and maximize battery performance and lifespan, it is necessary to directly observe the electrochemical reaction within composite electrodes during (dis)charging and establish appropriate design guidelines based on these observations. However, conventional characterization techniques have been unable to directly observe the time-varying electrochemical reactions that take place three-dimensionally and inhomogeneously within electrodes at both the particle scale (~few micron) and electrode scale (~hundreds of microns) during (dis)charging. Therefore, previous studies have been limited to obtaining averaged information on the overall electrochemical reactions within electrodes using conventional electrochemical measurements, estimating the electrochemical reactions in electrodes through numerical simulations, or conducting lower-dimensional (1D or 2D) observations of the 3D electrochemical reaction using X-ray or electron probe imaging techniques. Against this background, we have developed operando 3D imaging techniques for probing electrochemical reactions in SSB electrodes based on computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES)1–4. These techniques can track the spatiotemporal evolution of the electrochemical reaction within composite electrodes during (dis)charge cycles, thereby allowing for the analysis of electrochemical reactions in five dimensions (5D), encompassing 3D spatial coordinates, time, and chemical state information. As shown in Fig. 1a1, our technique based on micro CT-XANES enables the operando 3D imaging of the evolution of the electrochemical reactions at the electrode scale with a spatial resolution of a few micrometers and a temporal resolution of approximately 30 minutes. This allows for the evaluation of the state of charge (SOC) distribution at any arbitrary cross-section in the thickness and in-plane directions within the same bulk region in electrodes at any given time. Consequently, it enables the quantitative analysis of when, where, how, and why low SOC regions appear within the electrode1,2,4, and allows for uncovering the correlation between local capacity degradation during cycling and electrode microstructures or the reaction history in previous (dis)charge cycles3. Furthermore, our advanced technique based on nano CT-XANES enables the operando, 3D, and simultaneous observation of the intra- and inter-particle reaction distributions among thousands of active material particles within SSB electrodes with a spatial resolution of sub-micrometers and a temporal resolution of approximately 30 minutes, as illustrated in Fig. 1b. The obtained information allows for the quantitative and statistical analysis of the correlations between the reaction characteristics of each active particle and its morphological features, facilitating the data-driven determination of optimal active material particle parameters, such as particle size, shape, arrangement, and size distribution. In the presentation, we will demonstrate how our 5D electrochemical reaction analysis techniques can provide useful insights for battery research.

References

Y. Kimura et al., J. Phys. Chem. Lett., 11, 3629–3636 (2020).

Y. Kimura et al., ACS Appl. Energy Mater., 3, 7782–7793 (2020).

Y. Kimura et al., Small Methods, 7, 2300310 (2023).

S. Huang et al., J. Phys. Chem. C (2024) https://doi.org/10.1021/acs.jpcc.4c00318.

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