Unraveling Asymmetric Macroscopic Reaction Dynamics in Solid-State Li-S Batteries During Charge-Discharge Cycles: Visualizing Ionic Transport Limitations with 𝘖𝘱𝘦𝘳𝘢𝘯𝘥𝘰 X-Ray Computed Tomography

By eliminating liquid electrolytes, solid-state Li-S batteries (SSLSBs) deliver emerging high-energy storage solutions with intrinsic safety advantages. However, restricted access to in-battery reaction dynamics has obscured the mechanisms behind critical limitations in rate capability and cyclability. Here, 𝘰𝘱𝘦𝘳𝘢𝘯𝘥𝘰 X-ray computed tomography is employed to capture the evolving through-thickness sulfur-lithiation distribution within composite SSLSB cathodes during cycling, directly linking macroscopic ion transport to cell performance. The observations not only demonstrate that macroscopic ion transport severely restricts the rate capability of SSLSBs, but also disclose a previously unreported charge-discharge asymmetry in reaction heterogeneity that triggers irreversible Li₂S formation. Differential-evolution fitting of the reaction profiles quantified a deterioration in the effective ionic conductivity during charging, which drives the observed asymmetry and likely arises from electrochemical and/or microstructural degradation of the electrolyte. This study establishes a methodology for high-resolution visualization of reaction dynamics within composite cathodes, providing quantitative insights into the macroscopic transport bottleneck based on experimental observations, which are essential for guiding rational electrode design toward enhanced rate capability and cyclability.