All-solid-state batteries (ASSBs) using silicon (Si) anodes are among the most promising candidates for high-energy and long-lasting power sources, particularly for electric vehicles. Si can store more lithium than conventional graphite, but its volume expands by roughly 410% during charging. This swelling generates mechanical stress that cracks particles and weakens their contact with the solid electrolyte, disrupting the flow of ions and reducing efficiency.
To address this, a research group led by Professor Yuki Orikasa from the College of Life Sciences, Ritsumeikan University, along with Ms. Mao Matsumoto, a graduate student at the Graduate School of Life Sciences, Ritsumeikan University (at the time), and Dr. Akihisa Takeuchi from the Japan Synchrotron Radiation Research Institute, used operando synchrotron X-ray tomography with nanometer resolution to observe what happens inside these batteries as they charge and discharge in real time. This paper was made available online on October 8, 2025, and was published in Volume 19, Issue 41 of ACS Nano on October 21, 2025. "The insights obtained in this study, including the identification of nanoscale interfacial separation phenomena and their effect on ionic transport, deepen our understanding of the chemomechanical interplay in Si-based ASSBs and provide guidance for the design of more robust, high-capacity composite electrodes," said Prof. Orikasa.
The team built a specially designed, all-solid-state cell using a sulfide-based solid electrolyte, Li6PS5Cl, and optimized imaging optics that allowed three-dimensional (3D) visualization of the electrode's microstructure during cycling. These operando images captured how Si particles expand and shrink, forming shell-like voids around their surfaces as they delithiate. Conventional wisdom would suggest that such voids completely isolate Si from the electrolyte, blocking ion conduction. However, the researchers observed that portions of the solid electrolyte remained attached to the Si even after contraction. These residual layers act as tiny bridges, maintaining partial ionic contact and keeping the battery functional despite significant structural changes.
At higher resolution, the nano-computed tomography data revealed that the detachment of Si from the solid electrolyte is not uniform. Instead, it follows an anisotropic pattern. The separation begins along the sides of the Si particles where the pressure is lowest, while the regions compressed vertically remain largely connected. This directional delamination creates zones of preserved contact, enabling lithium ions to continue flowing through parts of the interface. Such partial connectivity explains why the battery continues to operate efficiently after the first few cycles, even though the contact between Si and electrolyte is far from perfect.
The analysis also found evidence of thin fragments of the sulfide electrolyte containing sulfur and phosphorus elements adhering to the silicon surface after cycling. These residual fragments provide additional anchoring points that link the shrunken Si to the surrounding electrolyte network. The discovery that small remnants of electrolyte can sustain conduction offers new insights into how ASSBs accommodate large mechanical strains without catastrophic failure. By combining these 3D visualizations with electrochemical data, the researchers traced the cause of the first-cycle capacity loss to the initial formation of interfacial voids. Once the void pattern stabilized and the anisotropic contact zones became established, subsequent cycles showed minimal degradation. Ms. Matsumoto concluded, "The findings suggest that not all interfacial separation is harmful; partial and directionally constrained delamination can coexist with stable performance if the electrolyte retains limited but continuous pathways for ion transport."
This work provides valuable design guidance for engineers developing next-generation ASSBs. Controlling pressure distribution within the electrode, selecting electrolyte materials that adhere well to silicon, and designing composite architectures that exploit anisotropic stress behavior could all help minimize performance loss. The study demonstrates that the mechanical and electrochemical interplay at the nanoscale ultimately governs macroscopic battery durability. Prof. Orikasa explained, "The operando X-ray tomography study revealed that the formation of shell-like interfacial voids around Si particles during the initial delithiation process results in irreversible capacity loss during the first cycle."
Through their detailed imaging of the silicon–electrolyte interface, the team has shown that mechanical imperfections can still coexist with electrochemical stability. This study illustrates how advanced visualization tools can uncover the hidden dynamics that make next-generation energy storage systems more resilient and efficient, guiding future innovations in electric vehicle and grid-scale battery technologies.
