Revealing the Impact of Graphite Reaction and Binder Structural Changes on the Degradation of Silicon-Graphite Composite Electrodes via Differential Capacity and Stress Analysis
Authors: Kim Sung Yeol , Kim Giyong 
Published: December 2024

Silicon-graphite (Si-Gr) composite electrodes, which are based on high-capacity silicon mixed with conductive graphite with low volume expansion, have garnered significant interest due to their excellent electrochemical performance and stability. However, increasing the silicon content in the composite electrodes accelerates degradation, leading to a limitation in achieving higher energy density. In addition, Si-Gr composite electrodes undergo multi-stage reactions of silicon and graphite, resulting in complex reaction kinetics and intricate volume change behaviors. Therefore, it is challenging to analyze the impact of each active material's reaction on the electrode degradation. Furthermore, as the degradation of composite electrodes has primarily been understood and studied in the context of silicon's significant volume expansion and degradation, there is a lack of research evaluating the influence of graphite reactions. Despite having a low volume expansion upon lithiation, graphite occupies a significant volume fraction in the composite electrode and exhibits a relatively consistent arrangement. Therefore, it is anticipated that graphite would also influence electrode structural changes and the consequent electrode defect formation.

In this study, we observe the average stress corresponding to the electrochemical reactions of the charge/discharge processes of Si-Gr composite electrodes in situ. The average stress can be understood as the internal pressure build-up within the electrode due to the volume expansion of the reactions. The stress and the reaction capacity were differentiated with respect to potential to analyze the contributions of each reaction on the volume change and the internal force evolution. Interestingly, graphite reactions generally induced higher stress than those of silicon during the lithiation process, and their stress per capacity increased as the State of Charge (SoC) increased. These results indicate that graphite can significantly contribute to the stress inside the electrode even though it exhibits a much lower expansion rate than silicon. Moreover, stress behavior that contradicts the silicon volume contraction was also observed during silicon delithiation. This result can be attributed to the stress-relieving phenomenon of the binder, highlighting the importance of the interaction between the active materials and binders. To observe the initial degradation of the composite electrode, repeated charge-discharge cycles were conducted, revealing intervals of unstable stress variation. Subsequent examination using ex-situ SEM showed that debonding and fracture at the interface between graphite and silicon/binder were the primary causes. Furthermore, EDS, XPS, and indentation analysis showed that such structural defects were caused by changes in the silicon-binder structure due to SEI formation. As confirmed by the micro indentation test, the viscoelastic binder and silicon were transformed into a brittle silicon-binder structure. Consequently, it failed to sufficiently dampen the forces generated by the active materials’ volume change resulting in the defects. These research findings contribute to a fundamental understanding of the stability of Si-Gr composite electrodes and our method can be expanded to various energy electrodes of different materials and compositions to enhance understanding of their behavior and contribute to their stability improvement.

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