Silicon-Graphite (Si-Gr) composite electrodes, composed of heterogeneous active materials, combine the high capacity of silicon with the conductivity and stability of graphite. These anodes, known for their excellent electrochemical performance and cycle stability, are gaining attention as potential replacements for graphite electrodes [1]. However, their long-term cycle stability is not as good as graphite's, and the higher the silicon content, the quicker the degradation occurs. Thus, it is necessary to improve this aspect [2]. The primary reason for the degradation of cycle stability in Si-Gr composite electrodes is the repeated growth of the Solid Electrolyte Interphase (SEI) during the cycling process [3]. Initially, the SEI forms around the active material, and as the cycling progresses, it extends over the entire electrode [4, 5]. During this process, the electrochemical performance of the active materials within the electrode decreases, and occasionally recovers. This fluctuation is influenced by the repeated growth of the SEI. When the residual stress inside the electrode exceeds the yield point, electrode cracking occurs. Unintentionally, these cracks shorten the ion transport pathways, which in turn can restore the electrochemical performance [5]. However, this interpretation is based on the observed capacity reduction and changes in electrode structure ex-situ. Therefore, there are inherent limitations in clearly delineating the step-by-step processes that result in internal structural changes and crack formation within the electrode. Additional analysis is required to further investigate these phenomena.In this study, to understand the internal structural changes of the Si-Gr composite electrode during high-speed, long-term cycling, we monitored the changes in electrode stress states in-situ during 200 cycles at 1C [6]. Additionally, by varying the silicon ratio, we aimed to comprehend the role of each active material over the long term. In Figure 1a, the observed capacity and stress changes in the 6% Si electrode are overlaid values of the two active materials' reactions. To analyze the stress behavior characteristics due to the volumetric behavior of each active material, as shown in Figures 1b and 1c, the capacity and stress were differentiated to distinguish the contributions of each material (dQ/dV, dσ/dV). Additionally, by calculating the differential stress to differential capacity (dσ/dQ), as shown in Figure 1d, we understood the changes in the electrode structure at the reaction point of Silicon (Si1 for Li3.75Si → LixSi). By further analyzing these results and compiling data from different ratios of active materials, we aim to analyze the changes in internal electrode structure and the causes of crack formation in long-term, based on high-speed charging/discharging. Figure 1
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