The impact of volume expansion on thermodynamic and kinetic properties of graphite/Si alloy composite anodes

Abstract

With the growing demand for high energy density and fast-charging/high-rate performance, understanding the thermodynamic and kinetic behavior of graphite/silicon composite anodes has become increasingly important. In this study, graphite/Si alloy composite electrodes containing 0–50 wt% silicon alloy were investigated using a three-electrode pouch cell combined with in situ dilation. Through this analysis, the changes in porosity after the formation cycles were determined, while the lithium-ion diffusion coefficient (D_Li⁺) was calculated based on the Weppner–Huggins equation. In addition, the resistance components were quantitatively evaluated by pulse polarization and electrochemical impedance spectroscopy (EIS). This study elucidates the effect of porosity evolution, induced by irreversible expansion with varying silicon content, on the electrode's electrochemical behavior. For the YNG-30% electrode, irreversible expansion during the formation process increased the porosity from 32.3% to 55.0%. As a result, the transport pathways for solvated Li⁺ were expanded, and the charge-transfer resistance (R_ct) decreased, leading to enhanced interfacial and interparticle diffusion in the graphite staging region. In contrast, in the silicon-dominant region, intrinsic thermodynamic and kinetic limitations resulted in higher overpotential at the beginning of charge and the end of discharge. Consequently, the role of silicon is not limited to increasing capacity but serves as a structural design element for achieving fast-charging and high-rate performance. These findings highlight that porosity optimization is a key factor for improving electrode performance.