Changes in Structure and Ionic Resistance of Lithium-Ion Battery Graphite Electrodes – Part I: Impact of Formation and SEI

With the interest in achieving higher gravimetric and volumetric energy densities in lithium-ion batteries, increasing the electrode loading and reducing its thickness—and consequently porosity—while maintaining low tortuosity is desirable. Achieving high power density requires a thorough understanding of these structural parameters of thick graphite electrodes and their changes during the formation of the solid-electrolyte-interphase (SEI). Graphite electrodes with loadings of ∼10 {mg cm}⁻² and pristine porosities ranging from 20–50% underwent formation at 45 °C in LP57 + 2% VC electrolyte. The ionic pore resistance after formation was quantified in non-blocking conditions using a μ-reference electrode and measuring electrochemical impedance spectroscopy at −5 °C. Upon formation, the ionic resistance of the graphite electrodes increased by up to 25%, which was quantitatively related to specific changes in electrode thickness, porosity, and tortuosity. The change in electrode thickness depended on the pristine porosity and increased by up to ∼11%. Mercury intrusion porosimetry revealed a decrease in porosity for initially high-porosity electrodes and an increase for those with low pristine porosity. The tortuosity of the electrodes increased by up to ∼10% upon formation. The thickness and density of the SEI were determined using a combination of pycnometry and mass-gain measurements, yielding 16–20 nm and ∼1.07 {g cm}⁻³, respectively.