Unveiling factors affecting the solid electrolyte interphase resistance: the effect of real surface area and particle size of electrode materials

This work identifies previously neglected factors and their impact on solid electrolyte interphase (SEI) formation on graphite and cathode electrolyte interphase (CEI) formation on lithium manganese oxide (LiMn2O4). These factors are active materials’ surface area and particle size. They were investigated by galvanostatic charging/discharging and electrochemical impedance spectroscopy (EIS) on the following examples: graphite (25.2 m2 g−1, 12.3 m2 g−1, 6.0 m2 g−1), LiMn2O4 (particle size < 0.5 µm) 11.7 m2 g−1, and LiMn2O4 (particle size < 5 µm) 3.3 m2 g−1, respectively. The test cells were worked in 1 M LiPF6 solution in a mixture of ethylene carbonate (EC) with dimethyl carbonate (DMC) (1:1) electrolyte. The other aspect of this study features the assessment of optimal current for the SEI layer formation. For this purpose, five electrode materials were cycled galvanostatically with current densities between 5 and 100 mA g−1. As observed, the lower current causes the formation of a more resistive SEI layer. Changes in SEI resistance do not necessarily lead to the corresponding changes in diffusion impedance. The current rate of the SEI formation cycle should be correlated not with the system capacitance but rather with the specific surface area of the active material.