The Effects of Silicon Anode Thickness on the Electrochemical Performance of Li-Ion Batteries

Abstract

The electrode configuration is an important element in the development of Li-ion cells. The energy density is proportional to the loading of the active material. Therefore, increasing the electrode thickness is the simplest way to achieve higher capacities. In this paper, we compare the effects of three different thicknesses of Ag-decorated Si electrode anode (HCSi) on the electrochemical performances such as the SEI layer formation, impedances, and mass capacitances. We prepared three different silicon electrode thicknesses to optimize the electrodes: 20, 40 and 60 µm and measured in situ galvanostatic electrochemical impedance spectroscopy (GEIS). Using GEIS, we studied the intercalation mechanism of Li+ ions in detail and found that despite having the same capacities (≈3500 mAh g−1), the thinnest electrode, HCSi20, allows diffusion of Li+ ions into the bulk, whereas thicker layers prevent smooth diffusion into the bulk of the silicon electrode due to increased layer resistance. The Voigt model was used to analyze the anomaly of the frequency dependence of the measured impedance, in which, the classical Randles circuit is connected in series with one or two R ‖ C parallel combinations. One R ‖ C circuit could be the result of the SEI formation, and the second R ‖ C circuit could be the contribution of Li. To increase the number of charge and discharge cycles, we improved the electrolyte by adding fluoroethylene carbonate (FEC), which reduced the capacity of the HCSi20 electrode to 50% of the initial capacity (≈3500 mAh g−1) after 60 cycles, whereas it dropped to 20% of the initial capacity after 10 cycles without the addition of FEC.