A comparative study on silicon-based negatrode materials in metallic cavity electrode and button half cell — Uncovering unseen microscopic and dynamic features

Abstract

The electrochemical characterization of lithium storage materials using the button cell is commonplace, but it is also tedious and time-consuming. Also, the results are often affected by the use of the binders and separator membranes, and by the electrode forming and cell assembly methods. To study the changes in materials before and after dis-/charging, one has to break up the button cell and disturb the packing structure of electrode. In this work, the metallic cavity electrode made of copper (Cu-MCE) was used to study silicon-based negative electrode (negatrode) materials during electrochemical de-/lithiation. The initial apparent reaction area (i.e. the contacting area between the Cu substrate and the active materials, 0.785 mm2) of the Cu-MCE was much smaller than that of the half-button cell (153.86 mm2), reducing significantly the overall current and hence polarization in the Cu-MCE. Powders of commercial silicon and phosphorus-doped silicon (P-doped Si) were tested in the Cu-MCE and a conventional button cell. Cyclic voltammograms (CVs) recorded using the Cu-MCE showed full activation in the first cycle, unlike the button cell whose CVs expanded continuously beyond 5 cycles. Current peaks on the CVs of the Cu-MCE agreed with the expected redox reactions but were more pronounced. The subtle differences between P-doped Si and pure Si could also be revealed by the Cu-MCE with the current peaks becoming more obvious, apparently due to modification in material structures and improved ion transport dynamics. The peak currents on the CVs of the Cu-MCE were plotted against the square root of scan rate (v1/2), showing non-linearity for the two oxidation peaks at 0.35 and 0.54 V, indicating both diffusion and surface of the delithiation processes. Linear plots were obtained for the two reduction peaks at 0.165 and 0.245 V with comparable slopes (−0.024 and 0.029 mA mV−1/2 s1/2), confirming diffusion control with insignificant polarization. However, similar analyses of the button cell revealed diffusion control in both oxidation and reduction, indicating slower dynamics with large polarization to delithiation. More importantly, the Cu-MCE can be inspected directly after dis-/charging without any disturbance, and provides unseen variation in the packing structure, particle morphology, and elemental information of the active materials. It is hoped that the higher accuracy, better details, and greater efficiency offered by the Cu-MCE for studying the intrinsic electrode reaction characteristics of Si-based electrode materials can be extended to other powdery materials for charge storage.