Impedance study on the correlation between phase transition and electrochemical degradation of Si-based materials

In order to explain the relationship between physical change and electrochemical degradation of Co–Co3O4 coated Si, impedance spectroscopy on Co–Co3O4 coated Si was conducted at various states during charge or discharge. Nyquist plots during Li+ insertion (charge) showed a unique behavior that below 70 mV vs. Li/Li+, the more Li+’s were inserted into the electrode, the larger its comprehensive resistance was getting. During Li+ extraction (discharge), electrode resistance was decreased after going through 0.43 V vs Li/Li+. When these data were fitted with the ordinary equivalent circuit which is composed of electrolyte resistance, charge transfer resistance and contact resistance, there was an abrupt augmentation of charge transfer resistance below 70 mV vs. Li/Li+ during charge, whereas there was its drastic diminishment between 0.2 and 0.5 V vs. Li/Li+ during discharge. Because these potential regions are each related to amorphous LixSi-to-Li15Si4 transition and vice versa, it could be shown that the formation and decomposition of Li15Si4 is responsible for the electrochemical degradation of Co–Co3O4 coated Si.