Phase transitions explanatory of the electrochemical degradation mechanism of Si based materials

Even if Co–Co3O4 coated Si showed a prominent improvement compared to Si in the capacity maintenance, it failed to thoroughly exclude the cyclic degradation that also consists in Si. Because the observation on dQ/dV curve of Co–Co3O4 coated Si manifested that Co–Co3O4 coated Si is charged or discharged only by alloying or de-alloying between Si and Li, some analyses were conducted to Co–Co3O4 coated Si to figure out how the degradation of Si based materials is specifically related to alloying or de-alloying between Si and Li. Most of the up-to-now reports have just tried to give us a simple explanation that the volume expansion during alloying or de-alloying between Si and Li, may have something to do with the cyclic degradation of Si based materials. Thickness variation measured at various SOC’s or DOD’s, informed that there is a narrow potential region closely linked to the primary volume expansion of Si based materials. Based on XRD analyses at different SOC’s or DOD’s, it came clear that Li15Si4 is formed and decomposed in this potential region. Thermodynamic point of view has indicated that some of potential plateaus during charge or discharge are developed by phase transitions from amorphous LixSi to Li15Si4 and vice versa. Because phase transitions are definitely followed by huge volume change as confirmed in thickness variation result, OCV’s and CCV’s of Co–Co3O4 coated Si during charge or discharge were compared to observe the change of electrode resistance. It let us know that the electrode resistance abruptly increases, wherever the volume expansion occurs with formation or decomposition of Li15Si4. Finally, the plateau-less discharge curves and improved cyclic property of Co–Co3O4 coated Si in the potential region between 70 mV and 1 V vs. Li/Li+, which can restrain the formation of Li15Si4, made sure that Li15Si4 is closely related to the cyclic degradation of Si based materials.