Analysis of the Deterioration Mechanism of Si Electrode as a Li-Ion Battery Anode Using Raman Microspectroscopy

Propylene carbonate (PC) and an ionic liquid consisting of 1-[(2-methoxyethoxy)­methyl]-1-methylpiperidinium (PP1MEM) and bis­(trifluoromethanesulfonyl)­amide (TFSA) were used as electrolyte solvents for Li-ion batteries, and the anode properties of Si electrodes were investigated using a thick film prepared by gas deposition without any binder or conductive additive. The Si electrode in PP1MEM-TFSA exhibited good cycle performance with a reversible capacity of 1050 mA h g^–1 even at the 100th cycle, whereas the Si electrode in PC showed a capacity of only 110 mA h g^–1. It is noteworthy that the electrode performance was significantly enhanced just by changing the electrolyte solvent to an ionic liquid even with the same Si used as the active material. Raman mapping analyses of the Si anodes after cycling were conducted to clarify the deterioration mechanisms of the electrodes. It was revealed that, in the case of PC, crystalline Si locally remained in the electrode after cycling, and Li–Si alloying and dealloying reactions occurred in limited regions. This led to the generation of intensive stress accumulation due to the extreme volume changes of Si in the regions inside the electrode, causing severe disintegration of the Si electrode. Consequently, the anode property of the Si electrode in PC resulted in very poor performance. In contrast to the behavior in the organic electrolyte, Li–Si reactions uniformly took place over the entire electrode in PP1MEM-TFSA, which relatively avoided any stress accumulation that could lead to electrode disintegration. This is considered to be the reason for the significant improvement in the cycle performance of the Si electrode using the ionic liquid instead of the conventional electrolyte