In situ SEM observation of a lithium deposition and dissolution process in an all-solid-state lithium metal battery using a sulfide-based solid electrolyte (SE) was carried out. We revealed visually that the morphology of lithium deposition varies with the operating current densities. At current densities higher than 1 mA cm(-2), local lithium deposition triggers large cracks, leading to a decrease in the reversibility of lithium deposition and dissolution. On the other hand, at a low current density of 0.01 mA cm(-2), its homogeneous deposition, which enables the reversible deposition and dissolution, hardly brings about the occurrence of unfavorable cracks. These results suggest that homogeneous lithium deposition on the SE and the suppression of the growth of lithium metal along the grain boundaries inside the SE are keys to achieve the repetitive lithium deposition and dissolution reaction without deterioration of the SE.
By exploiting characteristics such as negligible vapour pressure and ion-conductive nature of an ionic liquid (IL), we established an in situ scanning electron microscope (SEM) method to observe the electrode reaction in the IL-based Li-ion secondary battery (LIB). When 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide ([C2mim][FSA]) with lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) was used as the electrolyte, the Si negative electrode exhibited a clear morphology change during the charge process, without any solid electrolyte interphase (SEI) layer formation, while in the discharge process, the appearance was slightly changed, suggesting that a morphology change is irreversible in the charge-discharge process. On the other hand, the use of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][TFSA]) with Li[TFSA] did not induce a change in the Si negative electrode. It is interesting to note this distinct contrast, which could be attributed to SEI layer formation from the electrochemical breakdown of [C2mim](+) at the Si negative electrode|separator interface in the [C2mim][TFSA]-based LIB. This in situ SEM observation technique could reveal the effect of the IL species electron-microscopically on the Si negative electrode reaction.
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