Figure 4. EIS curves of anode-free NCM||Cu full cells that contains E1 and E2 before cycling (a), first charged (b), after first(c) and 100th (d) cycles.
To further demonstrate the effect of dual additives on the Li deposition/dissolution and SEI formation, in situ Raman was performed on the NCM||Cu cells with two different electrolytes (E1 and E2). The geometry of the in situ Raman cell is illustrated in Figure S2. NCM532, with a 10 mm diameter, was used as a cathode. A glass fiber separator soaked with 50 μL E1 electrolytes was placed on top of the cathode. 16 mm Cu mesh was placed on top of the separator. Then the glass window was pressed on the top. The cell was cycled at C/10 between 4.3 V and 3.0 V. Figure 5a shows the morphology of the formed Li metal structures during the first cycle. Initially, the Cu surface is shiny and smooth. After the first 30 min of plating, it was found that the deposited Li nuclei on the upper Cu surface were inhomogeneous (30-min image in Figure 5a). With Li plating proceeding, the plated Li metal show a porous structure (2h and 10 h image in Figure 5a). During the Li deposition process, the growth of inhomogeneous Li nuclei results in different Li growth rates at different locations. Then the Li metal grows into the free space with time. The deposited Li metal exhibit a highly porous structure due to the uniform Li growth process when the formed Li-metal structures fill the free space. As the Li stripping process begins, the dissolution rate of Li metal is also inconsistent at various points. That facilitates the formation of Li dendrites and dead lithium. It can be observed that when discharged to 3V, there is still a lot of Li metal on the copper surface as well as dendritic Li metal. That causes a significant initial capacity loss.
The potential curve of the first cycle and the corresponding in situ Raman spectra are shown in Figure 5b. With cycling, a peak at 1846 cm-1 starts to be observed, which is formed due to the symmetric stretching vibration of the triple bond of the acetylide anion in Li2C2.[34, 35]In addition, the signal at 1070 cm-1 is attributed to the symmetric stretching vibration of the carbonate anion from Li2CO3.[36]Li2CO3 is an SEI product originating from the decomposition of the organic carbonates in the electrolyte.[37] During the charging process, the intensity of SEI-induced spectra (1070 and 1846 cm-1) continuously grows. The intensity increase is due to the continuously formed SEI caused by the reaction between the electrolyte and newly-deposited Li metal. More importantly, during the discharge process, the SEI does not disappear, even when discharged to 3.0 V, which is another significant cause of the large decay of initial capacity and CE. The broad signals at 470 and 910 cm-1might be attributed to various origins. These signals could be from the electrolyte, e.g. the ring breathing modes of EC-lithium complexes R-O and C-C stretching.[35] Another possibility could be the decomposition of the polymer-based SEI, known as organic SEI.[37]
Figure 5c shows the optical microscopy images of Li metal plating/stripping behavior with E2 electrolyte. In the first 30 min of Li plating (Figure 5c), the Li nuclei were preferably deposited on the bottom of the copper surface. However, when charging to 4.3V, only a small amount of Li metal was found on the upper blank parallelogram area. That indicates more compact Li-metal structures formed on the bottom of the copper surface. In the stripping process, less rod-shaped Li metal was left. In addition, when discharged to 3V, the earlier-formed Li-metal structures on the upper Cu surface disappeared. This observation confirms that the dual additives improve Li plating and stripping reversibility. Dual additives facilitate the Li metal to form a more compact and smooth Li morphology.
Figure 5d shows in situ Raman spectra recorded during the first cycle. Similarly, SEI peaks at 1070 and 1846 cm-1appeared during the charging process. The SEI peaks can still be observed after discharge to 3.0 V, indicating that the formed SEI layers are stable. That could be well-protected plated Li metal. Compared to the E1 electrolyte, E2 can increase LiF content in the SEI components, thus improving stability. The theoretical Raman signal of LiF is at 228 cm-1. However, the signal is very weak and can hardly be traced in the Raman spectra because the formed LiF layer is very thin.[38] Nevertheless, in situ Raman measurements could effectively resolve the origins of the high-capacity loss and low CE of NCM||Cu cells by observing the formation of dead Li metal and SEI layers. The dual additives effectively guide the Li deposition in planar surfaces to form a more compact and smoother Li metal anode, which improves the stability of AFLBs.