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.