Figure 2. Electrochemical performance of
NCM||Cu cells at 0.5 mA·cm-2 current
density with E1 and E2. (a) Voltage profile using E1 and E2; (b)
Capacity retention and CE curves; (c) CV curves; (d) Rate capability at
various indicated C-rates; Voltage profiles of galvanostatic Li
deposition on a copper substrate at mA·cm-2 with E1(e)
and E2 (f). \(E_{\text{we}}\) refers to the potential of the Cu current
collector. A clear overpotential was observed, as circled by the dashed
line. The inset shows the overpotentials of Li nucleation.
The top and cross-sectional morphological analyses of the deposited Li
metal were performed. Figure 3a shows the morphology of the deposited Li
metal on Cu with E1 electrolyte. The top and cross-section show the
presence of tiny block crystal structures. After discharging, the
crystal structures disappeared, and the residual Li metal became porous
and non-compact (Figure 3b). After 100 cycles, many cracks are observed
on the surface of the residual Li metal (Figure 3c). Such cracks suggest
that the instability of the SEI is related to electrolyte composition.
The thickness of the Li residues is increasing, which is attributed to
the formation of dead Li metal and SEI layers, resulting in rapid
capacity loss.
In contrast, introducing dual additives makes the deposited Li
morphology after the initial charge more uniform (Figure 3d). It can be
related to the lower overpotential of Li nucleation, which is favorable
for the larger size of Li nuclei. The formed Li-metal structures in
E2-based cells are more compact than those with E1 (Figure 3e). More
importantly, the residual Li-metal structures in E2 cells exhibited
fewer cracks on the surface and were more compact after 100 cycles. In
addition, the thickness of the residue for E2 is smaller than for E1
(Figure 3f), suggesting that introducing dual additives improves the
stability of the formed SEI layers and suppresses the formation of dead
Li metal.
EDX measurements on the residual Li metal after 100 cycles were
performed to investigate further the effect of the dual additives on SEI
formation. As shown in Figure S1, C, O, F, and P elements were found in
the residuals (Figure S1a). These are attributed to the components of
formed SEI, such as Li2O,
Li2CO3, LiF,
LiPO2F2, and carbonate-based Li
salts.[30, 31] However, in comparison with the E1
electrolytes, the residues with dual additives contain arsenic, and the
content of F is significantly higher (Figure S1b). This observation
indicates that the additives also participate in forming SEI layers
during cycling. The dual additives promote the formation of
LixAs alloy phase and LiF on the formed Li-metal
surface,[32] enhancing the mechanical properties
of the formed SEI layers, improving the stability.