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.