Figure 3. Top (left panel) and cross-sectional (right panel) SEM images of electrodeposited Li films on Cu substrates in different states with two electrolytes: pristine electrolyte (E1) after initial charged (a), first discharged (b) and 100 cycles (c); optimized electrolyte (E2) after initial charged (d), first discharged (e) and 100 cycles (f).
EIS test was performed to understand the electrolyte/electrode interfacial behavior. The EIS spectra of the cells with E1 and E2 electrolytes were measured before cycling, after first charging, and after the 1st and 10th cycles. The Nyquist plots of the cells with E1 and E2 electrolytes are shown in Figure 4. They are with one semicircle at the high frequency, attributed to the Li+ diffusion through the SEI on the surface of the electrode.[33] Figure 4a-d shows the impedance of the cells with E1 and E2 electrolytes increase continuously with cycling. The impedance increase is due to the continuous generation of SEI as well as the formation of dead Li metal. However, the cell with the E2 electrolyte has a lower impedance than that with the E1 electrolyte (Figure 4c-d). The lower impedance of E2-based cells suggests that the dual additives can effectively reduce the cell’s impedance and thus improve cell stability. Dual additives reduce the barrier of Li nucleation and guide the Li metal to grow in structures with a smoother and denser morphology. Moreover, introducing F atoms makes the SEI layer contain more LiF inorganic components. That makes the formed SEI layer more stable and contributes to reducing the cell’s impedance, thus effectively improving the stability of the anode-free cell.