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.