Figure 1. Electrochemical performance of Cu||Li cells at 0.5 mA·cm-2 current density with E1 and E2. (a) Voltage profiles with E1 and E2; Polarization profile of plating/stripping process using E1 (b) and E2 (c); (d) Coulombic efficiency curves.
In contrast, under the same current density, the polarization voltage was 37 mV and 45 mV at the 2nd and 10th cycles for the cells with E1 electrolyte. The cells with E1 exhibit more significant polarization than E2 due to the faster growth of Li dendrites.[24, 25] The Li dendrites growth increases resistance due to an additional amount of SEI and dead Li metal formation. That leads to a decline in CE and capacity. Because the Li metal anode side has a surplus of Li-ions in the half Cu||Li cell, the CE indicates the Li loss on the Cu current collector due to the decomposition of electrolytes. The CE performance of Cu||Li cells at 0.5 mA·cm-2 for both E1 and E2 electrolytes is shown in Figure 1d. The initial CE of the cells with E2 electrolytes was 90% at 0.5 mA·cm-2, which is higher than the initial CE of the cells with E1 electrolytes (80%). Furthermore, after a few cycles, the CE of the cells with E1 electrolytes declined, and fluctuations started. In comparison, the CE is more stable and higher in cells with E2 electrolytes. Additionally, the capacity of Cu||Li cells with E2 electrolytes is more stable than that of cells with E1 electrolytes.
The NCM||Cu full AFLBs were also assembled with bare Cu as the anode current collector and LiNi0.5Co0.3Mn0.2O2(NCM532) as the cathode. E1 and E2 electrolytes were used to evaluate the full-cell performance. The tests show that the capacity loss of AFLBs is only around ∼14% in the first cycle, regardless of the electrolytes E1 and E2 (Figure 2a). This performance is much higher than the results with carbonate solvent (EC/EMC (3:7 wt %)), giving 77% capacity loss after the first charge,[26]. The initial discharge capacity of NCM||Cu cells with E2 is 3.98 mAh·cm-2. It is higher than the cells with E1 electrolytes (3.85 mAh·cm-2). With cycling, the capacity of NCM||Cu cells with E1 drops quickly. After 100 cycles, the discharge capacity decreased to 0.5 mAh·cm-2, much lower than that of cells with E2 (~2 mAh·cm-2). The capacity retention (Figure 2b) in E1 is below 15% after 100 cycles, comparable with earlier reported results.[25] The low CE of the cells with E1 electrolyte can be explained by the depletion of active Li metal due to the presence of free solvents. These solvents continuously react with active Li-ions during the continuous plating/stripping processes. Consumption of electrolytes could be another reason because the SEI layer formed in traditional carbonate electrolytes originates from the decomposition of electrolyte solvents.[27] On the contrary, the NCM||Cu cells with optimized electrolyte (E2) retained 75% after 50 cycles (Figure 2b) and even higher than 50% after 100 cycles. The E2-based cells exhibit a higher average CE of 98.3% after 100 cycles. From the first CV curves (Figure 2c), it was found that the current in the CV curve of E2 is higher than for E1 for the same voltage range (3 to 4 V). That confirms the presence of electrochemical reactions of the dual additives in this voltage range. The decomposition of the dual additives facilitates the formation of a more stable SEI layer and, therefore, a significant improvement in the stability of the anode-free cell. Figure 2d shows the influence of the current (0.1, 0.2, 0.5, 0.75, and 1 C-rate) on the discharge capacity of the anode-free cells with two different electrolytes. The initial discharge capacity of 4 mAh·cm-2 for the NCM electrode at 0.1 C-rate is reduced to 2 mAh·cm-2 at 1 C-rate. However, the capacity can almost be fully recovered to 3.5 mAh·cm-2at 0.1 C-rate after 15 cycles, indicating a better rate capability than the cells using E1 electrolytes. It is worth noting that the rate capability of the sample with E2 electrolytes is systematically better than for E1 electrolytes.
To further understand the synergetic effect of the dual additives of LiAsF6 and FEC on stabilizing the electrochemical performance of AFLBs, the Li nucleation overpotentials were measured. The Li nucleation overpotential probes the Li metal nucleation and growth behavior on Cu current collectors. The three-electrode system was used for measuring Li nucleation overpotentials with Cu as the negative electrode, NCM532 as the positive electrode, and Li foil as the reference electrode. The voltage profiles of Li metal deposited onto bare Cu foil during the first charge process in two different electrolytes are shown in Figure 2e-f. There is a significant voltage dip at the beginning of Li metal deposition (blue dashed circle marked), which is the Li nucleation overpotential. A flat voltage plateau follows this dip. The Li nucleation overpotential is defined as the difference between the bottom of the voltage dip and the flat part of the voltage plateau.[28] The voltage difference represents the energy necessary to overcome the heterogeneous nucleation barrier due to the large thermodynamic mismatch between Li metal and Cu. For E1, the overpotential is around 38 mV. In contrast, the overpotential for E2 is only 30 mV, indicating a lower nucleation barrier for Li metal with the dual additives of LiAsF6 and FEC.[29] The dual additives facilitate the deposition of Li metal on the Cu surface. That is also beneficial to form more compact Li dendrites.