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.