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Wang et al. Energy Mater 2023;3:300040 https://dx.doi.org/10.20517/energymater.2023.28 Page 9 of 14

Figure 4B shows that the DCIR of the cell with the fluorinated ether electrolyte is larger than that of the cell
with the carbonate electrolyte during the formation stage, and the growth rate of DCIR of the former is
much lower than that of the latter during the subsequent cycles. This indicates that in the cell with the
fluorinated ether electrolyte, both the CEI on the cathode and the SEI on the anode have been formed in the
formation stage, and both the interphases remain stable in the subsequent cycles, which are the key to the
cycling stability of the pouch cells with the fluorinated ether electrolyte. In contrast, the DCIR of the cell
with the carbonate electrolyte shows a large increase from the electrolyte assembly (formation) to the first
cycle and continues to increase in the subsequent cycles. This indicates that the complete formation of CEI
and SEI with this carbonate electrolyte occurs only after the completion of the first charge/discharge cycle.
Moreover, the stabilities of these two interphase layers are insufficient, as the carbonate electrolyte is
continuously decomposed, accompanied by the formation of new interphase layers.

Figure 4C shows that the cycle results at high temperatures (45 °C) are similar to those at room temperature
and that the cell with the fluorinated ether electrolyte still has higher first-cycle Coulombic efficiency and
capacity retention. In addition, it can be seen from Supplementary Figure 3 that although the initial DCIR of
the cell with the fluorinated ether electrolyte is slightly higher than that of the cell with the carbonate
electrolyte at 45 °C, the DCIR growth rate of the former (12.5% after 300 cycles) is smaller than that of the
latter (31.1% after 300 cycles). Besides, Figure 4D presents a comparison of DCIR cycled at different
temperatures, wherein it can be seen that the DCIR of the cells with the two control electrolytes increases
significantly at 0 °C, which is related to the limited internal charge transfer and lithium ion migration at low
temperatures. However, by comparison, the impedance value of the cell with the fluorinated ether
electrolyte is still smaller than that of the cell with the carbonate electrolyte, indicating that the fluorinated
ether electrolyte has application potential at low temperatures.

In order to further explore the gas production from the different components in the electrolyte during the
formation of the cathode and anode interphases, DEMS measurements were conducted on the different
electrolyte systems with Li||graphite and Li||NCM811 cells. As shown in Figure 5A, the Li||graphite cells
using both the fluorinated ether and carbonate electrolytes periodically generate H as the discharge process
2
progresses, while the cell with the ether electrolyte does not. Combined with the analysis of cell cycle
performance, it can be inferred that the generation of H is related to the formation of the SEI on the
2
graphite side in the fluorinated ether electrolyte. The addition of the TTME molecule enables the
fluorinated ether electrolyte to effectively generate a SEI, which facilitates the migration of lithium ions
towards the graphite side. The addition of film-forming additives makes the peak of H more noticeable in
2
the carbonate electrolyte group, and the highest peak in the first cycle can reach 9 nmol/min. However, the
ether electrolyte system does not produce H periodically because of the occurrence of irregular side
2
reactions on the graphite side.

The source of H is the reduction of proton hydrogen in the electrolyte. Some of the proton hydrogen comes
2
from trace amounts of water and alcohol impurities in the electrolyte, while the other part comes from the
decomposition of solvents . The proposed formation mechanism of H is displayed in
[27]
2
Supplementary Scheme 1. As for the ether electrolyte system, the possible reason for less H generation is
2
that the fragmentation of the graphite structure creates many binding sites for C atoms, which bind to
proton hydrogen in the electrolyte. It can be inferred from Figure 5B that the co-intercalation of DME with
graphite and the accompanied structural exfoliation and electrolyte decomposition process are the main
sources of CO gas generation. Previous reports suggest that the decomposition of EC is accompanied by the
[28]
generation of a small amount of CO . It can be inferred that the source of trace CO production in the
other two cells with the ether and fluorinated ether electrolytes is a consequence of the slight decomposition