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Page 4 of 14 Wang et al. Energy Mater 2023;3:300040 https://dx.doi.org/10.20517/energymater.2023.28

polyvinylidene difluoride at a weight ratio of 80:10:10. The pastes were cast onto Celgard 2400 separators to
2
form electrodes. The mass loading of graphite was controlled at ≈30 mg/cm , and the mass loading of
2
NCM811 was ≈18 mg/cm . Three layers of glass fiber (MA-EN-SE-01, Canrd) were used as the separators.
Li||NCM811 cells were cycled from 2.5 to 5.0 V, and Li||graphite cells were cycled from 2.5 to 0 V.

Characterizations
After 100 times cycling, the cells were disassembled in a glove box, and all the electrodes were washed with
the corresponding solvent (DME for TTME-d and DME-d systems, DEC for carbonate system) three times
before characterizations. The samples were transferred for characterizations under an Ar atmosphere
(without air contact). X-ray photoelectron spectroscopy (XPS) with a 200 μm X-ray gun source was
performed on an Escalab Xi+ (Thermo Avantage), and the C 1s signal of 284.8 eV was set as an internal
standard to correct the binding energies. The morphologies of graphite and cathode electrodes were
characterized with a field emission scanning electron microscope (SEM) (TESCAM MIRA3). Aberration-
corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM)
images were taken on Titan Cubed Themis G2 300. X-ray diffraction (XRD) patterns were characterized
with Rigaku SmartLab equipped using a Cu Kα radiation source in the 2θ range from 0 to 90°, with a speed
of 10°/min.

RESULTS AND DISCUSSION
Through the DFT calculation, the highest occupied molecular orbital (HOMO) energy value and the lowest
unoccupied molecular orbital (LUMO) energy value of the different solvents and additives were
investigated. As shown in Figure 1, the LUMO energy level of TTME (1.491 eV) is slightly lower than that
of DME (2.316 eV) but still higher than that of general carbonate solvents and additives, indicating its
strong reduction stability on the anode side. In addition, due to the high fluorine (F) substitution of the
TTME molecule, its HOMO energy level (-7.758 eV) is obviously lower than those of traditional ether
molecules and even lower than that of some carbonate solvents and additive molecules (DEC: -7.2779 eV,
VC: -6.999 eV). This shows that the substitution of hydrogen (H ) by F can improve the oxidation stability
2
of ether molecules.

For DME, although its reduction stability is the best among these solvents and additives, it would easily co-
intercalate with graphite during charge/discharge, resulting in the stripping of graphite and the
decomposition of the electrolyte [12-14] . This problem can be clearly seen from the CV curves in Figure 2A. As
the discharge progresses, the CV curve of the Li||graphite half-cell using the ether electrolyte begins to show
peaks when the voltage drops to ≈1.2 V, and as the discharge process continues, the peaks become larger,
and the shape of the curves is irregular. During the delithiation process, the internal reactions of the cell
with the ether electrolyte are also extremely unstable. As the voltage increases, small peaks not for the
charge-discharge platform continue to appear in the CV curve. In the subsequent four cycles, such irregular
small peaks are always observed, and none of them overlaps, indicating that various side reactions
constantly occur. This shows that DME cannot form a stable SEI film on graphite when it is solely used, and
the co-intercalation process with graphite leads to the collapse of the graphite structure and the
decomposition of the electrolyte.

The electrolyte developed in this work contains two ethers, DME and TTME, and the addition of TTME as
the co-solvent prevents the occurrence of the above problem. Considering the entire electrolyte system, the
addition of TTME weakens the interaction between DME solvent and Li ions, enhancing the desolvation
+
[19]
process of Li ions, which would be beneficial for inhibiting the co-intercalation with graphite . As can be
+
seen from the CV curves of the Li||graphite half-cell using the fluorinated ether electrolyte in Figure 2B,