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Page 10 of 18 Yang et al. Energy Mater 2023;3:300029 https://dx.doi.org/10.20517/energymater.2023.10

capacity retention of 51.6% after 1,000 cycles at the charge/discharge rate of 2 C/5 C, compared with
capacity retention of only 6% after 1,000 cycles in the base electrolyte with a more rapid capacity decay
during the first 300 cycles.

More details about the evolution of structure were investigated by XRD (in/ex-situ XRD). The evolution of
the (003) peak in in-situ XRD is related to the variation of the unit cell along the c-axis direction [21,76] . It can
be noted from the results of in-situ XRD [Figure 4A and B] that the interlayer spacing along the c-axis
direction expands as Li is continuously extracted (with charging voltage ≤ 4.16 V) and responds by the shift
+
of the (003) peak to a lower angle. When the charging potential is higher than 4.16 V, the sharp shift of the
(003) peak to a high angle indicates a rapid contraction of the layer spacing along the c-axis, corresponding
to the structural transformation from the H2 phase to the H3 phase, which leads to an increase in internal
[77]
stress and structural degradation . For the battery cycled in the LiDFOB-containing electrolyte, the
evolution angle of the (003) peak during the transition from the H2 phase to the H3 phase is observed to be
lower than that in the base electrolyte [Figure 4A and B], especially when the current rate increases from the
initial 0.1 C to 0.2 C in the second cycle (1.0774° vs. 1.1820°). This result indicates that the LiDFOB-derived
CEI film is robust enough to mitigate the irreversible structural degradation of the cathode, which could be
beneficial for improving the cycling stability of the battery. Similar conclusions can be drawn by testing the
structural evolution before and after long cycles by ex-situ XRD [Supplementary Figure 7]. The variation
range of the (003) peak in the base electrolyte (0.35°) after 300 cycles is larger than that in LiDFOB-
containing electrolyte (0.28°), suggesting the superior structural reversibility of the NCM85 cathode cycled
in the LiDFOB-containing electrolyte.

The phase transition process at high voltage is often accompanied by the oxidation of lattice oxygen to form
-[78]
-
O . The high catalytic activity of O and the released oxygen will exacerbate the electrolyte decomposition,
accompanied by the generation of CO . Therefore, the CO content obtained by in-situ DEMS is used to
[79]
2
2
monitor the gas evolution at the electrodes during cycling. It can be observed from Figure 4C that the
battery cycled in the base electrolyte displays a sharply rising CO signal when the charging cut-off voltage is
2
higher than 4.5 V, suggesting the intensification of the lattice oxygen loss and electrolyte decomposition. In
comparison, the CO evolution in the LiDFOB-containing electrolyte is significantly suppressed, indicating
2
that the introduction of LiDFOB can effectively stabilize lattice oxygen and suppress interfacial side
reactions [Figure 4D].
The morphologies and interfacial components of the cycled NCM85 electrode in the base and LiDFOB-
containing electrolytes were analyzed by the SEM (top-view and cross-section), TEM, and XPS, as shown in
Figure 5. Compared with the fresh NCM85 electrode with an intact structure and clean and smooth surface
[Figure 5A-C], severe interior cracks with partial crystal fragments can be observed on the particles after 200
cycles in the base electrolyte, and the electrode surface is also covered with an amorphous and
inhomogeneous interfacial film with a thickness of about 30-60 nm [Figure 5D-F], which mainly results
from the structural disruption of the NCM85 crystal and the deposition of electrolyte decomposition
products. In contrast, the morphology of the NCM85 cathode cycled in LiDFOB-containing electrolyte is
well maintained, and only a few cracks are discernible [Figure 5G and H]. Moreover, a relatively thin and
uniform CEI film derived from the LiDFOB additive was observed to cover the surface of the NCM85, with
a thickness of about 10 nm [Figure 5I], which can effectively protect the NCM85 electrode from electrolyte
erosion and particle cracking. In C 1s spectra of the XPS spectra [Figure 5J], the peaks located at 290 eV,
288.5 eV, 286.5 eV, and 284.7 eV are assigned to the Li CO , C=O, C-O, and C-C species, respectively, which
3
2
are mainly formed by the decomposition of organic solvent molecules (EC/EMC) in electrolyte [80-82] . The
deposits of C-O and C=O can also be observed at 533.4 eV and 531.8 eV in the O 1s spectra. The peaks