the redox reactions of the electrolyte system.
The CV curve of the cell with pristine Al 2 O 3
shows two cathodic peaks at ~1.56 and ~0.65 V
corresponding to reduction of LiNO 3 and
solvent, respectively ( 33 ). In addition to these
two peaks, one pronounced cathodic peak at
~1.11 V is observed in the CV curves of cells
with Al 2 O 3 -SAMs, which is assigned to the
reduction of TFSI−(Fig. 3A) ( 34 ). This peak of
TFSI−reduction attenuates greatly in the sub-
sequent cycles (fig. S20). The CV results dem-
onstrate that SAMs facilitate the preferential
reduction of LiTFSI. Impedance tests are also
performed on the cells before cycling and after
10, 50, and 100 cycles. The impedance of the
cell employing Al 2 O 3 -OOC(CH 2 ) 2 COOH re-
mains relatively stable after 100 cycles (Fig. 3B).
By contrast, both cells with Al 2 O 3 and Al 2 O 3 -
OOC(CH 2 ) 2 NH 2 experience more substantial
impedance increase (fig. S21 and table S1).
The impedance evaluations show that Al 2 O 3 -
OOC(CH 2 ) 2 COOH can deliver optimum effi-
ciency in enhancing interfacial stability.
We next investigate the chemical compo-
sition of the SEI by applying XPS on the Li
deposits after the first discharge (Fig. 3, C to
E). In the C 1s spectra, the peaks assigned to
C–C, C–O, C=O, C=O–C, and CF 3 originate from
the decomposition of the electrolyte. In the F 1s
spectra, there are two peaks of LiF and CF 3
species at 684.8 and 688.6 eV, respectively ( 35 ).
A high percentage of LiF (6.9%) is detected onthe surface of Li deposited in the presence of
Al 2 O 3 -OOC(CH 2 ) 2 COOH. In comparison, the
percentages of LiF are only 4.2 and 3.8% in
the cells equipped with Al 2 O 3 -OOC(CH 2 ) 2 NH 2
and Al 2 O 3 , respectively. If HOOC(CH 2 ) 2 COOH
molecules are added to the electrolyte as an
additive instead of forming ordered SAMs,
the Li deposit has only 4.2% LiF (fig. S22). In
addition, the XPS depth profiles of the Li
deposited with Al 2 O 3 -OOC(CH 2 ) 2 COOH show
that LiF appears on the SEI surface and its
content ratio gradually increases up to the
surfaceoftheLimetalanode(fig.S23).The
results indicate that the LiF generated by
SAMs can efficiently passivate the reactive
surfaces to reduce the initial side reaction
and can improve Li+diffusion throughout the
SEI ( 36 ). Moreover, as the temperature is in-
creased, the degradation kinetics may accel-
erate to facilitate LiF formation (fig. S24).
In our recent studies, we have investigated
the SEI in Li metal anodes at subangstrom
resolution using cryo–transmission electron
microscopy (cryo-TEM) ( 28 , 37 – 39 ). In this
study, we employ the same technique to di-
rectly visualize the nanostructure of the SEI.
Metallic Li is deposited on a Cu grid with a
capacity of 1 mA·hour cm−^2 at a current den-
sity of 1 mA cm−^2 for cryo-TEM observation.
In the presence of SAMs, the deposited Li
appears homogeneous with spherical mor-
phologies(Fig.4Aandfig.S25).VariousLi
deposits are visualized by the cryo–scanning
transmission electron microscopy (cryo-STEM),
and the corresponding distributions and
amounts of typical elements—such as C, O,
and F—in the SEI are analyzed (Fig. 4, B and C,
and figs. S25 to S28). Usually, the content of F in
the SEI induced by Al 2 O 3 -OOC(CH 2 ) 2 COOH is
much higher than those induced by Al 2 O 3 -
OOC(CH 2 ) 2 NH 2 and pristine Al 2 O 3 (Fig. 4D).
However, the SEI generated by the dispersed
HOOC(CH 2 ) 2 COOH additive without forming
ordered SAMs contains a relatively low amount
of F (fig. S29). Therefore, we have obtained high-
resolution TEM (HRTEM) images of the spe-
cific F-enriched SEI nanostructure generated
by Al 2 O 3 -OOC(CH 2 ) 2 COOH. The SEI exhibits
a classical mosaic structure that consists of an
amorphous phase and embedded Li, Li 2 O,
LiOH, and LiF nanocrystals (Fig. 4E). The
crystalline phases of Li, LiOH, and Li 2 O are
confirmed by matching the long-range–ordered
lattices with their known lattice planes (Fig. 4,
FtoH)( 39 ). Specifically, the calibrated inter-
planar spacing of 2.48 Å well matches the (110)
plane of metallic Li (Fig. 4F). More notably,
LiF nanoparticles with lattice corresponding
to the (111) plane can be clearly detected in the
SEI (Fig. 4I and fig. S30). In summary, both
the XPS and cryo-TEM results confirm the dis-
tribution and enrichment of LiF nanocrystals
in the Al 2 O 3 -OOC(CH 2 ) 2 COOH-induced SEI,
consistent with the simulated prediction.742 18 FEBRUARY 2022•VOL 375 ISSUE 6582 science.orgSCIENCE
Fig. 3. Analysis of the interfacial stability and SEI chemical composition.(A) First-cycle CV curves of
cells equipped with Al 2 O 3 , Al 2 O 3 -OOC(CH 2 ) 2 NH 2 , and Al 2 O 3 -OOC(CH 2 ) 2 COOH. (B) Electrochemical impedance
spectroscopy spectra of the cell equipped with Al 2 O 3 -OOC(CH 2 ) 2 COOH before cycling and after 10, 50, and
100 cycles. Z′, real part of complex impedance; Z′′, imaginary part of complex impedance; Rct, charge-transfer
resistance; Rb, bulk resistance; Wo, Warburg impedance; CPE, constant phase element. (CtoE) XPS spectra
of the SEIs in cells with (C) Al 2 O 3 , (D) Al 2 O 3 -OOC(CH 2 ) 2 NH 2 , and (E) Al 2 O 3 -OOC(CH 2 ) 2 COOH.
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