Al 2 O 3 -OOC(CH 2 ) 2 COOH remains uniform,
whereas the morphology of the Li deposited
with Al 2 O 3 becomes irregularly dendritic with
locally aggregated sediments after 200 cycles
(fig. S14). In addition, the thickness of deposits
is related to the generated inactive Li debris
termed“dead Li.”After 50 cycles, the thickness
of the deposit with Al 2 O 3 -OOC(CH 2 ) 2 COOH
is found to be ~36mm, smaller than those
with Al 2 O 3 -OOC(CH 2 ) 2 NH 2 (44mm) and Al 2 O 3
(69mm), indicating that Al 2 O 3 -OOC(CH 2 ) 2 COOH
can reduce the amount of dead Li (fig. S15).
Therefore, SAMs with ordered terminal groups,
especially Al 2 O 3 -OOC(CH 2 ) 2 COOH, are dem-
onstrated to greatly improve the CE and ex-
tend the cycle life of half cells.
Simulation of functional mechanism of SAMs
Density functional theory (DFT) and ab initio
molecular dynamics (AIMD) calculations are
carried out to explore the relationship between
the oriented functional groups of SAMs and
the electrochemical process. First, we illus-
trate the lowest unoccupied molecular orbital
(LUMO) and highest occupied molecular or-
bital (HOMO) energy levels of related mole-
cules (fig. S16), which are associated with the
electrolyte reaction activity in terms of frontier
molecular orbital theory ( 26 ). Our calculations
show that apart from the low-concentration
LiNO 3 , LiTFSI has the lowest LUMO energy
level (−1.03 eV) and can therefore be easily
reduced (i.e., accept electrons) during the bat-
tery cycling operation.
We construct four configurations based on
the corresponding experimental conditions
(see the supplementary materials for meth-
odological details) to capture the thermody-
namic behavior of LiTFSI. During the AIMD
equilibration process up to 12 ps, the forma-
tion of LiF is observed only in the case of
oriented carboxyl groups (fig. S17). To reveal
the LiF formation mechanism, we trace all
LiTFSI decomposition steps through the AIMD
simulation time (Fig. 2, C to F). The Bader
charge analysis reveals that ~−1|e| charge
is transferred from the carboxyl group to
the TFSI−anion at 50 fs, leading to cleavageof the N–S bond, as described in reaction
Eq. 1 ( 27 )N(SO 2 CF 3 ) 2 −+e−→NSO 2 CF 3 −+
SO 2 CF 3 − (1)Subsequently, the CF 3 −group is removed from
the SO 2 CF 3 −fragment at 200 fs, owing to the
acquisition of one electron (e−). This CF 3 −
group is further decomposed into CF and
F−at 450 fs, eventually resulting in LiF for-
mation. These reactions can be described as
followsSO 2 CF 3 −+e−→SO 2 −+ CF 3 − (2)CF 3 −+e−→CF + 2F− (3)F−+ Li+→LiF (4)This LiTFSI decomposition mechanism is in
good agreement with previous simulation
results ( 28 , 29 ). We attempt to understand
the role that high-density and highly ordered
functional groups play in this process. Elec-
trostatic potential and differential charge den-
sity calculations show that the directed polar
functional groups attract electrons from the
Al 2 O 3 substrate to the electrolyte (Fig. 2, G
and H). The red electrostatic potential isosur-
face indicates electrophilicity, and the blue
isosurface represents nucleophilicity ( 30 ). The
dipole moment induced by carboxyl groups
(higher molecular polarity compared with
amino groups) facilitates transfer of more
electrons from the Al 2 O 3 slab to the electro-
lyte environment than that induced by amino
groups (smaller molecular polarity) ( 31 ). Fur-
thermore, the differential charge density be-
tween Al 2 O 3 and functional groups shows more
obvious charge transfer from Al 2 O 3 to carboxyl
groups compared with pristine Al 2 O 3 or amino
groups (fig. S18), consistent with our electro-
static potential calculation results. In addi-
tion to the effect of induced dipole moment,
the dipole-dipole interaction between free ether-
based solvent molecules and carboxyl group–
terminated SAMs that can mitigate solvent
decomposition is also taken into consider-
ation ( 32 ). Detailed DFT calculations show
that such interaction may not be suitable for
the low-polarity ether-based electrolyte used
here (fig. S19). In short, the ordered orienta-
tion of the polar functional groups on the
Al 2 O 3 surface enables the same dipole direc-
tion, which attracts electrons from Al 2 O 3 to
the electrolyte environment. These excess
electrons thus facilitate the decomposition
of LiTFSI into F−species, finally forming LiF.Characterization of the LiF-enriched SEI
In view of the simulated predictions of SAM-
induced LiTFSI decomposition, we perform
cyclic voltammetry (CV) analysis to examineSCIENCEscience.org 18 FEBRUARY 2022•VOL 375 ISSUE 6582 741
Fig. 2. Electrochemical performances of LiÐCu half cells and simulations of the degradation mechanism
of LiTFSI.(A) CE comparison of the anodes in three 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 at current densities of 1, 2, and 3 mA cm−^2 with an areal capacity of 1 mA·hour cm−^2.
(B) Voltage profiles of the cell equipped with Al 2 O 3 -OOC(CH 2 ) 2 COOH at 1 mA cm−^2 of the 100th
and 200th cycles. (CtoF) Snapshots of AIMD simulations with instantaneous Bader charges (the unit
of charge is |e|) illustrating the degradation dynamics: (C) 0 fs, (D) 50 fs, (E) 200 fs, and (F) 450 fs.
(GandH) Electrostatic potential plotted on the isosurface ofr= 0.2|e| bohr−^3 (r, charge density)
of oriented (G) Al 2 O 3 -OOC(CH 2 ) 2 COOH and (H) Al 2 O 3 -OOC(CH 2 ) 2 NH 2.
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