alignment of SAMs (fig. S6). These results all
indicate that the specific molecules are suc-
cessfully grafted onto the surface of the Al 2 O 3 -
coated separator to generate SAMs with ordered
terminal groups.
Li plating and stripping behaviors with SAMs
To confirm the effect of SAMs on Li deposition,
electrochemical Li plating and stripping experi-
ments are carried out using typical coin cells of
bare Li and Cu coupled with an Al 2 O 3 -SAMs
separator. The results are plotted as coulombic
efficiency (CE) versus cycle number of cells at
different current densities with the same
plating capacity of 1 mA·hour cm−^2 (Fig. 2A).
Specifically, the cells equipped with Al 2 O 3 -
OOC(CH 2 ) 2 COOH and Al 2 O 3 -OOC(CH 2 ) 2 NH 2
exhibit stable CEs of ~97.7 and ~95.3%, respec-
tively, over 300 cycles under a current density
of 1 mA cm−^2. By contrast, the cell with bare
Al 2 O 3 shows inferior cycle life, and the CE
fades to 74.2% after 220 cycles. More specif-
ically, the cell with Al 2 O 3 -OOC(CH 2 ) 2 COOH is
stable with the overpotential of only ~15 mV
(Fig. 2B). When the current density is in-
creasedto2and3mAcm−^2 , the cells using
Al 2 O 3 -OOC(CH 2 ) 2 COOH also deliver more sta-
ble electrochemical cycling and still maintain
high CEs of 99.2% (2 mA cm−^2 ) over 300 cycles
and 99.4% (3 mA cm−^2 ) over 150 cycles, and
the cells using Al 2 O 3 -OOC(CH 2 ) 2 NH 2 have CEs
of 93.6% (2 mA cm−^2 ) after 200 cycles and
93.5% (3 mA cm−^2 ) after 150 cycles. By con-
trast, the cells with Al 2 O 3 exhibit rather un-
satisfactory cycling stability, showing CEs of
82.4% (2 mA cm−^2 )atthe200thcycleand70.9%
(3 mA cm−^2 )atthe150thcycle(Fig.2A).Asa
comparison, the individual HOOC(CH 2 ) 2 COOH
or HOOC(CH 2 ) 2 NH 2 molecules are directly
added into the electrolyte as additives, andthese dispersed molecules fail to form SAMs
because of the weak solvent polarity of the
ether-based electrolyte (figs. S7 and S8). The
CE of the cells using both individual additives
is apparently lower than that of the cell with
ordered SAMs (fig. S9). We also find that the
typical FTIR signals representing the inter-
actions between the head group of SAMs and
the Al 2 O 3 substrate remain evident after 5 or
even 100 cycles, indicating that the structure
of SAMs does not change substantially during
long-term battery cycling (figs. S10 and S11).
The morphologies of Li deposits are exam-
ined through scanning electron microscopy.
Li deposition onto the bare Cu electrode re-
sults in uneven striped Li with obvious cracks
and voids after 10 and 50 cycles, whereas
Li deposited in the presence of SAMs exhib-
its a smooth morphology (figs. S12 and S13).
The morphology of the Li deposited with740 18 FEBRUARY 2022¥VOL 375 ISSUE 6582 science.orgSCIENCE
Fig. 1. Schematic illustration of SAMs in
LMBs and characterization of Al 2 O 3 -
SAMs.(A) SAMs with carboxyl terminal
groups accelerate the reduction of LiTFSI
through dipole momentÐdirected electron
provision and thus generate a LiF-rich SEI.
PP, polypropylene. (BtoD) AFM images
showing the topographic features of
(B) Al 2 O 3 , (C) Al 2 O 3 -OOC(CH 2 ) 2 NH 2 , and (D)
Al 2 O 3 -OOC(CH 2 ) 2 COOH. (E) XPS full-scan
survey spectra of Al 2 O 3 , Al 2 O 3 -OOC(CH 2 ) 2 NH 2 ,
and Al 2 O 3 -OOC(CH 2 ) 2 COOH. a.u.,
arbitrary units. (F) FTIR spectra of Al 2 O 3
and Al 2 O 3 -OOC(CH 2 ) 2 COOH.
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