RESEARCH ARTICLES
◥BATTERIES
Self-assembled monolayers direct a LiF-rich
interphase toward long-life lithium metal batteries
Yujing Liu^1 †, Xinyong Tao^1 †, Yao Wang^1 †, Chi Jiang^1 , Cong Ma^1 , Ouwei Sheng^1 ,
Gongxun Lu^1 , Xiong Wen (David) Lou^2
High–energy density lithium (Li) metal batteries (LMBs) are promising for energy storage applications
but suffer from uncontrollable electrolyte degradation and the consequently formed unstable solid-
electrolyte interphase (SEI). In this study, we designed self-assembled monolayers (SAMs) with
high-density and long-range–ordered polar carboxyl groups linked to an aluminum oxide–coated
separator to provide strong dipole moments, thus offering excess electrons to accelerate the
degradation dynamics of carbon-fluorine bond cleavage in Li bis(trifluoromethanesulfonyl)imide.
Hence, an SEI with enriched lithium fluoride (LiF) nanocrystals is generated, facilitating rapid
Li+transfer and suppressing dendritic Li growth. In particular, the SAMs endow the full cells with
substantially enhanced cyclability under high cathode loading, limited Li excess, and lean electrolyte
conditions. As such, our work extends the long-established SAMs technology into a platform to control
electrolyte degradation and SEI formation toward LMBs with ultralong life spans.
L
ithium (Li) metal is designated as a prom-
ising anode material for next-generation
Li-based batteries because of its high
specific capacity (3860 mA·hour g−^1 ) and
low redox potential (−3.04 V versus the
standard hydrogen electrode) ( 1 , 2 ). However,
the practical application of Li anodes is limited
by dendritic Li growth ( 3 , 4 ), leading to safety
concerns and fast capacity fading of Li metal
batteries (LMBs) ( 5 , 6 ). Among the efforts to
inhibit the formation of Li dendrites, modifica-
tion or rebuilding of the solid-electrolyte inter-
phase (SEI) might be the most crucial ( 7 , 8 )
because the SEI, which is spontaneously de-
rived from the reaction between chemically
active Li metal and the electrolyte, is responsi-
ble for Li+transport and mechanical accom-
modation of rapid Li growth ( 9 , 10 ). Functional
fluorinated electrolyte constituents—such
as Li bis(trifluoromethanesulfonyl)imide
(LiTFSI) ( 11 ), Li bis(fluorosulfonyl)imide ( 12 ),
1,2-difluorobenzene ( 13 ), and fluoroethylene
carbonate ( 14 )—have been designed to perform
interface engineering to regulate the nano-
structure and chemical composition of the
SEI. The developed SEIs generated by these
strategies are all proven to involve the specific
constituent of lithium fluoride (LiF), which has
high interfacial energy, high chemical stability,
and a low Li+diffusion barrier ( 15 , 16 ). Gen-
erally, LiF is believed to be the decomposition
product of F-containing electrolyte ingredients
and contributes to boosting the cycle life of
LMBs. Therefore, precisely controlling the
electrolyte decomposition, particularly the
C–F dissociation chemistry, to construct a
LiF-rich SEI is a logically viable but still chal-
lenging method.
To regulate the electrolyte degradation pro-
cess, it is desirable to seek a strategy to im-
plement control of the redox state of the
electrolyte, focusing on the electronic proper-
ties of the anode interface related to the loss or
gain of electrons. As a reference, polar groups
(e.g., the carboxyl group) can promote the
cleavage of fluorinated bonds by changing
the kinetics of electron transfer ( 17 ). Thus,
how the degradation kinetics of fluorinated
constituents transform when these disordered
and dispersed functional groups become
ordered and close-packed is of interest. Self-
assembled monolayers (SAMs) have been ex-
tensively studied to construct surfaces with
highly oriented molecules and ordered ter-
minal groups and thus provide a convenient,
flexible, and universal platform through which
to tailor the interfacial properties of metals,
metal oxides, and semiconductors ( 18 , 19 ). As a
specific feature, long-range–ordered SAMs can
regulate or even determine the distribution of
surface dipoles relative to the molecular elec-
tronic structure and the orientation of terminal
groups ( 20 – 22 ). Thus, the SAM-induced dipole
moment may influence the kinetics of electron
transfer and change the electrochemical redox
dynamics of electrolytes to regulate the nano-
structure of the SEI. As such, SAMs can possibly
control the decomposition of fluorinated ingre-
dients contained in electrolytes through order-
ing of the terminal groups that determine the
surface electronic properties.In this study, we fabricated SAMs onto an
aluminum oxide (Al 2 O 3 )–coated polypropylene
separator [Al 2 O 3 -OOC(CH 2 ) 2 X] and employed
various terminal functional groups (X = NH 2 ,
COOH) to guide the smooth deposition of Li
metal. The simulation predicts that ordered
polar groups, particularly the carboxyl group,
expedite the decomposition of C–F bonds in-
volving the excess electrons induced by surface
dipoles (Fig. 1A). The proposed mechanism is
supported by atomic visualization and spectral
interpretation, in which many LiF nanocrys-
tals are identified in the SEI. Through the
generation of a LiF-rich SEI, half cells and full
cells both exhibit greatly improved cycling
stability.SAM fabrication and structural examination
An organic molecule [NH 2 (CH 2 ) 2 COOH or
HOOC(CH 2 ) 2 COOH] with a consistent chain
length is selected to establish SAMs on the
surface of an Al 2 O 3 -coated separator (figs. S1
and S2) through a facile soaking method.
The formation of SAMs originates from the
specific binding between the carboxyl group
and the hydroxyl-containing Al 2 O 3 surface
( 23 ). To verify that SAMs are successfully con-
structed on the Al 2 O 3 surface, atomic force
microscopy (AFM) is used to examine the
morphological evolution. In particular, the
moleculesaredenselypackedlikeafull
monolayer with ~20-Å topographic features
after modifying the Al 2 O 3 matrix (Fig. 1, B
to D), indicative of SAM establishment on
Al 2 O 3 as Al 2 O 3 -OOC(CH 2 ) 2 NH 2 and Al 2 O 3 -
OOC(CH 2 ) 2 COOH. In addition, the full x-ray
photoelectron spectroscopy (XPS) spectra
show that the C content is higher in both
Al 2 O 3 -OOC(CH 2 ) 2 NH 2 (23.95%) and Al 2 O 3 -
OOC(CH 2 ) 2 COOH (23.70%) than in bare Al 2 O 3
(18.41%) (Fig. 1E). More specifically, the peaks
at 284.8, 286.2, and 288.6 eV in the C 1s region
are assigned to C–C, C–O, and C=O, respec-
tively, and the peaks at 530.8 and 532.1 eV
in the O 1s region belong to Al–O–Al and Al–
O–C, respectively (fig. S3). The peak inten-
sities (and hence contents) of C=O and Al–
O–C in SAM-grafted Al 2 O 3 (Al 2 O 3 -SAMs) are
increased compared with those of pristine
Al 2 O 3 (fig. S4). In addition, the Fourier trans-
form infrared (FTIR) spectra of Al 2 O 3 -SAMs
exhibit the typical signals of C=O (1730 cm−^1 ),
N–H (1583 cm−^1 ), and C–O (1168 cm−^1 )( 24 , 25 ),
which belong to the terminal groups of the
SAMs (Fig. 1F and fig. S5). In particular, two
notable peaks at ~1638 and ~1443 cm−^1 appear
in Al 2 O 3 -SAMs, representing asymmetric and
symmetric bonding modes of the essential
(bidentate) interaction between the head-
group carboxylate and the Al 2 O 3 surface,
respectively (Fig. 1F and fig. S5) ( 23 ). Further-
more, the water wetting experiment demon-
strates an apparently increased contact angle
that can be attributed to the highly orderedRESEARCHSCIENCEscience.org 18 FEBRUARY 2022•VOL 375 ISSUE 6582 739
(^1) College of Materials Science and Engineering, Zhejiang
University of Technology, Hangzhou 310014, People’s
Republic of China.^2 School of Chemical and Biomedical
Engineering, Nanyang Technological University, 62 Nanyang
Drive, Singapore 637459, Singapore.
*Corresponding author. Email: [email protected] (X.T.); xwlou@
ntu.edu.sg (X.W.L.)
†These authors contributed equally to this work.