Ultrathin dendrimer–graphene oxide composite film for
stable cycling lithium–sulfur batteries
Wen Liua,1, Jianbing Jianga,1, Ke R. Yanga,1, Yingying Mia,b,1, Piranavan Kumaravadivelc, Yiren Zhonga, Qi Fana,2,
Zhe Wenga, Zishan Wua, Judy J. Chac,d, Henghui Zhoub, Victor S. Batistaa,3, Gary W. Brudviga,3, and Hailiang Wanga,3
a
Department of Chemistry and Energy Sciences Institute, Yale University, West Haven, CT 06516; bCollege of Chemistry and Molecular Engineering, Peking
University, Beijing 100871, China; cDepartment of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511; and dCenter for
Research on Interface Structures and Phenomena, Yale University, New Haven, CT 06511
Lithium–sulfur batteries (Li–S batteries) have attracted intense interest because of their high specific capacity and low cost, although they
are still hindered by severe capacity loss upon cycling caused by the
soluble lithium polysulfide intermediates. Although many structure
innovations at the material and device levels have been explored for
the ultimate goal of realizing long cycle life of Li–S batteries, it remains a major challenge to achieve stable cycling while avoiding
energy and power density compromises caused by the introduction
of significant dead weight/volume and increased electrochemical resistance. Here we introduce an ultrathin composite film consisting of
naphthalimide-functionalized poly(amidoamine) dendrimers and graphene oxide nanosheets as a cycling stabilizer. Combining the dendrimer structure that can confine polysulfide intermediates chemically
and physically together with the graphene oxide that renders the
film robust and thin (<1% of the thickness of the active sulfur layer),
the composite film is designed to enable stable cycling of sulfur cathodes without compromising the energy and power densities. Our
sulfur electrodes coated with the composite film exhibit very good
cycling stability, together with high sulfur content, large areal capacity, and improved power rate.
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lithium–sulfur battery ultrathin composite film
graphene oxide long cycle
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igh-energy-density batteries are demanded for electric transportation and stationary energy storage. Developing such a
new generation of batteries requires novel electrode (cathode and
anode) materials. Whereas lithium-ion batteries are widely used
nowadays in portable electronics and consumer products, further
increasing their energy density is a grand challenge as the state-ofthe-art cathode materials based on Li+ intercalation mechanisms
(e.g., LiCoO2, LiFePO4, and LiMn2O4) are approaching their capacity limits (1, 2). Sulfur, a light and abundant element capable of
gaining multiple electrons, is a promising alternative cathode material for high-energy-density rechargeable batteries (i.e., Li–S battery)
due to its high theoretical capacity of 1,672 mAh g−1 (3–6). However,
the cycle life of existing Li–S batteries still suffers from significant
capacity loss of their sulfur cathodes during cycling, due to dissolution and migration of the formed lithium polysulfide (LPS) intermediates (Li2Sx, 4 ≤ x ≤8) during the battery cycling process (7, 8).
Thus far, confining LPS has been regarded as one of the most
effective ways of increasing sulfur electrode cyclability. One major
approach features incorporation of polysulfide-confining components in electrode material structures or as bifunctional binders.
Various materials such as heteroatom-containing carbons (9–14),
metal oxides (15–18), metal sulfides (19, 20), metal–organic complexes (21, 22), and macromolecules containing N or O functional
groups (23–25) have demonstrated strong affinity to LPS and
consequently cycling stability improvement, although the chemical
interaction mechanisms between the electrode surface and LPS in
the presence of solvent molecules are still elusive. Another strategy
is to insert a polysulfide diffusion barrier interlayer in the battery
structure, which separates the polysulfide-confining function from
the electrode itself and can thus be applicable to essentially any
www.pnas.org/cgi/doi/10.1073/pnas.1620809114
type of sulfur cathode material. An ideal polysulfide control interlayer should have the following properties: (i) strong chemical
interactions with LPS in addition to physical confinement to effectively block polysulfide migration to the anode; (ii) low thickness
and light weight with highly exposed LPS interactive sites to minimize dead volume/weight; (iii) low electrochemical resistance to
ensure normal working voltage and good rate capability; and (iv)
well-defined surface site structures to elucidate the LPS binding
mechanisms on the molecular level. Whereas a number of interlayer structures based on carbon (26–29), polymers (30), metal
foams (31), oxide layers (32), and oxide/carbon composites (33, 34)
have been reported in the literature, they fall short in one or more
of these aspects (SI Appendix, Table S1).
In this work, we develop a composite thin film comprising
naphthalimide-functionalized poly(amidoamine) (PAMAM) G4
dendrimer (Naph-Den) and mildly oxidized graphene oxide
(mGO) as an LPS-confining interlayer to realize high-performance
sulfur cathodes that can be stably cycled. In the film structure,
the three components have distinct functionalities: the amidecontaining dendrimer molecules effectively trap polysulfides
via strong chemical binding which is further enhanced by
the branched dendrimer structure; the terminal naphthalimide
groups attached to the dendrimer structure interact with
mGO via π–π stacking and enable composite formation; the
mGO nanosheets impart mechanical strength and durability to the
Significance
The promise of lithium–sulfur batteries for future electric
transportation and stationary energy storage is being limited
by their poor cycling stability. Previous approaches to improvement often involve incorporating additional components
with significant dead weight or volume in battery structures.
We develop an ultrathin functionalized dendrimer–graphene
oxide composite film which can be applied to virtually any
sulfur cathode to alleviate capacity fading over battery cycling
without compromising the energy or power density of the
entire battery. The design provides a new strategy for confining lithium polysulfide intermediates and thus stabilizing lithium–sulfur batteries. It also brings a suitable platform for
elucidating the underlying materials and surface chemistry.
Author contributions: W.L., J.J., Q.F., and H.W. designed research; W.L., J.J., K.R.Y., Y.M., P.K., Y.Z.,
Z. Weng, and Z. Wu performed research; P.K. and J.J.C. contributed new reagents/analytic tools;
W.L., J.J., K.R.Y., Y.M., Y.Z., Z. Weng, Z. Wu, V.S.B., and G.W.B. analyzed data; and W.L., K.R.Y.,
H.Z., V.S.B., G.W.B., and H.W. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1
W.L., J.J., K.R.Y., and Y.M. contributed equally to this work.
2
Present address: School of Chemistry and Chemical Engineering, Southeast University,
Nanjing, Jiangsu 211189, China.
3
To whom correspondence may be addressed. Email: [email protected], victor.
[email protected], or [email protected].
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1620809114/-/DCSupplemental.
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Edited by Hongjie Dai, Stanford University, Stanford, CA, and approved February 17, 2017 (received for review December 18, 2016)
100-nm-thick film. The composite film enables a Li–S battery
with very good cycling stability and improved rate capability.
With a sulfur content of 76 wt % in the electrode material and a
sulfur mass loading of 2 mg cm−2 on the electrode, the cathode
exhibits a capacity decay rate as low as 0.008% per cycle over
500 cycles. As a result of the negligible thickness/weight of the
composite film, the battery energy density is hardly affected. The
well-defined molecular structure of the dendrimer facilitates
mechanistic investigation of LPS binding. X-ray photoelectron
spectroscopy (XPS) studies and density-functional theory (DFT)
calculations suggest that the Li ions of LPS bind strongly to the
O atoms in the amide groups of the dendrimer.
Results
The PAMAM G4 dendrimer was first functionalized by reacting
the terminal amine groups with 1,8-naphthalic anhydride (Fig. 1A).
Successful decoration was confirmed by UV-vis (SI Appendix, Fig.
S1) and 1H NMR (SI Appendix, Fig. S2) spectra. Mixing the NaphDen N,N-dimethylformamide (DMF) solution with an mGO DMF
suspension forms a sticky slurry (Fig. 1B) that can be conveniently
casted onto substrates using doctor blading to afford a thin film
(Fig. 1C). The atomic force microscopy (AFM) image shows that
the thickness of the film is about 100 nm (Fig. 1D). We coated the
Naph-Den–mGO slurry onto a prepared GO–S (SI Appendix, Fig.
S3) electrode surface (Fig. 1E). As a proof of concept, the GO–S
electrode material had a sulfur content of 65 wt % and the sulfur
mass loading on the cathode was 1 mg cm−2. Fig. 1F shows the top
view of the GO–S electrode coated with the Naph-Den–mGO film
(Naph-Den–mGO/GO–S), where a thin surface layer is observed.
C, N, O, and S elements are identified by energy-dispersive X-ray
spectroscopy (EDX), which confirms the existence of the N-containing dendrimer in the surface layer (SI Appendix, Fig. S4). Fig.
1G shows the side view of the Naph-Den–mGO/GO–S electrode,
where the Naph-Den–mGO layer can be clearly distinguished from
the underlying GO–S layer. The Naph-Den–mGO film has a
thickness of ∼90 nm shown in the enlarged image, which is a
negligible fraction (<1%) in volume compared with the active GO–S
layer.
The Naph-Den–mGO/GO–S electrode was evaluated in a coin cell
with a Li metal anode. The cell was first discharged and recharged at
various current rates (0.1–10 C), and then tested for consecutive longterm cycling at 1.0 and 2.0 C. Specific capacities of 1,472, 1,255, 1,083,
and 943 mAh g−1 (based on sulfur mass) were observed at 0.1, 0.2,
0.5, and 1.0 C together with well-defined voltage profiles (Fig. 2 A
and B). Remarkably, reversible capacities of 785 and 530 mAh g−1
were obtained at high rates of 2.0 and 5.0 C (Fig. 2 A and B). As a
comparison, the GO–S cathode without the Naph-Den–mGO film
exhibited comparable capacities at low rates but much lower capacities at high rates (Fig. 2B and SI Appendix, Fig. S5). At 5.0 C the
specific capacity was already as low as 143 mAh g−1.
The cell was then cycled at 1.0 C with a starting capacity of
830 mAh g−1. After 560 cycles at 1.0 C, the Naph-Den–mGO/GO–S
electrode retained a capacity of 757 mAh g−1, corresponding to an
average capacity loss of 0.016% per cycle (Fig. 2C). After that the
cell was further cycled for another 665 cycles at 2.0 C, during
which the capacity dropped from 668 to 562 mAh g−1, giving an
average capacity decay of 0.024% per cycle (Fig. 2C). Stable
charging and discharging voltage profiles with well-defined voltage
plateaus were recorded throughout the cycling process (SI Appendix, Fig. S6). In comparison, the GO–S cathode without the
Naph-Den–mGO film showed inferior cycle life. The capacity
decreased 0.054% per cycle at 1.0 C, and then 0.073% per cycle at
2.0 C (Fig. 2 C and D).
We further increased the sulfur content in the GO–S electrode
material to 76 wt % (SI Appendix, Fig. S7) and the sulfur mass
loading on the electrode to 2 mg cm−2. The Naph-Den–mGO/
GO–S electrode still had better kinetics than the GO–S electrode as evidenced by the voltage profiles with flatter discharging
plateaus (SI Appendix, Fig. S8), although both electrodes manifested compromised specific capacities and rate capabilities as
Fig. 1. Illustration of Naph-Den synthesis and sulfur
cathode fabrication. (A) Molecular structure illustration of Naph-Den. (B) Digital photos of Naph-Den
DMF solution, mGO DMF suspension, and NaphDen–mGO slurry. (C) A thin Naph-Den–mGO film on
a glass slide, prepared by casting the slurry on copper foil and then transferring the film. (D) AFM image and height profile for the Naph-Den–mGO film,
showing an average thickness of 97 nm with thicker
folded edges. (E) Digital photo of a GO–S electrode
coated with the Naph-Den–mGO layer. (F) SEM image of the surface of the Naph-Den–mGO/GO–S
electrode. (G) SEM side views of the Naph-Den–
mGO/GO–S electrode, with the enlarged image
showing the thickness of the Naph-Den–mGO film.
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Liu et al.
the sulfur content and mass loading increased. Notably, the
Naph-Den–mGO/GO–S electrode still exhibited excellent cycling
stability. Starting from 727 mAh g−1 at 1.0 C, the capacity remained
698 mAh g−1 after 500 cycles, giving an average capacity loss as low
as 0.008% per cycle (Fig. 3A). Importantly, the discharge voltage
did not decay over the 500 cycles (Fig. 3B). Without the NaphDen–mGO film, the GO–S electrode experienced much faster
capacity decay (0.068% per cycle) accompanied by decreasing
discharging voltages (Fig. 3 A and C). Parallel testing demonstrated
consistency in electrochemical performance of the Naph-Den–
mGO/GO–S cathodes. Capacity decay of about 0.01% per cycle
over 1,000 discharging–recharging cycles can be realized (SI Appendix, Fig. S9). As sulfur mass loading further increased to
3.5 mg cm−2, the Naph-Den–mGO/GO–S electrode was able to
deliver a stable specific capacity of ∼1,000 mAh g−1 at 0.2 C,
corresponding to a high areal capacity of 3.5 mAh cm−2 (SI Appendix, Fig. S10). At 0.5 C, ∼750 mAh g−1 and 2.8 mAh cm−2 could
be achieved with negligible capacity fading over cycling (SI Appendix, Fig. S11). The Naph-Den–mGO film can also enhance
cycling performance of other sulfur electrodes. Protected by the
composite film, a carbon nanotube buckypaper electrode loaded
with 6 mg cm−2 of sulfur exhibited stable capacity of ∼750 mAh g−1
at 0.5 C (SI Appendix, Fig. S12).
Electrochemical impedance spectroscopy (EIS) was performed for the Naph-Den–mGO/GO–S and GO–S electrodes
before and after long-term cycling. The Nyquist plots are shown
in Fig. 4A. Typically the depressed semicircle in the high-to
medium-frequency region reflects charge transfer resistance
(Rct) and the inclined line in the low-frequency region is related
to mass transfer process (35, 36). The GO–S cathode had an
initial Rct of 25 Ω, which rose to 175 Ω after 1,275 discharging–
charging cycles. Applying the Naph-Den–mGO film drastically
suppresses the increase of charge transfer resistance over cycling.
The Rct of the Naph-Den–mGO/GO–S cathode only increased
to 45 Ω from the initial 32 Ω over 1,275 cycles. The Naph-Den–
mGO/GO–S electrode after long-term cycling was imaged with
SEM. The surface of the cycled electrode remained smooth and
clean without chunk precipitates (Fig. 4B). The EDX spectrum
(SI Appendix, Fig. S13) revealed C, N, O, F, and S elements on
the electrode surface, which indicated the robustness of the
Naph-Den–mGO film. The side-view image of the cycled electrode further confirmed the integrity of the surface film over
cycling (Fig. 4C). It was observed that the thickness of the film
Liu et al.
Fig. 3. Electrochemical performance of the Naph-Den–mGO/GO–S electrode with higher sulfur content and mass loading. (A) Cycling performance
at 1.0 C. (B) Discharging–charging voltage profiles of the Naph-Den–mGO/
GO–S electrode before and after 500 cycles at 1.0 C. (C) Discharging–
charging voltage profiles of the GO–S electrode before and after 500 cycles
at 1.0 C. The GO–S electrode material has a sulfur content of 76 wt %, and
the sulfur mass loading on the cathode is 2 mg cm−2.
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Fig. 2. Electrochemical performance of the Naph-Den-mGO/GO-S electrode
compared with the GO–S electrode. (A) Representative discharging and
charging voltage profiles for the Naph-Den–mGO/GO–S electrode at various
rates. (B) Discharging specific capacities at various rates. (C) Cycling stability
for 560 cycles at 1.0 C and consecutive 665 cycles at 2.0 C. The GO–S electrode
material has a sulfur content of 65 wt %, and the sulfur mass loading on the
cathode is 1 mg cm−2.
increased to 400 nm due to uptake of sulfur species during the
cycling process. The elemental distributions of C and S in the
vertical cross-section of the electrode are shown in Fig. 4D. It is
clear that the sulfur species are well confined by the surface film.
The electrochemical stability of the composite film was also
verified by cyclic voltammetry (CV) measurements: In the potential window of 1.7–2.7 V vs. Li+/Li (the operating voltage
range of sulfur cathodes), the film showed no obvious redox
peaks during repeated CV cycles (SI Appendix, Fig. S14).
To further examine the LPS-confining effect of the NaphDen–mGO film, Li j GO–S and Li j Naph-Den–mGO/GO–S
cells were disassembled after 250 cycles, and the used Li anodes
were carefully analyzed with SEM and EDS. From the digital
photos of the disassembled cells (SI Appendix, Fig. S15 A and B),
it is obvious that the separator of the cycled Li j GO–S cell
contains a significant amount of LPS (yellow), whereas the
separator of the cycled Li j Naph-Den–mGO/GO–S cell is almost
free of LPS. Combined SEM imaging and EDS mapping (SI
Appendix, Figs. S15–17) shows that the cycled Li anode paired
with the GO–S cathode is composed of loosely packed small Li
particles with a high amount of sulfur species, whereas that
paired with the Naph-Den–mGO/GO–S cathode features
densely packed, larger Li granules containing a much smaller
amount of sulfur. These data, taken together, reveal that the
shuttle effect has been substantially suppressed by deploying the
Naph-Den–mGO interlayer.
The polysulfide-confining Naph-Den–mGO film promotes
high electrochemical performance of sulfur cathodes. The composite film not only prevents LPS from diffusing away from the
cathode and thus avoids active material loss and Coulombic efficiency decrease, but it also affords a uniform environment on
the cathode surface for facile LPS redox conversion and thus
ensures a high degree of utilization of active material and low
electrochemical resistance throughout the long-term cycling
process. As a result, the Naph-Den–mGO/GO–S electrode has a
long cycle life with a capacity decay of <0.01% per cycle, which
substantially outperforms any other sulfur cathode with interlayer protection (SI Appendix, Table S1). It is also arguably
the best cycling stability among all sulfur cathodes reported to
date (SI Appendix, Table S2). It is worth mentioning that our
Fig. 4. Characterizations of the Naph-Den-mGO/
GO-S electrode after long-term cycling. (A) Nyquist
plots of the EIS spectra of the Naph-Den–mGO/GO–S
cathode measured at 2.4 V before and after
1,275 cycles, in comparison with the GO–S cathode.
(B) Top-view SEM image of the cycled electrode.
(C) Side-view SEM image of the cycled electrode.
(D) EDX elemental mapping of the vertical crosssection of the cycled electrode.
Naph-Den–mGO film is only 100 nm thick; its small volume and
light weight allow for cycling stability enhancement without compromising the battery energy and power densities. This is difficult
to achieve with previously reported interlayers that are usually
thicker or heavier by at least one order of magnitude (SI Appendix,
Fig. S18). With N- and O-containing functional groups embedded
in its branched structure, the Naph-Den is expected to interact
with LPS strongly. In a control experiment, we evaluated a GO–S
electrode coated with an mGO film without the dendrimer component. The capacity fading over cycling (0.049% per cycle) was
substantially faster than the Naph-Den–mGO/GO–S electrode,
although slightly slower than the GO–S electrode without any film
coated (SI Appendix, Fig. S19). It is thus clear that the Naph-Den
is the central component of the composite film for confining
polysulfides. The following part of the article will discuss the
molecular origins of the binding between the Naph-Den and LPS.
Discussion
We first used XPS to probe the chemical interactions between
Naph-Den and LPS using Li2S4 as a model compound. The S 2p
core-level spectrum of Li2S4 exhibits two sets of doublets located
at 163.1/164.3 eV and 161.4/162.6 eV (Fig. 5A), with binding
energy splittings of 1.2 eV, which are attributed to the bridging
and terminal sulfur (SB0 and ST-1) atoms, respectively. The
spectral features agree well with previous results reported elsewhere (37). In the presence of Naph-Den, the S 2p doublets both
shift to lower binding energy (Fig. 5A), indicating increased valence electron density on the S atoms upon interaction with
Naph-Den. The binding energy of the N 1s electrons in the
Naph-Den remains almost unchanged upon interaction with
Li2S4 (Fig. 5B), which suggests a minor contribution from the N
sites in the dendrimer structure for LPS binding. This is distinct
from previous reports where N atoms in the carbon materials
bind LPS strongly (14). The O 1s spectrum of Naph-Den exhibits
two components at 531.8 and 533.5 eV (Fig. 5C), which could be
due to the O atoms of the amide and imide groups in the NaphDen structure. In the presence of Li2S4, the O 1s peak shifts to
lower binding energy, indicating that the O atoms interact
strongly with LPS. Further XPS studies reveal that the PAMAM
dendrimer interacts with Li2S4 in a similar manner as the NaphDen (SI Appendix, Fig. S20). It is thus likely that the O atoms of
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the amide groups in the PAMAM structure are the major interactive sites responsible for Naph-Den binding with LPS.
We then performed DFT calculations to examine the LPS
binding mechanism. We modeled LPS with LiSSH to include S
atoms representing both the terminal and bridging S atoms in
LPS. Different from many previous reports based on binding
energy (ΔEB) (14, 23), we focused on the binding free energy
(ΔGB) to evaluate the interaction strength between LiSSH and
different binding sites (BSite) and/or solvent molecules (Sol).
ΔGB is defined as follows:
ΔGB = GðHSSLiSolm BSiten Þ− GðHSSLiÞ− mGðSolÞ− nGðBSiteÞ,
where HSSSLiSolmBSiten is the complex of LiSSH with m solvent
molecules and n binding sites. The use of ΔGB in place of ΔEB
brings the advantage of including both thermal and entropy corrections, which is important for accurate modeling of the interaction strength. For example, we calculated the ΔEB of LiSSH to the
1,3-dioxolane (DOL) solvent molecule to be −20.7 kcal mol−1 (SI
Appendix, Fig. S21), in excellent agreement with previously
reported values for similar systems (14, 38). However, taking entropy into consideration, ΔGB for binding of LiSSH to DOL is −
10.6 kcal mol−1, which differs substantially from ΔEB. We found
that ΔGB for binding of LiSSH to another solvent molecule, dimethoxyethane (DME), is −20.6 kcal mol−1, significantly more negative than that for DOL (Fig. 6 A and B), suggesting that LiSSH
prefers to be solvated by the chelating ligand DME. ΔGB for
binding of LiSSH to two DME molecules is −24.3 kcal mol−1
(Fig. 6C), corresponding to ΔG of −3.7 kcal mol−1 for HSSLiDME to bind a second DME molecule. Therefore, LiSSH prefers
to be solvated by two DME molecules in the battery electrolyte,
consistent with previous reports on DME-solvated Li complexes in
both solution (39) and crystals (40, 41).
Now we turn to the binding of LiSSH to Naph-Den. There are
three sites in the Naph-Den molecular structure that can bind
LPS, namely the O atoms in the amide groups (O1 site), the N
atoms in the tertiary amine groups (N1 site), and the O atoms in
the terminal imide groups (Ot site), as shown in SI Appendix, Fig.
S22. SI Appendix, Fig. S23 shows that the binding between LiSSH
and a functional group is hardly affected by other atoms beyond
Liu et al.
the functional group, and thus demonstrates the validity of our
model. As shown in Fig. 6 D–F, ΔGB values for binding of LiSSH
to the N1, O1, and Ot sites in Naph-Den are −13.2, −19.0, and
17.3 kcal mol−1, respectively. All are smaller than that to DME,
indicating that the three types of binding sites are unlikely to
replace the DME ligand in HSSLi(DME). ΔGB values for
binding of LiSSH to the N1, O1, and Ot sites in the presence
of one DME molecule are calculated to be −22.8, −27.0,
and −24.5 kcal mol−1, respectively (Fig. 6 G–I). The ΔGB values
for binding of HSSLi(DME) to the N1, O1, and Ot sites can thus
be derived to be −1.2, −6.4, and −3.9 kcal mol−1, respectively.
Compared with the ΔGB values for binding of HSSLi(DME) to a
second DME molecule (−3.7 kcal mol−1), our results suggest that
it is thermodynamically favorable for the O1 site to replace a
DME molecule in HSSLi(DME)2 (ΔG = −2.7 kcal mol−1) and
thus bind LPS. Our results also suggest that the N1 site is unlikely
to bind the solvated LiSSH.
Considering that the samples for XPS study are under high
vacuum condition in which the volatile organic solvents are likely to
be removed from the samples, we include analysis of interactions
Fig. 6. DFT calculations of binding free energies (ΔGB). Optimized geometries for the binding of LiSSH to (A) one DOL molecule, (B) one DME molecule,
(C) two DME molecules, (D) N1 site, (E) O1 site, (F) Ot site, (G) N1 site and one DME molecule, (H) O1 site and one DME molecule, and (I) Ot site and one DME
molecule, and corresponding binding free energies in kilocalorie per mole. Gray, white, red, blue, yellow, and purple balls represent C, H, O, N, S, and Li
atoms, respectively.
Liu et al.
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Fig. 5. XPS analysis of the interactions between Naph-Den and LPS. (A) S 2p core-level spectra of Li2S4 before and after interacting with Naph-Den. (B) N 1s
and (C) O 1s core-level spectra of Naph-Den before and after interacting with Li2S4.
between LiSSH and Naph-Den in the absence of solvent molecules.
The calculated binding free energies suggest that the O1 site should
still be the dominant binding sites for LPS (SI Appendix, Fig. S24
A–C). Based on the optimized structure for LiSSH binding to the
O1 site of the Naph-Den, we further calculated the partial atomic
charges for the S, O, and N atoms in the structure to correlate with
the binding energy shifts measured by XPS (42, 43). As shown in SI
Appendix, Fig. S24D, the Mulliken charges on the O atom of the
Naph-Den O1 site and on the S atoms of the LiSSH all become more
negative (with increased electron density) upon interaction, which
well explains the experimentally observed red shifts of O 1s and S 2p
binding energies in the XPS spectra. It is, therefore, confirmed that
the O atoms of the amide groups in the Naph-Den structure are the
dominant sites that bind LPS.
In summary, we have designed and developed an ultrathin dendrimer–GO composite film to mitigate the polysulfide shuttling
problem and stabilize the cycling of Li–S batteries without compromising their energy and power densities. The dendrimer molecules
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Materials and Methods
Material synthesis, electrode preparation, materials characterization, electrochemical measurements, and computational methods are detailed in
SI Appendix.
ACKNOWLEDGMENTS. K.R.Y. and V.S.B. acknowledge computer time from the
National Energy Research Scientific Computing Center and the Yale High
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University. Y.M. acknowledges the support from China Scholarship Council.
Computational and synthetic work was supported by the US Department of
Energy Office of Science, Office of Basic Energy Sciences, under Award DE-FG0207ER15909 and by a generous donation from the TomKat Charitable Trust.
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Liu et al.
1
Supporting Information Appendix for
An Ultrathin Dendrimer-Graphene Oxide Composite Film for Stable Cycling LithiumSulfur Batteries
Wen Liu1,ƚ, Jianbing Jiang1, ƚ, Ke R. Yang1,ƚ, Yingying Mi1,2,ƚ, Piranavan Kumaravadivel1,3, Yiren
Zhong1, Qi Fan1,4, Zhe Weng1, Zishan Wu1, Judy J. Cha1,3,5, Henghui Zhou2, Victor S. Batista*,1,
Gary W. Brudvig*,1, Hailiang Wang*,1
1
Department of Chemistry and Energy Sciences Institute, Yale University, 810 West Campus
Drive, West Haven, CT 06516, USA
2
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
3
Department of Mechanical Engineering and Materials Science, Yale University, 15 Prospect
Street, New Haven, CT 06511, USA
4
Current address: school of Chemistry and Chemical Engineering, Southeast University,
Nanjing, Jiangsu 211189, China
5
Center for Research on Interface Structures and Phenomena, Yale University, 15 Prospect
Street, New Haven, CT 06511, USA
ƚ
These authors contributed equally
*
E-mail: [email protected]; [email protected]; [email protected]
1
2
MATERIALS AND METHODS
Synthesis of Naph-Den and preparation of Naph-Den-mGO slurry
All chemicals and solvents were commercially available and used as obtained without further
purification. Following a reported procedure (1) with modifications, a mixture of PAMAM G4
dendrimer (25 mg, 1.76 μmol) and 1,8-naphthalic anhydride (26.8 mg, 135 μmol) in DMF (1.5
mL) was stirred at 90 °C for 48 h in the dark under a nitrogen atmosphere. The crude NaphDen product was purified by dialysis using a regenerated cellulose membrane with a molecular
weight cutoff of 12–14 kDa in dimethyl sulfoxide (DMSO) for three days. The light yellow
solution in the dialysis bag was dried under vacuum for four days to yield the Naph-Den as a
light yellow brown semi-solid (40 mg, 85%): 1H NMR (400 MHz, CDCl3), δ 8.26 (br, 252H,
naphthyl-H and amido-H, overlapped), 7.94 (br, 128H, naphthyl-H), 7.67 (br, 128H, naphthylH), 4.02 (br, 128H, CH2 adjacent to naphthyl unit), 2.90 (s, 256H, -CH2-CO-), 2.74 (s, 256H, CH2-NH-CO-), 2.09 (br, 356H, -CH2-N-); λabs (DMSO) 330 nm. The Naph-Den-mGO slurry was
prepared by vortex mixing a 6 mg mL-1 mGO (prepared with a modified Hummers method in
which 0.5 g KMnO4 for 1 g Graphite (2)) DMF suspension with a 6 mg mL-1 Naph-Den DMF
solution.
Fabrication of Naph-Den-mGO film protected cathode
The GO-S composite material was prepared by decomposing sodium polysulfide (NaSx) in the
presence of GO (prepared with a modified Hummers method in which 4 g KMnO4 used for 1 g
Graphite (3)) in a water/ethanol mixed solvent. The obtained GO-S was mixed with carbon
black and polyvinylidene difluoride (PVDF) in a mass ratio of 80:10:10 in N-methylpyrrolidone
(NMP) to make a slurry that was then casted onto a carbon-coated Al foil (MTI Corporation,
USA) and dried at 50 °C in a vacuum oven for 2 h. The sulfur electrode film was compressed
and punched into circular pellets with a diameter of 14 mm. 20 μL of the Naph-Den-mGO
slurry (6 mg mL-1) was then cast onto the sulfur cathode by doctor blading, followed by drying
at 50 °C in a vacuum oven for another 2 h. The BP-S electrodes were prepared by immersing
BP (NTL-12214, 45 gsm MWNT Blend, Nanotech Labs, Inc.) in a 0.04 g ml-1 S8/DMF solution
followed by drying in a vacuum oven at 70 oC. The electrodes were then punched into circular
2
3
disks with a diameter of 14 mm. After that, the electrode disks were sealed in a Teflon lined
stainless steel autoclave and heated at 155 oC for 12h. The Naph-Den-mGO/BP-S electrodes
were prepared by casting the Naph-Den-mGO slurry onto the BP-S electrodes following the
same procedure as for preparing the Naph-Den-mGO/GO-S electrodes.
Materials characterization
1
H NMR spectra were recorded with an Agilent 400 MHz NMR instrument. Chemical shifts are
reported as ppm from the internal reference tetramethylsilane (TMS). Absorption spectra
were recorded with a Varian Cary 50 Bio UV-visible spectrophotometer. SEM imaging and EDX
measurements were performed with a Hitachi SU8230 field emission SEM microscope. TEM
images were taken on a JEM-1400 Plus TEM microscope. XPS measurements were performed
with a PHI Versa Probe II with Al Kα radiation. The XPS samples were prepared by soaking
material loaded silicon chips for 24 h in a 0.01 mol L-1 Li2S4 solution prepared by dissolving
stoichiometric amounts of elemental sulfur and lithium sulfide (Li2S) in a mixed solvent of DME
and DOL (1:1 by volume), followed by drying in an argon-filled glove box. The samples were
then transferred into the XPS chamber without exposing to air, using an air-tight vessel.
Electrochemical measurements
CR2032 coin cells were assembled in an argon-filled glove box with the prepared sulfur
electrodes as cathode, lithium disks as anode, and a porous membrane (Celgard 3501) as the
separator. The electrolyte was lithium bis(trifluoromethane) sulphonimide (1.0 M) in a solvent
mixture of DOL and DME (1:1 by volume) containing lithium nitrate (2 wt.%). Galvanostatic
discharging and charging were performed on an Arbin BT-2143 battery analyzer at various
rates (1 C = 1600 mA g-1) in the potential window of 1.7-2.7 V vs Li+/Li. The EIS data were
recorded using a 760D electrochemistry workstation (CH Instruments) at 2.4 V over the
frequency range of 100 kHz to 10 mHz. To assess the electrochemical stability of the NaphDen-mGO film, the Naph-Den-mGO slurry was coated onto Al foil and then cut into electrode
disks with a diameter of 14 mm. CR2032 coin cells were assembled using the Naph-Den-mGO
electrode as cathode and Li foil as anode. CV data were recorded with the Arbin BT-2143
battery analyzer at a scan rate of 0.05 mV s-1.
3
4
Computational details
The DFT study was performed with the B3LYP functional (4, 5) implemented in Gaussian 09
Rev. D.01 (6). Dispersion interactions were considered with Grimme’s D3 correction (7). A
pruned grid having 99 radial shells and 590 angular points per shell, called “ultrafine” grid in
Gaussian 09, was used for the non-analytic integral evaluations. The 6-31G(d) basis set (8, 9)
was used for the geometry optimization, Mulliken population analysis, and the vibrational
frequency analysis required for free energy calculations. The calculated vibrational
frequencies were used for the computation of molecular partition functions to evaluate
thermal contributions to the free energy at 298.15 K by employing the ideal gas, rigid rotator,
harmonic oscillator approximation (10). The 6-311+G(2df,p) basis set (11) was used in singlepoint calculations for refined energetics. The thermal contribution to the free energies from
the frequency analysis was added to the refined energetics for all final composite free
energies reported in the manuscript. In the calculation of binding free energies, the 1 M
standard state was used for all species.
4
5
SUPPORTING FIGURES
Figure S1 UV-Vis absorption spectra. Both the dendrimer (PAMAM) and its naphthalimide
functionalized product Den-Naph (PAMAM-Naph) were measured in DMSO at a concentration
of 0.5 mg mL-1.
Figure S2 1H NMR spectrum of Den-Naph in DMSO-d6.
5
6
Figure S3 Morphology and elemental distribution of GO-S cathode material with 65 wt.%
sulfur content. SEM images of GO-S at low (a) and high (b) magnification. (c, d, e) HAADF
image and EDX maps of GO-S.
Figure S4 SEM and EDX analysis for sulfur electrodes. (a, b) SEM image and corresponding
EDX spectrum of the GO-S electrode. (c, d) SEM image and corresponding EDX spectrum of
the Naph-Den-mGO/GO-S electrode.
6
7
Figure S5 Representative discharging-charging voltage profiles of GO-S at different C rates.
Figure S6 Discharging and charging voltage profiles of the GO-S electrodes with and without
the Naph-Den-mGO film at different stages of cycling stability test. The sulfur content in the
GO-S composite is 65 wt.%, and the total sulfur loading on the electrode is 1 mg cm-2.
Figure S7 Morphology of the GO-S cathode material with 76 wt.% sulfur content. SEM
images of the GO-S at low (a) and high (b) magnification.
7
8
Figure S8 Electrochemical performance of the Naph-Den-mGO/GO-S electrode compared
with the GO-S electrode. (a) Discharge capacities and (b) representative discharging-charging
voltage profiles of the GO-S electrode at different C rates. (c) Discharge capacities and (d)
representative discharging-charging voltage profiles of the Naph-Den-mGO/GO-S electrode at
different C rates. The sulfur content in the GO-S composite is 76 wt.%, and the sulfur mass
loading on the electrode is 2 mg cm-2.
Figure S9 Parallel cycling stability test of the Naph-Den-mGO/GO-S electrodes. The sulfur
content in the GO-S composite is 76 wt.%, and the sulfur mass loading on the electrode is 2
mg cm-2.
8
9
Figure S10 Cycling performance of Naph-Den-mGO/GO-S electrode at 0.2 C. The sulfur
content in the GO-S composite is 76 wt.%, and the sulfur mass loading on the electrode is 3.5
mg cm-2.
Figure S11 Electrochemical performance of the Naph-Den-mGO/GO-S electrode with high
sulfur mass loading. (a) Representative discharging-charging voltage profiles. (b) Discharging
capacities at different C rates. (c) Cycling stability at 1.0 C and 0.5 C. The sulfur content in the
GO-S composite is 76 wt.%, and the sulfur mass loading on the electrode is 3.5 mg cm-2.
9
10
Figure S12 SEM images and electrochemical performance of the Naph-Den-mGO/BP-S
electrode. (a) Top-view and (b) cross-section SEM images of the Naph-Den-mGO/BP-S
electrode. (c) Enlarged cross-section SEM image showing the Naph-Den-mGO film on the
electrode surface. (d) Enlarged cross-section SEM image showing the carbon nanotubes
coated with sulfur particles. (e) Cycling stability of the BP-S and Naph-Den-mGO/BP-S
electrodes. (f) Discharging-charging voltage profiles of the Naph-Den-mGO/BP-S electrode.
Figure S13 SEM and EDX characterization of the Naph-Den-mGO/GO-S electrode after
cycling (1275 cycles).
10
11
Figure S14 Cyclic voltammograms of the Naph-Den-mGO film in the Li-S battery electrolyte
Figure S15. Postmortem analysis of the Li anodes paired with the GO-S and Naph-DenmGO/GO-S cathodes in cells after 250 cycles. Photographs of the disassembled cells using (a)
GO-S and (b) Naph-Den-mGO/GO-S as cathodes. SEM images and the corresponding EDS
spectra and elemental maps of the cycled Li anodes paired with the (c) GO-S and (d) NaphDen-mGO/GO-S cathodes.
11
12
Figure S16. Cross-section SEM images of the Li anodes paired with the (a, b) GO-S and (c, d)
Naph-Den-mGO/GO-S cathodes after 250 cycles.
Figure S17. SEM-EDS analysis of the cross sections of the Li anodes paired with the (a) GO-S
and (b) Naph-Den-mGO/GO-S cathodes after 250 cycles.
12
13
Figure S18 Figures of merit for Li-S battery cathode interlayers reported in the literature
(Table S1). (a) Plot of cathode capacity loss upon cycling against interlayer thickness. (b) Plot
of cathode capacity loss upon cycling against interlayer mass (percentage of sulfur active
material). Our Naph-Den-mGO film outperforms any other interlayer reported to date.
Figure S19 Electrochemical performance of the mGO/GO-S electrode. (a) Cycling stability at
1.0 and 2.0 C rates. (b) Representative discharging-charging voltage profiles at different C
rates. (c) The discharging-charging voltage profiles before and after cycling at 1.0 and 2.0 C.
The sulfur content in the GO-S composite is 65 wt.%, and the total sulfur loading on the
electrode is 1 mg cm-2.
13
14
Figure S20 XPS analysis of the interaction between lithium polysulfide and the unmodified
dendrimer (PAMAM). (a) S 2p core level spectra of free Li2S4 and Li2S4 adsorbed on PAMAM.
(b) N 1s and (c) O 1s core level spectra of the PAMAM before and after its interaction with
Li2S4.
14
15
Figure S21 Theoretical calculations of binding energy (ΔEB). Optimized geometries for the
binding of LiSSH to (a) DOL, (b) DME, (c) two DME molecules, (d) N1 site, (e) O1 site, (f) Ot site,
(g) N1 site and one DME molecule, (h) O1 site and one DME molecule, and (i) Ot site and one
DME molecule, together with the corresponding binding energies in kcal/mol. Grey, white,
red, blue, yellow, and purple balls represent C, H, O, N, S, and Li atoms, respectively.
15
16
Figure S22 Structure of the PAMAM G2 dendrimer and three possible binding sites for
lithium polysulfides. Grey, white, red, and blue balls represent C, H, O, and N atoms,
respectively.
Figure S23 Optimized geometries for the binding of LiSSH to the O1 sites. Models include (a)
1 bond, (b) 2 bonds, (c) 3 bonds, (d) 4 bonds, and (e) 5 bonds beyond the amide group. The
calculated binding free energies and key bond lengths are shown in kcal/mol and Å. Grey,
white, red, blue, yellow, and purple balls represent C, H, O, N, S, and Li atoms, respectively.
16
17
Figure S24 Theoretical calculations in the absence of solvent molecules. Optimized
geometries for the binding of LiSSH to (a) N1 site, (b) O1 site, and (c) Ot site of Naph-Den, with
the binding free energies in kcal mol-1; Mulliken charges (d) before and (e) after LiSSH binding
to the O1 site of Naph-Den. Grey, white, red, blue, yellow, and purple balls represent C, H, O,
N, S, and Li atoms, respectively.
17
18
SUPPORTING TABLES
Table S1. Performance comparison with sulfur cathodes protected by different interlayers
reported in the literature (green: average capacity decay <2% per 100 cycles; yellow: 2-10%;
red: >10%).
Interlayer
Sulfur
Interlayer
Interlayer
loading
thickness
mass
(mg cm-2)
（μm）
Cycling stability
Cycles
(mg cm-2)
Capacity
Ref.
loss
per 100 cycles
Naph-Den-mGO
2.0
0.1
~0.05
500
0.8%@1 C
This work
TiO2-CNT
0.8
35
0.5~0.6
500
5%@1 C
(12)
Carbon/MWCNT paper
1.8~2.7
~60
\
100
6.5%@1 C
(13)
GO-TiO2
1.2
~3
0.15
250
7.2%@0.5 C
(14)
GO
1~1.2
0.75
0.29
400
8%@1 C
(15)
N,P co-doped graphene
2.1
\
1.0
500
9%@1 C
(16)
Graphene
3~4
20
1.3
300
10%@1 C
(17)
Carbon nanofibers
1.4
~280
4.2
100
10%@1 C
(18)
Ethylenediamine-functionalized rGO
3.6
0.1
0.54
200
10%@0.2 C
(19)
Microporous carbon paper
\
\
\
100
15%@1 C
(20)
Ni foam
\
~90
\
50
17%@0.2 C
(21)
GO/Nafion
1.2
0.13
0.0532
200
18%@0.5 C
(22)
SWCNT
1.5
~9
0.14
300
18%@0.2 C
(23)
GO
1~1.5
~6
0.12
100
23%@0.1 C
(24)
N doped graphene paper
1.06
~8
0.5
100
23%@0.2 C
(25)
rGO-carbon black
3
\
3
100
29%@0.2 C
(26)
Al2O3-activated carbon cloth
12
500
~2
40
\
(27)
MWCNT
\
40~50
0.6~0.8
100
38%@1 C
(28)
18
19
Table S2. Performance comparison with state-of-the-art high-performance sulfur cathodes
reported in the literature (green: average capacity decay <2% per 100 cycles; yellow: 2-10%;
red: >10%).
Cathode
Sulfur
Sulfur loading
content in
on
material
(mg cm-2)
65%
1.0
830
560
1.6%@1 C
76%
2.0
727
1000
1%@1 C
76%
3.5
741
200
1.1%@0.5 C
Covalent bond glued sulfur nanosheets
83%
1.2
715
500
2.1%@0.625C
(29)
S-TiO2 yolk-shell
71%
0.4-0.6
1030
1000
3.3%@0.5C
(30)
CoS2-graphene/S
75%
0.4
1000
2000
3.4%@2C
(31)
MnO2 nanosheets/S
75%
0.7-1.0
870
2000
3.6%@2C
(32)
Mesoporous carbon sphere-CNT/S
70%
5
1200
200
5%@0.2C
(33)
Hollow carbon nanospheres/S
67%
\
842
500
5.3%@1C
(34)
Si/SiO2@C-S
70%
3.1-3.5
890
500
6.3%@2C
(35)
Ti4O7/S
60%
1.5-1.8
850
500
6%@2C
(36)
ethylenediamine-functionalized rGO/S
69%
/
700
350
7%@0.5C
(37)
S/Co-N-Graphitic carbon
70%
2.5
795
500
9%@1C
(38)
Graphene-S sandwich
70%
3~4
1000
300
10%@1C
(17)
Dual confined N-doped double-shelled
63%
3.9
838
200
19%@0.5C
(39)
Naph-Den-mGO/GO-S
hollow carbon spheres/S
19
electrode
Cycling stability
Capacity
Cycles
(mAh g-1)
Ref.
Loss per 100
cycles
This
work
20
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