insights into the absorption mechanism of carbon nanotube paper-titanium … · ·...

Nano Res

1

Insights into the absorption mechanism of carbon

nanotube paper-titanium dioxide as a multifunctional

barrier for lithium-sulfur batteries

Guiyin Xu1, Jiaren Yuan1, Xinyong Tao2, Bing Ding1, Hui Dou1, Xiaohong Yan1,3 (), Yang Xiao1, Xiaogang Zhang1 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0812-0

http://www.thenanoresearch.com on May 9, 2015

© Tsinghua University Press 2015

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Nano Research

DOI 10.1007/s12274-015-0812-0

Nano Res

2

Address correspondence to Xiaogang Zhang, email1: [email protected]; Xiaohong Yan, email2: [email protected].

TABLE OF CONTENTS (TOC)




Guiyin Xu1, Jiaren Yuan1, Xinyong Tao2, Bing Ding1, Hui

Dou1, Xiaohong Yan1,3*, Yang Xiao1, Xiaogang Zhang1*

1Nanjing University of Aeronautics and Astronautics,

China.

2Zhejiang University of Technology, China.

3Nanjing University of Posts and Telecommunications,

China.

High-performance lithium-sulfur batteries: a multifunctional carbon

nanotube paper/titanium dioxide barrier has been designed to effectively

reduce the loss of active materials and restrain the diffusion of lithium

polysulfides to the anode, thereby improving the cycling stability of

lithium-sulfur batteries.

Xiaogang Zhang1, http://xgzhang.nuaa.edu.cn/





Guiyin Xu1, Jiaren Yuan1, Xinyong Tao2, Bing Ding1, Hui Dou1, Xiaohong Yan1,3 (), Yang Xiao1, Xiaogang

Zhang1 ( )

Received: day month year

Revised: day month year

Accepted: day month year

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

titanium dioxide,

carbon nanotube paper,

multifunctional barrier,

adsorption,

lithium-sulfur batteries

ABSTRACT

Lithium-sulfur batteries are highly attractive energy storage devices due to

their low cost, high specific capacity and energy density. However, the

insulation of sulfur and high solubility of lithium polysulfides result in the low

utilization of active materials and poor cycling performance. Herein, we

discover a multifunctional carbon nanotube paper/titanium dioxide barrier can

effectively reduce the loss of active materials and restrain the diffusion of

lithium polysulfides to the anode, thereby improving the cycling stability of

lithium-sulfur batteries. Using carbon nanotube paper/titanium dioxide as the

multifunctional barrier, the activated carbon/sulfur cathode with a high sulfur

content of 70% delivers a stable cycling performance and high Coulombic

efficiency (~99%) over 250 cycles at a current rate of 0.5 C. The improved

electrochemical performance is attributed to the synergistic effects of the carbon

nanotube paper and titanium dioxide, involving the physical barrier, the

chemical adsorption due to the binding formation of Ti-S and S-O, and the

other unique interactions between titanium dioxide and sulfur species.

1 Introduction

Lithium-sulfur (Li-S) batteries are considered to

be one of the most promising energy storage

devices in next generation high energy power

system, owing to their high specific capacity (1675

mAh g-1) and energy density (2600 Wh kg-1) [1-3].

However, the insulation of sulfur leads to the low

utilization of active materials, which seriously

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2 Nano Res.

impedes the rapid development of Li-S batteries

[4-6]. In view of the sulfur insulation, researches

usually improve the electrical conductivity of active

materials by adding carbon materials [7-10].

Furthermore, the high solubility of lithium

polysulfides in the organic electrolyte results in the

parasitic reaction of lithium polysulfides with the

lithium anode, which is the main factor to restrict

the practical application of Li-S batteries [11]. There

have been many strategies to absorb the active

materials by physical absorption with porous

carbon materials or chemical adsorption with the

functional group of graphene oxide [12, 13].

Conducting polymers have also been used to coat

the active materials to immobilize lithium

polysulfides and accommodate the volume

expansion of sulfur during discharge process

[14-16]. Recently, titanium dioxide (TiO2) was found

to be another promising candidate to improve the

cycling stability for Li-S batteries [17-19]. Cui et al.

[20] reported a sulfur-TiO2 yolk-shell

nanoarchitecture cathode, which could

accommodate the volume change of the active

materials during the charge/discharge process.

Simultaneously, the intact TiO2 shell could mitigate

the dissolution of lithium polysulfides during

repeated cycles. In addition, our previous work

found that the surface of TiO2 was positively

charged [21], demonstrating that mesoporous

hollow TiO2 spheres had not only the confinement

effect for the active materials, but also the surface

activity to capture polysulphide anions.

Although porous carbon materials could adsorb

sulfur during the impregnation process, it is

unavoidable that lithium polysulfides could be

desorbed from porous carbon materials in a long

term due to the weak interaction between the active

materials and porous carbon materials [22-25]. That

may be the reason that most published studies

rarely reported results more than 100 cycles for this

kind of Li-S batteries. Besides, the coating of

conducting polymers for sulfur cathodes could

increase the resistance of Li-S batteries. The

sulfur-TiO2 yolk-shell nanoarchitecture cathode had

an initial specific capacity of 1030 mAh g-1 (a typical

sulfur mass loading of 0.4-0.6 mg cm-2) at 0.5 C,

which is about 61% of the theoretical specific

capacity. The low initial specific capacity could be

attributed to the low electrical conductivity of the

sulfur-TiO2 composite, leading to the low utilization

of sulfur. More recently, a carbon paper was used as

an interlayer to alleviate the diffusion of lithium

polysulfides from the cathode to anode [26-28], thus

mitigate the shuttling effect and lithium anode

corrosion. This may be an optimizing strategy to

improve the cycle life under much higher loading of

sulfur. Therefore, the interesting applications of

functional interlayers in Li-S batteries need further

research and development.

Herein, we design TiO2 supported on

free-standing carbon nanotube paper as the

multifunctional barrier to suppress the diffusion of

lithium polysulfides for Li-S batteries.

Simultaneously, a high content of sulfur (70%) is

loaded to the activated carbon (AC) with a high

specific surface area and large pore volume as the

cathode, which could further increase the

utilization and reduce the loss of active materials.

The electrochemical results demonstrate that Li-S

batteries with the multifunctional carbon nanotube

paper/TiO2 (CNTP/TiO2) barrier have stable cycling

performance and high Coulombic efficiency.

Moreover, we investigate the special interactions

between TiO2 and sulfur species with scanning

transmission electronic microscope (STEM), X-ray

photoelectron spectroscopy (XPS), Raman spectra,

and the density functional theory (DFT) calculation.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

2 Results and discussion

Figure 1. Illustration for the conventional structure (a) and

special construction (b) of Li-S batteries. Scanning electron

microscopy (SEM) images of (c) CNTP (the inset is digital

photograph of CNTP), (d, e) CNTP/TiO2 surface (the inset is

digital photograph of CNTP/TiO2 and leaves), (d) CNTP/TiO2

cross-section.

In general, the conventional structure of Li-S

batteries is composed of the porous carbon/sulfur

(C/S) cathode, separator, and lithium anode (Figure

1a). The preparation process of porous carbon is

complex and time-consuming [29], which is not

conducive to the practical application. The

commercial AC possesses advantages, such as low

cost, excellent electrical conductivity, high specific

surface area (1964.8 m2 g-1, Table S1), large pore

volume (0.91 cm3 g-1) and unique pore-size

distribution [30]. Therefore, AC was selected as the

host for loading a high content of sulfur in this

work. Based on the Brunauer-Emmett-Teller (BET),

transmission electron microscopy (TEM), X-ray

diffraction (XRD) and thermal gravimetric (TG)

observations (Figure S1-4), the majority of sulfur is

confined into AC and only a small part of sulfur (5

wt%) is homogeneously dispersed outside AC. This

small part of sulfur could transform to lithium

polysulfides and prior dissolve into the electrolyte

during the discharge process. The dissolved lithium

polysulfides in-situ formed on the electrode have

the common-ion effect to prevent the further

dissolution of lithium polysulfides from the cathode

[3, 31], which is similar with the addition of lithium

polysulfides into electrolyte [32-34].

However, the AC/S composite could only

improve the electrochemical stability for Li-S

batteries in short-term cycles. As we all known,

trees can block rain from falling to the ground

because of the obstruction from rich leaves and

branches (the inset of Figure 1e). Moreover, leaves

can adsorb the rain due to the effect between cells

and H2O. Inspired by the nature phenomenon, the

CNTP/TiO2 barrier like trees could increase the

diffusion resistance of lithium polysulfides to the

lithium anode (Figure 1b). TiO2 on the CNTP like

leaves can effectively block and adsorb lithium

polysulfides, which is favorable for the free lithium

polysulfides to controllably deposit on the

interlayer. Therefore, the soluble lithium

polysulfides in the electrolyte can be mitigated to

the anode, stored and reused by the CNTP/TiO2

barrier in the special structure of Li-S batteries. In

addition, the wastage of electrolyte seriously affects

the cycling stability of Li-S batteries. The

hydrophobic CNTP/TiO2 interlayer could adsorb

the organic electrolyte, which can be used as the

electrolyte reservoir to improve the electrochemical

performance for Li-S batteries.

In order to construct an ideal interlayer, the

flexible, mechanically strong, and electrical

conductive CNTP is used as the substrate to grow

TiO2 (Figure 1c) [35-37]. The as-prepared sample

shows a unique structure and remarkable

mechanical property (the inset of Figure 1d). The

needle-shaped rutile TiO2 (JCPDS No. 21-1276,

Figure S5) is homogeneously grown on the surface

of CNTP (Figure 1d-f). Importantly, the majority of

pore size in CNTP/TiO2 falls in the range of 2-10 nm


4 Nano Res.

(Figure S6), which allows fast diffusion of lithium

ions [38]. In addition, Energy dispersive X-ray

spectroscopy (EDS) elemental maps (Figure S7a-e)

and STEM images (Figure S7f-h) further confirm the

homodisperse of carbon, titanium and oxygen in

CNTP/TiO2 and the uniform distribution of

needle-shaped TiO2 on CNTP.

Figure 2. (a) Typical CV curves of AC/S composite electrode

with the CNTP/TiO2 barrier at a scan rate of 0.2 mV s-1. (b)

Galvanostatic charge/discharge profiles in different cycles of

AC/S composite electrode with the CNTP/TiO2 barrier at 0.5 C.

(c) Cycling performance of AC/S composite electrode, AC/S

composite electrode with the CNTP barrier, and AC/S

composite electrode with the CNTP/TiO2 barrier at 0.5 C.

The CNTP/TiO2 composite is placed between the

AC/S cathode and the separator acting as a

multifunctional barrier for the diffusion of

polysulfides. Li-S batteries with the CNTP/TiO2

barrier were cycled between cut-off potentials of

1.7-3.0 V to decrease the contribution of TiO2 to the

total capacity, which is very small in this voltage

range [39]. The cyclic voltammetry (CV) curves

show typical characteristics of the sulfur oxidation

and reduction during charge/discharge process

(Figure 2a) [40, 41]. The reduction peak at ~2.3 V

can be assigned to the open ring reduction of S 8 to

lithium polysulfides (Li2Sn, 4≤n≤8). The strong

cathodic peak at ~2.0 V involves a strong reduction

of Li2Sn to Li2S2/Li2S. The redox peak currents and

potentials show no obvious change in the

successive five cycles, indicating good cycling

stability and reactive reversibility of the AC/S

composite electrode with the CNTP/TiO2 barrier.

Two stable oxidation peaks are observed at ~2.3 and

~2.4 V in the subsequent anodic scan, which are

attributed to the oxidation of Li2S/Li2S2 to Li2Sn and

Li2Sn to S8, respectively. The galvanostatic

charge/discharge curves of the AC/S composite

electrode with the CNTP/TiO2 barrier at 0.5 C (1

C=1675 mAh g-1) show two apparent plateaus in the

discharge curve and two plateaus in the charge

curve (Figure 2b), which corroborate with the

observation of the CV curves.

The initial discharge capacity of AC/S is 1084.9

mAh g-1 (a typical sulfur mass loading of ca 0.98 mg

cm-2) at 0.5 C, much larger than the capacity of the

sulfur cathode (667 mAh g-1) in our previous report

[22]. The improved electrochemical performance for

Li-S batteries is attributed to the intimate electric

contact of sulfur with AC, thus the utilization of

sulfur is increased. However, the capacity

degradation (142.4 mAh g-1 after 250 cycles) still

occurs because soluble lithium polysulfides diffuse

to the anode and are reduced by the metal lithium

anode. In fact, the solubility of the polysulfides is

not the fundamental obstruction for the practical

application of Li-S batteries, which could be

alleviated by the common-ion effect. The key

problem is the shuttle effect that the soluble lithium

polysulfides react with the metal lithium anode in a

parasitic reaction. Thus, 3.5 wt% sulfur on the

surface of the AC/S cathode firstly transforms to

lithium polysulfides formed in-situ on the cathode

and the capacity obviously fades during the first 30

cycles (Figure 2c). Then a barrier is used to restrain

the diffusion of lithium polysulfides to the anode in

this research. Li-S batteries using CNTP as a

physical barrier have a capacity of 627.0 mAh g-1

after 30 cycles and the capacity decays to 364.2 mAh

g-1 after 250 cycles at 0.5 C. With the multifunctional

CNTP/TiO2 barrier, the discharge capacity of Li-S


5 Nano Res.

batteries only decreases from 791.7 mAh g-1 to 575.8

mAh g-1 from the 30th to 250th cycle at 0.5 C,

showing a low capacity decay rate as small as 0.1%

per cycle. Even at a high current rate of 1 C, lithium

polysulfides could in-situ form on the cathode

during the first 15 cycles (Figure S8). Li-S batteries

can deliver a discharge capacity of 767.9 mAh g-1 at

15th cycle and still remain a high capacity of 508

mAh g-1 at 100th cycle with the Coulombic

efficiency as high as 98%, representing much better

cycling stability compared with previous reports

[42]. These results demonstrate that the

multifunctional CNTP/TiO2 barrier effectively

intercepts the diffusion of dissolved lithium

polysulfides to the lithium anode. Therefore, the

multifunctional barrier of CNTP/TiO2 plays an

important role in enhancing the electrochemical

performance for Li-S batteries.

Figure 3. SEM and EDS characterization of CNTP/TiO2 after

250 cycles at 0.5 C (charged state): (a) SEM image of the

cycled CNTP/TiO2. (b-f) EDS elemental maps for the region

shown in (a). (g) STEM image of the cycled TiO2 on CNTP and

corresponding elemental mapping images of (h) titanium, (i)

oxygen and (j) sulfur.

Afterwards, the multifunctional effect mechanism

is researched by characterization of the CNTP/TiO2

barrier after 250 cycles. The CNTP/TiO2 barrier after

250 cycles was washed with 1, 3-dioxolane and 1,

2-dimethoxyethane in the glovebox. The emergence

of some particles on the multifunctional CNTP/TiO2

barrier suggests the deposition of sulfur particles

(Figure S9). And the needle-shaped TiO2 still

remains during repeated cycling. EDS elemental

maps (Figure 3a-f) indicate the uniform distribution

of carbon, titanium, oxygen and sulfur,

demonstrating the sulfur species is adsorbed by the

CNTP/TiO2 barrier. STEM image of the cycled TiO2

on CNTP and corresponding elemental mapping

images (Figure 3g-j) further confirm the

homodisperse of titanium, oxygen and sulfur.

Notablely, sulfur area overlays well with the

titanium mapping image, which indicates that TiO2

is favorable for the selective deposition and

adsorption of the sulfur species. Moreover, Ti 2p

spectra of the CNTP/TiO2 barrier have a 0.2 eV shift

to lower binding energy after 250 cycles at 0.5 C

(Figure 4a, c), showing the interaction between the

sulfur species and TiO2 [43]. Although Ti-S binding

is not found in the Ti 2p XPS spectra for the

relatively high intensity of the Ti-O binding, the

different sulfur bindings of the CNTP/TiO2 barrier

after 250 cycles at 0.5 C are observed in the S 2p XPS

spectra (Figure 4b) [18]. The peaks at 169.3 and

168.4 eV correspond to the S-O bonding. The peaks

at around 166 eV can be identified to the S-S

bonding from lithium polysulfides. The peak at

164.8 eV is attributed to the Li-S bonding. The last

peaks located at about 163 eV are contributed to the

Ti-S bonding. Moreover, the peak changes in the

Raman spectra of the CNTP/TiO2 barrier before

cycling and after 250 cycles are owing to the

interactions between TiO2 and the sulfur species

(Figure 4d). Thus, these observations combining

with the electrochemical performance demonstrate

the CNTP/TiO2 barrier is a promising candidate for

Li-S batteries.


6 Nano Res.

Figure 4. (a) Ti 2p XPS spectra for the CNTP/TiO2 barrier

before cycling. (b, c) S 2p and Ti 2p XPS spectra for the

CNTP/TiO2 barrier after 250 cycles at 0.5 C. (d) Raman spectra

of the CNTP/TiO2 barrier before cycling and after 250 cycles at

0.5 C.

In order to further unveil the absorption

mechanism of lithium polysulfides on rutile TiO2

matrix, the interactions between the sulfur species

and rutile TiO2 (110) surface were investigated by

the DFT calculation. The main theoretical capacity

of Li-S batteries is from the redox reactions of Li2S4

to Li2S2/Li2S [44]. Therefore, the Li2Sn (n=1, 2, 4)

molecules were selected for the theoretical

calculation. The representative structures of Li2Sn

trapped on TiO2 (110) surface are thermally stable

with negative absorption energy (Figure 5). The

length of Li-S bonds is in a range of 2.35~2.55 Å ,

demonstrating that Li2Sn molecules are adsorbed on

TiO2 matrix. The adsorption form of Li2S, which is

immobilized nearly on the bridge sites of Ob, is

different because the Li-S bond with a length of 3.37

Å is absent (Figure 5b). The DFT calculation results

show that the strong Ti-S interaction exists in all

structures, which could be caused by the similar

ionic bonding properties of TiO2 and Li2Sn [45]. The

S-O bond appears in the structure of Li2S adsorbed

on the nearby bridge sites of oxygen atoms (Ob,

Figure S10). Those calculation results are in good

agreement with the XPS analysis above and

demonstrate that lithium polysulfides can be well

confined on the TiO2 substrate by the chemical

binding.

Figure 5. The representative geometries after fully optimizing

show the interactions between the Li2Sn (n=1, 2, 4) molecules

and rutile TiO2 (110) surface (the fixed two bottom tri-layers

depicted in Figure S10 are not presented for the sake of clarity):

(a, b) Li2S adsorbed on TiO2 surface. (c, d) Li2S2 immobilized

on TiO2 surface and (e, f) Li2S4 trapped on TiO2 surface.

The effects of the multifunctional CNTP/TiO2

barrier for Li-S batteries are elucidated by the

schematic diagram (Figure 6). First, the CNTP/TiO2

barrier is a physical barrier to alleviate the diffusion

of lithium polysulfides to the lithium anode. Thus,

it could restrict active materials to in-situ form

lithium polysulfides on the cathode and block

lithium polysulfides from reacting with the metal

lithium. Meanwhile, the CNTP/TiO2 barrier can

function as the second current collector during

charge/discharge process. Second, the positively

charged TiO2 on the CNTP could attract the

negatively charged polysulphide anions by

electrostatic adsorption. Third, the interaction

between Lewis acidic Ti (IV) center and the


7 Nano Res.

polysulfides base could attract the escaped lithium

polysulfides. More importantly, TiO2 could

chemically absorb sulfur species by the formation of

Ti-S and S-O bonds. Consequently, Li-S batteries

with the multifunctional CNTP/TiO2 barrier display

a long life performance.

Figure 6. Illustration for the effects of the CNTP/TiO2 barrier.

3 Conclusions In summary, the AC/S composite combining with the

multifunctional CNTP/TiO2 barrier for Li-S batteries

exhibits a high reversible capacity and cycling

performance. AC could load a high content of sulfur,

greatly enhance electron transport and increase the

utilization of active materials. Meanwhile, the

multifunctional CNTP/TiO2 barrier could trap

lithium polysulfides and improve the

electrochemical performance for Li-S batteries in

long-term cycles. Therefore, the multifunctional

CNTP/TiO2 barrier is an ideal candidate for

long-cycle Li-S batteries. The concept can be

extended to other oxides (SiO2, TinO2n-1, Al2O3,

Mg0.6Ni0.4O, etc.) and metal-organic frameworks,

which have low electrical conductivity to limit the

utilization of sulfur but possess the special

interactions with sulfur species.

Acknowledgements This work is supported by the National Key Basic

Research Program 973 (No. 2014CB239701), National

Natural Science Foundation of China (No. 21173120,

51372116, 11374162, 51032002), Natural Science

Foundation of Jiangsu Province (No. BK2011030), the

Fundamental Research Funds for the Central

Universities of NUAA (NP2014403), a Project

Funded by the Priority Academic Program

Development of Jiangsu Higher Education

Institutions. The authors also thank the support of

computer resources from National Supercomputing

Center in Shenzhen.

Electronic Supplementary Material: Supplementary

material (detailed description of the experimental

procedures and calculations) is available in the

online version of this article at

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Nano Res.

Electronic Supplementary Material




Guiyin Xu1, Jiaren Yuan1, Xinyong Tao2, Bing Ding1, Hui Dou1, Xiaohong Yan1,3 (), Yang Xiao1, Xiaogang

Zhang1 ( )

Supporting information to DOI 10.1007/s12274-****-****-*



Nano Res.

Experimental Section

Preparation of AC/S and CNTP/TiO2 composite

To prepare the AC/S composite, the mixture of AC (Fuzhou Yihuan Carbon Co. Ltd., China) and element

sulfur with the mass ratio of 7:3 was heated at 155 °C for 10 h.

TiO2 was synthesized by a hydrothermal method.[1] To prepare the CNTP/TiO2 composite, CNTP (Suzhou

Jiedi Nano Science and Technology Co. Ltd., China) punched into discs with a diameter of 1.88 cm was

soaked in a solution containing 15 ml absolute ethanol and 0.33 ml titanium tetrachloride for 12 h. CNTP was

then vacuum-dried at 60 °C for 24 h and heat-treated at 400 °C for 30 min in muffle furnace, thus CNTP was

coated with the TiO2 nanoparticle seeds. 1.3 ml titanium butoxide was added into a mixture solution of

hydrochloric acid (20 ml) and acetone (20 ml). This solution was stirred until it became clear. Then, the

solution and the as-prepared CNTP were transferred into a Teflon-line stainless-steel autoclave. The

hydrothermal treatment was conducted at 200 °C for 2 h. Then, the obtained CNTP/TiO2 was repeatedly

washed with water and absolute ethanol. Finally, CNTP/TiO2 was dried under a vacuum atmosphere at

60 °C for 12 h. The TiO2 loading content on CNTP was controlled to be ~2 mg cm-2.

Characterization

X-ray diffraction (XRD) patterns were measured on a Bruker-AXS D8 DISCOVER. Copper K line was

used as a radiation source with λ=0.15406 nm. Field emission scanning electron microscopy (FESEM) and

transmission electron microscopy (TEM) measurements were carried out with JEOL JSM-6380LV FE-SEM

and FEI TECNAI-20, respectively. Scanning transmission electronic microscope (STEM) was performed on a

Tecnai G2 F30. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Perkin-Elmer PHI

550 spectrometer with Al K (1486.6 eV) as the X-ray source. The Raman spectra of CSB and ACSB were

measured by a Jobin Yvon HR800 confocal Raman system with a 632.8 nm diode laser excitation on a 300 line

S mm-1 grating at room temperature. The N2 adsorption/desorption tests were determined by

Brunauer-Emmett-Teller (BET) measurements using an ASAP-2010 surface area analyzer. The pore size

distribution (PSD) was derived from the desorption branch of the isotherm with the Barrett-Joyner-Halenda

(BJH) method. Thermal gravimetric (TG) analysis was conducted on a TG-DSC instrument (NETZSCH STA

409 PC) under a N2 atmosphere at a heating rate of 10 °C min-1 from 30 to 500 °C.

Electrochemical characterization

Electrochemical characterization of the AC/S composite electrode, AC/S composite electrode with the

CNTP barrier, and AC/S composite electrode with the CNTP/TiO2 barrier was carried out by galvanostatic

cycling in CR2016-type coin cells. The working electrodes were prepared by a slurry coating procedure. The

slurry consisted of 70 wt% active material, 20 wt% acetylene black and 10 wt% polyvinylidene fluoride

(PVDF) dissolved in N-methyl pyrrolidinone (NMP), and was uniformly spread on an aluminium foil

current collector. Finally, the electrode was dried at 70 °C overnight. Each current collector contained ca 2.0

mg cm-2 active material. Test cells were assembled in an argon-filled glove box using Li foil as the counter

electrode and polypropylene (PP) film as the separator. The CNTP barrier or the CNTP/TiO2 barrier was

placed between the AC/S cathode and the separator. The electrolyte was 1 mol L-1 LiTFSI and 0.1 mol L-1

LiNO3 in a mixed solvent of 1, 3-dioxolane (DOL) and 1, 2-dimethoxyethane (DME) with a volume ratio of

1:1. The coin cells were galvanostatically charged-discharged at different current densities between 1.7 and

3.0 V (vs. Li/Li+) using a CT2001A cell test instrument (LAND Electronic Co.). The cyclic voltammetry (CV)

measurement was conducted with a CHI 600A electrochemical workstation at a scan rate of 0.2 mV s-1 in the

voltage range of 1.7 to 3.0 V (vs. Li/Li+).


Nano Res.

Computational Section

The structural optimization and static calculation were carried out by employing the Vienna ab initio

Simulation Package (VASP),[2] which was based on density functional theory (DFT) and the projected

augmented wave (PAW) method.[3, 4] Generalized gradient approximation (GGA) in the form of

Perdew-Burke-Ernzerhof (PBE) was chosen as exchange correlation potential.[5] The electron wave functions

were expanded by a plane wave basis set with a cutoff energy of 400 eV. In addition, the surface Brillouin

zone was sampled by using a 2×2 k-points mesh generated by Monkhorst-Pack scheme. All structures were

fully optimized until none of the forces exceeded 0.02 eV Å -1.

The lattice constants of the bulk rutile TiO2 are calculated to be a=4.606 Å and c =2.955 Å respectively,

which are consistent well with the experimental values of 4.594 Å and 2.958 Å .[6] The rutile TiO2 (110) surface

is modeled using a (4×2) surface unit cell with a slab geometry consisting of four tri-layers. A vacuum layer

(15 Å ) is set in the perpendicular direction of the surface to eliminate the interaction between the slabs and

their periodic image. The two bottom tri-layers depicted in the rectangle of Figure S10 are anchored at their

initial positions. This system is called a fixed double-layer model, which has been tested to be very effective

in simulating geometrical and electronic properties of TiO2 (110) surface.[7] We estimate the adsorption

energy (Ea) by static calculations to further analyze the stability of Li2Sx (x = 1, 2, 4) on TiO2 (110) surface,

defined as the difference between the total energy of Li2Sx on TiO2 (Etot) and the energy sum of Li2Sx (2 xLi SE )

and TiO2 (2TiOE ):

2 2( )

xa tot Li S TiOE E E E .


Nano Res.

Figure S1. (a) Transmission electron microscopy (TEM) image and (b) high-resolution transmission electron microscopy (HRTEM)

image of AC. (c) N2 adsorption/desorption isotherms at 77 K and (d) Pore size distribution (PSD) curves of AC and AC/S. PSD is

calculated using the Barrett-Joyner-Halenda (BJH) method.

AC has the lamellar structure (Figure S1a) with worm-like micropores (Figure S1b). The

adsorption/desorption isotherms of AC are typical type I isotherms, indicating the presence of micropores

(Figure S1c). A clear upward trend peak (< 2 nm) in the PSD curves further confirms the abundant

micropores in AC (Figure S1d). AC has a high specific surface area (1964.8 m2 g-1, Table S1) and large pore

volume (0.91 cm3 g-1), displaying potential applications for Li-S batteries. After confining sulfur, the specific

surface area and pore volume of activated carbon/sulfur (AC/S) decrease to 7.4 m2 g-1 and 8.6×10-3 cm3 g-1,

respectively.


Nano Res.

Figure S2. (a, b) TEM images of AC/S and the corresponding elemental mapping images for (c) carbon/sulfur, (d) carbon and (e)

sulfur.

The lamellar structure of AC is still maintained in the AC/S composite (Figure S2a). HRTEM image

confirms that no large bulk sulfur could be observed outside AC (Figure S2b). The elemental mapping

images (Figure S2c-e) of carbon/sulfur, carbon and sulfur further demonstrate the uniform distribution of

sulfur in AC, which ensures the intimate contact between AC and sulfur.


Nano Res.

Figure S3. XRD patterns of S, AC, and AC/S composite.

The broad signals around 24° and 44° in the XRD pattern of AC correspond to the (002) and (100) spacing

of the graphene stacks,[8] indicating that AC has excellent electrical conductivity. XRD pattern of AC/S

indicates that sulfur is in crystalline state to exist in the AC/S composite.[9] However, the diffraction intensity

of sulfur peaks in AC/S is lower than that in sulfur under the same condition, indicating the majority of

sulfur is confined into AC and only a small part of sulfur is dispersed outside AC.


Nano Res.

Figure S4. TG curve of the AC/S composite.

The TG curve shows two weight loss stages. One weight loss stage from around 200 to 300 °C reflects the

evaporation of sulfur from the outside and mesopores of AC. The other weight loss stage from around 300 to

480 °C reflects the evaporation of sulfur from micropores in AC. These demonstrate that sulfur and

micropores have a stronger interaction. For the AC/S composite, the corresponding weight loss is

approximately up to 70 wt%, which is consistent with the proportion of the initiate amount. The extra of ca. 5

wt% indicates that a small part of sulfur is homogeneously dispersed outside AC (Table S1).


Nano Res.

Figure S5. XRD patterns of CNTP and CNTP/TiO2.


Nano Res.

Figure S6. (a) N2 adsorption/desorption isotherms at 77 K and (b) PSD curves of CNTP and CNTP/TiO2. PSD is calculated using the

Barrett-Joyner-Halenda (BJH) method.


Nano Res.

Figure S7. (a) SEM image of CNTP/TiO2. (b-e) EDS elemental maps for the region shown in (a). (f) STEM image of TiO2 on the

CNTP and corresponding elemental mapping images of (g) titanium and (h) oxygen.


Nano Res.

Figure S8. Cycling performance of AC/S composite electrode with the CNTP/TiO2 barrier at a constant rate of 1 C.


Nano Res.

Figure S9. SEM images of CNTP/TiO2 after 250 cycles at 0.5 C (charged state). The deposition of sulfur particles is shown in the

dotted circle.


Nano Res.

Figure S10. The optimized geometry of a four tri-layer TiO2 (110) slab. A (4×2) supercell is chose for obtaining physically

appropriate results. The two bottom tri-layers enclosed in the rectangle are fixed.


Nano Res.

Table S1. Physical characteristics of AC and AC/S composite

Samples BET total surface

area (m2 g-1)

Total pore

volume (cm3 g-1)

Average pore size

(nm)

AC 1964.8 9.1×10-1 1.9

AC/S 7.4 8.6×10-3 4.7

The density of sulfur is 2.06 g cm-3. As calculated below, the 1.86 g sulfur is filled in 1 g AC. The mass

percent of sulfur in the AC/S composite is close to 65%.

3 3Sulfur content g = sulfur density g cm pore volume cm

sulfur content

Sulfur weight ratio in the AC/S composite % = 100%AC/S composite

AC/S composite= sulfur content AC content


Nano Res.

Table S2. Physical characteristics of CNTP and CNTP/TiO2 composite

Samples BET total surface

area (m2 g-1)

Total pore

volume (cm3 g-1)

Average pore size

(nm)

CNTP 114.4 2.6×10-1 9.0

CNTP/TiO2 59.5 1.1×10-1 7.3


Nano Res.

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[3] Kohn, W.; Becke, A. D.; Parr, R. G. Density functional theory of electronic structure. J. Phys. Chem. 1996, 100, 12974-12980.

[4] Blöchl, P. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953-17979.

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[6] Vinet, P.; Ferrante, J.; Smith, J. R.; Rose, J. H. A universal equation of state for solids. J. Phys. C: Solid State Phys. 1986, 19, L467.

[7] Hameeuw, K. J.; Cantele, G.; Ninno, D.; Trani, F.; Iadonisi, G. The rutile TiO2 (110) surface: Obtaining converged structural properties from first-principles calculations. J. Chem. Phys. 2006, 124, 024708.

[8] Wang, H.; Xu, Z.; Kohandehghan, A.; Li, Z.; Cui, K.; Tan, X.; Stephenson, T. J.; King’ondu, C. K.; Holt, C. M. B.; Olsen, B. C.;

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[9] Zhao, Y.; Wu, W.; Li, J.; Xu, Z.; Guan, L. Encapsulating MWNTs into hollow porous carbon nanotubes: A tube-in-tube carbon nanostructure for high-performance lithium-sulfur batteries. Adv. Mater. 2014, 26, 5113-5118.

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