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A high-capacity lithium–air battery with Pdmodified carbon nanotube sponge cathodeworking in regular air

0008-6223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.carbon.2013.05.066

* Corresponding authors: Fax: +86 27 87558241 (Y. Huang).E-mail address: huangyh@mail.hust.edu.cn (Y. Huang).

1 These authors contributed equally to this work.

Yue Shen a,1, Dan Sun a,1, Ling Yu a, Wang Zhang a, Yuanyuan Shang b, Huiru Tang c,Junfang Wu c, Anyuan Cao b,*, Yunhui Huang a,*

a State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering,

Huazhong University of Science and Technology, Wuhan, Hubei 430074, Chinab Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, Chinac State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics,

Chinese Academy of Sciences, Wuhan 430071, China

A R T I C L E I N F O A B S T R A C T

Article history:

Received 8 February 2013

Accepted 29 May 2013

Available online 13 June 2013

We report a lithium–air battery with a free-standing, highly porous Pd-modified carbon

nanotube (Pd–CNT) sponge cathode. The Pd-CNT sponge was synthesized through a chem-

ical vapor deposition growth followed with an electrochemical deposition process. To build

a whole lithium–air battery, the air cathode is integrated with a ceramic electrolyte-pro-

tected lithium metal anode and non-volatile ionic liquid electrolyte. The lithium anode is

stable during the operation and long-time storage and the ionic liquid is chemically inert.

By controlling the amount of ionic liquid electrolyte, the sponge is wet but not fulfilled by

the electrolyte. Such configuration offers a tricontinuous passage for lithium ions, oxygen

and electrons, which is propitious to the discharge reaction. In addition, the existence of Pd

nanoparticles improves the catalytic reactivity of the oxygen reduction reaction. The bat-

tery is durable to any humidity level and delivers a capacity as high as 9092 mA h g�1.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The lithium–air battery is receiving world-wide interest be-

cause its theoretical specific energy far exceeds the best of

lithium-ion batteries [1–5]. By carefully designing the porosity,

conductivity and catalytic reactivity of the air cathode, the

cathode capacity of the lithium–air battery with organic elec-

trolyte can reach 15000 mA h g�1 [6]. However, the lithium–air

battery is still far from practical application due to a lot of

problems such as low discharge rate, poor cyclability and

rigorous operation condition [7]. Most of the organic

electrolyte lithium–air batteries have to be operated in pure

oxygen atmosphere to avoid the fast oxidation of the lithium

anode in humid air [8–12]. Aqueous electrolyte lithium–air

batteries with ceramic electrolyte (lithium super ionic con-

ductor, LiSICON [13]) protected anode have been developed

to overcome this disadvantage [14–20]. Nevertheless, the en-

ergy density of an aqueous electrolyte battery is much lower

than the organic electrolyte system [21]. And the evaporation

of the aqueous electrolyte in open air condition is always a

problem. All solid-state lithium–air battery with LiSICON

powder as the cathode electrolyte is an optional choice

[22,23], but the ionic conductivity at the grain boundary of

the LiSICON powder is usually too low. Obtaining high

C A R B O N 6 2 ( 2 0 1 3 ) 2 8 8 – 2 9 5 289

capacity in regular air instead of pure oxygen or saturated

water vapor is a very important direction for development

of lithium–air batteries. A very recent work took advantage

of the p–p interaction between imidazolium ions and single-

walled carbon nanotubes to form a hydrophobic gel air cath-

ode and obtained a capacity as high as 10730 mA h g�1 at 50%

relative humidity (RH), which is very encouraging [24].

In this work, we report a novel lithium–air cathode with a

free-standing, highly porous and catalytic active Pd–modified

carbon nanotube (Pd–CNT) sponge. The air cathode is inte-

grated with a ceramic electrolyte-protected lithium metal an-

ode and non-volatile ionic liquid electrolyte. The battery

design makes it durable to any humidity level and delivers a

capacity as high as 9092 mA h g�1.

2. Experimental

2.1. Synthesis of the carbon nanotube sponges

The carbon nanotube (CNT) sponges were synthesized by

chemical vapor deposition (CVD) process [25]. Ferrocene in

dichlorobenzene solution (0.06 g ml�1) was continuously in-

jected into a 2-inch quartz tube housed in a resistive furnace

by a syringe pump at a feeding rate of 0.13 ml min�1. The

reaction temperature was set at 860 �C. Carrier gas, a mixture

of Ar and H2, was flowing at a rate of 2000 and 300 ml min�1,

respectively. A 2 inch · 1 inch quartz sheet was placed in the

reaction zone as the growth substrate. The sponge-like prod-

ucts were collected from the quartz substrate after CVD,

which reached a thickness of about 0.3 mm for a growth per-

iod of 1 h.

2.2. Electrochemical deposition of the noble metal catalyston the CNT sponges

As synthesized CNT sponges were immersed in a PdCl2(3.3 mg ml�1), HCl (0.1 mol l�1), polyethylene oxide (PEO, mol-

ecule weight � 600,000, 30 mg ml�1) aqueous solution. Two

pieces of the CNT sponges were set as the anode and cathode,

respectively, for the electrochemical deposition. The current

density was set at 0.302 mA mg�1 (carbon) for 10 min to make

the weight ratio between the Pd and CNT equal to 1:10. After

the deposition, the samples were washed with DI water and

were heated to 350 �C for 4 h in Ar–H2 atmosphere (5% H2,

pressure = 15 atm) to eliminate the PEO residue.

2.3. Assembly of the lithium–air battery

Copper current collect, Li–metal disk (1 mm thick), Celgard

3501 porous polymer separator immersed with LiTFSI–

PP13TFSI (0.3 mol kg�1, from Shanghai Chengjie Chemical

Co. Ltd.) and LiSICON plate (thickness = 150 lm, from Ohara

Inc.) were assembled layer by layer in the argon-filled glove

box. Epoxy resin was used as the seal around the edge of

the assembled anode. After that, the anode part was taken

out of the glove box. The Pd–CNT sponge (thickness = 0.3 mm)

was wetted with the ionic liquid electrolyte (30–40% volume

ratio) and put onto the LiSICON surface as the cathode. No

binder was used. The average loading density of the CNT

sponge (including noble metal catalyst) was about 1 mg cm�2.

Nickel foam was put on top of the CNT sponge as the cathode

current collector. The whole cathode was directly exposed in

the open air.

2.4. Electrochemical measurements

The cells were tested in a special blasting drying oven in

which there were cables connected to the battery tester. The

RH outside the oven was controlled with a humidity sensor,

a humidifier and a dehumidifier and the temperature outside

the oven was maintained at 25 �C with an air-conditioner. To

test the discharge performance in RH = 0% condition, the cell

was put in a beaker which had cotton and CaO powder above

the cell. The cells were discharged to 2.0 V to measure the ini-

tial capacity. To test the cyclability at low discharge depth, the

cells were firstly discharged to 2000 mA h g�1 and then cycled

at 1000 mA h g�1. The specific capacity was calculated by the

mass of the whole cathode (Pd + CNT). The electrochemical

impedance spectroscopy (EIS) of the cells was also measured

in the blasting drying oven at the working condition. The fre-

quency range applied was from 1 MHz to 30 mHz at potentio-

static signal amplitudes of 10 mV.

2.5. SEM, TEM, XRD characterizations

The morphology of the air cathode was observed with scan-

ning electron microscope (SEM, SIRION200) before and after

discharge. Transmission electron microscopy (TEM) observa-

tion was carried out on a JEOL 2100F microscope. The dis-

charge product was characterized by X-ray diffraction (XRD,

PANalytical B.V., Holland).

2.6. NMR analysis of the electrolyte

To prepare the sample, the whole air cathode with electrolyte

was soaked into dimethyl sulfoxide-D6 containing 0.03% tet-

ramethylsilane (Cambridge Isotope Laboratories, Inc.) to

make a solution. All 1H NMR (nuclear magnetic resonance)

spectra were acquired at 298 K on a Bruker AVIII 600 MHz

spectrometer equipped with an inverse cryoprobe using a sin-

gle pulse sequence with a recycle delay of 2.0 s (RD-90�-AQ).

Sixteen transients were collected into 32000 data points for

each spectrum with a spectral width of 20 ppm. 19F NMR spec-

tra were acquired with a single pulse sequence (RD-90�-AQ)

on a home-build 500 MHz spectrometer equipped with an

OneNMR probe (Varian Inc.). The 90� pulse length was 7.8 s

and 16 transients were also collected into 32 k data points

with a spectral width of 30 ppm.

3. Results and discussion

3.1. Battery configuration

Our battery design has overlapping features from both the or-

ganic electrolyte design and the aqueous electrolyte design.

The basic principle is like the organic electrolyte design: the

lithium ions come from the lithium metal anode transport

through the electrolytes to the porous air cathode and react

Fig. 2 – Photo of the lithium metal anode taken from the

battery after discharge–charge and 2 months storage.

290 C A R B O N 6 2 ( 2 0 1 3 ) 2 8 8 – 2 9 5

with the oxygen to form solid state discharge products;

whereas a ceramic lithium super ionic conductor (LiSICON)

plate was added in between the cathode and the anode, like

the aqueous electrolyte design, to protect the lithium metal

anode (Fig. 1). With this design, the lithium metal anode is to-

tally isolated from the oxygen and water in the air. So it is sta-

ble during the operation and longtime storage. Fig. 2 shows

that the surface of the lithium metal anode was still shiny

after discharge–charge and 2 months storage. (If the lithium

metal is exposed in humid air, the surface will become dark

after a few minutes.)

3.2. Structure of the Pd–CNT sponge

To meet the need for a conductive, catalytically active porous

framework of the air cathode, we synthesized Pd–CNT

sponges (Fig. 3) by CVD growth and electrochemical deposi-

tion. The CNT sponge [25,26] is a free-standing sponge-like

bulk material consisting of self-assembled, interconnected,

highly conductive (conductivity = 0.5–1 S cm�1) multi-walled

CNT skeletons, with a density close to the aerogels (den-

sity = 5–10 mg cm�3), porosity of more than 99% and average

pore size of 80 nm [25]. Besides, unlike other carbon nanotube

or carbon nanofiber materials that have a natural tendency to

aggregation and entanglement, the loose structure of CNT

sponge is quite robust. Its surface area is not so large (300–

400 m2 g�1) [25] comparing with active carbon (higher than

2000 m2 g�1) but the surface area is mainly from the macrop-

ores instead of micropores in active carbons. Considering the

solid product will easily fulfill the micropores in the discharge

process, the surface area from micropores is not important.

To improve its catalysis reactivity, traditional oxygen reduc-

tion reaction (ORR) catalyst Pd nanoparticles were deposited

on the surface of the CNT through electrochemical process.

The morphologies are shown in Fig. 3a and b. Comparing

Fig. 3b and c, we can see the Pd particles on the CNTs. The

uniform coating of Pd nanoparticles is also confirmed by en-

ergy dispersive X-ray spectroscopy (EDX) and elemental map-

ping image, as shown in Fig. 3e. The EDX-measured weight

Fig. 1 – Scheme of the battery configurat

ratio of Pd to C is 1:18. The observed Fe signal comes from

the catalyst used in the CNT growth process.

3.3. Liquid electrolyte selection

Ionic liquid lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)

in N-propyl-methyl-piperidinium-bis(trifluoromethylsulfo-

nyl)imide (PP13TFSI) solution (0.3 mol kg�1) was chosen as

the cathode electrolyte. This electrolyte has a very low vapor

pressure so that it will never dry out. It has excellent wettabil-

ity to the graphitic carbon surface. Since it is impossible to

measure the contact angle of the ionic liquid on CNT sponge,

we measured the contact angle of the ionic liquid on a com-

pressed graphite pellet surface that also consists of sp2 C

atoms, as displayed in Fig. 4. And more importantly, it doesn’t

contain ether or carbonate group which are easily attacked by

the nucleophilic O2� and decomposes [27–29]. The only possi-

ble point to be attacked by O2� in PP13TFSI is the N atom in

the center of the PP13 cation. However, since the alkyl groups

bonded to the N atom are very poor leaving groups, there

won’t be any reaction. It should be mentioned that dimethyl

sulfoxide (DMSO) based electrolytes could also be an optional

ion before (a) and after (b) discharge.

Fig. 3 – (a) SEM image (b and c) TEM images of the Pd–CNT sponge (b) and the pure CNT sponge (c). (d) Optical photo of a piece

of Pd–CNT sponge. (e) EDX of the Pd–CNT sponge and the corresponding SEM image and Pd elemental mapping image.

Fig. 4 – The contact angle between LiTFSI–PP13TFSI ionic

liquid and compressed graphite surface.

C A R B O N 6 2 ( 2 0 1 3 ) 2 8 8 – 2 9 5 291

choice [10,30–32]. In this case, we did not explore them in this

work considering the evaporation problem.

The volume ratio of ionic liquid electrolyte to CNT sponge

was controlled to 30–40% so that the electrolyte was able to

wet the surface of the Pd–CNT but not enough to fulfill the

pores. In such an air–cathode, the Li ions can easily move to

the surface of the Pd–CNT through the electrolyte. The highly

conductive multi-walled CNT network serves as the highway

for electron transferring. Meanwhile, the oxygen molecules

are free to diffuse through the unfilled pores. Even after dis-

charge, the solid discharge products can stay in the space in

between the CNTs. Therefore, a good discharge performance

can be expected.

3.4. Discharge performance of different air cathodes

We compared the discharge performance of CNT sponge

based air cathodes and conventional Super P at 55 �C and

50% RH (outside the oven), as shown in Fig. 5. The purpose

of heating the battery was to lower the resistance of LiSICON

and accelerate the electrochemical reaction. At a low dis-

charge current density (0.05 mA cm�2), the specific capacity

of pure CNT sponge without the catalyst was 6424 mA h g�1.

This value was almost three times of that of Super P – the

common carbon material used in the previously reported lith-

ium–air batteries [10]. But the discharge voltage plateau was

only 2.45 V, which was 0.2 V below Super P. A possible reason

is that CNT sponge has fewer defects on the surface than

super P. Previous studies showed that on graphene surface,

Li2O2 prefers to nucleate and grow near functionalized lattice

defect sites or dangling r-bonds (sp3 carbon atoms) at the

edges [6,33]. Similar defect structures are more likely to be

found on super P surface rather than the smooth and perfect

CNT surface. The deposition of Pd nanoparticles on the sur-

face of CNT sponges effectively make up this shortage. As a

noble metal catalyst, the Pd particles would catalyze the

ORR on its surface. Only a small amount of Pd can effectively

accelerate the nucleation and growth of the solid discharge

products. After Pd modification, the discharge voltage plateau

increased to 2.65 V, and at the same time the capacity in-

creased to 9092 mA h g�1. As the discharge current density in-

creases, the influence of the catalyst becomes more

significant. At a current density of 0.2 mA cm�2, the capacity

of pure CNT sponge decreased to 910 mA g�1, whereas the

capacity of Pd–CNT sponge remained at 4930 mA g�1.

3.5. Influence of the temperature and the environmentalhumidity

The influence of the temperature and the environmental

humidity to the discharge performance was studied, as pre-

sented in Fig. 6. Lowering the temperature will deactivate

some of the reactive points in the air cathode and increase

Fig. 6 – (a and b) Discharge curves of Pd–CNT sponge cathode

at different battery temperature (a) and different

environmental humidity (b). (c) Discharge curves of different

cathode materials at 25 �C, 50% RH.

Fig. 5 – Discharge curves of different cathodes at various

current densities: (a) 0.05, (b) 0.1 and (c) 0.2 mA cm�2.

292 C A R B O N 6 2 ( 2 0 1 3 ) 2 8 8 – 2 9 5

the resistance of LISICON, thus both of the discharge voltage

and the capacity become lower. The humidity in the air will

cause the formation of LiOH which leads to a higher discharge

voltage. So the discharge curve in dry air (filtered with CaO

powder) was about 0.1 V lower than that in humid air. The

capacity in 50 and 95% RH both exceeded 6500 mA g�1; the

capacity in dry air was also higher than 5500 mA g�1. Above

all, we used a LiSICON plate to completely isolate the lithium

metal anode and allow the solid LiOH formation at the cath-

ode. The discharge capacity is comparable to lithium–oxygen

batteries with only Li2O2 formation. With proper temperature

maintaining system, our battery can tolerate any moisture

level.

3.6. Characterization of the discharge products and EISanalysis

The discharge product on the cathode was investigated with

SEM (Fig. 7a) and XRD (Fig. 7b). The results show that the dis-

charge products consist mainly of LiOHÆH2O and Li2O2

Fig. 7 – (a) SEM image of the discharge products. (b) XRD pattern of the cathode material before and after discharge. The

vertical lines at the bottom indicate standard peak positions from powder diffraction files. (c) EIS of the battery at different

discharge states.

C A R B O N 6 2 ( 2 0 1 3 ) 2 8 8 – 2 9 5 293

nanocrystals uniformly grown on the surface of the CNT

sponge. The peaks from Li2O2 are about 0.2� shifted which

showed that there is a strain in the Li2O2 which comes from

the lattice mismatch between the LiOHÆH2O and the Li2O2.

Unlike the conventional LIBs, the termination of the dis-

charge process in Li–air batteries has nothing to do with the

insertion of Li ions into the cathode material, but is related

with the accumulation of the solid discharge products that

may hinder further electrochemical reaction. With the dis-

charge process going on, most of the reactive points on the

CNT sponge are covered with discharge products, so that

the charge transfer resistance becomes higher and the voltage

drops. This mechanism agrees well with EIS in Fig. 7c. The

charge transfer resistance, which is reflected by the radius

of the semi-cycle [34], increases with the discharge process.

3.7. Cyclability of the battery

The cycle performance of the battery is tested with different

discharge–charge modes. If the battery is fully discharged

and charged between 2.0 and 4.5 V, the capacity decreases

to less than 1/20 of the initial value after only four cycles

(Fig. 8). That is mainly because the discharge products are

in solid state. In the charging process, after the oxidation of

inner part of the discharge products, the outside part of the

Fig. 8 – The cycle performance of the battery with fully

discharge–charge.

discharge products would be out of contact with the catalyst

so that the electrochemical reaction is terminated.

If the discharge–charge depth is controlled to 1000 mA h g�1,

the cyclability is much improved. The discharge end potential is

remained at2.2 Vafter 16cycles (Fig. 9).There are alwaystwo po-

tential plateaus in the charging process, corresponding to the

oxidation of Li2O2 and LiOHÆH2O, respectively. The whole cycling

experiment lasted for about 1 week. During this period, it is pos-

sible for carbon dioxide in the air to react with the discharge

products and hence to form Li2CO3. Although there is no Li2CO3

peak in the XRD pattern of the discharge product, we do observe

Fig. 9 – The cycle performance of the battery at restricted

discharge–charge depth. (a) The discharge–charge curves. (b)

The voltage at the end of discharge in different cycles.

Fig. 10 – 1H NMR spectra (left column) and 19F NMR spectra (right column) of the cathode electrolyte before discharge (a) after

fully discharged to 2.0 V (b) and after fully charged to 4.5 V (c). The red numbers indicate signal assignments. The blue

numbers represent the integrated peak area. (For interpretation of the references to color in this figure legend, the reader is

referred to the web version of this article.)

294 C A R B O N 6 2 ( 2 0 1 3 ) 2 8 8 – 2 9 5

some small bubble formation when the discharged cathode is

treated with diluted HCl. Li2CO3 cannot be oxidized during the

charging process, which may be the reason of the poor cyclabil-

ity. Therefore, developing new technology to eliminate CO2 con-

tamination is very important for lithium–air batteries.

3.8. Stability of the ionic liquid electrolyte

The composition of the ionic liquid electrolyte before dis-

charge, after discharge and after 1 cycle of discharge–charge

was investigated with NMR spectroscopy (Fig. 10). The PP13

cation was characterized with 1H NMR and the TFSI anion

which does not contain any hydrogen was characterized with19F NMR. All the peaks from the organic electrolytes main-

tained the same after electrochemical reactions. The only dif-

ference in the spectroscopy before and after discharge–charge

was that the peak of H2O became stronger. No peak from any

electrolyte decomposition product was observable. This result

is quite encouraging since a lot of other organic electrolytes

such as ethers and organic carbonates decompose into Li2-

CO3, HCO2Li, CH3CO2Li, CO2 and other side-reaction products

after discharge [27–29].

4. Conclusions

The CNT sponge obtained from CVD growth is an excellent

cathode skeleton for lithium–air batteries due to its porous,

conductive, free-standing nature. Electrochemical deposition

of Pd effectively improves its catalytic activity. When Pd–

CNT sponge is wetted with an ionic liquid electrolyte and

integrated with a ceramic electrolyte protected Li–metal an-

ode, the battery can tolerate regular air with any humidity le-

vel and delivers a capacity as high as 9092 mA h g�1. Our

results indicate a promising way to achieve practically usable

lithium–air batteries with high capacity.

Acknowledgements

We acknowledge financial supports from the China Postdoc-

toral Science Foundation (2012M510178), Natural Science

Foundation of China (51202076, 20825520) and Ministry of Sci-

ence and Technology of China (2011YQ12003503). The authors

thank Analytical and Testing Center of HUST for XRD and

SEM measurements. A. Cao acknowledges financial support

from the Beijing Natural Science Foundation (Program No.

8112017) and Prof. Dehai Wu and Kunlin Wang for help in pre-

paring carbon nanotube sponge samples.

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