Tin oxide and carbon composite (Sn6O4(OH)4/AG) as the anode in a lithium ion battery

TSINGHUA SCIENCE AND TECHNOLOGYISSN 1007-0214 06/20 pp554-560Volume 10, Number 5, October 2005

Tin Oxide and Carbon Composite (Sn6O4(OH)4 /AG) as

the Anode in a Lithium Ion Battery

LIN Kezhi ( ), WANG Xiaolin ( )**

Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

Abstract: A tin oxide and carbon composite (Sn6O4(OH)4/AG) with a Sn content of 0.15-0.30 was prepared by

chemical deposition at normal pressures and temperatures. The structures of the artificial graphite (AG), the

Sn6O4(OH)4, and the Sn6O4(OH)4/AG were analyzed using X-ray diffraction. The electrochemical lithiation

was investigated by measuring the galvanostatic charge and discharge ratio. The electrochemical capacities

of the three materials during the first discharge were 310 mAh/g (AG), 616 mAh/g (Sn6O4(OH)4/AG), and

1090 mAh/g (Sn6O4(OH)4). The discharge capacity of the Sn6O4(OH)4/AG was larger than the simple sum of

the capacities provided by AG and Sn6O4(OH)4 with the same content. The cyclic performance of

Sn6O4(OH)4/AG was also better than that of Sn6O4(OH)4 for voltages of 0 to 3 V. The results imply that the

interaction between Sn and C in Sn6O4(OH)4/AG is very strong and effectively inhibits the volume expansion

of the Sn.

Key words: electrochemical lithiation; artificial graphite; Sn-based material

Introduction

A current area of intense research activity in lithium

ion batteries is the development of new anode

materials[1-3]

. The anode materials are usually

classified into three classes based on the

electrochemical lithiation mechanism. The first is

carbon materials[4-7]

which include graphitized carbon

(artificial graphite (AG), natural graphite, mesocarbon

microbeads, etc.), and disordered carbon (coke, pitches,

etc.). Different graphite intercalated compounds (GICs)

are formed by electrochemical lithiation of graphitized

carbon. Carbon materials usually have good cyclic

performance and little volume change during lithiation

and de-lithiation processes. However, the electroche-

mical capacity of the GICs in the first stage is only

372 mAh/g. The second class of materials includes tin,

silicon, aluminum, and their oxides. Tin and tin oxide

have received much attention[8-12]

. They can form

various alloys with lithium and have different electro-

chemical capacities. However, the volume changes of

the Sn and Sn oxide during electrochemical lithiation

and de-lithiation cause progressive cracking, which

induces capacity fading[13]

. The cyclic performance of

this second class of materials can be effectively

improved by reducing the dimension of the anode[9,11]

.

The third class of materials is a combination of the first

two kinds, especially the combination of carbon

materials and tin or tin oxide materials[14-19]

, which are

referred to as tin oxide and carbon composites in this

paper. The carbon materials in the tin oxide and carbon

composites act as a matrix coated with tin oxide. The

tin oxide and carbon composite has larger electro-

chemical capacity than the carbon material alone and

smaller volume charges than tin oxide during electro-

chemical lithiation and de-lithiation. The advantages of

combining the carbon and tin oxide materials into a

composite include better charge and discharge stability

and larger electrochemical capacities. The three classes

of materials are summarized in Table 1.

Received: 2004-10-16

To whom correspondence should be addressed.

E-mail: [email protected]; Tel: 86-10-62794741

LIN Kezhi et al Tin Oxide and Carbon Composite (Sn6 O4 (OH)4 /AG) Served as 555

Table 1 Anode materials for lithium-ion batteries and their preparation and electrochemical capacities

Anode material Preparation method Electrochemical capacities Reference

Disordered carbon Heating organic precursors at 700 Much larger than the theoretical capacity [4]

Natural graphite Madagascar natural graphite 360 mAh/g at 0 to 2 V [5]

Artificial graphite Timrex (Lonza) KS-44 300 mAh/g at 0 to 1.7 V [6]

MCMB (Mesocarbon microbead) Commercially available 330 mAh/g at 0 to 1 V [7]

Sn2P2O7 Solid state reaction at 900 in flowing nitrogen 530 mAh/g for the first cycle at 0 to 1.2 V [8]

SnO2 Low-pressure chemical vapor deposition at 675 500 mAh/g for the 40th cycle at 0.04 to 1.15 V [9]

Sn-Ni alloy Electrolytic deposition on a copper foil 80 mAh/g for the 40th cycle at 0.01 to 1.2 V [10]

SnS2

Sonichemical procedure in aqueous solution

(nano-scale)

400 mAh/g of annealed SnS2 for first cycle at 0 to

2 V [11]

Sn (33 wt.%)/ACF (Actived

carbon fibers)

ACF impregnated in SnSO4 solution and heat

treated at 1000200 mAh/g at 0 to 1.5 V [14]

Sn (20 wt.%) / pitch Pyrolysis of pitch containing various organotin

compounds at 800ºC 398 mAh/g for the first cycle at 0 to 1.5 V [15]

Sn (0.044 wt.%) / graphite Vapor phase and molten salt techniques at 400

with hydrogen

300 mAh/g for the first five cycles at 0.04 to

1.2 V [16]

SnO2 (56 wt.%) / sugar carbon Heat treating a mixture of colloidal SnO2 and

sucrose at 650300 mAh/g after 60 cycles at 0 to 1 V [17]

SnO2 (2 wt.%) / graphite Pechine process followed by heat treatment at

500

Maximum specific capacity: 350 mAh/g at 0 to

1 V [18]

SnSb0.5 (50 wt.%) / CNTs (Carbon

nanotubes prepared by CVD ) Chemical reduction in aqueous solution 400 mAh/g after 70 cycles at 0 to 2.5 V [19]

The preparation methods for the tin oxide and

carbon composites are complicated and most need heat

treatments at temperatures higher than 400 to obtain

better performance. The electro-chemical lithiation

voltage ranges are usually 0 to 1.2 V which is narrower

than that of carbon materials. The objective of this

work is to develop a new simple method to prepare a

tin oxide and carbon composites. The electrochemical

lithiation properties of the composite for the anode of

lithium ion batteries are then measured and compared

with those of AG and tin oxide materials over a wide

voltage range of 0 to 3 V. The lithiation mechanism of

the composite is also investigated using chronoam-

prometry.

1 Experimental

1.1 Preparation

The tin oxide and carbon composite and the tin oxide

were prepared by dropping an excess ammonia

solution of KBH4 into a hydrochloric solution of SnCl2

with or without AG with stirring at normal pressure

and room temperature. The ratio of tin to carbon in the

solution was in the range of 0.15-0.30. The solution

was then stirred for about 1 h. The AG used in the

experiment was obtained from the Osaka Gas

Company. The samples were washed, filtered, and

dried at 90 for 5 h to remove the adsorbed water.

1.2 Analysis of the structure and components

The structure and morphology of the three samples, the

AG, the tin oxide, and the tin oxide and carbon

composite, were analyzed by X-ray diffraction (XRD,

Bruker D8) and a scanning electron microscope (SEM,

KYKY2000). The structural stabilities of the samples

were analyzed by thermogravimetry analysis (TGA,

Shimadzu DTG60). The tin content in the tin oxide and

carbon composite was measured using X-ray

fluorometry (XRF, XRF-1700).

1.3 Electrochemical characterization

The electrochemical lithiation characteristics of the

samples were measured by using them as the anode of

a lithium ion battery. Lithium sheets were used as the

counter and the reference electrode. The electrolyte

was 1 mol/L LiPF6 /EC+DMC (1:1 by volume). The

Tsinghua Science and Technology, October 2005, 10(5): 554 560556

galvanostatic charge and discharge were carried out on

the batteries with current density of 30 mA/g. The

galvanostatic voltage was in the range of 0.005 V- 3 V.

The differential capacity profiles were obtained from

the galvanostatic data. The chronoamprometry

measurements used an EG&G Potentiostat/Galvonostat

(Model 283) system after five charge and discharge

processes. The sample content used for the electro-

chemical lithiation was 0.15 which corresponded to a

Sn6O4(OH)4 content of 0.18.

2 Results and Discussion

2.1 Morphology and structure

Figure 1 shows the XRD patterns of the two samples.

All peaks identified as * in the pattern for the Sn-based

compound matched well with the Bragg peaks of tin

oxide hydroxide, Sn6O4(OH)4 (JCPDS 46-1486). The

XRD patterns of the prepared samples were the same

as for the samples dried at 120 . The TGA of the

prepared samples showed no structural changes for

temperatures from 30 to 120 . Therefore, the Sn-

based material was referred to as Sn6O4(OH)4 and the

Sn-C complex as Sn6O4(OH)4/ AG.

Fig. 1 XRD patterns of Sn6O4(OH)4/AG and Sn6O4(OH)4

* and # indicate the characteristic diffraction peaks ofSn6O4(OH)4 and graphite.

The deposit reaction is 2

6 4 4 26Sn 12OH Sn O (OH) 4H O (1)

The d002 of AG in the Sn6O4(OH)4 /AG was 0.3346 mm,

which was smaller than the d002 of 0.3351 nm for the

original AG. The reason was not clear.

Figure 2 presents the SEM images of Sn6O4(OH)4/

AG and Sn6O4(OH)4. In Fig. 2a, the blocks were AG

with grains sizes of 10-40 µm. Sn6O4(OH)4 grains

smaller than 2 µm were deposited on the AG blocks.

Most of the grains were bound to the surfaces of the

AG blocks or the edges of the AG layers. The

remaining grains were distributed in the interspaces

between AG blocks. Some grains agglomerated

together to form larger particles with sizes of about 5

µm. The co-existence of larger agglomerates and

smaller particles enhanced the stacking density of the

material and the energy density when electrochemi-

cally lithiated. As seen in Fig. 2b, the Sn6O4(OH)4

grains had a uniform size of 1-2 µm, which was almost

as same as in the Sn6O4(OH)4 / AG.

Fig. 2 SEM images of Sn6O4(OH)4/AG (a) andSn6O4(OH)4 (b)

2.2 Electrochemical characterization

Figure 3 shows the differences of the electrochemical

characteristics for AG, Sn6O4(OH)4/AG, and Sn6O4(OH)4.

For AG during the first charging process, the voltage

plateau of 1.0 V to 0.3 V corresponded to the decom-

position of the electrolyte and the formation of the

solid electrolyte inter-phase (SEI) membrane. The

voltage plateau of 0.3 V to 0.005 V corresponded to the

formation of GICs[20]

. During the second charging

process, the voltage plateau at 1.0 V to 0.3 V

disappeared. The first and second discharge plots were

almost the same.

The Sn6O4(OH)4 on the AG was reduced during the

first lithiation reaction in the following reactions in the

voltage plateaus of 1.3 V and 1.0 V.


Fig. 3 Charge and discharge profiles of AG, Sn6O4(OH)4/AG, and Sn6O4(OH)4. The current density was 30 mA/g.

+

6 4 44Li Sn O (OH) 4e 4LiOH 4SnO 2Sn

2

(2)

22Li 2LiOH 2e 2Li O H (3)

The voltage plateau at 0.8 V corresponded to the

decomposition of the electrolyte at the surface of the

anode to form the SEI membrane. Because the

lithiation mechanism of tin oxide was not the same as

for the carbon material, there were more voltage

plateaus Sn6O4(OH)4/AG during the charging process.

There are several tin-lithium alloys which include

Li0.57-1.0Sn, Li1.0-2.33Sn, Li2.33-2.5Sn, Li2.33-2.5Sn, and

Li3.5-4.4Sn formed during the lithiation of tin or tin

oxide. The voltage plateaus from 0.7 V to 0.3 V

corresponded to the alloying reaction of Li+ with Sn.

The followed reaction at about 0.5 V may occur only

in the first cycle.

(4) +

22Li SnO 2e Li O Sn

The plateaus at 0.3 V to 0.005 V were the classic

potential plateaus corresponding to lithium intercalated

into the graphite to form different stages of GICs[20]

. In

addition to the discharge capacity provided by the AG,

the discharge capacity provided by the Sn6O4(OH)4

was about 400 mAh/g. The capacities of 100 mAh/g

were due to the breakage of the Li O bond in the Li2O

formed during the charging at voltages above 1.2 V.

The charge and discharge characteristics of

Sn6O4(OH)4 were similar to those of Sn6O4(OH)4 /AG

except for the reaction of Li+ with AG in the

Sn6O4(OH)4/AG. The discharge capacities of

Sn6O4(OH)4 were the largest of the three materials

during the first two cycles. The sum of the capacities

of AG (82%) and Sn6O4(OH)4 (18%) was about 400

mAh/g. The discharge capacities of the Sn6O4(OH)4/

AG at the first charge and discharge cycles were larger

than 400 mAh/g. The extra capacity may be due to the

interaction of Sn with C in the Sn6O4(OH)4/AG.

Therefore, the Sn6O4(OH)4 and the AG in the

Sn6O4(OH)4/AG are not just a simple mixture. More

investigations are needed to further understand the

reactions causing the increased capacity.

The electrochemical characterizations of the three

samples are further compared in the diffraction

capacity plots in Fig. 4. The peaks in the differential

capacity plots indicate that reactions occurred at those

voltages. The voltage peaks for AG indicate the

formation voltages of three stages of GICs. The peaks

for Sn6O4(OH)4 clearly show the characteristic reaction

of Sn with Li. The peak at 0.61 V indicates the classic

de-lithiation of Sn-Li alloy but there are no

corresponding peaks for the de-lithiation in

Sn6O4(OH)4 /AG. During the charging and discharging

of Sn6O4(OH)4, the peaks corresponding to the reaction

of Li+ with Sn are much sharper than for Sn6O4(OH)4/

AG perhaps due to the small amount of Sn and the lack

of Sn agglomerations in Sn6O4(OH)4 /AG.

Fig. 4 Differential capacity dQ/dV vs. voltage transferred from the data of Fig. 3 of AG, Sn6O4(OH)4/AG, and Sn6O4(OH)4


Figure 5 illustrates the cyclic performance of AG,

Sn6O4(OH)4 /AG, and Sn6O4(OH)4. In the first several

cycles, the discharge capacities of Sn6O4(OH)4 were

larger than those of Sn6O4(OH)4 /AG. However, after

the sixth cycle the discharge capacities of Sn6O4(OH)4 /

AG were larger than those of Sn6O4(OH)4 and the

coulomb efficiencies were approximately equal to

those of AG. At the tenth cycle, the discharge

capacities of the three materials were 220 mAh/g (AG),

451 mAh/g (Sn6O4(OH)4/AG) and 368 mAh/g

(Sn6O4(OH)4). The utilization of Sn in Sn6O4(OH)4/AG

was approximately 100%. Buiel and Dahn[5]

observed

that keeping the charging and discharging voltage in

the range of 1.3 V-0.4 V can reduce the formation of

lower Li-Sn phase alloys and decrease the volume

changes, thereby reducing the capacity loss during

cycling. However, Sn6O4(OH)4 loaded on AG

possessed a stable cycling performance even for

voltages of 0.005 V-3 V because the Sn6O4(OH)4 on

AG provided a sort of in situ buffer of Li2O formed

during the first charging process to accommodate the

Sn volume change. The interaction between AG and Sn

also restricts the Sn volume changes to some extent.

Fig. 5 Change of charge and discharge capacity vs. cycle number of AG, Sn6O4(OH)4/AG, and Sn6O4(OH)4

2.3 Mechanism of lithium intercalation into Sn6O4(OH)4 /AG

Chronoamprometry is an effective method to

investigate the diffusion of lithium into the host

materials. Figure 6 shows the function of id vs. t and

t–1/2 of lithium intercalated into Sn6O4(OH)4 /AG

measured in the voltage ranges of 0.535 V to 0.525 V

(1), 0.295 V to 0.285 V (2), and 0.15 V to 0.115 V (3)

using chronoamprometry. The initially large current

corresponded to the electrochemical reduction. For

t >100 s, id is related linearly to t –1/2 indicating that

diffusion controlled the whole process. The linear

region is in agreement with Cottrell’s equation[21]

.

(5) 1/ 2

d 0Li( / )i nFA C D t

whereLi

C is the variation of the Li+ concentration

and D0 is the diffusion coefficient of Li+ in

Sn6O4(OH)4 /AG. Though the volume changes are large

for the reaction of Li+ with Sn

[22], the Sn was only 15%

of the total Sn6O4(OH)4 /AG material, soLi

C can

be assumed to be approximately constant. The

electrical quantities for each potential step were

obtained by integrating the chronoamprometry plots.

Figure 6b shows id plotted versus t –1/2 obtained from

Fig. 6a. A linear regression fit of the data was used to

determine the slope . The following relationships

are based on Eq. (5) and Coulomb’s law:

d

1/ 2

i

t (6)

(7) intLiC Q

+

2

d

1/ 2

Li

iD

t C (8)

Substituting Eqs. (6) and (7) into Eq. (8) gives2

int

DQ

(9)

The experimental measurements of and can

be used in Eq. (9) to determine the ratios of the

diffusion coefficients for the different potentials. The

ratio of D

intQ

0.125 V: D0.295 V : D0.535 V was 0.56:1.12:1. The

voltage range of 0.535 V-0.525 V corresponded to the

formation of Li7Sn3[3]

. The voltage range of

0.125 V-0.115 V corresponded to the formation of the

second stage of GICs[20]

. If the volume changes are

considered, the ratios at 0.295 V and 0.535 V would be

even larger. D0 decreased as the lithium reacted with

the active materials. At the plateau of 0.295 V to 0.285

V which had no reaction, D0 was the largest of the

three values. D0 at 0.535 V larger than that at 0.125 V

may implies that the lithiation of Sn proceeds more

easily than that of AG, which may be the reason why

the capacity provided by AG in Sn6O4(OH)4 /AG was

less than the original AG.

If the volume change of the Sn6O4(OH)4 /AG is the

same as for graphite whose volume change from C to


LiC6 is 0.027 mol/cm3, the diffusion coefficient

obtained from Eq. (5) is approximately 1 10–7

–

1 10–8

cm2

/s.

Fig. 6 Function of id vs. t and t–1/2 of lithium intercalated into Sn6O4(OH)4 /AG measured at voltage ranges of 0.535 V-0.525 V (1), 0.295 V-0.285 V (2), and 0.125 V-0.115 V (3) using chronoamperometry

3 Conclusions

A tin oxide material (Sn6O4(OH)4) and a tin oxide and

carbon composite (Sn6O4(OH)4/AG) were prepared by

chemical deposition. The AG, Sn6O4(OH)4, and

Sn6O4(OH)4/AG were then used as anodes of lithium

ion batteries. The electrochemical tests results show

that the electrochemical discharge capacity of

Sn6O4(OH)4/AG is larger than those of AG and

Sn6O4(OH)4 after the seventh charge and discharge

cycle and that the coulomb efficiency was larger than

that of Sn6O4(OH)4. The electrochemical capacity of

Sn6O4(OH)4/AG was not the simple sum of the

capacities of the AG and Sn6O4(OH)4. The extra

capacity may result from the interaction between Sn

and C. The results show that the interaction effectively

restricts the volume change and the agglomeration of

Sn. Further investigations are needed to study the

coulomb efficiency enhancement. The complex of

Sn6O4(OH)4 /AG will be a good candidate for the

anode of lithium ion batteries.

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Tsinghua University and University of Waterloo Sign MOU

Tsinghua University and the University of Waterloo signed their first Memorandum of Understanding for

Educational and Scientific Cooperation on June 17, 2005.

As one of Canada’s leading research universities, the University of Waterloo was ranked most innovative among

47 universities in Canada in 2004, according to the national magazine Maclean’s.

Its School of Computer Science has been ranked first across Canada and is famous in the world.

This MOU will provide a platform for the exchange between the two universities and will promote the close

cooperation between the faculty members, departments and research institutes of both universities.

(From http://news.tsinghua.edu.cn)

Tin oxide and carbon composite (Sn6O4(OH)4/AG) as the anode in a lithium ion battery

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