TixSn1−xO3 solid solution as an anode material in lithium-ion batteries

Electrochimica Acta 72 (2012) 186–191

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Tix Sn1−x O3 solid solution as an anode material in lithium-ion batteries Jingdan Yan a,c , Huaihe Song a,∗ , Huijuan Zhang a , Jiayan Yan a , Xiaohong Chen a , Feng Wang a,∗ , Huiying Yang b , Manabu Gomi c a b c

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029 PR China Pillar of Engineering Product Development, Singapore University of Technology and Design, 279623, Singapore Graduate School of Engineering, Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan

a r t i c l e

i n f o

Article history: Received 8 February 2012 Received in revised form 26 March 2012 Accepted 3 April 2012 Available online 10 April 2012 Keywords: SnO2 TiO2 Li-ion batteries Anode materials Solid solutions

a b s t r a c t Tix Sn1−x O3 solid solutions were prepared by a hydrothermal process. The morphologies and structures of Tix Sn1−x O3 solid solutions were investigated by scanning electron microscope, transmission electron microscope and X-ray diffraction measurements. The electrochemical properties of Tix Sn1−x O3 solid solution electrodes with different Sn/Ti ratios were examined by a variety of electrochemical testing methods. It was found that, the Tix Sn1−x O3 solid solution showed not only higher specific capacity of 506 mAh g−1 after 30 cycles but also better cycle performance, superior than the pure SnO2 electrode, which can be ascribed to the stable cyclability of TiO2 and the high reversible capacity of nanosized SnO2 . The Tix Sn1−x O3 solid solutions would be a potential candidate as anode material for a new generation lithium ion batteries. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Recently, lithium-ion batteries are becoming widely used in various electronic devices, such as cellular phones, notebook computers, camcorders, electric tools and electric vehicles. Concomitantly, the common commercial Lithium-ion batteries with a graphite anode cannot satisfy the increased demands owing to the limited specific capacity of graphite, which stimulates researchers to explore novel electrode materials to substitute the traditional materials [1,2]. SnO2 and tin-based composite electrode materials have been investigated as possible candidates for the next generation of Liion batteries [3–6] due to their high lithium storage capacities and low potentials of lithium ion insertion. A SnO2 anode can give a theoretical specific capacity of 781 mAh g−1 , which is over twice as much as the graphite anode (372 mAh g−1 ) [7]. However, a major problem of using SnO2 as an anode material for lithium-ion batteries is the large volume change during the alloying and de-alloying processes. The volume change will lead to the mechanical stress and further induce a rapid decay in mechanical stability. The electrode will thus suffer from cracking and crumbling, resulting in the loss of

∗ Corresponding authors. Tel.: +86 10 64434916; fax: +86 10 64434916. E-mail addresses: [email protected] (H. Song), [email protected] (F. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.04.018

capacity [3,8–10]. In order to improve the stability and cycle life of the SnO2 electrode, different strategies have been undertaken. For example, SnO2 nanofibers and thin-films [11], SnO2 /NiO nanomaterials [12], SnO2 –PPy composite [13], and SnO2 /carbon composites [14] have been tried to effectively accommodate the volume change and improve the cycling stability. Nanosized titanium dioxide has attracted great attention as an alternative anode material owing to its low cost, low toxicity, high reversibility, better safety and higher insertion ratio compared with other materials in lithium ion batteries [15–19]. However, its specific capacity is lower than that of other metal oxides like SnO2 and Fe2 O3 . If TiO2 is combined with other oxides possessing high specific capacity such as SnO2 , we may be able to get a new electrode material with a high specific capacity as well as a stable cyclability. Roginskaya et al. [20] reported the thin-film nanostructured electrodes with the composition of SnO2 –TiO2 . The titania nanotubes coverd tin nanowires also showed excellent cycle stability. [21] Furthermore, Uchiyama et al. [22] investigated the lithium insertion behavior of nanosized rutile-type Tix Sn1−x O2 (x = 0–1.0) solid solutions and found that there is no structural change of rutile-type crystal structure in the potential range of 1.2–3.5 V, which provides us a solid support for the design and synthesis of a series of TiO2 solid solution electrode materials. Herein, a type of solid solution, Tix Sn1−x O3 is synthesized using a hydrothermal treatment. The morphologies, structures and the possibility of Tix Sn1−x O3 as an anode material for lithium-ion batteries have been examined.

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2. Experimental 2.1. Preparation of Tix Sn1−x O3 Tix Sn1−x O3 was prepared under solvothermal conditions by using alcohol and de-ionized water (1:1 volume ratio) as a mixed solvent. SnCl4 ·5H2 O and TiCl3 were added into the solvent with a series of mole ratios (Sn:Ti = 3:1, 1:1 and 1:3). Then, hexamethylenetetramine was added as a precipitator into the solution under magnetic stirring at room temperature. After 1 h, the solution was put into a Teflon-lined autoclave, heated up to 90 ◦ C at the rate of 1 ◦ C/min, and then kept at the temperature for 1 h. Afterward, the temperature was increased to 190 ◦ C at the rate of 1 ◦ C/min, and maintained at this temperature for 2 h. After cooling to room temperature, the precipitation was centrifuged and washed by de-ionized water and alcohol until no Cl− was examined in the solution. Then by drying at 60 ◦ C for 12 h the Tix Sn1−x O3 precursor was obtained. The precursor was calcined at 300 ◦ C and 550 ◦ C for 1 h in air, respectively. The final Tix Sn1−x O3 products were denoted as SxTy-Z according to the molar ratio and the final heat treatment temperature. Here S represents SnO2 , T is TiO2 , x and y are the molar ratios of Sn:Ti, and Z is the calcined temperature. For example, S3T1-300 means the sample was prepared at the mole ratio of Sn:Ti = 3:1 and final heat treated at 300 ◦ C. The pure SnO2 and TiO2 were also synthesized using the above method. 2.2. Characterization The products were characterized by X-ray diffraction (XRD) on a Rigaku D/max-2500B2+/PCX system operating at 40 kV and ˚ over the range of 20 mA using Cu-K␣ radiation ( = 1.54056 A) 5–90◦ (2) at room temperature. The morphologies of the samples were observed by transmission electron microscope (TEM, Hitachi H-800) and field-emission scanning electron microscope (FE-SEM, Hitachi S-4700) equipped with energy dispersive X-ray spectroscope (EDS). The samples for TEM observation were prepared by dispersing the products in ethanol with an ultrasonic bath for 15 min, then placing a few drops of the resulting suspension onto a copper grid. 2.3. Electrochemical measurements The electrochemical properties of the above samples as anode materials for lithium-ion batteries were measured in a twoelectrode system. Lithium sheet was used as counter electrode, and the working electrode was comprised of an active mass (60 wt%), carbonaceous additive (acetylene black, 20 wt%) and poly(vinylidene difluoride) (PVDF, 20 wt%) binder. The electrolyte was 1 M LiPF6 solution in a 1:1 (volume) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) from Merck Co. The cells were galvanostatically charged (Li insertion) and discharged (Li extraction) in the voltage ranging from 0.01 to 2.50 V versus Li/Li+ at a current density of 0.2 mA/cm2 . The specific capacities were calculated according to the mass of active materials in electrodes, not including the masses of carbon black and binder PVDF. 3. Results and discussion 3.1. Morphologies and structures of the materials The XRD patterns of the samples are shown in Fig. 1(a). It can be seen that the diffraction peak positions of solid solution are in good conformability with that of rutile SnO2 and no other peaks are detected. But as more Ti is added, the peak intensity decreased, indicating that the crystallinity decreased. There are no diffraction

Fig. 1. (a) XRD patterns of samples calcined at 300 ◦ C, (b) XRD patterns of S3T1, S3T1-300 and S3T1-550, and (c) EDX pattern of S1T1-300.

peaks of single TiO2 , suggesting that Ti atom has entered the crystal lattice of SnO2 and forms the solid solution with (does not cause the change of lattice structure of) SnO2 . As we have known, TiO2 and SnO2 are both tetragonal systems, and the lattice parameters of TiO2 are: a = b = 3.7852 nm and c = 9.5139 nm, which are close to that of SnO2 : a = b = 4.7382 nm and c = 3.1871 nm. Moreover the ionic radius of Ti4+ is 0.068 nm, closing to that of Sn4+ 0.071 nm. Thus, tin atom can be substituted by Ti atom in the lattice to form a substitutional solid solution. From the EDS detection (the spectra for S1T1-300 is shown in Fig. 1(c)), the mole ratio of Sn:Ti in sample S3T1-300, S1T1-300 and S1T3-300 are 3.3:1, 1.52:1 and 1:1.91, respectively, slightly different from the original addition ratios, implying that some Ti elements have lost during synthesis. According to Scherrer formula, the diameters of the Tix Sn1−x O3 solid solutions are less than 10 nm by roughly calculated.

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Fig. 2. SEM images of (a) SnO2 , (b) S3T1-300, (c) S1T3-300, (d) S3T1-550, (e) S1T3-550, and (f) TEM image of S3T1-550.

Fig. 1(b) shows the XRD patterns of S3T1 calcined at different temperatures. There is almost no difference in XRD patterns between S3T1-300 and S3T1. While S3T1-550 exhibits very sharp and strong peaks, implying the higher crystallinity at hightemperature heat treatment. Although S3T1 and S3T1-300 are both amorphous, 300 ◦ C heat treatment is enough to remove the adsorbed water and water of crystallization, which formed in the hydrothermal process and will take great influence on the electrochemical properties of electrodes. As seen in Fig. 2(a)–(c), the particles heated at 300 ◦ C are almost spheres and agglomerates with the diameters less than 10 nm as observed, which are in good agreement with that calculated by XRD patterns. There is no essential difference between the pure SnO2 and Tix Sn1−x O3 samples with different mole ratios of Sn:Ti. Fig. 2(d), (e) and (f) show the SEM images of S3T1-550 and

S1T3-550, and the TEM image of S3T1-550, respectively. S3T1550 shows a honeycomb-like structure, while S1T3-550 exhibits a stick-like structure. They obviously show growth of particle size by high-temperature heat-treatment. 3.2. Electrochemical properties of Tix Sn1−x O3 solid solutions The CV curves of pure SnO2 and S1T1-300 are shown in Fig. 3(a) and (b), respectively. The curves of the two samples are very similar, suggesting that the lithium-ion insertion/desertion mechanisms in SnO2 and Tix Sn1−x O3 are almost the same. It should be mentioned that the insertion/desertion behaviors of TiO2 anode usually takes place at above 1.5 V [23]. However, there is no obvious redox peak at above 1.5 V in CV curves for Tix Sn1−x O3 solid solution (in Fig. 3(b)), suggesting that the capacity contribution from TiO2

J. Yan et al. / Electrochimica Acta 72 (2012) 186–191

0.006

a

2

0.000

c

a'

-0.002

c1

c2

-0.004 2nd

a

-0.006

b

1

c'

b'

0.002

Current(mA)

Current(mA)

0.004

189

b

c'

b'

0

-2

c2

a'

-1

c1 b

2nd

-3

c

a

-4

-0.008 1st

1st

-5

-0.010 0.0

0.5

1.0

1.5

2.0

0.0

2.5

0.5

1.0

1.5

2.0

2.5

Voltage(V)

Voltage(v) Fig. 3. CV curves of (a) SnO2 and (b) S1T1-300.

should be neglectable. In Fig. 3(a) and (b), an irreversible reduction peak around 1 V (peak c) during the first cycle is observed, which is no longer present in subsequent cycles, but instead is separated into two peaks labeled c1 and c2. The reactions occur according to the following reactions: 2Li+ + SnO2 + 2e− → Li2 O + SnO, 2Li+ + SnO + 2e− → Li2 O + Sn (1) Peaks b, b1 and a, a1 appear below 0.75 V which should be attributed to the reversible process of alloying and de-alloying. The respective reactions are: Peaks b and b : Li+ + Sn + e− ↔ LiSn

2.5

a

S3T1-300

TiO2 S1T3-300

S1T1-300 −

3.4Li + LiSn + 3.4e ↔ Li4.4 Sn

2.0

(3)

Reaction (1) is generally considered as an irreversible process, thus the total reversible capacity is often defined as 781 mAh g−1 by considering only reactions (2) and (3) [4,5,24]. The cells were charged and discharged between 0.01 V and 2.5 V at a constant current density of 0.2 mA/cm2 and their first charge/discharge voltage profiles are shown in Fig. 4(a). In the first discharge (Li-insertion) profile, there is no obvious voltage plateau which is probably due to the amorphous structure of samples. For the SnO2 sample, the voltage decreases quickly from 2.5 V to 1.0 V. Then slows down after 1.0 V, corresponding to the alloying process, which is in good agreement with CV test. In the first charge profile, curves of both the SnO2 and solid solutions are quite different from that of TiO2 [15,16]. The first charge (Lidesertion) capacity decreases as more Ti is added. For the first cycle, the SnO2 sample shows a discharge and charge capacity of 1500 mAh g−1 and 891 mAh g−1 , respectively. The first coulombic efficiency for SnO2 sample is 59.4%, which is the largest in all the samples. Fig. 4(b) shows the voltage profiles of cells for the first two circles. The S3T1-300 shows a lower first discharge capacity than the other two samples. We inferred that for S3T1, the existence of adsorbed water and water of crystallization increased the first discharge capacity. But it is known that, this kind of capacity is irreversible. Furthermore, the existence of adsorbed water and water of crystallization may decrease the cycle stability, which is confirmed in Fig. 5(a). In Fig. 5(a), the charge capacity of S3T1 decreased from 644 to 417 mAh g−1 after 30 cycles. S3T1-550 shows higher first discharge capacity than the amorphous sample S3T1-300. But in Fig. 5(a), its charge capacity is only 382 mAh g−1 after 30 cycles. The bad cycle stability would be related with the large particle size and the well-crystal structure, which possibly

SnO2

1.5 1.0 0.5 0.0 0

500

1000

1500

2000

2500

-1

Capacity(mAh·g ) 2.5

S3T1-300

b

S3T1-550 S3T1

2.0

Voltage(V)

+

(2)

a :

Voltage(V)

Peaks a and

suffer lager volume change than amorphous electrodes during the charge/discharge processes [25]. S3T1-300 electrode shows better cycle stability. It means that amorphous structure may show some advantage on relieving mechanical stress. Meanwhile heating at 300 ◦ C can reject the influence of adsorbed water and crystal water. Thus the sample calcined at 300 ◦ C shows better cycle performance. Fig. 5(b) shows the cycle performance of various electrodes at the current density of 0.2 mA/cm2 . For pure SnO2 , the reversible capacity decreases from the original 891 mAh g−1 to 314 mAh g−1 with a 64.8% loss after 30 cycles. The capacity loss is huge as

1.5 1.0 0.5 0.0

2nd

1st 0

1000

2000

3000

4000

-1

Capacity(mAh·g ) Fig. 4. (a) The first charge/discharge profiles of samples calcined at 300 ◦ C, and (b) the first twice charge/discharge profiles of sample S3T1, S3T1-300 and S3T1-550.

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J. Yan et al. / Electrochimica Acta 72 (2012) 186–191

a

700 600

S3T1-300

500 400

S3T1-550

300

S3T1 0

5

10

15

20

25

30

b

1400 -1

800

Capacity(mAh·g )

-1

Capacity(mAh·g )

1600

S3T1-550, charge S3T1-550, discharge S3T1-300, charge S3T1-300, discharge S3T1, charge S3T1, discharge

900

35

1200 1000

SnO2

800 600

S3T1-300

400

S1T1-300

200

S1T3-300 TiO2

0 0

5

10

Cycle number

15

20

25

30

35

Cycle number

c -1

Capacity(mAh·g )

500 SnO2

400

S3T1-300 300

S1T1-300 200

S1T3-300 0

10

20

30

Cycle number Fig. 5. Cycle performances with voltage ranging from 0.01 to 2.50 V of (a) sample S3T1, S3T1-300 and S3T1-550, (b) samples calcined at 300 ◦ C, and inset (c) cycle performances with voltage ranging from 0.01 to 0.75 V of samples calcined at 300 ◦ C.

16

S1T1-300

S3T1-300

14 12 10

-Zre(W)

expected with more than 2% per cycle. The capacity of solid solution retains better than that of pure SnO2 . As more Ti is added, the capacity decreases while the stability increases. S3T1-300 keeps the capacity of 506 mAh g−1 after 30 cycles with a 1.15% loss per cycle. S1T1-300 and S1T3-300 keep the capacities of 382 and 346 mAh g−1 , respectively, with only 0.5% loss per cycle. As we discussed, Ti plays very little roles in lithium storage. Thus calculated by the Sn percentage in solid solution (the mole ratio of Sn:Ti from EDS result), the theoretical capacity of S3T1, S1T1 and S1T3 should be 672, 579 and 388 mAh g−1 when regarding the theoretical capacity of SnO2 as 781 mAh g−1 , respectively. As discussed before, the reversible capacity of SnO2 and solid solutions are mainly attributed to the reversible process of alloying and de-alloying with the voltage range from 0.01 to 0.75 V. As a result, in Fig. 5(c), we show the charge capacities of samples during 0.01–0.75 V in 30 cycles. It can be found that the reversible capacity of pure SnO2 electrode quickly decreases from 472 to 229 mAh g−1 , while the Tix Sn1−x O3 solid solutions exhibit very stable cycle performance, in which S3T1 possesses the highest reversible capacity of 350 mAh g−1 . Although the curve of S3T1 is not as flat as that of the other samples, which perhaps due to larger structure change, it is clear that the capacity retention of Tix Sn1−x O3 is better than that of pure SnO2 . Fig. 6 shows the Nyquist comlex plane impedance plots of three electrodes. It is clear that in medium-frequency region the diameter of the semicircle for S3T1-300 electrode is smaller than those of S1T3-300 and S1T1-300 electrodes, indicating S3T1-300 electrode has lower charge-transfer impedance than S1T3-300 electrode. For further study, the AC impendence spectra were modeled by the modified Randles equivalent circuit, as shown in Fig. 7. Re is the electrolyte resistance. Cf and Rf are the capacitance and resistance of the solid electrolyte interface, respectively. Cdl and Rct are the double-layer capacitance and charge-transfer resistance,

8 6

S1T3-300

4 2 0 -2 0

5

10

15

20

25

30

35

Zre(W) Fig. 6. AC impedance spectra of S1T3-300, S1T1-300 and S3T1-300 electrodes.

Fig. 7. Randles equivalent circuit.

J. Yan et al. / Electrochimica Acta 72 (2012) 186–191 Table 1 Kinetic parameters of S1T3-300, S1T1-300 and S3T1-300 electrodes.

S1 T3 -300 S1 T1 -300 S3 T1 -300

Acknowledgments

Re ()

Rf ()

Rct ()

i0 (×10−4 , A cm−2 )

3.70 4.07 3.39

7.64 5.84 4.28

19.78 14.3 2.54

13.0 18.0 101.4

respectively. Zw is the Warburg impedance which is related to the diffusion of lithium ions in the electrodes. The exchange current density i0 is calculated according to Eq. (1) [26]. RT i0 = nFRct

191

(4)

The kinetic parameters of S1T3-300, S1T1-300 and S3T1-300 electrodes are shown in Table 1. The S3T1-300 electrode shows lower Re , Rf and Rct values than S1T3-300 electrode. Meanwhile the value of exchange current density increased to 101.4 from 13 A cm−2 , indicating that the electrochemical activity of S3T1-300 electrode is much higher than that of S1T3-300 and S1T1-300 electrode, which is important for anode material. As we confirmed above, the solid solution samples showed the similar lithium-storage mechanisms as pure SnO2 . Thus Sn would be considered as the active element for electrode actions, while TiO2 does not contribute to capacity. If some active Sn is replaced by Ti in the cubic of SnO2 , the capacity for lithium insertion will decrease. However, with Ti atom doping, better stability is attached which indicates that the structure of solid solution samples is kept better than that of pure SnO2 during the cycle. The balance between specific capacity and cycle stability should be found by controlling the atomic ratio of Ti:Sn in the solid solution. Further investigations are carried out in our laboratory.

This work was supported by the Foundation of Excellent Doctoral Dissertation of Beijing City (YB20081001001). One of the authors: H. Y. Yang thank the financial support from international design center (IDC) of Singpaore University of Technology and Design. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

4. Conclusions A group of special Tix Sn1−x O3 solid solutions are prepared by hydrothermal process. The electrochemical performance for lithium-ion batteries indicated that SnO2 /TiO2 electrodes exhibit a good cycle performance and high specific capacity, which is hopeful as the potentially novel anode material for lithium-ion batteries.

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