SnO2 composite materials as an anode for lithium-ion batteries

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Synthesis of TiO2(B)/SnO2 composite materials as an anode for lithium-ion batteries Hsin-Yi Wu, Min-Hsiung Hon, Chi-Yun Kuan, IngChi Leu

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Cite this article as: Hsin-Yi Wu, Min-Hsiung Hon, Chi-Yun Kuan, Ing-Chi Leu, Synthesis of TiO2(B)/SnO2 composite materials as an anode for lithium-ion batteries, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.04.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of TiO2(B)/SnO2 composite materials as an anode for lithium-ion batteries Hsin-Yi Wua, Min-Hsiung Hona, b, Chi-Yun Kuanc, and Ing-Chi Leud, *

a

Department of Materials Science and Engineering, National Cheng Kung University,

Tainan, Taiwan (R.O.C.)

b

Research Center for Energy Technology and Strategy, National Cheng Kung

University, Tainan, Taiwan (R.O.C.)

c

Technical Department, ThinTech Materials Technology Co., Ltd., Kaohsiung,

Taiwan (R.O.C.)

d

Department of Materials Science, National University of Tainan, Tainan, Taiwan

(R.O.C.)

* Corresponding author: [email protected] (Ing-Chi Leu)

1

Abstract A TiO2(B) nanosheets/SnO2 nanoparticles composite was prepared by the hydrothermal and chemical bath deposition (CBD) methods, and its electrochemical properties were investigated for use as the anode material of a lithium-ion battery. The as-prepared composites consisted of monoclinic-phase TiO2(B) nanosheets and cassiterite structure SnO2 nanoparticles, in which SnO2 nanoparticles were uniformly decorated on the TiO2(B) nanosheets. The TiO2(B)/SnO2 composites showed a higher reversible capacity and better durability than that of the pure TiO2(B) for use as a battery anode. The composite electrodes exhibiting a high initial discharge capacity of 2239.1 mAh g-1 and a discharge capacity of more than 868.7 mAh g-1 could be maintained after 50 cycles at 0.1C in a voltage range of 1.0 - 3.0 V at room temperature. The results suggest that TiO2(B) nanosheets coated with SnO2 could be suitable for use as a stable anode material for lithium-ion batteries. In addition, the coulombic efficiency of the nanosheets remains at an average of 93.1% for the 3rd to 50th cycles.

Keywords: SnO2, TiO2(B) nanosheet, hydrothermal synthesis, lithium-ion battery

2

1. Introduction Due to their high energy density and excellent long cycling performance, rechargeable lithium-ion batteries (LIBs) have attracted enormous attention in the last few years for large scale applications. For example, if electric vehicles are to become viable alternatives to those that run on fossil fuels, further development of energy storage materials that offer high capacity is needed [1]. To promote the performance of lithium-ion batteries, either new materials with better performance must be developed or the materials currently used need to be improved [2, 3]. The use of tin-based materials as anodes has attracted particular interest because of their high specific lithium storage capacities. SnO2, with a theoretical capacity of 782 mAh g-1, has been extensively studied as an alternative material to the currently commercially employed graphite. However, the application of tin-based anodes is significantly hampered by the poor cycling performance, which is caused by the significant volume change during the alloying and dealloying processes of Li-Sn. Several strategies have been proposed in recent years to minimize the mechanical stress in the electrode caused by the volume expansion. The current strategies to overcome the Sn-related problems adopt two approaches: reducing the alloy particle size and using composite materials [4-7]. In most cases, the specific capacity and rate of lithiation and delithiation increase 3

when the materials are nanostructured. These, enhancements are explained in terms of higher surface areas, shorter Li+ diffusion paths and different surface energies for nanostructured materials allowing for more facile lithiation and delithiation [1]. In addition, using composite architectures can improve the Li+ storage capacity and cyclability of SnO2-based nanomaterials because of the buffering and mechanical support functions of the TiO2(B) during charge/discharge cycling [8, 9]. It is expected that the use of composite materials made of tin oxide and other specially-structured nanomaterials, which can alleviate the large volume expansion of tin oxide would be a promising way to minimize the mechanical stress in the electrode caused by volume expansion. Among the most studied polymorphs, nanostructured TiO2(B) has the highest capacity with promising high rate capabilities. TiO2(B) is able to accommodate one Li+ per Ti, giving a capacity of 335 mAh g-1 for nanotubular and nanoparticulate TiO2(B) [1]. As the demand for alternative electrode materials for lithium-ion batteries is rapidly increasing, TiO2(B) is drawing a large amount attention as an anode material due to its safety, abundance in nature, chemical stability, and non-toxicity. The TiO2(B) also has a small volume expansion ratio (3%) during intercalation/extraction of the Li-ion process, and exhibits good cyclic stability [10]. TiO2 nanosheets can thus offer a large surface area and excellent pathways for Li-ion 4

to transfer through interfaces. In recent years, various types of TiO2 and SnO2 composite electrodes have been prepared and enhanced electrochemical properties have been reported for them. However, the theoretical specific capacity of TiO2(B) is very low, which hinders its practical use in Li ion batteries. It is thus of interest to prepare a new TiO2(B)/SnO2 nanocomposite by using Ti and Sn precursors, in an attempt to overcome these obstacles. Here, the TiO2(B) can not only facilitate the lithium transport, but also works as a mechanical support which can effectively buffer the volume changes of tin during lithium ion intercalation/deintercalation. In addition, the supporting effect of TiO2(B) could prevent the tin crystals from agglomerating during cycling [11]. Control of the active material particle size to minimize catastrophic particle pulverization is even more important in producing a composite electrode for practical use. TiO2(B) nanosheets are prepared in this paper by an alkaline hydrothermal method starting from TiO2 particles (P25). TiO2(B)/SnO2 composite nanomaterials are then

．

prepared using SnCl4 5H2O and urea by a chemical bath deposition (CBD) method on the TiO2(B) nanosheets. It is important to understand the effect of the nanostructures in the TiO2(B)/SnO2 composites on the resulting electrochemical performance of the battery. Electrochemical measurements show that all the TiO2(B) samples with SnO2 decoration demonstrate greatly enhanced lithium storage properties with excellent 5

capacity retention at a current rate of 0.1 C after 50 charge–discharge cycles. The excellent discharge performance and good cyclability of these nanostructures revealing their potential for use as anode materials in lithium ion batteries, combing the advantages of the high specific capacity of Sn with the structural stability of TiO2(B).

2. Experimental TiO2(B) nanosheets were prepared by hydrothermal reaction of TiO2 powders (P25, anatase and rutile phases in ratio of about 3:1; Degussa) with a 6 M sodium hydroxide (NaOH) aqueous solution at 180oC for 12 h [12]. After reaction, the sample was rinsed with a 10 M hydrochloric acid (HCl) solution until a pH value of 7.5 was reached and the centrifugal sediments were rinsed with deionized (DI) water, and then dried at 65°C for 12 h. A CBD method was employed for the synthesis of SnO2 nanoparticle-decorated TiO2(B) nanosheets. Various amounts (0.085 g, 0.17 g, 0.255 g, and 0.34 g) of SnCl4

．

5H2O and 1.35 g of urea were added into 50 ml deionized water with 0.4 g of TiO2(B) nanosheets and the reactants were mixed by magnetic stirring for 30 min at 80oC. After reaction, the samples and centrifugal sediments were rinsed with deionized water, and then dried at 65°C for 12 h. A schematic illustration of the material 6

preparation process is shown in Fig. 1. The phase identification and crystal structure determination of the nanocomposites were performed using X-ray diffraction (XRD, Cu Ka, k = 1.54178 Å; Rigaku). The morphology and nanostructure of the nanocomposites were evaluated using a transmission electron microscope (TEM, JEM-2100F; JEOL). The amount of SnO2 decoration on TiO2(B) was determined by energy dispersive X-ray spectroscopy (EDS, JEM-2100F; JEOL) and inductively coupled plasma-mass spectroscopy (ICP-MS, AGILENT 7500CE). Electrochemical tests were performed using a CR2032 type coin cell. The electrochemical properties of the cell with the prepared anodes were examined by compressing a mixture of the as-prepared TiO2(B)/SnO2 composite material, carbon black (Super P, TIMCAL), and binder (Poly (acrylic acid)) in a weight ratio of 7: 2: 1 and pasting the mixture on copper foil. Pure lithium metal foil was used for the counter and reference electrodes. LiPF6 (1 M) in ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1:1 was used as the electrolyte (BASF Battery Materials Co., Ltd.). In order to examine the redox characteristics of the composite material anode in the lithium ion battery, the cells were galvanostatically charged and discharged over a voltage of 1.0 V to 3.0 V vs. Li/Li+ using a battery cycler (Arbin Inc., USA).

7

3. Results and discussion The preparation of composite electrodes in this work involves the hydrothermal synthesis of TiO2(B) nanosheets and CBD of SnO2 nanoparticles. The XRD patterns of the synthesized nanosheets are shown in Fig. 2(a), and all spectra are assigned to TiO2(B) (JCPDS 74-1940). Figure 2(b) shows the SEM morphology of the TiO2(B) nanosheets synthesized in 6 M NaOH aqueous solutions at 180°C for 12 h. It can be seen that some fractions of the nanosheets in the products have their edges rolled up due to surface tension. The obtained selected area electron diffraction (SAED) pattern can be indexed to TiO2(B) with the monoclinic space group C2/m, as shown in Fig. 2(c). Regarding the crystal structure of the SnO2 particles prepared by CBD, because of the small crystallite size and the low temperature process employed in the present study, only broadened peaks can be obtained in the XRD patterns in Fig. 3, where the diffraction patterns can be indexed to SnO2 (JCPDS card No. 46-1088). The morphology of composites is confirmed by the TEM images as shown in Fig. 4(a) and (b). These show that the SnO2 particles produced using the CBD method are successfully coated on the TiO2(B) nanosheets, and that the nanoparticles are randomly distributed on the surface of these. To study the effect of the amount of SnO2 decoration on the electrochemical properties of the composites, and determine the optimal amount of SnO2 with regard to enhancing the capacity of TiO2(B), 8

．

different amounts of SnCl4 5H2O concentrations were employed to obtain in different amounts of SnO2 in the final nanocomposites. In order to further confirm the composition of the TiO2(B)/SnO2 composite, the presence of elements was determined by performing EDS spot analysis under TEM observations (shown in Fig. 4(c)). The EDS mappings from a selected area of the nanocomposite cluster show that the main metallic elements are Sn and Ti. In order to determine the amount of SnO2 nanoparticles deposited on the TiO2(B) nanosheets, a blank experiment without TiO2(B) nanosheets was carried out. Figure 5 shows that the obtained net weights of SnO2 particles are 0 g, 0.03 g, 0.06 g, 0.12 g and 0.16 g with different SnCl4

．5H O concentrations of 0 mM, 4.85 mM, 9.70 mM, 2

14.55 and 19.40 mM, respectively, added in the starting solutions. It can be seen that

．

the deposition weight of SnO2 increased along with the SnCl4 5H2O concentration in the solutions. Moreover, ICP-MS was also been used to analyze the chemical composition of the composites, and the calculated Sn/Ti ratio could be used to estimate the amount of SnO2 decorated on TiO2(B) nanosheets. Figure 6 shows the Sn/Ti weight ratio of TiO2(B)/SnO2

composite

nanosheets

made

from

solutions

with

different

．

concentrations of SnCl4 5H2O. Based on this figure, the Sn/Ti ratios are 0.068, 0.273,

．

0.896 and 1.248 with increasing concentrations of SnCl4 5H2O in the solution, form 0 9

mM to 19.50 mM. The analysis of the amount of deposited Sn and the result of the ICP experiments highlight that the Sn/Ti ratio is dependent on the concentration of SnCl4

．5H O in the solution. 2

Figure 7(a) and (b) show the battery characteristics of capacity and coulombic efficiency at 0.1 C after different number of cycles. The charge/discharge cycling performances of the TiO2(B)/SnO2 composites from different SnCl4 5H2O

．

concentrations of 4.85 mM, 9.70 mM, 14.55 mM and 19.40 mM are shown in Fig. 7(a). The voltage range is set from 1 V to 3 V and charge/discharge rate is 0.1 C. The TiO2(B)/SnO2 composite electrodes exhibit a good cycling stability and the charge capacity remains at about 815.8 mAh g-1, 493.9 mAh g-1, 725.7 mAh g-1 and 421.0 mAh g-1 (the average of all circles) for different SnCl4-5H2O concentrations. The capacity retention (last/initial capacity) is about 31.2 %, 27.4 %, 27.6 % and 19.4 % after 50 cycles, respectively. Recent studies showed that the TiO2(B)/SnO2 composite material can slightly reduce the capacity retention (> 50%), and thus improve the capacity stability. The results of the current work show that SnO2 decoration can improve capacitance of the cell, and the degree of to which this is enhanced increases along with the amount of SnO2 that is decorated on the nanosheets. While the capacity is rather stable for up to 50 cycles, the energy density is not so high which might be due to the volume expansion caused by SnO2 addition [13, 14]. When 9.70 mM SnCl4 5H2O was used to prepare the electrode, the resulting composite has a high reversible capacity (Sn/Ti ratio of about 0.273:1). However, Fig. 7(a), shows that in the first cycle the TiO2(B)/SnO2 composite nanosheet has a high capacity, together with a high initial irreversible capacity. The TiO2(B)/SnO2 composite electrode materials from different SnCl4 5H2O concentrations deliver initial charge capacities of 2175.9 mAh -1

．

g , 2239.1 mAh g-1, 1439.5 mAh g-1 and 1717.7 mAh g-1, and discharge capacities of 1033.8 mAh g-1, 868.7 mAh g-1, 592.2 mAh g-1 and 471.9 mAh g-1, with coulombic efficiencies of 47.5 %, 38.8 %, 41.1 % and 27.5 %, respectively. The lower coulombic efficiency at the first cycle is a result of the reduction of SnO2 nanoparticles to Sn, which occurs with Li2O formation as shown in Eq. (2) [15, 16]. In general, the SnO2 + 4 Li + + 4e − → Sn + 2 Li2O reaction is irreversible.

Li + + e− + electroyte → SEI ( Li )

(1)

SnO2 + 4 Li + + 4e − → Sn + 2 Li2O

(2)

10

．

Sn + xLi + + xe− ↔ Lix Sn ( 0 ≤ x ≤ 4.4 ) TiO2 + xLi + + xe− ↔ LixTiO2 ( 0 ≤ x ≤ 0.5)

(3) (4)

However, the LixSn alloy compound forms during the Li-ion intercalation process as shown in Eq. (3), and this leads to a volume change of 300 % [13, 14]. As mentioned previously, the irreversible capacities could be attributed to the reduction of SnO2 and the formation of a solid electrolyte interphase film (SEI), as indicated in Eq. (1) [17]. The large charge in volume would generate a large internal stress, leading to cracking of the electrode, loss of electrical contact, large initial irreversible capacity [13, 18-22], and eventually rapid fading of capacity. As a result, when the addition of SnO2 is increased, the structure would be destroyed due to the charge in volume. After three charge/discharge cycles, the average coulombic efficiency becomes 93.1 %, and

．

the capacity reaches to a stable value when the SnCl4 5H2O concentration is 9.70 mM. Moreover, the TiO2(B)/SnO2 nanosheets composite anode delivers a high charge/discharge capacity of ~689.6 mAh g-1, even after 50 cycles. The SnO2 particles are mainly deposited on the sheet or in the interior of the tubes (i.e., sheets with edges rolled up) of the TiO2(B) nanosheets, and the change in volume of SnO2 in the discharge and charge processes can be accommodated by the nanosheets. Therefore, it is apparent that the TiO2(B) sheets provide a supporting function 11

which alleviates the pulverization and drastic volume variation of the SnO2. Fig. 7(c) shows cyclic voltammogram (CV) of the TiO2(B)/SnO2 nanocomposites at a scan rate of 0.1 mV s-1 in the voltage range 0.0-3.0 V. In the curve, the cathodic/anodic peaks at 0.012 V, 0.175 V, 0.267 V and 0.575 V, are mainly related to the formation of various LixSn phases such as LiSn to Li22Sn5 during the charge-discharge process, while the peak at around 0.835 V is possibly derived from Li2O formation and electrolyte decomposition [23], which causes a large irreversible capacity in the first few cycles that gradually disappears in the subsequent cycles. There are two obvious cathodic/anodic peak pairs at 1.596 V and 1.776 V, which is probably attributable to lithium ion insertion into/extraction out of TiO2(B) [21, 24, 25]. The results indicate that SnO2 is truly involved in the electrochemical reaction and the TiO2(B)/SnO2 composite electrode exhibits a greatly improved cycling performance and a higher reversible specific capacity. It is also interesting to find that some studies [11, 26] reported in the literature demonstrated the capacities beyond the theoretical charge/discharge performance; however, the underlying mechanism remains to be understood. The results of recent studies of nanocomposites composed of SnO2 and TiO2 are listed in Table 1. It is interesting to note that the nanocomposites prepared in the current study have capacities far better than those reported in the literatures, even 12

though our method for preparing the nanocomposite is relatively simple. The capacity obtained in this work is higher than those reported in studies that used a similar current density, that could be a result of the higher mass ratio of SnO2 in our electrode compared to that of TiO2(B). Although the charge capacity of the composite declined significantly in the first cycle, still has excellent cycling performance. The results thus indicate that the modification of TiO2(B) nanosheets with SnO2 nanoparticles are effective in maintaining cycling performance. To further improve the performance, more attention may be paid to optimizing the configuration of the TiO2(B)/SnO2 nanosheets composite and enhancing the conductivity.

4. Conclusions TiO2(B)/SnO2 nanocomposite materials are successfully synthesized by a simple

．

CBD method using SnCl4 5H2O solution and urea as raw materials. These composites show enhanced cycling performance when used as anode materials in Li-ion batteries. The TiO2(B)/SnO2 nanocomposites have an average reversible capacity of 650.5 mAh g-1 (with 9.70 mM SnCl4

．5H O) and a better cyclability compared with the blank 2

TiO2(B) nanosheet. The coulombic efficiency rate of the TiO2(B)/SnO2 sheet nanocompsite after 50 cycles is 91.9 % at the charge-discharge rate of 0.1 C. It is 13

demonstrated that the capacity and electrochemical characteristics of TiO2(B) nanosheets could be improved by decorating SnO2 nanoparticles on the surfaces of TiO2(B) nanosheets. This TiO2(B)/SnO2 nanocomposite is thus a promising potential anode material for use in lithium-ion batteries, even though the composition and structure of these materials require further improvement by optimizing the fabrication processes for the composite.

Acknowledgements This study was supported by the Ministry of Science and Technology, Republic of China (MOST 100-2221-E-006-123-MY3 and MOST 100-2628-E-024-001-MY2), which is greatly appreciated.

14

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Fig. 1. Schematic illustration of the process for synthesizing the TiO2(B) nanosheet and TiO2(B)/SnO2 nanocomposite. nanocomposite

19

(a)

(b)

(c)

Fig. 2. (a) XRD pattern of TiO2(B) nanosheets prepared from 6 M NaOH at 180oC for 12 h (30 kV/20 mA), (b) TEM image of TiO2(B) nanosheets prepared from 6 M NaOH at 180oC for 12 h, and (c) SAED pattern.

20

Fig. 3. XRD patterns for SnO2 nanoparticles fabricated under different SnCl4 concentrations of (a) 4.85mM, (b) 9.70mM, (c) 14.55mM and (d) 19.40mM.

21

．5H O 2

(a)

(b)

(c)

Fig. 4. (a) and (b) TEM images of the TiO2(B)/SnO2 nanocomposites with a SnCl4 5H2O concentration of 19.40 mM and (c) EDS pattern.

22

．

Fig. 5. The amount of SnO2 deposited with different concentrations of SnCl4

23

．5H O. 2

Fig. 6. ICP analyses of TiO2(B)/SnO2 nanocomposite fabricated with different SnCl4 5H2O concentrations.

24

．

(a)

(b)

25

(c)

Fig. 7. Electrochemical performance of TiO2(B)/SnO2 nanostructure (a) variation of the discharge–charge specific capacity versus the cycle number at a rate of 0.1 C for the TiO2(B)/SnO2 nanocomposite, (b) coulombic efficiency and (c) cyclic voltammogram of TiO2(B)/SnO2 nanostructure electrode between 0.0 and 3.0 V at a scan rate of 0.1mV s-1.

26

Table 1. List of recent works using TiO2/SnO2 composites as the lithium-ion battery electrode. Materials

Capacity after 30

Publication

cycles

year [Ref.] -1

SnO2 nanoparticles on TiO2(B) nanosheets

~868 mAh g

This work

SnO2/TiO2 nanocomposite

-1

581 mAh g

2012 [18]

TiO2 nanocones@SnO2 nanoparticles

350 mAh g-1

2012 [19]

-1

2013 [20]

SnO2@TiO2 double-shell nanotubes

-1

232 mAh g

2013 [21]

TiO2(B)@SnO2 core-shell hybrid nanowire

463 mAh g-1

2011 [26]

mesoporous C-TiO2-SnO2 nanocomposites

-1

2012 [27]

-1

2010 [28]

SnO2@TiO2 hollow microtubes

805 mAh g

378 mAh g

SnO2@TiO2 double shelled nano-spheres

128 mAh g

27