Materials Letters 181 (2016) 289–291
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Novel mesoporous TiO2 spheres as anode material for highperformance lithium-ion batteries Jianqiang Guo n, Jing Li n, Yeju Huang, Min Zeng, Rufang Peng Institute of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 April 2016 Accepted 13 June 2016 Available online 14 June 2016
Mesoporous TiO2 spheres are successfully synthetized as high performance anode materials for lithiumion batteries. The obtained mesoporous TiO2 spheres exhibit excellent rate capability and good cycling stability, which shows initial discharge capacity of 315 mAh/g and remains 208 mAh/g after 150 cycles. This improved electrochemical performances of TiO2 spheres is ascribed to the synergetic effects of their unique sphere structure and mesopore structure. & 2016 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Energy storage and conversion Mesoporous TiO2 spheres Anode material
1. Introduction Titanium based materials have been widely studied as a promising anode material in lithium-ion batteries [1–5]. Especially, anatase TiO2 has attracted extensive attention due to its low cost, environmentally friendliness and safety . In spite of these considerable advantages, many issues still hinder the commercialization of anatase TiO2 material. It is well known that one of the major challenges is the poor cycle performance caused by severe aggregation during Li þ insertion/extraction processes [7–9]. Therefore, the key factor to improve the performance of anatase TiO2 material is to suppress the aggregation of anatase TiO2. In this paper, we use mesoporous TiO2 spheres as anode material for lithium-ion batteries. This mesoporous sphere structure can buffer structural strain caused during Li þ insertion and extraction, improving the structural stability and cycle performance. Besides, the mesopore in the TiO2 spheres can provide transportation channel for the charge transfer in the electrolyte. As a result, the as-prepared mesoporous TiO2 spheres exhibit high electrochemical performance as anode materials for lithium-ion batteries.
2. Experimental 2.1. Preparation of mesoporous TiO2 spheres 7.95 g of HDA was dissolved into 800 mL of ethanol, followed n
Corresponding authors. E-mail addresses: [email protected]
(J. Guo), [email protected]
http://dx.doi.org/10.1016/j.matlet.2016.06.059 0167-577X/& 2016 Elsevier B.V. All rights reserved.
by adding 3.20 mL of KCl solution (0.1 M). After that, 18.10 mL of TIP was dropped to this solution under magnetic stirring. The white precursor suspension was left for 18 h and then centrifuged, and the products (named as precursor A) were washed with ethanol three times and dried at room temperature. To prepare the mesoporous TiO2 spheres, a solvothermal treatment of the precursor A was conducted. 1.6 g of precursor A was immersed into the 20 mL ethanol and 10 mL deionized water mixture with an ammonia concentration of 0.22 M. Then the solution was transferred into PTFE tank and heated at 160 °C for 16 h. After cooling down to room temperature, the powder was washed by ethanol dried at 60 °C for 24 h. As a result, the mesoporous TiO2 spheres was received. 2.2. Material characterization The morphology and crystal structure information were obtained from scanning electron microscopy (SEM, Ultra55), X-ray diffraction (XRD, D8 Advance, Cu Kα radiation). The N2 adsorption–desorption were determined by Brunauer–Emmett–Teller (BET) measurements using an Quantachrome instrument (Quabrasorb SI-3MP) surface area analyzer. 2.3. Electrochemical measurements Electrochemical measurements were carried out via CR2016 coin type test cells type test cells assembled in an argon-ﬁlled glove box using lithium metal as both counter and Reference electrodes. The working electrodes were prepared by mixing TiO2, carbon black and polyvinylidene ﬂuoride binder with a weight ratio of 70:20:10. The mixed slurries were immediately casted onto copper foil, dried in oven at 80 °C for 2 h, and then left at
J. Guo et al. / Materials Letters 181 (2016) 289–291
room temperature overnight. A Celgard 2400 membrane was used as a separator. The electrolyte consisted of a solution of 1 M LiPF6 in a 1:1(v/v) EC/DMC. The discharge–charge measurements were performed on Land CT2001A tester (Wuhan, China)at the constant current mode over the range of 1.0–3.0 V.
3. Results and discussions Fig. 1 shows the morphologies of the TiO2 material at different magniﬁcations. As shown in Fig. 1(a), the as-synthesized TiO2 materials display a nano-sphere structure. With the increase of magniﬁcation (Fig. 1(a)), it can be clearly observed that the TiO2 spheres are composed of small nanoparticles. The nitrogen adsorption–desorption isotherms of TiO2 spheres and pore size distribution curves of TiO2 spheres are derived from BET measurements. As shown in Fig. 1(a), N2 adsorption–desorption isotherm of TiO2 spheres can be identiﬁed as type IV isotherm in the IUPAC classiﬁcation with a typical mesopore hysteresis loop . Besides, the pore size distribution curve of the TiO2 spheres host (the inset) indicates that the size of the pores mainly range from 6 to15 nm, which can improve the charge transfer capability of electrolyte, reduce the Liþ diffusion paths and increase the electronic conduction as well as structural stability . Fig. 1(b) shows the XRD pattern of the TiO2 spheres. All of the diffraction peaks are well matched with the standard card (JCPDS no. 21-1272). Besides, the sharp diffraction peaks indicate the good crystallization of the as-prepared TiO2 spheres. The TiO2 sample is also conﬁrmed as a high purity because there is no diffraction peaks of other impurity phase. Some electrochemical measurements were carried out to
evaluate the performance of the TiO2 spheres. The ﬁrst cycle charge/discharge curves of TiO2 spheres at a current rate of 0.2 C are shown in Fig. 2(a). Two voltage plateaus appear at 1.7 and 2.0 V during the discharging and charging processes, which are in accord with the previous reports . Additionally, it can be seen that the TiO2 spheres delivered a speciﬁc discharge capacity of 315 mAh/g and a high coulombic efﬁciency value of 94% at the initial cycle at a current rate of 0.2 C. Fig. 2(b) shows the cycling performance of TiO2 spheres at a constant current rate of 0.2 C. As shown in Fig. 2(b), the speciﬁc capacity of the TiO2 spheres starts at 315 mAh/g and still remains at 208 mAh/g after 150 cycles, with a capacity loss of 34%. Simultaneously, the coulombic efﬁciency is as high as 99%, suggesting excellent electrochemical reversibility during the Li þ insertion and extraction process. To further demonstrate the superior performance TiO2 spheres, the rate performance were tested at different current rates from 0.2 to 20 C. As shown in Fig. 3, the TiO2 spheres delivered high speciﬁc capacities of 105 mAh/g and 78 mAh/g at the current density of 5 C and 10 C, respectively. Even when the current rate was improved to 20 C, the speciﬁc capacity still remain to be 63 mAh/g. In addition, the capacity was recovered when the current rate turned back to 1 C, indicating excellent rate capability and good cycling stability of the TiO2 spheres. Based on the above results, the high performance of mesoporous TiO2 spheres can be attributed to the following advantages: (a) the sphere structure can buffer structural strain caused during Liþ insertion and expulsion, improving the structural stability and cycle performance, (b) the mesopore in TiO2 spheres is beneﬁcial for improving the electronic conductivity by accelerating the charge transfer in the electrolyte.
Fig. 1. SEM images of the TiO2 spheres (a) and (b) at different magniﬁcations. (c) Nitrogen adsorption–desorption isotherms of TiO2 spheres, the inset shows the pore size distribution, (d) XRD patterns of TiO2 spheres.
J. Guo et al. / Materials Letters 181 (2016) 289–291
Fig. 2. (a) The ﬁrst charge/discharge curves of TiO2 spheres at a current rate of 0.2 C, (b) the cycling performance of TiO2 spheres at a constant current rate of 0.2 C.
Fig. 3. The rate capability of TiO2 spheres at different current rates from 0.2 to 20 C.
4. Conclusions In summary, mesoporous TiO2 spheres are successfully synthetized via a solvothermal method. The as-prepared mesoporous TiO2 spheres shows high speciﬁc capacitance with a discharge capacity of 315 mAh/g and a coulombic efﬁciency of 94% at the ﬁrst cycle and excellent cycling performance with a capacity loss of 34% after 150 cycles. Therefore, the mesoporous TiO2 spheres is favorable for the development of the alternative anode materials for LIBs.
Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant no. 14Zg1102).
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