C composite and its lithium storage

Accepted Manuscript The preparation of a new quasi-elliptic TiO2/SnO2/C composite and its lithium storage Zhangmin Hong, Qinghua Tian, Wei Zhang, Li Yang PII: DOI: Reference:

S0167-577X(18)31618-5 https://doi.org/10.1016/j.matlet.2018.10.054 MLBLUE 25095

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

9 August 2018 30 September 2018 8 October 2018

Please cite this article as: Z. Hong, Q. Tian, W. Zhang, L. Yang, The preparation of a new quasi-elliptic TiO2/ SnO2/C composite and its lithium storage, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet. 2018.10.054

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The preparation of a new quasi-elliptic TiO2/SnO2/C composite and its lithium storage Zhangmin Honga, Qinghua Tiana*, Wei Zhanga and Li Yangb* aDepartment

of Chemistry, School of Sciences, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China

bSchool

of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China

*Corresponding author e-mail address: [email protected], [email protected]

Abstract: Herein, a new quasi-elliptic nanostructure of composite composed of innermost quasielliptic TiO2, outmost carbon coating and sandwiched SnO2 nanoparticles has been successfully fabricated by a well-designed approach for the first time. This unique architecture provides the structural stability of SnO2 nanoparticles with double assurance, as well as improves the conductivity of whole composite. As a result, the as-prepared TiO2/SnO2/C composite exhibits superior lithium storage. Moreover, the preparation of elliptic TiO2/SnO2/C composite can further enrich the variety of the morphologies of TiO2/SnO2/C composites. Keywords: Crystal structure; Functional; Composite; Anode; Lithium-ion batteries 1 Introduction Currently, as a crucial semiconductor sensor material, SnO2 has also attracted increasing attention in lithium-ion batteries (LIBs) field owing to high theoretical capacity of 782 mAh g-1 [1, 2]. However, the practical application of SnO2 anode in LIBs is still not implemented due to two mian issues. One is the poor electric conductivity, which severely restricting the rate capacity; The other is the large volume change (358%) accompanying the structural transformation during lithiation/delithiation process, which leading to the capacity fade quickly and cycling stability terrible. Fortunately, like other new metal oxide anodes, the above shortcomings of SnO2 anode can be well addressed via fabricating nanosized SnO2/C composites [3-13]. Moreover, it is suggested that integrating nanosized SnO2 into a C/TiO2 composite matrix can further improve the

cycling performance of electrodes. Consequently, various types of TiO2/SnO2/C composites have been prepared and exhibited better performance compared to the individual SnO2 counterparts [14-16]. However, the nanostructures of these reported TiO2/SnO2/C composites are mainly focused on the one-dimensional wires or tubes, two-dimenisonal sheets and three-dimensional spheres due to the spherical, lamellar and linear templates or components are readily available. In addition to these nanostructures, new nanostructures such as elliptic and cuboidal TiO2/SnO2/C composites are rarely reported due to their preparation are more difficult. Herein, a nanostructure of TiO2/SnO2/C composite with a novelly quasi-elliptic morphology (TiO2/SnO2/C NEs) has been successfully fabricated by a well-designed strategy for the first time, as shown in schematic (Fig. 1). The TiO2/SnO2/C NEs consists of innermost quasi-elliptic TiO2, outmost carbon coating and sandwiched SnO2 nanoparticles. Moreover, it exhibits outstanding lithium storage as a LIB anode.

Fig. 1 The preparation schematic of TiO2/SnO2/C NEs composite. Notes: HTO is the H2Ti3O7.

2 Experimental section 2.1 Sample preparation The HTO was prepared according to our previous report [17]. 0.2 g of HTO was dispersed into 60 ml deionized water via ultrasonic. Then, 0.1 g K2SnO3•3H2O was added under stirring and then stirring for another 30 min. The above suspention was transferred into a 100 ml Teflon-liner of stainless steel autoclave and kept in an oven for 24 h at 180 °C. After autoclave cooled down to room temperature, collection of the as-prepared TiO2/SnO2 NEs precipitation was achieved by

centrifugation and wash using ethanol and deionized water thoroughly, and then dried at 70 °C overnight. The TiO2/SnO2 NEs was re-dispersed into 60 ml deionized water, and then addition of 1.2 g glucose under stirring and then stirring for another 30 min. The suspension was transferred into a 100 ml Teflon-liner of stainless steel autoclave and kept in an oven for 24 h at 180 °C. After autoclave cooled down to room temperature, the collection and dry process of the as-prepared dark brown precipitation was the same with TiO2/SnO2 NEs. Finally, the TiO2/SnO2/C NEs was obtained by carbonization of as-prepared precipitation at 500 °C under Ar atmosphere for 3 h. 2.2 Material and electrochemical characterizations Transmission electron microscopy (TEM, JEOL JEM-2010) with a energy dispersive X-ray detector (EDX), X-ray diffraction (XRD, Rigaku, D/max-Rb using Cu Kα X-ray radiation), Raman spectroscopy (Renishaw in Via), X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD instrument with using aluminum Kα X-ray radiation) and thermogravimetric analysis (TGA, SDT Q600 V8.2 Build 100) were carried out on samples to research their physical and chemical properties. As-prepared sample, acetylene black (AB) and sodium carboxymethyl cellulose (CMC) with a mass ration of 7/2/1 were used to make the working electrode. The mass loading of TiO2/SnO2/C NEs active material in electrode is about 0.77 mg cm-2. The electrodes were assembled into 2016type coin cells with lithium foil, Cellgard 2400 membrane, and 1 M LiPF6 (dissolved in a mixture of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1) used as counter/reference electrodes, separator, and electrolyte, respectively. The CT2001a cell test system (LAND Electronic Co.) was used to test the galvanostatic discharge/charge cycling of cells with a voltage range of 0.01 to 3.0 V at room temperature. CHI660D electrochemical workstation

was applied to perform cyclic voltammetry (CV) test and electrochemical impedance spectrum (EIS) collection at a scan rate of 0.3 mV s-1 between 0.01 and 3.0 V, and at an ac perturbation of 5 mV in the frequency range from 100M to 0.01 Hz at room temperature, respectively. 3 Results and discussion Fig. 2a shows the EDX spectrum of TiO2/SnO2/C NEs. The observed peaks for C, O, Sn and Ti elements indicate TiO2/SnO2/C NEs consists of C, O, Sn and Ti elements. And the atomic percentage of each element in TiO2/SnO2/C NEs has also been given in Table S1. Fig. 2b further gives the XRD pattens. Obviously, the XRD patterns of TiO2/SnO2/C NEs can be indexed into two crystalline phases, one is the rutile SnO2 (JCPDS card No. 41-1445) and, the other is the anatase TiO2 (JCPDS card No. 21-1272) [16]. Fig. 2c shows the Raman spectrum. Two peaks observed at 1342 and 1583 cm-1 correspond to characteristic D and G band of carbonaceous materials, respectively. The content of carbon is estimated to be 53% (Fig. 2d). Besides, XPS characterization was used to determine the chemical states of Ti, Sn, O and C elements of TiO2/SnO2/C NEs, as shown in Fig. S1. Thus, it is concluded that the TiO2/SnO2/C NEs consists of SnO2, TiO2 and carbon.

Fig. 2 (a) EDX spectrum, (b) XRD patterns, (c) Raman spectrum and (d) TG curve of TiO2/SnO2/C NEs.

In order to provide insight into the microstructures of TiO2/SnO2/C NEs and TiO2/SnO2 NEs, TEM characterization was conducted on them. We can see from Fig. 3a that the TiO2/SnO2 NEs is

composed of quasi-elliptic nanorods with a length of about 100 - 200 nm and a width of about 30 50 nm. By further magnified TEM observation (Fig. 3b and Fig. S2a), we can find that the surface of quasi-elliptic nanorods is deposited by smaller nanoparticles. Fig. 3c gives the HRTEM image. A clear interplanar spacing of 0.35 nm is observed on the elliptic nanorod and almost throughouts the nanorod, corresponding to the d101-spacing of anatase TiO2 [16]. Another clear interplanar spacing of 0.33 nm is observed on the nanoparticle, corresponding to the d110-spacing of rutile SnO2 [16]. Then, Fig 3(d, e) and Fig. S2b show the different magnified TEM iamges of TiO2/SnO2/C NEs. Expectably, the TiO2/SnO2/C NEs well retains the microstructure of TiO2/SnO2 NEs besides one more outmost carbon coating. As seen from the HRTEM image (Fig. 3f), the SnO2 nanoparticles are sandwiched between inner TiO2 and outer carbon (the thickness is about 3.5 nm). Therefore, it is demonstrated that the TiO2/SnO2/C NEs is composed of SnO2 nanoparticles sandwiched between innermost quasi-elliptic TiO2 nanorods and outmost carbon coatings.

Fig. 3 TEM images of (a-c) TiO2/SnO2 NEs and (d-f) TiO2/SnO2/C NEs.

Then, the electrochemical characterization was performed on TiO2/SnO2/C NEs. Fig. 4a reveals the CV plot of TiO2/SnO2/C NEs electrode. A pair of cathodic/anodic peaks at about 1.71/2.12 V correspond to the reversible lithium insertion into/abstraction from elliptic TiO2 (TiO2 + xLi+ + xe-

↔ LixTiO2) [18]. Then, a weak peak at about 1.08 V mainly corresponds to the irreversible formation of solid electrolyte interface (SEI) film and partially reversible reduction reaction of SnO2 nanoparticles by lithium [19]. Therefore, a weak anodic peak corresponding to the partially reversible oxidation reaction of Sn into SnO2 or SnO is seen at about 1.13 V. Moreover, a sharp cathodic peak and a corresponding anodic peak observed at about 0.68-0.01 and 0.51 V respectively corresponds to the reversible alloying/dealloying reaction between Sn and lithium (Sn + xLi+ + xe- ↔LixSn (0 ≤ x ≤ 4.4) ) [20]. So, the CV plot indicates that the TiO2/SnO2/C NEs has a typical lithium storage behaviour of TiO2/SnO2/C composites. After first cycle the following four cycles almost overlap, indicating the good reversible lithium storage performance. Fig. 4b displays the galvanostatic discharge/charge profiles of TiO2/SnO2/C NEs electrode at 200 mA g-1. Obviously, they well match with the CV result. Additionally, it is found that the capacity decreases before 50 cycles and then steadily increases until to 450 cycles, indicating outstanding cycling stability. We can see from Fig. 4c that the TiO2/SnO2/C NEs exhibits superior lithium storage performance, giving capacity of 470.8 mAh g-1 with a coulombic efficiency of about 99% after even 450 cycles. The initial coulombic efficiency is only 40.8%, which is mainly due to the irreversible formation of SEI film and partially reversible reaction between SnO2 and lithium during first cycle [20]. However, as seen from Fig. 4c that the cycling performances of both control samples of TiO2/SnO2 NEs and SnO2 NPs (Fig. S3) are far inferior to TiO2/SnO2/C NEs. And the charge transfer resistances of these three electrodes are also compared (Fig. 4d). Obviously, the TiO2/SnO2/C NEs (90 Ω) and SnO2 NPs (81.5 Ω) electrodes deliver much smaller diameters of the high frequency semicircles than that of TiO2/SnO2 NEs (238 Ω), testifying former two electrodes have lower charge transfer resistances. And the electrochemical performance of

pure TiO2 NEs has been studied by EIS and rate tests, as shown in Fig. S4. Obviously, the performance of pure TiO2 NEs is much inferior to TiO2/SnO2 NEs and TiO2/SnO2/C NEs. It is indicated that the carbon coating and smaller size of SnO2 NPs have positive effects on the improvement of electrochemical reaction kinetics. Moreover, the influences of reaction time and temperature on the microstructure and electrochemical performance of TiO2/SnO2/C NEs have been preliminarily studied as shown in Fig. S5. It is indicated that the reaction time and temperature have important effect on the performance of TiO2/SnO2/C NEs composites.

Fig. 4 (a) CV plot and (b) galvanostatic discharge/charge profiles of TiO2/SnO2/C NEs electrode at 200 mA g-1; (c) Cycling performance at 200 mA g-1 and (d) Nyquist plots of TiO2/SnO2/C NEs, TiO2/SnO2 NEs and SnO2 NPs electrodes; (e) Rate capabilities of TiO2/SnO2/C NEs and TiO2/SnO2 NEs electrodes.

Finally, Fig. 4e gives the rate capabilities of TiO2/SnO2/C NEs and TiO2/SnO2 NEs electrodes. As expected, the TiO2/SnO2/C NEs has better rate capacity than TiO2/SnO2 NEs. Thus, it is demonstrated that thus superior performance of TiO2/SnO2/C NEs electrode should be attributed to the synergistic effect of SnO2, TiO2 and carbon. One hand, the SnO2 nanoparticles can not only reduce the absolute volume change but also shorten the diffusion distance of lithium-ions and electrons; On the other hand, the SnO2 nanoparticles are further sandwiched between innermost TiO2 and outmost carbon that can not only immobilize the SnO2 nanoparticles to prevent them from aggregation, but also accommodate the volume change of SnO2 nanoparticles by the

synergistic effect of stable TiO2 and flexible carbon, as well as the good conductive carbon can improve the conductivity of whole composite. 4 Conclusions In summary, a new quasi-elliptic nanostructure of TiO2/SnO2/C NEs has been successfully prepared by a well-designed approach for the first time. This particular architecture with a synergistic effect of components provides the SnO2 nanoparticles with outstanding structural stability and improved electrochemical kinetics. As a result, the TiO2/SnO2/C NEs exhibits superior lithium storage, obtaining capacity of 470.8 mAh g-1 after even 450 cycles. Acknowledgments We are grateful for financial support from Natural Science Foundation of Zhejiang Province (No. LQ18B030008). References [1] Z. H. Wen, G. Wang, W. Lu, Q. Wang, Q. Zhang, J. H. Li, Growth Des. 7 (2007) 1722. [2] Y. Deng, C. Fang, G. Chen, J. Power Sources 304 (2016) 81. [3] W. Zhou, L. J. Lin, W. J. Wang, L. L. Zhang, Q. Wu, J. H. Li, L. Guo, J. Phys. Chem. C 115

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Graphical Abstract

The as-prepared new quasi-elliptic TiO2/SnO2/C composite composed of innermost elliptic TiO2, outmost carbon coating and sandwiched SnO2 nanoparticles exhibits superior lithium storage, as well as enriches the variety of the morphologies of TiO2/SnO2/C composites.

Highlights 1. A new quasi-elliptic TiO2/SnO2/C composite was prepared for the first time. 2. Thus architecture provided the SnO2 with double structure protection. 3. The composite exhibited improved cycling stability.