Cu2[email protected]2 core-shell nanocube composite as improved performance anode materials for lithium-ion batteries

Accepted Manuscript Cu2O@TiO2 core-shell nanocube composite as improved performance anode materials for lithium-ion batteries S.M. Yin, L.W. Ye, Y.F. Yuan, M. Zhu, C.B. Chen, Y.C. Wu, S.Y. Guo PII: DOI: Reference:

S0167-577X(18)30700-6 https://doi.org/10.1016/j.matlet.2018.04.096 MLBLUE 24263

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

9 February 2018 8 April 2018 23 April 2018

Please cite this article as: S.M. Yin, L.W. Ye, Y.F. Yuan, M. Zhu, C.B. Chen, Y.C. Wu, S.Y. Guo, Cu2O@TiO2 core-shell nanocube composite as improved performance anode materials for lithium-ion batteries, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.04.096

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Cu2O@TiO2 core-shell nanocube composite as improved performance anode materials for lithium-ion batteries S. M. Yin, L. W. Ye, Y. F. Yuan, M. Zhu, C. B. Chen, Y. C. Wu, S. Y. Guo College of Machinery and Automation, Zhejiang Sci-Tech University, Hangzhou 310018, China

Abstract: Uniform Cu2O nanocubes were large-scale synthesized through a liquid-phase reduction method. TiO2 was further coated onto Cu2O nanocubes to form Cu2O@TiO2 core-shell composite based on hydrolysis reaction of titanium isopropoxide. As-synthesized Cu2O@TiO2 were characterized by XRD, SEM and TEM. As anode materials for lithium ion batteries, Cu2O@TiO2 core-shell composite showed higher discharge capacity and more stable cycling performance than the uncoated Cu2O nanocubes, exhibiting the strong composition effect. Keywords: Cu2O; TiO2; Composite materials; Energy storage and conversion

1. Introduction Lithium ion batteries (LIBs) have drawn tremendous research attention due to their wide application in various fields, such as portable electronics, electric vehicles and energy storage. As a crucial component, electrode material plays a vital role in pursuing high-performance LIBs. Therefore, it is very urgent to develop new, economical, abundant, simply made and safe anode materials with the improved electrochemical performances to supplement or replace traditional C-based anode materials. Transition metal oxides have been extensively studied as alternative anode materials for LIBs[1-4]. Among these transition metal oxides, Cu2O with theoretical capacity of 375 mAh g−1 has attracted much attention[5]. Unfortunately, rapid capacity fading and poor cycling stability seriously limit its application. During the charge/discharge reaction, Cu2O undergoes relatively large volume fluctuations, which results in material pulverization and hence rapid capacity fading[5-7]. It is noted that nanometer structuring and compositing of the electrode materials have been 

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E-mail address: [email protected] (Y. F. Yuan) 1

proved to be two promising avenues to boost the electrochemical performance by effectively alleviating the stress-induced structural variation during long-term electrochemical reactions. Consequently, Cu2O nanocubes, octahedral[6], hollow nanospheres[7] as well as Cu2O-CuO-RGO composite[8], Cu2O beads/wrinkled grapheme[9], and so on, have been extensively reported as anode materials for LIBs. In this work, from the angle of material structure design of Cu2O, Cu2O nanocubes coated with TiO2 were prepared and the effect of the TiO2 coating on electrochemical performance of Cu2O was studied. 2. Experimental section Typically, 17.7 g Sodium citrate was dissolved into 200 ml distilled water with constant stirring for 20 min. Then, 0.5 ml 1.2 M CuSO4 solution was added to the solution mixture under stirring. After 5 min, 0.5 ml 4.8 M NaOH solution was quickly poured into the above mixture and stirred for another 5 min. Finally 0.5 ml 1.2 M ascorbic acid was put to the solution mixture and stirred for 30 min. The resulting precipitate was collected by centrifugation, washed several times with distilled water and absolute ethanol, and dried in vacuum at 60°C. 0.1 g as-prepared Cu2O was again dispersed into 30 ml ethanol. 0.05 ml titanium isopropoxide (TIP, 97%) was added to the dispersion under stirring. After 10 min, 0.5 ml distilled water was added dropwise into the dispersion and stirred for one hour. The resulting product was collected by centrifugation and washed several times with distilled water and absolute ethanol, dried in vacuum at 60°C. Finally, the sample was calcined at 450°C for 120 min under an argon gas flow at 60 ml min-1 in a tube furnace with the heating rate of 5°C min-1. The morphology and microstructures of the samples was examined by scanning electron microscopy (SEM, vltra55) and Transmission Electron Micro-scope (TEM, JEM-2100). The crystalline phase was investigated by X-ray diffraction (XRD, ARLXTRA). The working electrodes were prepared by dispersing active material, acetylene carbon black, polyvinylidene fluoride (PVDF) at a weight ratio of 8:1:1 in N-methylpyrrolidinone. The slurry was uniformly pasted onto Ni foam and dried at 2

100°C for 12 h under vacuum. The weight of the active materials in each electrode was about 1 mg. The cells were assembled in a glovebox with lithium metal as the counter/reference electrode and Celgard separator. The electrolyte was 1 M LiPF6 with the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v). The charge/discharge measurements were carried out at the current density of 100 mA g-1 in the voltage range of 0.01-3 V using Battery Testing System (Neware) at room temperature. 3. Results and discussion Fig. 1a shows the XRD pattern of the resulting product, and it can be seen that these diffraction peaks at 29.6°, 36.4°, 42.3°, 61.4° and 73.5° can be well attributed to (110), (111), (200), (220), (311) crystal planes of Cu2O cubic phase (JCPDS 65-3288), respectively. The diffraction peak at 32.7° is ascribed to (020) crystal plane of TiO2 (JCPDS 13-1970). There are no other noticeable peaks belonging to the impurities, which indicates the high purity of the product. Fig. 1b gives SEM image of the synthesized Cu2O, and lots of cubes can be clearly seen. These Cu2O cubes have well-defined crystal structure as well as good size and shape uniformity. Their surfaces seem not to be flat but the edges are arcs with smooth transition. Their edge lengths are about 200 nm. The growth mechanism of Cu2O cubes in the liquid-phase reduction method has been described elsewhere[10]. After the cubes are coated by TiO2 that comes from the hydrolytic decomposition of TIP[11], the surface of Cu2O cubes becomes flat and meanwhile their edges become incisive (Fig. 1c). TEM image (Fig. 1d) clearly reveals the relationship between Cu2O cubes with TiO2. A layer of intact compact uniform TiO2 tightly coats a Cu2O nanocube, presenting a typical core-shell composite structure. The layer thickness of TiO2 is about 40-50 nm. To reveal the electrochemical performance of Cu2O@TiO2 core-shell composite, galvanostatic charge/discharge test with a voltage window of 0.01−3 V vs Li/Li+ was performed and shown in Fig 2. Fig. 2a compares the cycling performance of Cu2O@TiO2 with pure Cu2O nanocubes and highlights the improved discharge capacity and cycling stability of Cu2O@TiO2. At the 1st cycle, Cu2O@TiO2 delivers 435 mAh g-1 of discharge capacity much higher than 297 mAh g-1 of pure Cu2O 3

nanocubes. After 15 cycles, discharge capacity of Cu2O@TiO2 slowly declines to 362 mAh g-1, which is still higher than 340 mAh g-1 of pure Cu2O nanocubes that is its maximum discharge capacity for pure Cu2O nanocubes. Subsequently, the discharge capacity of Cu2O@TiO2 fluctuates around 362 mAh g-1 until the 79th cycles, then begins to decline slowly. At the 100th cyles, Cu2O@TiO2 still shows 330 mAh g-1 of discharge capacity. The average discharge capacity of Cu2O@TiO2 over 100 cycles is 358 mAh g-1, surpassing 336 mAh g−1 of porous Cu2O film[12], 324 mAh g−1 of Cu2O rod-like arrays[13], 142 mAh g−1 of Cu2O nanocubes[14], 107 mAh g−1 of electrodeposited

Cu2O

coatings[15],

but

lower

than

592

mAh

g−1

of

CuO/Cu2O@CeO2[16] and 550 mAh g−1 of Cu2O/CuO/reduced grapheme oxides[17]. For comparison, the discharge capacity of pure Cu2O nanocubes slowly decreases after the 15th cycles, and then the decline accelerates after the 54th cycle. At the 100th cycle, its discharge capacity is 202 mAh g-1 that is only 61% of Cu2O@TiO2. The average discharge capacity of Cu2O@TiO2 based on 100 cycles is 358 mAh g-1 and increases 22.6% in comparison with pure Cu2O nanocubes. The corresponding value of the latter is only 292 mAh g-1. The theoretic specific capacity of TiO2 is 335 mAh g-1, which means that TiO2 does not have the ability to raise specific capacity of the composite. Conversely, this also indicates that the improvement of discharge capacity of the composite should be due to the fact that the TiO2 coating improves electrochemical performance of Cu2O nanocubes. The cycling stability of Cu2O@TiO2 is also improved obviously, which should be owing to the coating effect of TiO2 that effectively tolerates large internal stress and volume fluctuations of Cu2O during the charge/discharge cycles, accordingly increases the structure stability and cycling stability of Cu2O. It can be clearly seen from Fig. 2a that the coating effect of TiO2 on Cu2O is remarkable and intensive. Fig. 2b compares the charge/discharge curves of Cu2O@TiO2 core-shell composite with pure Cu2O nanocubes. Not only their charge curves but also their discharge curves show similar inflection points, which indicates that their electrochemical behaviors are similar basically. Nevertheless every transition zones of Cu2O@TiO2 is lengthened obviously. This means the electrochemical reaction extent of Cu2O after 4

the coating of TiO2 is enhanced. The charge/discharge curves of other cycles display the same phenomenon. 4. Conclusion Cu2O nanocubes are successfully coated by a layer of TiO2, forming a typical core-shell composite structure. The coating of TiO2 remarkably improves discharge capacity and cycling stability of Cu2O nanocubes, presenting the strong composition effect. This provides one new route to study the application of Cu2O in LIBs, including not only the new strategy, but also new materials. Acknowledgments This work was funded by 521 talents-cultivated project of Zhejiang Sci-Tech University; National Natural Science Foundation of China (No. 51302247 and 51602286); Zhejiang Top Priority Discipline of Textile Science and Engineering Visiting, Zhejiang Sci-Tech University (2014KF14); Natural Science Foundation of Zhejiang province (No. LY18E010004). References [1] Q. Q. Xiong, J. J. Lou, Y. J. Zhou, S. J. Shi, Z. G. Ji, Mater. Lett. 210 (2018) 267-270. [2] Y. F. Yang, D. Cheng, S. J. Chen, Y. L. Guan, J. Xiong, Electrochim. Acta 193 (2016) 116-127. [3] D. Cheng, Y. F. Yang, Y. B. Luo, C. J. Fang, J. Xiong, Electrochim. Acta 176 (2015) 1343-1351. [4] Q. Q. Xiong, H. Z. Chi, J. Zhang, J. P. Tu, J. Alloys Compd., 688 (2016) 729-735. [5] Y. M. Yang, K. Wang, Z. H. Yang, Y. M. Zhang, H. Y. Gu, W. X. Zhang, E. Li, C. Zhou, Thin Solid Films 608 (2016) 79-87 [6] M. C. Kim, S. J. Kim, S. B. Han, D. H. Kwak, E. T. Hwang, D. M. Kim, G. H. Lee, H. S. Choe, K. W. Park, J. Mater. Chem. A 3 (2015) 23003–23010. [7] Shilpa, P. Rai, A. Sharma, RSC Adv. 6 (2016) 105231-105238. [8] L. N. Sun, Q. W. Deng, Y. L. Li, L. B. Deng, Y. Y. Wang, X. Z. Ren, P. X. Zhang, Electrochim. Acta 222 (2016) 1650-1659. [9] G. Ananya, N. Pranati, S. Ramaprabhu, Int. J. Hydrogen Energy 41 (2016) 5

3974-3980. [10] Y. B. Xiong, L. H. Yan, T. Chen, Mater. Lett. 172 (2016) 109-111. [11] B. Y. Guan, L. Yu, J. Li, X. W. Lou, Sci. Adv. 2 (2016) e1501554. [12] J. Y. Xiang, X. L. Wang, X. H. Xia, L. Zhang, Y. Zhou, S. J. Shi, J. P. Tu, Electrochim. Acta 55 (2010) 4921-4925 [13] Y. M. Yang, K. Wang, Z. H. Yang, Y. M. Zhang, H. Y. gu, W. X. Zhang, E. Li, C. Zhou, Thin solid films 608 (2016) 79-87. [14] H. D. Liu, Z. L. Hu, R. Hu, B. T. Liu, H. B. Ruan, L. Zhang, W. Xiao, Int. J. Electrochem. Sci., 11 (2016) 2756-2761. [15] G. K. Kiran, T. R. Penki, P. V. Kamath, N. Nunichandraiah, J. Solid State Electrochem. 20 (2016) 555-562. [16] L. J. Wang, X. J. Wang, Z. H. Meng, H. J. Hou, B. K. Chen, J. Mater. Sci. 52 (2017) 7140-7148. [17] S. H. Wu, G. L. Fu, W. Q. Lv, J. K. Wei, W. J. Chen, H. Q. Yi, M. Gu, X. D. Bai, L. Zhu, C. Tan, Small 14 (2018) 1702667.

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Figure Caption Fig. 1 (a) XRD pattern of Cu2O@TiO2 composite; SEM images of (b) pure Cu2O nanocubes and (c) Cu2O@TiO2 composite; (d) TEM image of Cu2O@TiO2 composite. Fig. 2 (a) Cycling performance and the 1st charge/discharge curves of Cu2O@TiO2 composite and pure Cu2O nanocubes.

Cu2O

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Figure 1

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500

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0 0

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Figure 2

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Highlights

► Cu2O nanocube is coated by a layer of intact compact uniform TiO2 film ► Cu2O@TiO2 composite shows higher discharge capacity than the uncoated Cu2O ► Cu2O@TiO2 composite shows more stable cycling performance than the uncoated Cu2O

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