Heterogeneous triple-shelled TiO2@NiCo2O4@Co3O4 nanocages as improved performance anodes for lithium-ion batteries

Accepted Manuscript Heterogeneous Triple-shelled TiO2@NiCo2O4@Co3O4 Nanocages as improved performance anodes for lithium-ion batteries L.W. Ye, Y.F. Yuan, D. Zhang, M. Zhu, S.M. Yin, Y.B. Chen, S.Y. Guo PII: DOI: Reference:

S0167-577X(18)31342-9 https://doi.org/10.1016/j.matlet.2018.08.131 MLBLUE 24842

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

17 May 2018 25 July 2018 23 August 2018

Please cite this article as: L.W. Ye, Y.F. Yuan, D. Zhang, M. Zhu, S.M. Yin, Y.B. Chen, S.Y. Guo, Heterogeneous Triple-shelled TiO2@NiCo2O4@Co3O4 Nanocages as improved performance anodes for lithium-ion batteries, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.08.131

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Heterogeneous Triple-shelled TiO2@NiCo2O4@Co3O4 Nanocages as improved performance anodes for lithium-ion batteries L. W. Yea, Y. F. Yuana, D. Zhangb, M. Zhua, S. M. Yina, Y. B. Chena, S. Y. Guoa a

College of Machinery and Automation, Zhejiang Sci-Tech University, Hangzhou 310018, China

b

Hang Zhou City of Quality and Technical Supervision and Testing Institute, Hangzhou 310019, China

Abstract: ZIF-67-derived NiCo2O4@Co3O4 double-shelled nanocages was coated by a layer of TiO2 shell, forming a heterogeneous triple-shelled cage-in-cage structure. As-synthesized triple-shelled nanocages were characterized by XRD, SEM and TEM. As anode materials for lithium ion batteries, triple-shelled nanocages show improved cycling stability and discharge capacity, which could be due to the cage-in-cage hollow nanostructure and beneficial effect of the TiO2 shell. Key word: TiO2; NiCo2O4@Co3O4; Composite materials; Energy storage and conversion

1. Introduction Many efforts have been devoted to developing lithium ion battery (LIB) because of their potential applications. Transitional metal oxides can meet the requirements of next-generation LIBs with high energy and high power density, and have been suggested as promising advanced anodes[1-3]. In particular, NiCo2O4 and Co3O4 possess high theoretical capacity and have been reported a lot[4-6]. Recently, hollow micro/nanostructures have attracted intensive interest for their intriguing structural features and great potential in a myriad of applications including energy storage, catalysis and drug delivery[7]. Hollow materials are appropriate electrode materials for LIBs because hollow structures possess large interfacial surfaces that allow Li+ to easily access the nanoshells from both sides, resulting in a dramatically shortened diffusion path and improved kinetics that are favorable for high rate capacity, better cycle performance, and improved storage capacity. Accordingly, multishelled Co3O4[8], TiO2 hollow microspheres[9], hollow NiCo2O4 nanoboxes[10], and so on, have been extensively reported as anode materials for LIBs. Shell quantity and shell composition are the most important factors influencing the 

Corresponding author. Tel.: +86-571-8684-3343

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

electrochemical performance of hollow materials. The multiple layers of shells and multiple compositions of hollow materials could bring about structure merits and synergetic effects, which might offer vast opportunities and hold great potentials for improved physical/chemical properties. For example, three-layered TiO2@Carbon@MoS2 hierarchical nanotubes manifested remarkable lithium storage performance with good rate capability and long cycle life[11]. Herein, this work reported a heterogeneous triple-shelled TiO2@NiCo2O4@Co3O4 nanocage materials prepared by metal organic framework derivation combined with hydrolysis reaction of titanium isopropoxide (TIP), and explored its LIB application.

2. Experimental 2.1. Materials synthesis Preparation of ZIF-67. 2 mmol of Co(NO3)2·6H2O and 8 mmol of 2-methylimidazole were respectively dissolved in 50 mL methanol under stirring for 30 min. Then, 2-methylimidazole solution was quickly poured into the solution of Co(NO3)2 and the mixed solution was aged for 24 h at room temperature without stirring. The purple precipitate was collected by centrifugation, washed with methanol three times, and finally dried at 70 °C for 12 h. Synthesis of double-shelled NiCo2O4@Co3O4 Nanocage. 40 mg ZIF-67 was first dispersed in 25 mL of ethanol containing 80 mg of Ni(NO3)2·6H2O. After stirring for 30 min, the ZIF-67/Ni−Co LDH precursor were formed, collected by centrifugation, washed with ethanol several times, finally dried at 70°C for 12 h. Then, the double-shelled NiCo2O4@Co3O4 nanocage were obtained by annealing the precursor in air at 350°C for 2 h with the heating rate of 1°C min−1. Synthesis of triple-shelled TiO2@NiCo2O4@Co3O4 nanocage. 100 mg NiCo2O4/Co3O4 nanocage was again dispersed into 30 ml of ethanol. 0.1 ml TIP was added to the dispersion under intensely stirring. After 10 min, 1 ml distilled water was added dropwise into the dispersion and stirred for an hour. The resultant product was collected by centrifugation, washed several times with distilled water and ethanol, dried at 70°C. Finally, the product was calcined at 450 °C for 120 min under an argon gas flow at 60 ml min -1 in a tube furnace with heating rate of 5°C min-1. 2

2.2. Material characteristics and electrochemical measurements Crystalline structures and morphology of as-prepared products were characterized with powder X-ray diffraction (XRD ARLXTRA), scanning electron microscopy (SEM, vltra55), and transmission electron microscopy (TEM, JEM-2100). Electrochemical experiments were performed using CR2025 coin-type cells assembled in a glovebox with lithium foil as the counter/reference electrode and Celgard 2400 as the separator. 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 evenly pasted onto Ni foam and dried at 100°C for 12 h under vacuum. The weight of the active materials in each electrode was about 1 mg. The electrolyte was 1 M LiPF6 with the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v). The galvanostatic charge/discharge measurements were performed through Battery Testing System (Neware) at room temperature.

3. Results and discussion Fig. 1a is SEM image of the purple ZIF-67 particles. They are solid structure, and show a regular rhombic dodecahedral morphology with smooth surface. The average particle size is approximately 800 nm. After reaction with Ni(NO3)2 in ethanol solution for 30 min, ZIF-67 still maintains the polyhedral shape, although changes into two-layered structure. The surface layer is rougher Ni-Co precursor constructed by small nanosheets and the internal is residual ZIF-67. After annealing in air, the polyhedral shape is still kept, but the surface precursor changes to NiCo2O4 that still shows the rougher surface, while the internal changes to hollow Co3O4[12]. Accordingly, double-shelled NiCo2O4@Co3O4 nanocage forms. A broken particle clearly reveals the double-shelled structure of the nanocage (Fig. 1b). Fig. 1c presents TEM image of double-shelled nanocages. The cage-in-cage structure is distinctly shown. The size of the inner cage is about 500 nm and the external 700-800 nm. The inner cage does not lie in the center because it is independent and freedom. Through the hydrolytic decomposition of TIP, TiO2 is coated on the surface of NiCo2O4, forming the third layer of shell (Fig. 1d). The surface of the nanocage becomes smooth, differing from rougher surface of NiCo2O4 before the coating. Fig. 1e show the corresponding TEM image. It is clear that TiO2 is not an 3

independent existing shell and it tightly attaches to the NiCo2O4 shell. There is no interspace. This leads to the increasing in the wall thickness of the external nanocage, differing from the thin wall of single NiCo2O4 shell in Fig. 1c. Besides, it can be seen that TiO2 is compact uniform, and it forms an intact shell. Fig. 1f gives X-ray diffraction pattern of the resultant product. These diffraction peaks can be well attributed to Co3O4 phase (JCPDS JCPDS 74-1656), NiCo2O4 (JCPDS 73-1702) and TiO2 (JCPDS 13-1970), which indicates that the resultant product is composed of Co3O4, NiCo2O4 and TiO2. To further reveal triple-shelled structure in TiO2@NiCo2O4@Co3O4 TSNC, elemental mapping analysis is carried out (Fig. 2). It can be seen that Ti, Ni and Co mapping coincide well with triple-shelled structure. To reveal LIB application of TiO2@NiCo2O4@Co3O4 heterogeneous triple-shelled nanocages (TSNC), galvanostatic charge/discharge test with a voltage window of 0.01−3.0 V vs Li/Li+ was performed (0.1C). Fig. 3a compares the cycling performance of TSNC with NiCo2O4@Co3O4 double-shelled nanocages (DSNC) and highlights the improved discharge capacity and cycling stability of TSNC. At the 1st cycle, TSNCs delivers 2000 mAh g-1 of discharge capacity much higher than 1718 mAh g-1 of DSNC. The theoretical capacity of NiCo2O4 and Co3O4 are both 890 mAh g-1. The discharge capacity of both materials at the 1st cycle is remarkably higher than the theoretical value, showing high electrochemical activity of hollow nanocage materials. The excessive high discharge capacity is unsustainable. After the initial 15 cycles, the discharge capacity of TSNC declines to 750 mAh g-1, and subsequently fluctuates around 750 mAh g-1 until the 200th cycles. In comparison, the discharge capacity of DSNC declines quickly until the 150th cycle, and subsequently maintains 370 mAh g-1. The average discharge capacity of TSNC based on 200 cycles is 852 mAh g-1 while DSNC is 691 mAh g-1. The increasing rate is 23.3%. The discharge capacity and cycling performance of TSNC surpass double-shelled and triple-shelled Co3O4[6], Co3O4 nanoparticles[13], NiCo2O4 nanorods[14] and Co3O4@TiO2 core-shell nanofibers[15]. The nanocage structure endows the tremendous specific surface area of NiCo2O4 and Co3O4. Hollow structure can accommodate the electrolyte in its interior, so both sides of the shells participate in electrochemical reaction. The thin shell wall shortens the diffusion route of Li+, accelerating electrochemical reaction kinetics. Hence both of them show high discharge capacity during the initial cycles. The third shell TiO2 plays critical role on cycling stability of the nanocages. TiO2 possesses low volume 4

expansion (< 4%), high chemical stability. It tightly attaches to the NiCo2O4 shell and effectively tolerates large internal stress and volume fluctuations of NiCo2O4 during the charge/discharge cycles, accordingly increases the structure stability and cycling stability of NiCo2O4. Consequently, TSNC ends earlier its declining tendency and maintains a relatively high electrochemical active level. The discharge specific capacity of TSNC includes the mass of TiO2 whose theoretical capacity is only 335 mAh g-1, much lower that those of NiCo2O4 and Co3O4. However, it can be seen that in the ahead 15 cycles and the later 130 cycles, TSNC shows higher discharge capacity than DSNC. This means that the TiO2 shell is not a burden on TSNC and its beneficial effect is predominant. This would be synergetic effect. Fig. 3b and c give the charge/discharge curves of TSNC and DSNC. Their charge and discharge curves present similar inflection points. The difference lies in the fact that every transition zones of TSNC is lengthened obviously. This means their similar electrochemical behaviors, but the enhanced electrochemical reaction extent of TSNC. 4. Conclusion Heterogeneous triple-shelled TiO2@NiCo2O4@Co3O4 nanocages show typical cage-in-cage structure. The internal cage is Co3O4 and the external is NiCo2O4 coated by the third shell TiO2. Heterogeneous triple-shelled nanocages show the improved cycling stability and higher discharge capacity due to the hollow nanocage structure and the beneficial effect of TiO2, and is promising anode materials for LIBs. This work broadens the study on hollow nanostructures in LIBs. 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); Natural Science Foundation of Zhejiang province (No. LY18E010004). References [1] Y. F. Yang, D. Cheng, S. J. Chen, Y. L. Guan, J. Xiong, Electrochim. Acta 193 (2016) 116-127. [2] Q. Q. Xiong, J. J. Lou, Y. J. Zhou, S. J. Shi, Z. G. Ji, Mater. Lett. 210 (2018) 267-270. [3] D. Cheng, Y. F. Yang, Y. B. Luo, C. J. Fang, J. Xiong, Electrochim. Acta 176 (2015) 1343-1351. 5

[4] J. Leng, Z. X. Wang, X. H. Li, H. J. Guo, H. K. Li, K. M. Shih, G. C. Yan, J. X. Wang, J. Mater. Chem. A 5 (2017) 14996–15001. [5] T. Li, X. H. Li, Z. X. Wang, H. J. Guo, Y. Li, J. X. Wang, J. Mater. Chem. A 5 (2017) 13469–13474. [6] X. Y. Yao, C. Y. Zhao, J. H. Kong, D. Zhou, X. H. Lu, RSC Adv. 4 (2014) 37928–37933. [7] X. H. Xia, S. J. Deng, S. S. Feng, J. B. Wu, J. P. Tu, J. Mater. Chem. A 5 (2017) 21134-21139. [8] J. Y. Wang, N. L. Yang, H. J. Tang, Z. H. Dong, Q. J. M. Yang, D. Kisailus, H. J. Zhao, Z. Y. Tang, D. Wang, Angew. Chem. Int. Ed. 52 (2013) 6417-6420. [9] H. Ren, R. B. Yu, J. Y. Wang, Q. Jin, M. Yang, D. Mao, D. Kisailus, H. J. Zhao, D. Wang, Nano Lett. 14 (2014) 6679−6684. [10] J. F. Chen, Q. Ru, Y. D. Mo, S. J. Hu, x. H. Hou, Phys. Chem. Chem. Phys. 18 (2016) 18949-18957. [11] S. B. Wang, B. Y. Guan, L. Yu, X. W. Lou, Adv. Mater. 29 (2017) 1702724. [12] H. Han, B. Y. Guan, B. Y. Xia, X. W. Lou, J. Am. Chem. Soc. 137 (2015) 5590-5595. [13] J. Yang, X. H. Liu, J. L. Tian, X. Ma, B. F. Wang, W. J. Li, Q. G. Wang, RSC Adv. 7 (2017) 21061–21067. [14] Z. C. Ju, G. Y. Ma, Y. L. Zhao, Z. Xing, Y. H. Qiang, Y. T. Qian, Part. Part. Syst. Charact. 32 (2015) 1012–1019. [15] X. L. Tong, M. Zeng, J. Li, Z. J. Liu, J. Alloy. Compd. 723 (2017) 129-138. Figure Captions Fig. 1 (a) SEM image of ZIF-67; (b) SEM and (c) TEM images of NiCo2O4@Co3O4 double-shelled nanocages; (d) SEM, (e) TEM images and (f) XRD pattern of triple-shelled TiO2@NiCo2O4@Co3O4 nanocages. Fig. 2 Elemental distributions for an individual TiO2@NiCo2O4@Co3O4 nanocage. (a) SEM image, (b) O mapping, (c) Ti mapping, (d) Co mapping, (e) Ni mapping. Fig. 3 Cycling performance of (a) triple-shelled nanocages and (b) double-shelled nanocages; first three charge/discharge curves of (c) triple-shelled nanocages and (d) double-shelled nanocages. 6

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Highlights

► Heterogeneous triple-shelled TiO2@NiCo2O4@Co3O4 nanocages with cage-in-cage structure ► Triple-shelled nanocages show improved cycling stability and discharge capacity ► Cage-in-cage hollow nanostructure and beneficial effect of the TiO2 shell

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