Shuoyu Li, Yuyi Liu, Peisheng Guo and Chengxin Wang

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A Fast and Universal Approach to Encapsulating Transition Bimetal Oxide Nanoparticles in Amorphous Carbon Nanotubes under Atmospheric Environment based on Marangoni Effect Shuoyu Li, Yuyi Liu, Peisheng Guo, and Chengxin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08225 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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ACS Applied Materials & Interfaces

A Fast and Universal Approach to Encapsulating Transition Bimetal Oxide Nanoparticles in Amorphous Carbon Nanotubes under Atmospheric Environment based on Marangoni Effect

Shuoyu Li, Yuyi Liu, Peisheng Guo and Chengxin Wang* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials and Engineering, The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen (Zhongshan)University, Guangzhou 510275, China Abstract Transition metal oxides nanoparticles capsuled in amorphous carbon nanotubes (ACNTs) are attractive anode materials of lithium ion batteries (LIBs). Here we firstly designed a fast and universal method with a hydromechanics conception which is called Marangoni flow to fabricating transition bimetal oxides (TBOs) in ACNTs composite with a better electrochemistry performance. Maragoni flows can produce a liquid column with several centimeters high in a tube with one side immersion in the liquid. The key point to induce a Marangoni flow is to make a gradient of the surface tension between the surface and the inside of the solution. With our research, we control the gradient of the surface tension by control the viscosity of a solution. In order to show our method could be generally used, we synthesis two anode materials such as (a) CoFe2O4@ACNTs, (b) NiFe2O4@ACNTs. All of these have a similar morphology which is ~20µm length and with a diameter of 80-100 nm of the ACNTs and the particles (inside ACNTs) is smaller than 5 nm. Specially there are amorphous carbon between the nanoparticle. All the composite materials showed outstanding electrochemistry performance which have a high capacity and cycling stability for what after 600 cycle the capacity changed less than 3%.

Key Words: Marangoni Flows; surface tension; viscosity; carbon nanotubes; lithium battery; transition metal oxide;

*Corresponding author: Fax: +86-20-8411-3901; e-mail: [email protected]

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Introduction Rechargeable LIBs are becoming a key-enabling power source for electric industry which have shown superiority of high energy density, environmental friendliness, and long lifespan.1 Considering their satisfied safety, reliability, flat discharge potential, and low electrochemical potential with respect to lithium metal, graphitized carbon materials have been one of the most widely used anode materials for LIBs.

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Nevertheless, the reversible capacity of a graphitized

carbon materials anode is ~372 mAhg-1.3 So it is very necessary to develop mew electrodes using inexpensive, safe, and serviceable materials with an excellent charge/discharge rate capacity and high reversible capacity.4 Consequently, transition bimetal oxides (TBOs) attracted wide attention as promising candidates for next generation anode materials. The TBOs are able to insert/extract numbers of Li+ per formula unit with a unique lithium storage mechanism reaction (MOx + xLi+ +xe-→ M + xLi2O) result in a prominent larger capacity.5 But larger capacity means more Li+ take per transition metal or carbon, this usually associate with a large volume expansion which will result in a destruction of the TBOs structure when electrochemical cycling.6 Moreover, in sharp contrast to interlayer/interstitial, an abrupt structural change takes place upon electrode discharging for metal oxide undergoing conversion, consequently, it will weaken the electrical contacts between active material and current collector.7 Some research has focused on optimizing the particle size of the TBOs or cladding graphitized carbon nanotubes(CNTs) outside. But graphitized CNTs will interdict the inserting/extracting process of Li+ in the direction perpendicular to the CNTs wall.8 On the other hand, it is very difficult to get TBOs nanoparticles with small size and homogeneous distribution inside the CNTs.9 Otherwise, the TBOs nanoparticles will aggregate to a bulk after several times of redox reaction. In conclusion, a structure that provide short Li+ ion diffusion and accept large volume expansion is desired, so the transition metal oxide and amorphous carbon nanotubes (ACNTs) composite material was designed.10 Here we firstly find a new fast way to fabricating several kinds of TBOs nanoparticles capsuled in ACNTs based on a theory of hydromechanics which is named Marangoni effect. In this way, the TBOs were capsuled in ACNTs homogeneously, each TBOs nanoparticle is divided by amorphous carbon around so that it is impossible for small TBOs nanoparticles to birdnesting, on the other hand, the amorphous carbon around and the 2


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ACNTs also make a great contribution to stop the structure distortion in inserting/extracting process of Li+.11 In generally, the surface tension mainly depends on the chemical composition at the interface. A flow induced by surface tension gradients in a solution is termed Marangoni convection.12 Consequently, Marangoni flows may come from the gradients of solute concentration at the interface.13 When the minor component in the solvent mixture has a lower surface tension than the major solvent, a Marangoni flow with a direction counter to that of the convective flow can be induced.14 Based on the theory above, we build an experiment to induce a Marangoni flow by adjust the viscosity to control the surface tension. As a result, we get a flow in the nanotube template with the direction antigravity. As shown in scheme 1(a), we used two transition metal chlorides (MClx) as the solute and an ionic liquid 1-allyl-3-butyl imidazolium bromide ([ABlm]Br) as the solvent, then put an anodic aluminum oxide (AAO) template float on the prepared solution. When the Mx+ hydrolyzed, the solution surface in the nanotube absorbed a mass of hydrone in the air, consequently, the solution surface in the nanotubes become attenuation with a higher surface tension than the ropy solution with a lower surface tension which has no connection with air, as a result, a surface tension gradient would induce a Marangoni flow from down to upside in the nanotube which can quickly fill the nanotubes with the compound solution. Scheme1(b) shows the simulated state of the Marangoni flow in the AAO nanotube half a minute later and the optical photograph at the upside of the AAO. Compared with the optical photograph in scheme 1(c), we can clearly see that the solution under the AAO template was carried upon the other side of the AAO template through the nanotubes in the AAO template by the Marangoni flow. Scheme1(d) and 1(e) shows the experiment process and the simulation of the microstructure of the TBOs@ACNTs composite material. The TBOs nanoparticles and the ACNTs would formed at the same time with a very simple muffle furnace. This mixed solution will release bulk of gases when heating in muffle furnace which is very beneficial that both the wall of ACNTs and TBOs particles are porous enough, the conclusion is rigorously proofed by the BET result in Fig. S4. Both those porous structure and small size of particles are very helpful for the Li+ to shuttle back and forth, reduce the diffusion length, and reaction will go much sufficiently. In addition, our research not only provide a new valuable composite material of TBOs and amorphous carbon, but also created a 3



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new way which is simple to execution due to a common hydromechanics theory. Experimental We used the commercial AAO template which is bestrewed channels with the diameter of 80-100nm. The solution was prepared with 1ml [ABlm]Br and 300µl deionized water at first, keep stirring with a glass rob lightly for 5minutes until the water completely dissolved with [ABlm]Br. Then add the weighed MClx (FeCl3+CoCl2, FeCl3+NiCl2) at second and keep stirring for 5 minutes until the powder completely dissolved. Then fall the solution into a quartz crucible which is opening capacious enough to put the AAO template in. When the ropy solution was evenly spread in the quartz crucible, put an AAO template on the liquid surface slightly so that the AAO template could float like a flat base boat on the solution and keep a close integration with the solution. After a half minute, it will be sweat on the up side of the AAO template (shown in scheme1b) which has no contact with the solution by the effect of Marangoni flow. That means the solution have suffused the channels of AAO template. Then put the quartz crucible with solution and AAO template into a muffle furnace which has been heated to 600℃ for 4minutes to lead the solution solidify and formed cellular liked with porous amorphous carbon. Then take the quartz crucible and AAO template to room temperature and wait for cool, afterwards take off the AAO template and clean the redundant amorphous carbon which deposited at the underside of the AAO template. Finally, put the cleared AAO template in 5mol/L sodium hydroxide solution, the template will dissolve and only the TBOs@ACNTs residual. After centrifugal cleaning the sodium hydroxide solution, there are only pure TBOs@ACNTs composite material obtained. Structure characterization The microstructures ware represented by a transmission electron microscopy (TEM, FEI Tecnai G2 F30) at 300kV. The specimens for TEM observation were prepared by dripping the product which has been dispersed in ethanol onto a carbon-coated Cu grid. X-ray diffraction (XRD) was represented by a D-MAX 2200 VPC diffractometer with Cu Kα radiation (λ = 1.54056 Å）over the angular range 10-70° (2θ). For X-ray photoelectron spectroscopy(XPS) analysis, an ESCALab250 with Al Kα radiation (15kV 150W) was used. Thermogravimetric analysis(TGA) measurements were carried out from 25℃ to 800℃ at a heating rate of 10℃ min-1 in air atmosphere using a TG-209 thermal analyzer.

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Electrochemical measurements The coin-type two-electrode half cells was assembled to measure the electrochemical properties of the TBOs@ACNTs as an anode material of lithium ion batteries. The mixed electrode material slurry which was brushed on a copper foil served as the working electrodes were fabricated by mixing 20 wt.% Super P as the conductive agent, 70 wt.% TBOs@ACNTs as the active material with and 10 wt.% polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP) as a binder. The brushed copper foils were dried in vacuum drying oven at the temperature of 90 ℃ and the time of 6 h for solvent evaporation, then cut into circular sheet (14 mm in diameter), subsequently, pressed under a pressure of 2MPa and then dry for another 2 h. At last, the coin cells were assembled in a glove box which is Ar-filled. The final loading mass of active materials on electrode is ~0.5mg. The volume of electrolyte used in every battery was 20µL, the electrolyte was dissolving 1 M LiPF6 in a solvent which is consist of propylene carbonate, dimethyl carbonate and ethylene carbonate at a mixture of 1:1:1(v/v/v). The separator was using Celgard 2400 membrane and the counter/reference electrodes was using metallic lithium foil. The cells were measured in the voltage range of 0.005-3 V versus Li+/Li by a battery test system. Results and Discussion Fig. 1(a) shows the morphology image and powder X-ray diffraction patterns of the CoFe2O4@ACNTs composite material. The XRD pattern of the CoFe2O4@ACNTs indexed to the cobalt iron oxide [JCPDS file no. 22-1086]. Fig. 1(b) shows the morphology image and powder X-ray diffraction patterns of the NiFe2O4@ACNTs composite material and SEM image. The XRD pattern of the NiFe2O4@ACNTs indexed to the trevorite [JCPDS file no. 86-2267]. From the SEM imagesFig.1(a)(b) it can be seen that the difference of metal ion did no influence at the morphology too much. That means the method based on Marangoni flow can be general use in such kinds of composite material preparation. Most of the ACNTs are well formed with a uniform length of ~20µm, And the TBOs particles distributed homogeneously inside of the ACNTs. Fig.2 shows different magnification TEM images and EDS mappings of (a) CoFe2O4@ACNTs, (b)NiFe2O4@ACNTs, which it can be seen that all the ACNTs are filled with 5



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nanoparticles homogeneously. From the high magnification TEM images it shows that the diameter of the particles inside the ACNTs of all four materials is ~5nm. And more importantly, there are amorphous carbon between the TBOs nanoparticles and covered the nanoparticles. This special structure has a huge influence on the electrochemical performance of the TBOs@ACNTs because this structure can effectively prevent the TBOs nanoparticles growth and aggregation.15 From the high resolution TEM image of the TBOs@ACNTs composite material, parallel crystal lattice fringes of the nanoparticles are clear to distinguish. It can be seen that each CoFe2O4, NiFe2O4 nanoparticles is independent. The inter-plane distance of two nanoparticles is calculate from the Fourier transformation which is corresponding to the (400) and (220) lattice plane of the cobalt iron oxide, the (400) and (311) lattice plane of the trevorite, these are well agreed with the XRD results. According to previous reports, attributed to their different Gibbs free energies which is root in excess surface contribution of the forms of iron oxide, the TBOs nanoparticle is more stable in lithium battery performance when the particle size is smaller than 16nm.16 The smaller crystallite size of TBOs nanoparticles indicates that the spatial confinement effect of ACNTs hold back the growth and aggregation of the TBOs nanoparticles during high temperature processing.17 In the element mapping images of Fig.2 it is obvious that carbon not only formed the ACNTs, but also distributed all over the nanotube, the transition metal elements distribution in the nanotube as well.18 It is clear to resolution that the TBOs is cotton like and fluffy with countless small pores. It is clear that the TBOs is completely packaged in the ACNTs instead of mixed in the ACNTs’ wall. This amazing split phase phenomenon in a mixed solution when heating is due to the difference between the curing temperature of the ionic liquid and the form of TBOs.19 XPS analysis is performed to analyze element binding configuration of the TBOs@ACNTs composite material. In Fig. S1 (a1~a5) shows the XPS spectrum of survey spectrum, Co2p, Fe2p, O1s, and C1s, which are well agreement with those of CoFe2O4.17 In Fig. S1 (b1~b4) shows the XPS spectrum of survey spectrum, Ni2p, Fe2p, O1s, and C1s which are well agreement with those of NiFe2O4.20 Fig.3 shows the image of scanning transmission electron microscope (STEM) image of the (a) CoFe2O4@ACNTs, (b) NiFe2O4@ACNTs, in which shows clearly that the nanoparticles are distribution in ACNTs discretely and homogeneously, the space between the nanoparticles is clear 6


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to see, just like a hollow out. Fig.S2 shows thermogravimetric (TG) analyses of (a) CoFe2O4@ACNTs, (b) NiFe2O4@ACNTs in air, from which shows a weight loss of (a) 59.7 wt.% (b)60.3 wt.% for the TBOs@ACNTs composite material and an oxidation temperature of ~420 ℃ for the ACNTs and the amorphous carbon filled between the interspace of TBOs nanoparticles. Otherwise, there are ~9wt% water in the air adsorbed in the sample. Therefore, the TBOs nanoparticles content of the thirsty sample is ~30wt%. Since the TG data and STEM, it is proved that there is amorphous carbon between and covered the TBOs nanoparticles,21 this is very helpful to enhance the conductivity and stability of cycling by stop the particles birdnesting.22 The images also identified again that the method with Marangoni flow is generally and subtle.23 The discharge-charge curves of the CoFe2O4@CNTs at 100mAg-1 are shown in Fig. 4(a1). the region of 3.0-0.75V is corresponding to the course of lithium insert in CoFe2O4 nanoparticles and the reductive process of Fe3+ to Fe0 and Co2+ to Co0.24 The reversible reaction process of CoFe2O4 with metallic lithium is: CoFe2O4 + 8Li → Co + 2Fe + 4Li2O Co + 2Fe + 4Li2O ↔ CoO + Fe2O3 + 8Li Fig. 4(a2) shows the CV curves of CoFe2O4@CNTs, there are two cathodic peaks appeared at 0.54 V and 1.44 V (vs. Li/Li+) in the first cycle, in the subsequent cycles the cathodic peaks moved to 0.81V and 1.52 V (vs. Li/Li+). The poignant cathodic peak at 0.54 V (R1) is corresponding to the reduction of Co2+ and Fe3+ to their metallic states.25 After the second cycle, two cathodic peaks located at 0.77(R2) and 1.52 V(R3) are relative to the reversible reductive reaction of Fe2O3 and CoO.26 The anodic peak at 1.31 V(O1) and 1.71 (O2) at the first cycle might due to the oxidation of the metallic iron and cobalt to Co2+ and Fe3+ respectively, which shifted a little in the subsequent cycles.27 In

similarity

with

the

mechanism

of

CoFe2O4,

the

whole

electrochemical

lithiation-delithiation process of NiFe2O4 is: NiFe2O4 + 8Li → Ni + 2Fe + 4Li2O Ni+ 2Fe + 4Li2O ↔ NiO + Fe2O3 + 8Li The discharge-charge curves of the CoFe2O4@CNTs at 100mAg-1 are shown in Fig. 4(b1). The CVs of the NiFe2O4@ACNTs for the first three cycles at a rate of 0.2mVs-1 are shown in Fig. 4(b2), the peak at 1.25V(O1) and 1.75(O2) are corresponding to the oxidative reaction and peaks 7



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at 0.62V(R1), 0.81(R2) and 1.51V(R3) corresponding to the reduction reaction of Ni2+ and Fe3+ respectively.28 The galvanostatic cycling performance of CoFe2O4@ACNTs at a current density of 500mAg-1 is shown in Fig.5(a), which shows ~550 mAhg-1 at first ten cycles and keep ~520 mAhg-1 after 600 cycles. The galvanostatic cycling performance of NiFe2O4@ACNTs at a current density of 500mAg-1 is shown in Fig.5(b), which shows ~670 mAhg-1 at first ten cycles and keep ~700 mAhg-1 after 600 cycles. The slight capacity decrease illustrated that the structure of the amorphous carbon around the TBOs nanoparticles and the ACNTs improves the electrochemical performance.29 Otherwise, the obvious large capacity loss between the first and second cycle for the electrode due to the formation of a SEI layer on the surface of the electrode in the first discharge step.30 Consisting of ~30wt% of TBOs ,all of the TBOs@ACNTs have shown an outstanding electrochemistry performance.31 Above all, we can conclude that both the two kinds of TBOs@ACNTs have shown an improved electrochemical performance, the CoFe2O4@ACNTs presented a lower capacity but a better cycle stability than the [email protected] The rate capacity of the TBOs@ACNTs is further measured. The capacity retention and consecutive cycling performance of the CoFe2O4@ACNTs under different current densities are shown in Fig. 5(c). The CoFe2O4@ACNTs show a high reversible charge capacity of 1161mAhg-1 at 60 mAg-1. Meanwhile, a stable capacity which is more than 266 mAhg-1 remained when the current rate rises to 1200 mAg-1, nearly 22.9% of its capacity at 60 mAg-1. Noteworthy, a reversible capacity of 1117 mAhg-1 is observed again when the current rate come back to 60 mAg-1, which indicated a nice reversibility and rate capacity of the [email protected] The rate capacity of the NiFe2O4@ACNTs is shown in Fig. 5(d), it can be seen that the NiFe2O4@ACNTs shows a high reversible charge capacity of 960mAhg-1 at 60 mAg-1. Meanwhile, a stable capacity which is more than 364 mAhg-1 remained after the current rate change to 3000 mAg-1, nearly 37.9% of its capacity at 60 mAg-1. Obviously, a reversible capacity of 951mAhg-1 is observed again when the current rate come back to 60 mAg-1, which indicated a nice reversibility and rate capacity of the [email protected],35 Conclusion

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An electrode of TBOs@ACNTs is prepared by a new method with the Marangoni effect, the TBOs nanoparticles which with a diameter of ~5nm was encapsulated uniformly and homogeneously in the hollow cores of flexible amorphous ACNTs whose diameter is 80-100nm. Excellent electrochemical performance of this special structure material means a great success in the theory of the material preparation, this new method with Marangoni effect can expend to the preparation of many other composite materials as well, and the most importantly, this new method do not ask for complex progress or priceless instruments, which means very easy to operate and low cost. It could be predicated that this new method will play a protagonist role in composite material with nanometer-size for application in energy storage, catalysis, sensing, etc.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (U1401241). Supporting Information The supporting information contains the XPS analyze of the two bimetal oxides and ACNTs combine materials, the TG analysis, the morphology of CoFe2O4@ACNTs (SEM image) after 600 cycles, the BET analysis and a table for the comparative of the cycling performance between our bimetal oxides@ACNTs and other bimetal oxides.

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16. Zeng, Z.; Zhao, H.; Lv, P.; Zhang, Z.; Wang, J.; Xia, Q. Electrochemical Properties of Iron Oxides/Carbon Nanotubes as Anode Material for Lithium Ion Batteries J. Power Sources 2015,274, 1091-1099. 17. Li, Z. H.; Zhao, T. P.; Zhan, X. Y.; Gao, D. S.; Xiao, Q. Z.; Lei, G. T. High Capacity Three-Dimensional Ordered Macroporous CoFe2O4 as Anode Material for Lithium Ion Batteries Electrochim. Acta 2010,55 (15), 4594-4598. 18. Lim, J. A.; Lee, W. H.; Lee, H. S.; Lee, J. H.; Park, Y. D.; Cho, K. Self-Organization of Ink-jet-Printed Triisopropylsilylethynyl Pentacene via Evaporation-Induced Flows in a Drying Droplet Adv. Funct. Mater. 2008,18 (2), 229-234. 19. Zou, M.; Li, J.; Wen, W.; Chen, L.; Guan, L.; Lai, H.; Huang, Z. Silver-Incorporated Composites of Fe2O3 Carbon Nanofibers as Anodes for High-Performance Lithium Batteries J. Power Sources 2014,270, 468-474. 20. Li, C.; Gu, L.; Tong, J.; Tsukimoto, S.; Maier, J. A Mesoporous Iron-Based Fluoride Cathode of Tunnel Structure for Rechargeable Lithium Batteries Adv. Funct. Mater. 2011,21 (8), 1391-1397. 21. Yan, W.; Cao, X.; Tian, J.; Jin, C.; Ke, K.; Yang, R. Nitrogen/Sulfur Dual-Doped 3D Reduced Graphene Oxide Networks-Supported CoFe2O4 with Enhanced Electrocatalytic Activities for Oxygen Reduction and Evolution Reactions Carbon 2016,99, 195-202. 22. Lavela, P.; Tirado, J. L. CoFe2O4 and NiFe2O4 Synthesized by Sol–Gel Procedures for Their Use as Anode Materials for Li Ion Batteries J. Power Sources 2007,172 (1), 379-387. 23. Tabuchi, M.; Kitta, M.; Kageyama, H.; Shibuya, H.; Imaizumi, J. Mn Source Effects on Electrochemical Properties of Fe -and Ni-Substituted Li2MnO3 Positive Electrode Material J. Power Sources 2015,279, 510-516. 24. Hayashi, A.; Muramatsu, H.; Ohtomo, T.; Hama, S.; Tatsumisago, M. Improvement of Chemical Stability of Li3PS4 Glass Electrolytes by Adding MxOy (M = Fe, Zn, and Bi) Nanoparticles J. Mater. Chem. A 1, 6320-6326 (2013). 25. Du, N., Zhang, H., Chen, B, D., Wu, J, B., Ma, X, Y., Liu, Z, H., Zhang, Y, Q., Yang, D, R., Huang, X, H., Tu, J, P. Porous Co3O4 Nanotubes Derived from Co-4(CO) (12) Clusters on Carbon Nanotube Templates: A Highly Efficient Material for Li-Battery Applications Adv. Mater. 19, 4505 (2007). 26. Jin, S.; Yang, G.; Song, H.; Cui, H.; Wang, C. Ultrathin Hexagonal 2D Co2GeO4 Nanosheets: Excellent Li-Storage Performance and ex Situ Investigation of Electrochemical Mechanism ACS Appl. Mater. Interfaces 2015,7 (44), 24932-43. 27. Liu, Y.; Zhang, N.; Yu, C.; Jiao, L.; Chen, J. MnFe2O4@C Nanofibers as High-Performance Anode for Sodium-Ion Batteries. Nano Lett. 2016,16 (5), 3321-8. 28. Lu, Q.; Fang, J.; Yang, J.; Feng, X.; Wang, J.; Nu, L, Y. A Polyimide Ion-conductive Protection Layer to Suppress Side Reactions on Li4Ti5O12 Electrodes at Elevated Temperature. RSC Adv. 2014,4 (20), 10280. 29. Xin, S.; Guo, Y, G.; Wan, L, J. Nanocarbon Networks for Advanced Rechargeable Lithium Batteries Acc. Chem. Res., 2012, 45, 1759. 30. Nai, J, W.; Tian, Y.; Guan, X.; Guo, L. Pearson’s Principle Inspired Generalized Strategy for the Fabrication of Metal Hydroxide and Oxide Nanocages J. Am. Chem. Soc. 2013, 135, 16082.

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31. Nai, J, W.; Yin, H, J.; You, T, T.; Zheng, L, R.; Zhang, J.; Wang, P, X.; Jin, Z.; Tian, Y.; Liu, J, Z.; Tang, Z, Y.; Guo, L. Efficient Electrocatalytic Water Oxidation by Using Amorphous Ni–Co Double Hydroxides Nanocages Adv. Energy Mater. 2015, 5, 1401880. 32. Xu, Q.; Li, J, Y.; Sun, J, K.; Yin, Y, X.; Wan, L, J.; Guo, Y, G. Watermelon-Inspired Si/C Microspheres with Hierarchical Buffer Structures for Densely Compacted Lithium-Ion Battery Anodes Adv. Energy Mater. 2016, 1601481. 33. Zhao, H, W.; Yue, Y, H.; Zhang, Y, W.; Li, L, D.; Guo, L. Ternary Artificial Nacre Reinforced by Ultrathin Amorphous Alumina with Exceptional Mechanical Properties Adv. Mater. 2016, 28, 2037. 34. Zhou, X, S.; Wan, L, J.; Guo, Y, G. Binding SnO2 Nanocrystals in Nitrogen-Doped Graphene Sheets as Anode Materials for Lithium-Ion Batteries Adv. Mater. 2013, 25, 2152. 35. Zhou, X, S.; Dai, Z, H.; Liu, S, H.; Bao, J, C.; Guo, Y, G. Ultra-Uniform SnOx/Carbon Nanohybrids toward Advanced Lithium-Ion Battery Anodes Adv. Mater. 2014, 26, 3943.

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(a) (a)

Half a minute later

(b)

The optical photograph of AAO

several minutes later

(c)

3 minutes

3 minutes with nothing upside

30 minutes

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(d) 600℃ ℃ for 4min

(e)

After centrifuge cleaning

Scheme 1 the schematic of the producing process of the Marangoni flows in nanotubes. (a) the simulation of the Marangoni flow when the AAO just put on the ropy solution surface. (b) the simulated state of the Marangoni flow in the AAO nanotube half a minute later and the optical photograph at the upside of the AAO. (c) the final simulated state of the AAO template and the optical photograph after different time of the AAO and the white napkin paper on the AAO template. (d) the heat treatment process of TBOs@ACNTs. (e) the structure representation of TBOs@ACNTs.

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Fig.1 The morphology and XRD pattern of the (a) CoFe2O4@CNTs; (b) NiFe2O4@CNTs.

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(a)

(b)

Fig.2 TEM image， Fourier transformation and EDS mapping of the (a) CoFe2O4@ACNTs; (b) NiFe2O4@ACNTs. The resolution image is after Gatan image filter Gantan.

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Fig.3 The image of scanning transmission electron microscope (STEM) image of the (a) CoFe2O4@CNTs; (b)NiFe2O4@CNTs.

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Fig.4 Galvanostatic charge–discharge voltage profiles of the CoFe2O4@ACNTs(a1) and NiFe2O4@ACNTs(b1) in the voltage range of 0.05–3.0V (vs. Li+/Li) at a current of 100 mA g-1; Cyclic voltammograms of the CoFe2O4@ACNTs(a2) and CoFe2O4@ACNTs(b2) for the initial five cycles at a scan rate of 0.2mV s-1.

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(b)

Fig.5 (a) cycling performance of the CoFe2O4@ CNTs at a current density of 0.5A g-1; (b) cycling performance of the NiFe2O4@ CNTs at a current density of 0.5A g-1; cycling performance under different current densities of (c) CoFe2O4@CNTs, (d) NiFe2O4@CNTs.

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Shuoyu Li, Yuyi Liu, Peisheng Guo and Chengxin Wang

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