Multilayer CuO@NiO Hollow Spheres: Microwave-Assisted Metal

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Multi-Layer CuO@NiO Hollow Spheres: MicrowaveAssisted Metal-Organic-Framework Derivation and Highly Reversible Structure-Matched Stepwise Lithium Storage Wenxiang Guo, Weiwei Sun, and Yong Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b05610 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 7, 2015

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Graphical abstract Multi-layer CuO@NiO hollow sphere is synthesized by a microwave-assisted metal organic framework (MOF) approach and shows excellent electrochemical performances as an anode for lithium ion batteries.

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Multi-Layer CuO@NiO Hollow Spheres: Microwave-Assisted Metal-Organic-Framework Derivation and Highly Reversible Structure-Matched Stepwise Lithium Storage Wenxiang Guo, Weiwei Sun and Yong Wang* Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shangda Road 99, Shanghai, P. R. China, 200444 Email: [email protected] Abstract: A unique CuO@NiO microsphere with three-layer ball-in-ball hollow morphology is successfully synthesized by Cu-Ni bimetallic organic frameworks. The beforehand facile microwave-assisted production of Ni organic framework sphere is used as the template to induce the morphology control of bimetallic oxides. Designed by the controlled surface cationic exchange reactions between Cu and Ni ions, there is an elemental gradient (decreased amount of CuO but increased amount of NiO) from the shell to the core of the microsphere product. This ternary metal oxide hollow structure is found to be very suitable to solve the critical volume expansion problem, which is critical for all high-capacity metal oxide electrodes for lithium ion batteries. A reversible larger-than-theoretical capacity of 1061 mAh·g-1 can be retained after repetitive 200 cycles without capacity fading compared to the initial cycle. These excellent electrochemical properties are ascribed to the step-by-step lithium insertion reactions induced by the matched CuO@NiO composition from the shell to the core and facilitated lithium/electron diffusion and accommodated volume change in porous bimetallic oxides microsphere with multiple-layer yolk-shell nanostructure. Keywords: ball-in-ball, hollow sphere, CuO, NiO, metal organic framework, 2


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lithium-ion battery

Hollow particles with versatile composition and size have attracted significant attention in various applications such as energy storage, catalysis, drug delivery, etc.1,2 Different synthesis approaches such as templating method, Kirkendall effect, galvanic replacement, self-assembly, thermal decomposition have been proposed to construct a desirable shell structure with interior void space.3-5 Among these methods, templating method is believed to be a universal and straightforward approach for the control of morphology and size with high uniformity. Metal organic frameworks (MOFs) are microporous materials composed of metal ions and organic ligands. Recently, MOFs have been proposed as an effective template to fabricate porous materials for applications in gas separation, catalysis, gas sensor, and energy storage.6-9 The removal of solvent and gasification of organic ligands during thermal treatment in air can produce numerous nanoporous spaces in the final metal-based products.

Transitional metal oxides (TMOs) are a class of important materials for energy-related applications such as lithium ion batteries, supercapacitors, and fuel cells.10-13 With respects to their Li-storage properties, they can exhibit ~2-3 times higher theoretical capacity than commercial graphite anode, therefore they have been suggested as promising advanced anodes to meet the requirements of next-generation lithium ion batteries with high energy density and power.14-16 In particular, CuO and NiO can deliver large theoretical capacities of 674 and 718 mAh·g-1 respectively when used as 3


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anodes for LIB.17-25 However all transitional metal oxides still suffer dissatisfactory cycling performances which is mainly due to large volume expansion/retraction associated with the lithium insertion and extraction process.26,27 An effective strategy is to design porous TMO’s electrode nanostructure to solve this problem.28-30 The carefully-crafted pores can render the electrode fast ion and electron diffusion and even large Li-ion storage. More importantly, the large volume expansion associated with lithium insertion can be accommodated in these inherent void spaces of the electrode.31-34

In order to realize structure merits and synergetic effect, multiple layers of shells and multiple compositions of hollow particles are highly desirable.35-41 This may offer vast opportunities and hold great potentials for improved physical/chemical properties. Herein, this work reports a new MOF-derived multiple-layer hollow structure of ternary metal oxide (CuO@NiO) and its lithium ion battery application. Ni-organic framework sphere42,43 was prepared with the assistance of microwave irradiation and then converted to Cu/Ni-organic frameworks by controlled cationic exchange43-47 between Cu2+ and Ni2+. The subsequent thermolysis of organic ligands in air would lead to interesting multiple-layer CuO@NiO hollow spheres. In this structure, a yolk-shell sphere is encapsulated further inside a hollow sphere. It is worth mentioning that the Cu content is decreased, but the Ni content is increased from the exterior shell to the interior core of the product due to the gradient of ion exchange rate from the shell to the core. This structure characteristic coincides with the 4


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sequence of the lithium insertion (first in CuO at ~1.2 - 1.1 V and then in NiO at ~0.7 - 0.6 V). 48-50 The stepwise lithium insertion and MOF-derived porous multi-shell structure lead to very good lithium storage properties. An extremely large reversible capacity of 1061 mAh·g-1 can be retained after 200 cycles of lithium insertion and extraction.

Results and Discussion The structures of Cu-Ni-BTC MOF precursors and their corresponding CuO@NiO composites were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. The XRD patterns of Cu-Ni-BTC and CuO@NiO are compared with Ni-BTC and NiO as shown in Figure 1 and Figure S1. All diffraction peaks of Cu-Ni-BTC can be assigned to Cu3(BTC)2(H2O)3.51,52 It is worth noting that the Ni-BTC composite is largely amorphous with no obvious peaks detected in the XRD patterns, which is in accordance with previous Ni-BTC MOFs.42,43 For the final product by calcining the Cu-Ni-BTC precursor, two types of characteristic peaks (CuO and NiO) are observed. These characteristic peaks can be readily assigned to the standard NiO phase (PDF 47-1049) and CuO phase (PDF 44-0937). No other redundant peaks are detected and the CuO@NiO product is determined to be a mixture of NiO and CuO. In order to further determine the composition of CuO@NiO, XPS results are shown in Figure 2. It is observed from Ni 2p spectrum that the characteristic 2p1/2 peak and its satellite peak are located at 871.3 and 879.3 eV respectively, while the 2p3/2 peak and its satellite peak appear at 853.5 5


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and 860.7 eV respectively, which further confirms the presence of NiO.53,54 The Cu 2p spectrum in Figure 2c reveals a main 2p3/2 peak at 934.2 eV and two satellites peaks (941.5 and 943.9 eV) with higher binding energy values, which demonstrates the existence of Cu2+ from CuO phase.49 Peaks of O 1s appeared at 531.2 and 529.3 eV can be assigned to the O2- from CuO and the mixture of NiO and CuO respectively.49,53 These features also offer evidences to reveal the composition of NiO and CuO in CuO@NiO product, which is in agreement with XRD results. The C peak shown in the full spectrum (Figure 2a) belongs to extraneous hydrocarbon from XPS instrument.55 The molar ratio of Cu to Ni is estimated to be ~0.59 : 1 by XPS on the microsphere surface. Elemental analysis results (N: 0.00%, C: 0.00%, H: 0.085%) reveal the absence of C and N in CuO@NiO product, indicating the BTC ligands have been completely decomposed in the calcination process.

The thermogravimetric analysis (TGA) of Cu-Ni-BTC to CuO@NiO was investigated and is shown in Figure S2. The weight loss from room temperature to 300 oC is ~28.1%, which can be assigned to the removal of the absorbed water and DMF solvent. A significant weight loss (~44.6%) in 300-400 oC should be attributed to the decomposition of MOF skeleton and the gasification of BTC ligands. It is worth mentioning that Cu and Ni ions are converted to CuO and NiO during heating process in air. When the temperature continues to increase, there is no detected weight loss. It is indicated that the Cu-Ni-BTC MOF has been completely transferred to CuO@NiO composite, which is stable at a high temperature. Therefore a temperature of 500 oC 6


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was selected to obtain CuO@NiO composite in this work. Figure S3 shows the nitrogen adsorption/desorption isotherm curves of CuO@NiO. The CuO@NiO microsphere shows a BET surface area of 16.3 m2·g-1 with mesopore size distribution centered at ~3.5 nm and ~20.3 nm.

The morphologies of Cu-Ni-BTC MOF precursor and CuO@NiO product were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in SEM images of Figure 3a, c, Cu-Ni-BTC exhibits microsphere morphology (~1-1.2 µm in size). After annealing the Cu-Ni-BTC precursor, the obtained CuO@NiO product is shown in SEM images of Figure 3b, d. The microsphere morphology is largely maintained with slightly reduced microsphere size (~0.9-1 µm in size), which should be ascribed to the removal of organic ligands and the resultant shrinkage effect in the annealing process in air. The high-magnification SEM image (Figure 3d) reveals that there is possibly a small ball encapsulated inside each of these microspheres under electron imaging. Elemental mapping images of the CuO@NiO microsphere are shown in Figure 3e-h. Several elements such as Ni, Cu and O are distributed in the ternary metal oxide. The EDS results indicate that the molar ratio of Cu to Ni is ~0.42 : 1, which is substantially smaller than the XPS surface result ( Cu: Ni = ~0.59 : 1). It is possibly because the EDS can measure the material up to several hundred nanometers in depth, which is much deeper than that of XPS (around several nanometers).

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TEM image of Figure 4a shows the Ni-BTC microsphere precursor with smooth surface and its derived hollow NiO microsphere is shown in Figure 4b. The obtained NiO microsphere exhibits an interesting ball-in-ball yolk-shell structure.42 In comparison, Figure 4c and Figure 4d-f show the Cu-Ni-BTC precursor and its derived main product of CuO@NiO in this work. Compared to Ni-BTC with smooth surface in Figure 4a, the surface of Cu-Ni-BTC becomes quite rough as shown in Figure 4c and Figure S4a-b. This is because some Cu ions are introduced into Ni-BTC microspheres based on a surface cation exchange mechanism with the assistance of microwave irradiation, therefore the surface composition and morphology of Cu-Ni-BTC are not as homogenous as that of Ni-BTC. Compared to the yolk-shell NiO in Figure 4b, unique multi-layer CuO@NiO hollow spheres (Fig. 4d, e) are obtained from the Cu-Ni-BTC precursor. This microsphere consists of three layers, which can be labeled as 1st ball, 2nd ball and 3rd ball in two images. The overall microsphere size is estimated to be ~900 nm, which is slightly reduced compared to the NiO microsphere. HRTEM image of CuO@NiO microsphere is shown in Figure 4f. Interplanar distances of 0.21 and 0.25 nm can be indexed to the (200) plane of NiO (PDF 47-1049) and the (002) plane of CuO (PDF 44-0937). The crystal lattices of two types of metal oxides are overlapped to some extents in the joint section, indicating a mechanically robust nanoparticle-integrated structure. The SAED pattern of CuO@NiO is shown in the inset of Figure 4f, which can further confirm the existence of two types of NiO and CuO crystals. Based on the energy dispersive spectra (EDS) results in Figure 4g, the content of Ni is increased from the exterior 1st ball to the core 8


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3rd ball, which can be explained by the surface exchange reactions between Cu and Ni ions. More Ni ions on the exterior surface are substituted by Cu ions compared to the interior Ni ions in the Cu-Ni-BTC MOF.

The effect of experimental conditions on the microwave-assisted synthesis of Cu-Ni-BTC

MOFs

and

derived

CuO@NiO

products

were

explored.

If

Ni(NO3)2·6H2O, Cu(NO3)2·3H2O and H3BTC are dissolved in the solution and react at the same time under one-pot microwave irradiation at 150 oC, only nanoparticle products are obtained as shown in Figure S5a-b. It is indicated that the introduction of Cu2+ would affect the growth process of Ni-BTC microsphere and Cu-BTC MOFs trend to form nanoparticle morphology, which has been reported in a previous report.56 The influence of microwave-irradiation temperature for MOF precursors on the product morphology was also investigated at a lower temperature (120 oC) and a higher temperature (180 oC) for the second-step microwave irradiation process. The obtained products were named as CuO@NiO120 and CuO@NiO180 respectively and are shown in SEM images of Figure S6a-b. After calcination, the obtained CuO@NiO120 exhibits a mixture of aggregated spheres and nanoparticles (Figure S6a), which indicates that CuO nanoparticle and NiO microsphere are obtained separately. MOF-derived CuO nanoparticle has been reported at this temperature in a previous study.57 When the temperature is increased to 180 oC, Cu2+ ions have a fast reaction rate with Ni3(BTC)2 and the obtained product becomes unstable. Therefore the CuO@NiO180 product consists of broken microspheres and a number of particles 9


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(Figure S6b). It is indicated that the temperature is important for the formation of uniform CuO@NiO spheres by controlled substitution of nickel ions by copper ions in Ni-Cu-BTC MOFs. Furthermore, the XRD patterns of CuO@NiO120, CuO@NiO150 and CuO@NiO180 are compared in Figure S6c. Several characteristic peaks of CuO@NiO120 and CuO@NiO180 are located at same positions as those of CuO@NiO150.

Based on the above results, the preparation process of CuO@NiO multi-layer hollow microspheres is illuminated in Scheme 1. The preparation process of NiO yolk-shell structure is used for comparison. The yolk-shell structure of NiO is formed by heterogeneous decomposition of Ni-BTC MOFs. There is a large temperature gradient from the shell to the core of MOFs in the decomposition process.42 The MOF surface is decomposed first and forms an exterior shell. With prolonged thermal treatment, the core of MOF is also decomposed. Two opposite forces may be present at this stage.58,59 They are the adhesion force (Fad) between surface layer and inner core with outward direction and the contraction force (Fco) by decomposition of inner core. The shell and core layers are formed and separated under these two forces. For the Cu-Ni-BTC composite, the Cu2+ ions have replaced some Ni2+ ions from the exterior surface to the interior core. In a similar process, the CuO@NiO shell is formed first. However induced by the different decomposition temperature between Cu-BTC and Ni-BTC, the interior Cu-Ni-BTC ball would undergo similar stepwise decomposition process during the thermal treatment, forming the secondary shell layer and the 10


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subsequent core layer. As a result, three-layer hollow CuO@NiO microspheres are formed as the final product when all Cu-Ni-BTC precursors are converted completely to CuO and NiO. The generated mesopores among metal oxide nanoparticles and in the interior space of the microsphere may hold great potential for applications in electrochemical energy storage.

The multi-layer CuO@NiO hollow spheres were fabricated as anode materials for lithium ion batteries and their electrochemical performances are shown in Figure 5. There are two steady discharge plateaus in the initial discharge (lithium insertion) curve. The first platform at ~1.2 - 1.1 V and the second platform at ~0.7 - 0.6 V can be mainly assigned to the lithium insertion into CuO and NiO respectively.48-50 The electrolyte decomposition and the formation of solid electrolyte interface (SEI) film may also occur accompanying the reduction reaction of metal oxide.48 This phenomenon has been widely accepted to be the main reason for the irreversible capacity loss in the first cycle. These two stepwise lithium insertion reactions can be described as follows:

CuO + 2Li+ + 2e ↔ Cu + Li2O

(Equation 1)

NiO + 2Li+ + 2e ↔ Ni + Li2O

(Equation 2)

In the charge (lithium extraction) curve, the oxidation reactions of two metals are observed in a long voltage slope. There are two reduction peaks centered at 1.08 and 11


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0.62 V in the cathodic scan of CV curves, which agree well with two discharge plateaus in the first-cycle discharge curve. However, because the reversible oxidation reactions of Ni and Cu take place in a wide overlapped voltage range,48,49 two corresponding broad peaks (centered at ~2.21 and 2.36 V) are observed in the anodic scan for the CuO@NiO anode.

Cycling performances of multilayer CuO@NiO anode are shown in Figure 5c. The first-cycle discharge and charge capacities are 1218 mAh g-1 and 856 mAh g-1. There is a gradual increase of reversible charge capacity in the initial 45 cycles, indicating an activation process for this multilayer hollow sphere. This phenomenon has also been observed for previous transitional metal oxides.60-63 After 45 cycles, the CuO@NiO anode exhibits very stable cycling performances in 200 cycles. The reversible capacity can be retained at 1061 mAh·g-1 after 200 cycles of discharge and charge. Coulombic efficiencies are also approaching ~100% except for the first few cycles. This reversible capacity is larger than the theoretical capacities of CuO (674 mAh·g-1) or NiO (718 mAh·g-1). It should be mainly ascribed to the unique multi-layer hollow structure of CuO@NiO microspheres, which provide a large electrochemically active surface area, facilitated lithium diffusion and more active sites for lithium ion storage.

In comparison, NiO yolk-shell composite exhibits an

initial reversible charge capacity of 1063 mAh·g-1, which decreases to 669 mAh·g-1 after 100 cycles. The introduction of Cu element has been demonstrated to be very

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helpful for the property improvement. The Nyquist plot is shown in Figure 5d. The charge-transfer resistance (Rct) of the CuO@NiO was calculated to be 271 Ω for the first cycle. After the lithium insertion and extraction at a constant current of 100 mA·g-1, Rct was decreased to 252 Ω after three cycles. A very small Rct value of 28 Ω was determined after 200 cycles. It is indicated that an activation process during cycling is present and the charge-transfer is facilitated after the activation process during repetitive cycling.

The electrochemical performances of the multi-layer CuO@NiO hollow microsphere anode are also compared with previous Cu-Ni-O ternary oxides, NiO, and CuO structures (hollow sphere, nanosheet, nanorod, nanocube, nanotube, nanoflower, etc) in Table S1, Supporting Information. In general, the CuO@NiO hollow microsphere exhibits very outstanding Li-ion storage properties among these relative nanostructures. For example, comparatively small reversible charge capacities of 563 and 566 mAh·g-1 after 50 cycles are observed for CuO microsphere / NiO nanosheet composite64 and CNT supported Cu-Ni-O composite65 respectively. The hollow ternary metal oxide structure is demonstrated to be a very suitable structure design for lithium ion batteries. Excellent electrochemical properties in terms of large capacity and excellent cycling stability should be largely ascribed to the special porous multi ball-in-ball structure and the synergistic effect of dual-metal oxides. MOF-derived synthesis approaches lead to the dual-modal pores in the CuO@NiO product:

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inter-particle mesopores and large macroporous void space (~100 - 200 nm in size) inside microsphere. These pore surfaces are usually hydrophobic due to the organic ligand precursor, which can favor the electrolyte diffusion. Mesopores can be used to hold electrolyte, provide large electrochemically active surface area, and generate more active sites for lithium ion storage. Macroporous space can facilitate the electrolyte infusion and the ion/electron transport and provide fast pathway to the accessibility of mesopores. The two layers of porous thin shell composed of numerous integrated CuO/NiO nanoparticles also provide reduced pathway for the diffusion of electrolyte, ion and electron. These mesopores and macropores in the hierarchical CuO@NiO microspheres both offer void space for accommodating the large volume change and maintaining the structure stability during repetitive cycling with lithium ions. Compared with pristine NiO or CuO nanostructures listed in Table S1, the ternary metal oxides composition in CuO@NiO can achieve a synergistic effect. Because lithium insertion is found to occur mainly at a higher voltage of ~1.2 - 1.1 V for CuO and a lower voltage of ~0.7 - 0.6 V for NiO, the inactive component can serve as “volume-buffering reservoir” to relieve the mechanic stress associated with the lithium insertion into another active component at a certain voltage. It is noted that the volume expansion process associated with lithium insertion is the most critical stage for the structure stability of the electrode. These structure merits with multi-layer porous ternary metal oxide spheres should give rise to long-term structure stabilization. Figure 5e shows the TEM image of the ternary metal oxides hollow spheres. Although the hollow structure is hard to be determined due to the presence of 14


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a large amount of conducting agent (carbon black), PVDF binders and the electrolyte, the nanoparticle-assembled microsphere morphology is largely retained during repetitive cycling.

Conclusions In summary, multi-layer hollow CuO@NiO spheres were synthesized successfully by a metal-organic-framework (MOF) derived approach. Two-step microwave irradiation processes were used to obtain the Cu-Ni bimetallic MOFs, which were then converted to multi-shell porous CuO@NiO microsphere by a heterogeneous post-annealing process in air. The CuO@NiO product consists of three layers of ball-in-ball structure and interconnected CuO/NiO nanoparticles were integrated to form this multi-shell ball-in-ball structure. There is also an elemental gradient (more Cu on the exterior surface, but more Ni in the interior core) in the product, matching the lithium insertion sequence into two metal oxides (CuO and NiO). Induced by these structure features, the multi-shell CuO@NiO porous microsphere exhibits a large higher-than-theoretical capacity of 1061 mAh·g-1 after 200 cycles of discharge and charge, which is very outstanding among various NiO or CuO electrode structures.

Methods Synthesis of Cu-Ni-BTC MOF In a typical synthesis, 0.2726 g nickel nitrate hexahydrate [Ni(NO3)2·6H2O] or 0.1313 g 1,3,5-benzenetricarboxylic acid (H3BTC) was separately dissolved in 10 ml 15


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N,N-dimethylformamide (DMF) with magnetic stirring at room temperature. The Ni(NO3)2 solution was added slowly into the H3BTC solution and stirred further for 30 min. The mixture was moved into a specialized glass tube and reacted to Ni-BTC MOFs at 150 oC for 30 min under microwave-irradiation with continuous magnetic stirring in a single mode microwave reactor (Nova, EU Microwave Chemistry).42 0.1133 g Cu(NO3)2·3H2O (the molar ratio of Ni2+ to Cu2+ is 2:1) was added into the above Ni-BTC microsphere dispersion in DMF and stirred for 30 min. The mixed solution was irradiated again in the microwave reactor at 150 oC for 30 min. After centrifugal washing with ethanol and dying at 60 oC in electrical oven, yellow green powder (Cu-Ni-BTC) was collected.

Synthesis of multi-layer hollow CuO@NiO sphere The obtained Cu-Ni-BTC power was ground in an agate mortar and then spread in a porcelain boat. The porcelain boat was put into an electrical furnace and annealed in air at 500 oC for 2 h with a heating rate of 5 oC min-1. After cooling to room temperature, the dark red powder (multi-layer hollow CuO@NiO) was collected.

The irradiation temperatures in the microwave-assisted synthesis of Cu-Ni-BTC were adjusted to be 120 and 180 oC with similar other experimental conditions. The obtained CuO@NiO products were labeled as CuO@NiO120 and CuO@NiO180 respectively.

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Material Characterization The CuO@NiO materials were characterized by X-ray diffraction (XRD, Rigaku D/max-2550V, Cu Ka radiation), field-emission scanning electron microscopy (FE-SEM, JSM-6700F) with an energy dispersive X-ray spectrometer (EDS) and transmission electron microscopy/selected area electron diffraction (TEM/SAED, JEOL JEM-200CX and JEM-2010F). The surface compositions of CuO@NiO products were evaluated by X-ray photoelectron spectra (XPS, PHI ESCA-5000C). The specific surface area and porous structure were evaluated by an accelerated surface area and porosimetry analyzer (Micromeritics Instrument Corp, ASAP 2020 M+C, Nitrogen). Thermogravimetric analysis (TGA) of the product was performed in air on a NETZSCH STA 409 PG/PC instrument and elemental analysis was performed on Vario MICRO.

Electrochemical Measurements Electrochemical performance was measured by assembling the Swagelok-type half cells at room temperature (25 oC). The galvanostatic charge-discharge tests were performed on a LAND-CT2001 test system at a current density of 100 mA g-1 in 5 mV and 3.0 V. The working electrodes were composed of 80 wt.% active materials (CuO@NiO and NiO microspheres), 10 wt.% conductive agent (acetylene black) and 10 wt.% poly(vinylidene difluoride) (PVDF) as binder. The loading amount of the anode was kept at ~2 mg cm-2 on copper foil. Lithium foil was used as the reference electrode and the electrolyte was 1M LiPF6 in ethylene carbonate (EC) and diethyl 17


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carbonate (DEC) (1/1, w/w). Cyclic voltammetry (0.1 mV·s-1) and Nyquist plots (100 kHz-0.01 Hz) were carried out on a CHI660D electrochemical workstation.

Acknowledgement The authors gratefully acknowledge the follow-up Program for Professor of Special Appointment in Shanghai (Eastern Scholar), the National Natural Science Foundation of China (51271105 and 51201095), Shanghai Municipal Education Commission (11SG38,13YZ012) and Innovative Research Team (IRT13078) for financial support. The authors also thank Lab for Microstructure, Instrumental Analysis and Research Center, Shanghai University, for materials characterizations.

Supporting Information Available Supporting Information (SI) available: XRD patterns, TGA curve, and nitrogen adsorption isotherms curve, TEM/SEM images of Cu-Ni-BTC MOFs and CuO@NiO products and table of electrochemical properties comparison. This material is available free of charge via the Internet at http://pubs.acs.org.

References and Notes

1. Wang, Z. Y.; Zhou, L.; Lou, X. W. Metal Oxide Hollow Nanostructures for Lithium-Ion Batteries. Adv. Mater. 2012, 24, 1903-1911. 2. Wu, H. B.; Chen, J. S.; Hng, H. H.; Lou, X. W. Nanostructured Metal Oxide-Based Materials as Advanced Anodes for Lithium-Ion Batteries. Nanoscale 2012, 4, 2526-2542. 3. Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; 18


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Figure 1. XRD patterns of CuO@NiO and NiO microsphere.

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Figure 2. XPS spectra of CuO@NiO: (a) full spectrum, (b) Ni 2p, (c) Cu 2p, (d) O 1s.

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Figure 3. (a, c) SEM images of Cu-Ni-BTC spheres. (b, d) SEM images of CuO@NiO spheres with hollow structure. (e-h) Elemental mapping images of CuO@NiO.

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Figure 4. TEM images of (a) Ni-BTC, (b) NiO, (c) Cu-Ni-BTC, (d, e) CuO@NiO. (f) HRTEM image of CuO@NiO. (g) EDS spectrum of different layers of CuO@NiO. 27


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Scheme 1. Schematic illustration showing the cationic exchange process of metal organic framework (MOF) and its conversion to multi-layer hollow structure.

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Figure 5. Electrochemical performances of multi-layer CuO@NiO spheres: (a) cycle voltammogram profile, (b) first cycle discharge (lithium insertion) and charge (lithium extraction) curve, (c) cycling performance at a current of 0.1 A·g-1, and (d) Nyquist plots for the first, third and 200 cycles. (e) TEM image of the anode after 200 cycles.

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Multilayer CuO@NiO Hollow Spheres: Microwave-Assisted Metal

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