High rate Li-ion storage properties of MOF-carbonized derivatives coated on MnO nanowires

Abstract

Recently, metal–organic framework (MOF) derived porous carbon-based composites have become one of the most advanced electrode materials for high performance energy storage systems. In this work, zeolitic imidazolate framework (ZIF) types of MOF strung by MnO₂ NWs, forming an interesting structure like Chinese candied hawthorn fruit on a stick, are used as precursors to prepare C/Co-coated MnO nanowires (C/Co-MnO NWs). It is interesting and exciting to observe that the simultaneously formed carbon coating derived from the ZIFs significantly promotes the cyclic and rate performances of manganese oxide because of the synergistic effect of the highly conductive uniform carbon coating and the high capacity contribution from the MnO NWs. The obtained C/Co-MnO NWs could deliver 848.4 and 718 mA h g⁻¹ at 500 and 5000 mA g⁻¹ after 40 charge/discharge cycles, respectively, which is superior to other reported MOF-derived nanostructured materials, and makes it a very promising candidate anode material for future high-power lithium ion batteries.

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Introduction

Worldwide recognition of the importance of green energy storage and distribution for people's daily lives and the atmospheric environment has now emerged alongside the decay of the urban environment. The incomplete combustion of fossil fuels to power traditional vehicles and to supply heat for space heating is considered as one of the main reasons for the more and more serious fog and haze conditions in large cities, especially in developing countries. Therefore, the ever-increasing demand for electrical vehicles (EVs) and low cost green energy harvest stations is accelerating the development of advanced energy storage and energy conversion systems. Currently, lithium-ion batteries (LIBs) are the leading power sources for EVs and still predominate in the battery market for portable electronic devices and green energy distribution because of their high energy density, long life span, lack of memory effect, and environmental benignity.^1–3 However, traditional LIBs that use graphite as the anode material are not able to satisfy the rapidly expanding demand due to the limited theoretical capacity of graphite (372 mA h g⁻¹).^2–6 Thus, one of the urgent research interests for materials scientists delving into next generation energy storage technology is to develop a novel low cost anode material with higher capacity and better cycle stability at higher power states.

Nowadays, other than various advanced carbon materials, nanostructured silicon-based hybrids and nanostructured transition metals and their oxides or sulfides have attracted intensive interest because of their much higher theoretical lithium ion storage capacities.^5–16 However, their semi-conductive or even insulating nature and huge volume change during the (de)lithiation process are two of their intrinsic drawbacks as anode materials for LIBs. Therefore, Mn-based oxides, such as MnO, MnO₂ and Mn₃O₄,^16–25 have been extensively studied as promising candidate anode materials due to their smaller volume change relative to Si materials and their environmentally benign nature. However, in the same way as for other transition metal oxide anode materials for LIBs, the rate capability and cyclic stability of Mn-based oxide anodes also have to be solved before their practical application. As for the less conductive Mn-based oxides, the significant volume change during the charge–discharge process will again lead to the pulverization of active particles and their detachment from the conductive network or current collector, in turn increasing the system charge transfer resistance and reducing the availability of the active substance.

In recent years, great efforts have been made by materials scientists to solve the aforementioned problems encountered by Mn-based oxide anodes. For example, binary manganese oxides (M_xMn_yO_z) have been developed as promising anode candidates with enhanced electrochemical performance,²⁰ since most research results indicate that binary metal oxides exhibit better conductivity and structure stability than mono-metal oxides. The development of nanostructured hierarchical Mn-based oxides is also one of the important strategies to promote the cyclic and rate capability of Mn-based anode materials.¹⁶ Furthermore, constructing a uniform 3 dimensional conductive network via the introduction of uniform surface coating and formation of a nanostructured composite with carbon is considered as one of the most important and effective ways to achieve superior rates and cyclic stabilities for Mn-based oxide anode materials.^17–21 The introduction of carbon materials, such as carbon nanotubes, graphene and nitrogen-doped carbon coatings, can not only help to accelerate charge transport, but is also helpful for the formation of a stable solid electrolyte interphase and the remission of inner stress caused by the huge volume change.

In the last few years, metal–organic frameworks (MOFs), constructed with metal ions or clusters and polyfunctional organic ligands, have attracted increasing attention both from fundamental scientific research and for practical multifunctional applications.^9–29 Zeolitic imidazolate framework (ZIF) types of MOFs, such as ZIF-67 and ZIF-8,^9–14 have high porosity and are considered as promising MOFs for commercial applications. Carbonized derivatives of ZIFs show especially good potential for applications in LIBs. For instance, ZIF nanocrystal derived carbon anode materials show good cyclic stability and superior rate capability.^27–29 However, the very low tap density of highly porous nanostructures, together with their relatively low capacity, limits their practicability as anode materials for commercial LIBs. Nevertheless, the unique structure and composition of ZIFs and their carbonized derivatives are very helpful to design different nanostructured transition metals and their oxides and to improve their rate and cyclic stability.

Based on the above concerns, an interesting structure of ZIF-67 MOFs on MnO₂ nanowires (NWs) is designed and prepared by a controllable way in this work, as inspired by the interesting structure of candied hawthorn fruit on a stick. Furthermore, the simultaneously formed carbon coating derived from ZIF-67 is designed to promote the cyclic and rate performances of manganese oxide. Benefiting from the synergistic effect of the highly conductive uniform carbon coating and the high capacity contribution from MnO, the obtained C/Co-MnO NW composite shows a high reversible capacity of 848.4, 836.5, 834 and 718 mA h g⁻¹ at 500, 1000, 2000 and 5000 mA g⁻¹ after 40 charge/discharge cycles, respectively.

Experimental

Materials and synthetic procedures

General procedure for the synthesis of MnO₂ NWs. The MnO₂ NWs were synthesized by a graphene oxide (GO) modified hydrothermal method referred to in ref. 30. The typical processes are as follows: initially, GO powder, prepared by our modified Hummers’ method,⁸ was dispersed into ultra-pure water to obtain a 2 mg mL⁻¹ GO aqueous solution. Subsequently, 0.35 g manganese sulfate monohydrate (MnSO₄·H₂O, Guangdong Xilong Chemical) was dissolved into 7.5 mL GO aqueous solution to form a clear solution, labeled solution A. Meanwhile, 0.5 g potassium permanganate (KMnO₄, Shanghai Lingfeng Chemical) was dissolved into 10 mL distilled water to form a clear solution, B. Subsequently, solution B was added dropwise into solution A under magnetic stirring, following the reaction below:

2KMnO₄ + 3MnSO₄ + 2H₂O → 5MnO₂ + K₂SO₄ + 2H₂SO₄

A dark brown precipitate gradually appeared with the oxidation of the Mn²⁺ precursor by the strong oxidizer. After 15 min ultrasonication treatment, the obtained brown dispersion was sealed into a 50 mL Teflon-lined stainless steel autoclave and put into a blowing drying box preheated to 140 °C. After 2 h hydrothermal reaction, the solid products were collected and washed several times with distilled water and ethyl alcohol through centrifugation. After drying at 60 °C for 24 h in a vacuum drying oven, the MnO₂ NWs were finally obtained and kept for the next step.

Controllable self-assembly process for the unique structure of ZIF-67 on MnO₂ NWs. The typical self-assembly process for the unique structure of ZIF-67 on MnO₂ NWs (ZIF-67-MnO₂ NWs) is as follows: at the beginning, 30 mg nanowires were uniformly dispersed into 20 mL methanol with the aid of ultrasonication. Then, 100 mg polyvinylpyrrolidone (PVP, Shanghai Aladin Ltd) was dissolved into the MnO₂ NW dispersion to modify the surface of the NWs. Subsequently, 100 mg cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, Wuxi Yasheng Chemical) was added into the above solution, marked as solution A; finally, 10 mL solution B (0.2 g mL⁻¹ 2-methylimidazole (Shanghai Aladin Ltd) in methanol) was mixed into solution A drop by drop. After settling for 24 h, a purple solid product was collected by centrifugation and washed with methanol three times. After drying at 60 °C for 24 h in a vacuum drying oven, the final product of ZIF-67-MnO₂ NWs was obtained and kept for the next step. The controlled ZIF-67-MnO₂ NW mixture was prepared by the same process mentioned above without using PVP. The controlled ZIF-67 pure MOFs were prepared by the same process mentioned above without using PVP or the MnO₂ NWs.

Preparation of the carbonized derivatives from ZIF-67 and ZIF-67 on MnO₂ NWs. In order to obtain the final carbon derivatives, the as-prepared ZIF-67 and ZIF-67 on MnO₂ NWs were carbonized at 300 and 550 °C for 3 h with a temperature ramp of 1 °C min⁻¹ under N₂ atmosphere in a horizontal tube furnace, respectively. After naturally cooling to room temperature, the black remainders, namely C/Co and C/Co-MnO NWs, were finally obtained as anode materials for LIBs. For comparison, the controlled mixture of MnO₂ NWs and Co/C derivatives was also prepared by physically mixing the as-prepared MnO₂ NWs (75 wt%) and carbonized ZIF-67 (25 wt%) via ultrasonication in ethanol, followed by drying the dispersion at 60 °C for 10 h in a blowing drying oven.

Characterization

The morphologies of the MnO₂ NWs, ZIF-67, ZIF-67-MnO₂ NWs, C/Co and C/Co-MnO NWs were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) at an acceleration voltage of 3 kV and field emission transmission electron microscopy (FE-TEM, JEOL 2010F) at an accelerating voltage of 200 kV. Nitrogen adsorption/desorption isotherms were obtained at 77 K using an automated adsorption apparatus (Micromeritics ASAP2020). The surface area was calculated based on the Brunauer–Emmett–Teller (BET) equation. The X-ray diffraction patterns of the MnO₂ NWs, ZIF-67, ZIF-67-MnO₂ NWs, C/Co and C/Co-MnO NWs were measured on an X-ray diffractometer (RIGAKU, RINT-ULTIMA III) using Cu Kα radiation (λ = 1.54051 Å). The diffraction patterns were recorded in the 2θ range of 5–75° with a step size of 0.01°. The elemental mapping of the C/Co-MnO NWs was characterized by energy-dispersive X-ray spectroscopy (EDS, Oxford instruments X-Max). The Co and Mn content within the as-prepared C/Co-MnO NWs was measured by following the general rules of JY/T 015-1996 for inductively coupled plasma-atomic emission spectrometry (ICP-AES, Prodigy, Teledyne Leeman Labs).

To investigate the electrochemical performances, the as-prepared active materials, including the MnO₂ NWs, ZIF-67, ZIF-67-MnO₂ NWs, C/Co and C/Co-MnO NWs, were homogeneously mixed with a conductive additive (acetylene black) and binder (PVDF) in a weight ratio of 75%/15%/10% in a suitable amount of N-methylpyrrolidinone solvent to form a uniform slurry. The working electrodes were prepared by coating the obtained slurry on copper foil, which acted as a current collector. The electrode was then punched out into disks 10 mm in diameter. The average mass loading of the active materials was about 1.27 mg cm⁻². Two-electrode lithium ion batteries were assembled in an ultrapure Ar gas-filled glove box to investigate the lithium ion storage performance of the MnO₂ NWs, ZIF-67, ZIF-67-MnO₂ NWs, C/Co and C/Co-MnO NWs. The electrolyte used was 1 mol L⁻¹ LiPF₆ in ethylene carbonate (EC) + dimethyl carbonate (DMC) + ethyl methyl carbonate (EMC) in a volume ratio of 1 : 1 : 1. Lithium discs were used as the counter electrodes. Cyclic voltammetry (CV) and galvanostatic charge and discharge measurements were carried out in a voltage range of 0.01 to 3 V vs. Li/Li⁺ at a current density ranging from 0.5 to 5 A g⁻¹, respectively. Electrochemical impedance spectroscopy was carried out in a frequency range of 0.01 Hz to 100 kHz, and the perturbation amplitude was controlled at 5 mV. Galvanostatic charge/discharge tests were performed on a battery testing system (CT2001A, Wuhan Land).

Results and discussions

Morphology and structure

ZIF-67. In this work, one type of ZIF series MOF, namely ZIF-67, was successfully prepared as a control electrode material and an important building block for the nanostructure of MOFs on MnO₂ NWs. The typical polyhedron morphology of the controlled ZIF-67 shown in Fig. 1a is similar to that in other reported work.³¹ The obtained ZIF-67 polyhedrons have a narrow particle size ranging from 500–800 nm. The X-ray diffraction pattern, shown in Fig. 2a (red), matches well with that of the reported structure of ZIF-67.³¹ The nitrogen adsorption/desorption isotherm and pore-size distribution curve in red presented in Fig. 2c and d indicate the microporous characteristics of ZIF-67 with a BET specific surface area of 1158.4 m² g⁻¹. This observation is also similar to that in other reported studies.

Fig. 1 Scanning electron microscopy (SEM) images of the as-prepared (a) ZIF-67, (b) MnO₂ NWs, (c and d) ZIF-67-MnO₂ NWs, (e) carbonized derivative of ZIF-67 (C/Co) and (f) carbonized derivative of ZIF-67-MnO₂ NWs (C/Co-MnO NWs).

Fig. 2 (a and b) X-ray diffraction (XRD) patterns, (c) nitrogen adsorption/desorption isotherm profiles and (d) pore-size distribution curves of the as-prepared MnO₂ NWs (in black), ZIF-67 (in red), ZIF-67-MnO₂ NWs (in blue), carbonized derivative of ZIF-67 (C/Co) (in green) and carbonized derivative of ZIF-67-MnO₂ NWs (C/Co-MnO NWs) (in pink).

MnO₂ NWs. Like ZIF-67, the MnO₂ NWs were also prepared as an important building block for the nanostructure of MOFs on MnO₂ NWs. With the modification of GO, the MnO₂ NWs prepared through the modified hydrothermal reaction have a large ratio of length to diameter. The length is about 2–3 μm, as seen in Fig. 1b, while the diameter is around 10–30 nm, as shown in Fig. 3d. The MnO₂ NWs’ BET specific surface area of 77.327 m² g⁻¹, obtained from the nitrogen adsorption/desorption isotherm shown in Fig. 2c (black), further reflects the nanoscale nature of the product. The X-ray diffraction pattern given in Fig. 2a (black) indicates that the obtained NW is potassium doped MnO₂, K_2−xMn₈O₁₆ (PDF#44-1386), which belongs to the monoclinic I2/m (12) space group. The high resolution TEM images and the composition analysis results provided in Fig. 3e and f and Fig. S2d (ESI†) further confirm the observed structure and composition of the obtained MnO₂ NWs. The measured lattice fringes (Fig. 3e and f) with interplane distances of 0.314, 0.2772 and 0.1847 nm can be indexed to the (30−1), (110) and (41−1) crystallographic planes of the monoclinic K_2−xMn₈O₁₆ structure, respectively.

Fig. 3 High resolution transmission electron microscopy (HR-TEM) images of the as-prepared carbonized derivative of ZIF-67 (C/Co) (a–c), ZIF-67-MnO₂ NWs (d–f), and carbonized derivative of ZIF-67-MnO₂ NWs (C/Co-MnO NWs) (g–i).

ZIF-67-MnO₂ NWs. With the aim of exploiting the advantages of both MOFs and manganese oxide NWs, a unique structure of MOFs on NWs has been designed and successfully developed in this work. During the preparation process, PVP was used to modify the surface conditions of MnO₂ NWs. With the help of surface modification, ZIF-67 MOFs were uniformly grown on the surface of MnO₂ NWs. Although the developed Chinese candied hawthorn fruit on a stick structure of the ZIF-67 on MnO₂ nanowires (ZIF-67-MnO₂ NWs) is similar to previously reported work,³² the design mentality, the preparation process and the final products in this work are different from the research done in ref. 32. As shown in Fig. 1c and d and 3d, MnO₂ NWs strung all ZIF-67 polyhedrons to form a unique structure like that of Chinese candied hawthorn fruit on a stick or a pearl necklace and lamb skewer. Interestingly, the ZIF-67 polyhedrons on MnO₂ NWs are only about 100 nm, which is much smaller than the pure ZIF-67 polyhedrons (500–800 nm) grown in the same process and the reported ZIF-67 on MnO₂ NWs prepared with the aid of polyetherimide (PEI).³² It is also valuable to note that the sample prepared without using PVP is a mixture of ZIF-67 and MnO₂ NWs, as shown in Fig. S1a (ESI†). Although the size of the obtained ZIF-67 particles is also slightly reduced to 300–400 nm, they are still larger than the ZIF-67-MnO₂ NWs. The results mentioned above strongly indicate that the obviously heterogeneous nucleation phenomenon on the PVP modified surface of the MnO₂ NWs is responsible for the uniform growth of the much smaller particle size of the ZIF-67 polyhedrons on MnO₂ NWs. The X-ray diffraction pattern presented in Fig. 2a is the combined pattern of both ZIF-67 and MnO₂, which further confirms the successful growth of the unique structure of ZIF-67 MOFs on MnO₂ NWs (ZIF-67-MnO₂ NWs). The large BET specific surface area (520.975 m² g⁻¹) and the microporous structure, observed in Fig. 2c and d, further indicate that the as-prepared ZIF-67-MnO₂ NWs maintain the advantageous nature of ZIF-67.

C/Co and C/Co-MnO NWs. In order to obtain highly conductive materials for LIBs with high rates, both ZIF-67 and ZIF-67-MnO₂ NWs were carbonized at a temperature of 550 °C for 3 h under Ar gas atmosphere. As shown in Fig. 1e, the ZIF-67 polyhedrons became withered, like Chinese traditional wontons. The XRD pattern and the high resolution TEM images shown in Fig. 2b and 3a, b and c, respectively, indicate that the carbonized derivative of ZIF-67 is a composite of carbon and cobalt nanoparticles (C/Co). As indicated by the high resolution TEM images shown in Fig. 3b and c, the measured lattice fringes with an interplane distance of 0.206 nm can be indexed to the (111) crystallographic plane of the cubic cobalt, which belongs to the crystal space group of Fm3̄m (225). This observation indicates that the cobalt components within ZIF-67 are reduced to elemental metallic cobalt nanoparticles after a carbothermic reduction reaction. Furthermore, the cubic metallic cobalt particles dispersed into the carbon matrix are about 2–5 nm. During the carbonization process, the structure becomes dense. As shown in Fig. 2c and d, the specific surface area and the pore volume decrease from 1158.4 m² g⁻¹ and 0.66 cm³ g⁻¹ for ZIF-67 to 193.755 m² g⁻¹ and 0.078 cm³ g⁻¹ for the C/Co composites, respectively.

Similar to the obtained C/Co composites, the carbonized derivative of the ZIF-67-MnO₂ NWs becomes a complex composite of carbon, cobalt nanoparticles and MnO NWs (C/Co-MnO NWs) with a small amount of Mn and CoC_x formed during the carbonization process, as indicated by the pink color X-ray diffraction pattern shown in Fig. 2b. As shown in Fig. S1b (ESI†), the morphology of ZIF-67 on MnO₂ NWs is still maintained after 3 h annealing at 300 °C. However, after being calcined at 550 °C for 3 h, the ZIF-67 polyhedrons became molten and formed a uniform carbon coating on the surface of the MnO NWs reduced from the MnO₂ NWs, which can be observed from the SEM image shown in Fig. 1f and the high resolution TEM images in Fig. 3g–i, and further confirmed by the element mapping results shown in Fig. S2 (ESI†). The measured lattice fringes (Fig. 3h and i) with interplane distances of 0.2612, 0.2174 and 0.1445 nm can be indexed to the (111), (200) and (311) crystallographic planes of the cubic MnO structure in the space group of Fm3̄m (225), respectively. The uniform carbon coating, about 2–3 nm in thickness, can be observed from the inset of Fig. 3h and is good for the construction of a 3D conductive network. As for the Mn and CoC_x impurity phases, it is very difficult to define them from the high resolution TEM images together with MnO, due to their small amounts and similar interlayer spaces. Based on the distribution of Co shown in Fig. S2c (ESI†), cobalt mainly gathers at the locations of polyhedron sites. The ICP-AES measurement result indicates that the elemental content of Co and Mn within the finally obtained C/Co-MnO NWs is about 16.0% and 43.8%, respectively. The nitrogen adsorption/desorption isotherm and pore-size distribution curves presented in Fig. 2c and d confirm that the BET specific surface area of the C/Co-MnO NWs further decreased to 57.8 m² g⁻¹, which is good for obtaining a higher volumetric energy density.

Lithium ion storage properties

In this work, galvanostatic charge/discharge measurements at different current densities, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out in lithium half-cells to investigate and understand the electrochemical behaviour of the as-prepared active materials. Fig. 4 and 5 indicate that the MnO₂ NWs, ZIF-67-MnO₂ NWs, C/Co, and C/Co-MnO NWs are electrochemically active, and capable of lithium ion storage. As shown in Fig. 4a, the typical CV curves of the second cycle scanning between 0.01 and 3.0 V at 0.1 mV s⁻¹, which are similar to those of reported MnO/C materials,^17,18 demonstrate that the electrodes made of C/Co and C/Co-MnO NWs show a smaller voltage gap between the reduction and oxidation peaks. Meanwhile, the CV curve of the C/Co-MnO NWs shows a larger peak current density than that of C/Co, resulting from a higher fraction of high capacity transition metal components. A pair of weak typical redox peaks could be found from the CV curve of C/Co shown in Fig. 4a, which indicates that Co has a relatively minor contribution to the capacity of the C/Co electrode. Therefore, the CV analysis results clarify that the C/Co-MnO NWs show better electrochemical performance due to the synergistic effect of the MnO NWs and their uniform MOF derived carbon coating. It can be found from Fig. 4b that the C/Co and C/Co-MnO NWs show much lower charge transfer resistance than the MnO₂ NWs and ZIF-67-MnO₂ NWs. After fitting with the equivalent circuit shown in the inset of Fig. S3c (ESI†), the calculated resistance, including the electrolyte resistance and the contact resistance of the active materials to the current collector, is around 4.2 Ω. The charge transfer resistance (R_ct), which is equal to the diameter of the semicircle in the high frequency range, is about 65.9 Ω. Furthermore, in the Bode plots given in Fig. S3d (ESI†), the characteristic frequencies f₀ at a phase angle of −45° for the C/Co-MnO NWs, C/Co, ZIF-67-MnO₂ NWs, and MnO₂ NWs are 0.071, 0.067, 0.026 and 0.010 Hz, corresponding to time constants τ₀ (τ₀ = 1/f₀) of 14.1, 14.9, 38.5 and 100 s, respectively. The values of the C/Co-MnO NWs are much smaller than those of the materials mentioned earlier, indicating that the C/Co-MnO NWs have a faster charge–discharge rate than the other materials. This observation further supports the fact that the improved conductivity of the C/Co-MnO NWs, arising from the uniform carbon coating on the MnO NWs, is responsible for its better electrochemical performance.

Fig. 4 (a) Cyclic voltammetry (CV) curves, (b) electrochemical impedance spectroscopy results, (c) the charge/discharge profiles at the 40th cycle of the as-prepared MnO₂ NWs (in black), ZIF-67-MnO₂ NWs (in blue), carbonized derivative of ZIF-67 (C/Co) (in green) and carbonized derivative of ZIF-67-MnO₂ NWs (C/Co-MnO NWs) (in pink), and (d) the charge/discharge profiles at the 40th cycle of the C/Co-MnO NWs at a current density from 500 to 5000 mA g⁻¹.

Fig. 5 (a and b) The cyclic performances of the C/Co-MnO NWs at a current density of 500 and 5000 mA g⁻¹, respectively, (c) comparison of the rate and capacity retention after 50 cycles of the as-prepared MnO₂ NWs (in black), ZIF-67-MnO₂ NWs (in blue), C/Co (in green) and C/Co-MnO NWs (in pink), and (d) comparison of the rate and capacity retention after 50 cycles of the as-prepared C/Co-MnO NWs with other reported similar materials, such as C/nano-Si,⁹ MOF-derived porous Co₃O₄,¹⁴ ZnO/Co₃O₄,¹⁵ MnO@C,¹⁷ MnO₂/MWCNT,²¹ and Co-BDC MOF.³⁴

The typical charge/discharge profiles presented in Fig. S3 (ESI†) and Fig. 4c and d for the first cycle and the 40th cycle, respectively, also evidently prove that the electrode made of C/Co-MnO NWs delivers an excellent rate capability and a much higher capacity than other electrodes prepared with other controlled materials. The Li//C/Co-MnO NW half-cell exhibits a much higher capacity, higher coulombic efficiency for the first cycle and better cyclic performance than the Li//MnO₂ NW half-cell, which is promoted by the ZIF-67-derived surface layer of C/Co. With respect to ZIF-67-derived C/Co, a typical discharge plateau and reduction peak clearly observed at a voltage of 0.5 V should result from the reduction of Mn²⁺ to Mn⁰. The corresponding redox reaction of Mn²⁺/Mn⁰ should account for the higher capacity of the C/Co-MnO NWs over C/Co. This lithium ion storage mechanism is similar to that in other reported MnO@C composites.^17,33 Moreover, the synergistic effect of the MnO NW and its uniform MOF derived carbon coating is responsible for its superior lithium ion storage performance, compared to the ZIF-67-MnO₂ NWs. This result is consistent with the CV analysis results, seen in Fig. 4a and c. As shown in Fig. 4d, the specific charge capacities of the C/Co-MnO NWs are 848.4, 836.5, 834 and 718 mA h g⁻¹ at 500, 1000, 2000 and 5000 mA g⁻¹ after 40 charge/discharge cycles, respectively, which confirm the superior rate capability of the as-prepared C/Co-MnO NWs. Fig. 5 and Fig. S4 (ESI†) further exhibit that the as-prepared C/Co-MnO NWs not only deliver a much better rate and cyclic performance than the controlled C/Co, MnO₂ NWs, ZIF-67-MnO₂ NWs and C/Co-MnO₂ physical mixture at a constant current density of 500, 1000, 2000 and 5000 mA g⁻¹, but also show a superior rate and cyclic performance than other reported similar materials (see Fig. 5d), such as C/nano-Si,⁹ MOF-derived porous Co₃O₄,¹⁴ ZnO/Co₃O₄,¹⁵ MnO@C,¹⁷ MnO₂/MWCNT,²¹ Co₃O₄/MnO₂³² and Co-BDC MOF.³⁴

Furthermore, the as-prepared C/Co-MnO NWs also present a much better rate capability than the controlled mixture of C/Co-MnO₂ NWs, as seen in Fig. 5 and Fig. S4a and b (ESI†). As shown in Fig. 1f and 3h, the in situ formed uniform carbon coating fully covered the surface of the MnO NWs. It has a strong interaction and created an integral electrical contact between MnO and the carbon coating. A similar mechanism could also be observed from the reported core–shell ZnO/ZnFe₂O₄@C mesoporous nanospheres.³⁵ However, there is only point electrical contact between the physically mixed MnO₂ NWs and the large size C/Co particles derived from ZIF-67, as shown in Fig. S4a and b (ESI†). Therefore, the synergistic effects of much improved conductivity and relatively suppressed volume change resulting from the integrally strong carbon coating are responsible for the much superior rate and cyclic performance of the C/Co-MnO NWs compared to the controlled mixture of C/Co-MnO₂ NWs.

Conclusions

In this work, a novel and interesting structure of ZIF-67 MOFs on MnO₂ nanowires (NWs), like candied hawthorn fruit on a stick, is designed and successfully prepared by a controllable multi-step chemical method. The observed experimental results demonstrate that the polyvinylpyrrolidone (PVP) modified surface is helpful for the growth of ZIF-67 on the MnO₂ NWs and for minimizing the MOF size from 500 to 100 nm. It is also interesting and exciting to find that the simultaneously formed carbon coating derived from ZIF-67 significantly promotes the cyclic and rate performances of manganese oxide. Benefiting from the synergistic effects of the highly conductive uniform carbon coating and the high capacity contribution from MnO, the obtained C/Co-MnO NWs deliver 848.4, 836.5, 834 and 718 mA h g⁻¹ at 500, 1000, 2000 and 5000 mA g⁻¹ after 40 charge/discharge cycles, respectively. The achieved high rates and much enhanced cyclic performances make the present C/Co-MnO NWs superior to other reported MOF-derived nanostructured materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51402155), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) (YX03002), National Synergistic Innovation Center for Advanced Materials (SICAM) and Foundation of NJUPT (NY214021).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qm00178a

‡ Dr Huang and Mr Gong made equal contributions to this paper.

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