C Microspheres Fabricated from MOF Template

Oct 3, 2017 - Mesoporous Mn3O4/C Microspheres Fabricated from MOF Template as Advanced Lithium-Ion Battery Anode. Hai-Jun Peng†, Gui-Xia Hao‡, Zha...
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Mesoporous MnO/C Microspheres Fabricated from MOF template as Advanced Lithium-Ion Battery Anode Hai-Jun Peng, Gui-Xia Hao, Zhao-Hua Chu, Jia Lin, Xiao-Ming Lin, and Yue-Peng Cai Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00978 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

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Mesoporous Mn3O4/C Microspheres Fabricated from MOF Template as Advanced Lithium-Ion Battery Anode Hai-Jun Peng,† Gui-Xia Hao,‡ Zhao-Hua Chu,‡ Jia Lin,† Xiao-Ming Lin,*,†,§ and Yue-Peng Cai*,† †

Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, School of Chemistry and Environment, South China Normal University, Guangzhou 510006, P. R. China ‡ College of Chemistry and Environmental Engineering, Hanshan Normal University, Chaozhou ,Guangdong 521041, P. R. China § Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education School of Chemistry and Environment, South China Normal University, Guangzhou 510006, P. R. China ABSTRACT: A three-dimensional (3D) metal–organic framework (MOF), namely {[Mn4(PBA)4(H2O)6·5H2O]}n (Mn-PBA), has been successfully constructed from 5-(4-pyridin-3-yl-benzoylamino)-isophthalic acid ligand (H2PBA) and Mn(II) ions under solvothermal condition. Structural analysis reveals that there exists 1D hexagonal channels in the 3D structure along the b-axis. Mesoporous Mn3O4/C composites were fabricated by the direct thermolysis of Mn-PBA at 500 ºC under air atmosphere. When tested as a lithium ion battery anode material, Mn3O4/C electrode delivers an excellent capacity of 1032 mAh g-1 at 200 mA g-1 after 500 cycles along with remarkable rate capacity, which is supposed to benefit from the unique microshperes characteristic and large accessible specific area. Owing to the good cycling stability and high capacity, the Mn3O4/C electrode can be regarded as a promising anode material for LIBs.

1. INTRODUCTION With the rapid progress of advanced electronic equipments, such as high-speed cameras, flexible telephones, portable laptop computers and hybrid electric vehicles, it is necessary to develop high performance and high availability energy transformation systems. Lithium ion batteries (LIBs) with cycling stability, light-weight and high energy density are regarded as promising, clean and renewable energy sources.1-3 Nevertheless, because of the easy exfoliation, poor rate capacity and cycling stability, low theoretical capacity (372 mAh g-1) and inherent safety risk, commercial graphite material could not achieve high power density goal of newgeneration LIBs.4,5 In recent years, transition metal oxides (TMOs), such as Co3O4,6 Fe3O4,7 ZnO,8,9 CuO,10 MnO,11-13 have been widely researched as anode materials for their satisfactory energy storage performances. Among various TMOs, Mn3O4 have attracted extensive attention owing to its low voltage potential (~0.5 V vs. Li+/Li), high theoretical capacity, low-cost, natural abundance and environmental compatibility.14-16 However, the use of pure Mn3O4 is limited by large capacity loss arising from volume expansion during electrochemical reaction, low rate capability and poor electrical conductivity (usually between 10-5 to 10-6 S m-1).17 In order to tackle these problems, various strategies have been developed. One effective method is to combine with diverse carbon-based materials, such as conducting polymer graphene, carbon nanowire, carbon tube and carbon fibers, which are electronically conductive and light-weight.18,19 In addition,

fabrication of porous structure with large specific surface areas have also been regarded as an effective approach to improve the capacity retention by accommodating the volume change during Li-ion insertion/extraction,20,21 which can provide more reaction sites and enlarge contact areas between the electrolyte and active materials. Particularly, metal-organic frameworks (MOFs), constructed from organic ligands and metal ions or metal clusters, are emerging as a porous organic-inorganic hybrid material with tunable porosities and large surface areas.22-24 Recent years, MOFs have been used as either precursors or templates to form metal oxides and carbon materials via thermal treatment.25,26 Despite these great successes, it still remains significant challenge to develop promising electrode materials with reversible and maximum lithium storage. In our previous work, we had synthesized a Pb-MOF, in which organic moiety could provide probable Li+ binding sites for the reversible lithium storage.27 By utilizing a new Cd-MOF as template, porous carbon with microtube shape was fabricated and delivered a discharge capacity of 741 mAg h-1 at 100 mA g−1.28 As a continuation of our work, herein, we used a linear ligand 5-(4-Pyridin-3-yl-benzoylamino)-isophthalic acid (H2PBA) to construct a porous Mn-based MOF. Delightedly, a well-ordered crystalline 3D porous Mn-PBA was successfully synthesized and then was transformed into porous Mn3O4/C composites by annealing under air atmosphere. Furthermore, we also evaluated the electrochemical performances as an anode material for LIBs. 1

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2. EXPERIMENTAL SECTION 2.1. Materials Characterization. Functional groups of the materials were assessed using fourier transform infrared spectroscopy (Thermo Nicolet NEXUS 670 FT-IR, USA). Elemental analyses were performed using a PerkinElmer 240 elemental analyzer (USA). Crystalline phase was identified by a Bruker D8 Advance X-ray diffraction patterns diffractometer (BRUKER-AXS, Germany) equipped with a Cu target tube and a graphite monochromator. Raman spectra were obtained from Renishaw inVia confocal Raman microscope (UK). Netzsch Thermo Microbalance TG 209 F1 Libra (Germany) was used for thermogravimetric analyses (TGA) in order to know the thermal behaviour (mass) of the material. The sorption isotherms were assessed using a Belsorp max gas sorption analyzer (Japan) at 77 K. The surface morphology and architecture were recorded using field emission scanning electron microscopy (FESEM, TESCAN Maia 3, Czech). Xray photoelectron spectroscopy (XPS) was recorded by ESCALAB 250Xi XPS spectrometer (USA). Varian Mercury Plus 300 MHz spectrometer was used to record 1H NMR spectra (USA). 2.2. Preparation of the Mn-PBA. The synthetic route of H2PBA ligand was given in Figure S1 and the obtained ligand was further confirmed by H1 NMR spectrum (Figure S2). A mixture containing H2PBA (72.4 mg, 0.2 mmol), Mn(NO3)2·6H2O (112 mg, 0.4 mmol), H2O (12 mL) and dimethylformamide (12 mL) was sealed in a glass reaction bottle. The system was heated at 100 ºC for 3 d and then cooled to room temperature. Pale yellow crystals were obtained by filtration and washed with DMF. FT-IR (KBr, cm1 ): 3266 (m), 2373 (w), 2036 (w), 1616 (vs), 1548 (w), 1394 (w), 904 (w), 617 (vs) (See Figure S3). Anal. calcd for C80H78N8O31Mn4 (1867.26): C 51.49, H 4.18, N 5.99 %; found: C 51.43, H 4.22, N 5.97 %. 2.3 Preparation of Mn3O4/C material. The as-prepared Mn-PBA samples were calcined directly in a tubular furnace at 500 ºC under an air atmosphere for 5 h. The resulting products were obtained after 3 h and then washed with anhydrous methanol and deionized water before drying in an oven at 100 ºC to obtain the final products. 2.4 Preparation of carbon material. The as-made MnPBA samples were thermally treated at 700 ºC in nitrogen atmosphere. Subsequently, the obtained sample were washed by HF acid for 3 days to remove metal composite and dried under vacuum to generate the carbon material. 2.5 Electrochemical Measurements. The working electrode was composed of the active material (70 wt %), acetylene black (20 wt %) and polyvinylidene difluoride (PVDF) (10 wt %) dissolved in N-methylpyrrolidinone (NMP). Each constituent was milled well to form slurry that was pressed onto a cleaned copper foil current collector and dried at 100 ºC for 12 hours under vacuum. The loading amount of the active material on the area was about 1 mg cm-2. Electrochemical test CR2025-type coin cells were prepared in an Ar-filled glove-box (< 0.01 ppm) using a porous polypropylene membrane (Celgard-2400) as the separate, lithium metal foil (Tianjin Lithium Industry Co., Ltd.) as the counter/reference electrodes and 1.0 M LiPF6 in mixed ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and ethylene carbonate (EC) (EMC:DEC:EC, 1:1:1 v/v/v) as the

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electrolyte. The galvanostatic charge/discharge tests were carried out at current density of 200 mA g-1 at room temperature (25 ºC) via a multi-channel LAND CT2001 test system (Nanjing, China) between 0.01 to 3.0 V (vs. Li+/Li). Cyclic Voltammetry (CV) curves were performed using a CHI605C electrochemical workstation at a scanning rate of 0.1 mV s-1 (voltage range: 0.01−3.0 V). The electrochemical impedance spectra (EIS) were tested on the same workstation for various electrodes at 25 ºC over the frequency range from 100 kHz to 0.01 Hz. 2.6 X-ray crystallographic studies. X-ray reflection intensities were performed with a Bruker APEX II diffractometer at 296 K using graphite monochromatic Mo-Ka radiation (λ=0.71073Å). The empirical absorption corrections were used to correct the reflections. The structure was solved by the direct method and refined by full-matrix least-squares on F2 using SHELXL programs (SHELXTL-2016).29 Isotropic displacement parameter was used to place the organic hydrogen atoms in calculated positions. The details of data collection, crystal parameters, and refinement are summarized in Table S1. Table S2 presents the selected bond lengths and bond angles. CCDC 1536622 contains the supplementary crystallographic data. 3. RESULTS AND DISCUSSION 3.1. Structure and Characterization of Mn-PBA MOF. Structural analysis displays Mn-MOF crystallizes in monoclinic C2/c space group and asymmetric unit contains two Mn2+ ions, two PBA2- ligands, three coordinated H2O molecules and two and a half free H2O molecules. As depicted in Figure 1a, Mn(1) atom is coordinated by one N atom, three O atoms from four different PBA2- ligands and one O atom from water molecule. Mn(2) is also five coordinated in a distorted square pyramidal geometry. The basal plane is defined by one nitrogen atom (N1), one oxygen atom from H2O molecule and two carboxylate oxygen atoms from two different ligands, and the apical position is occupied by one oxygen atom from a H2O molecule. As seen in Figure 1b, the neighboring Mn(II) ions are connected together through the carboxylate groups of PBA2- ligands to form a 1D grid-like chain. These organic chains are further linked by PBA2ligands using the two carboxylate groups with bidentate bridging coordination mode to give rise to a 3D framework (Figure 1c). This 3D structure possesses 1D hexagonal channel along the b-axis (Figure 1d). These channels are occupied by solvent molecules, giving a solvent accessible volume of approximately 19.7 % (1610.1 Å3 of the 8162.06 Å3 unit cell volume), calculated by using PLATON/SOL program.30 Power X-ray diffraction (PXRD) pattern of as-prepared MnPBA was shown in Figure S4. Most of the peak positions in the as-made samples and simulated one are quite well matched, indicating phase purity of the bulk samples. The difference in diffraction intensity may be due to the preferred orientation effect of the powder particles. Infrared spectrum (IR) reveals that the adsorption peak around 3266 cm-1 for Mn-PBA assigned to the v(OH) vibrations of the H2O molecules. Strong peaks at 1616 and 1393 cm-1 corresponded to asymmetric and symmetric vibrations for carboxylate groups (Figure S5). Thermal stability of the Mn-PBA was also measured under air atmosphere. The curve in Figure S6 shows three main mass 2

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losses. The first weight loss (exp. 4.86 %) between 30 and 150 ºC corresponds to the release of five free H2O molecules (calcd. 4.82 %). The second weight loss (exp. 5.65%) in the range of 150 to 220 ºC is ascribed to the loss of six coordinated H2O molecules (calcd. 5.78%). However, when the temperature exceeds 300 ºC, a large weight loss appears, indicating that the framework has collapsed. According to the TGA result, Mn-PBA samples in our case were calcined at 500 ºC by one-step thermal treatment under a flow of air to ensure the complete conversion of precursors to the final products.

Figure 2. PXRD pattern (a) and Raman spectra (b) of Mn3O4/C samples.

To study the chemical bonding state and chemical composition of the Mn3O4/C samples, X-ray photoelectron spectroscopy (XPS) was applied. Figure 3a shows an overall XPS curve of the Mn3O4/C composites, suggesting that the samples are composed of O, C, and Mn elements. Figure 3b displays the high-resolution XPS spectrum of Mn 2p, where two peaks located at 653.3 and 641.7 eV can be attributed to Mn 2p1/2 and Mn 2p3/2 levels, respectively.34 The C 1s spectrum in Figure 3c include three peaks, corresponding to different chemical states of carbon: 288.6 eV (C=O), 285.7 eV (C-O) and 284.7 eV (C-C).35 The O 1s spectrum consists of three binding energy peaks locating at 532.6, 531.3, and 530.0 eV (Figure 3d).36 The porous nature of the as-synthesized Mn3O4/C samples was measured by the N2 adsorptiondesorption characterization. As depicted in Figure 3e, it presents a representative type-IV with the hysteresis loops at a high relative pressure between 0.7 and 1.0, implying the mesoporous characteristic with void spaces in the microspheres, which is further confirmed by TEM images (Figure S7 and S11b). The corresponding Brunauer-EmmettTeller (BET) surface area is calculated to be 137 m2 g-1 with a total pore volume of 0.48 cm3 g-1. The pore-size distribution calculated by Barrett-Joyner-Halenda (BJH) method suggests the existence of three kinds of mesopores with an average size of 5, 17 and 40 nm. Such high surface areas can reduce volumetric expansion and facilitate the electrolyte diffusion during the long-term cycles, which will favorable for improving the cycling stability during the charge/discharge process.37,38

Figure 1. (a) Coordination environment of Mn atom. (b) 1D grid-like organic chain in Mn-PBA. (c) 3D framework with 1D channel viewed along the b-axis (the metal centers are highlighted using polyhedra). (d) 3D packing structure with 1D channel.

3.2. Characterization of the Mn3O4/C composites. XRD analysis in Figure 2a confirmed that the obtained product after pyrolysis was composed of tetragonal Mn3O4 phase (JCPDS No. 24-0734, space group: I41/amd), indicating the successful synthesis of Mn3O4 samples.31 Raman spectrum in Figure 2b shows that two wide peaks at 1598 and 1356 cm-1 result from G (ordered sp2 bonded carbon) and D (disorder or defects of carbon) bands of carbon, respectively. The IG/ID value of Mn3O4/C sample is 1.19, indicating that the carbon in Mn3O4/C material has a high graphitization degree.32 The strong peak at 655 cm-1 and a weak peak at 363 cm-1 can be attributed to Mn3O4.33

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Figure 3. XPS spectra of Mn3O4/C sample. (a) Survey curve. (b) Mn 2p, (c) C 1s, and (d) O 1s spectra. (e) N2 adsorption/desorption isotherm of Mn3O4/C sample. (f) Pore size distribution.

reveal the existence of C, O and Mn elements in the structure (Figures 3e-g), which is consistent with the XPS analysis. 3.3. Electrochemical performances. The electrochemical properties of the as-synthesized Mn3O4/C composites were systematically studied. Cyclic voltammetric (CV) curves of Mn3O4/C electrode were tested for the first three cycles with a scan rate of 0.1 mV s-1. As shown in Figure 5a, a reduction peak at 0.79 V in the first cathodic sweep was observed due to the reduction of Mn3+ to Mn2+ (Mn3O4+2Li++2e-→3MnO+Li2O) and the generation of a solid electrolyte interface (SEI) layer.39,40 This peak disappears in the following cycle. The reduction peak appeared at 0.21 V, which shifted to about 0.43 V in the second cycle, involving the formation of Li2O and manganese metal (MnO+2Li++2e-→Mn+Li2O).41-43 In the anodic scan performance, the oxidation peak appeared at 1.18 V due to the oxidation reaction of Mn0 to Mn2+/Mn3+ and Li2O decomposition (3Mn+8Li2O→Mn3O4+8Li++8e-),44 which remained in the further cycles. The overlapping curves were observed from the second cycle, demonstrating good reversibility and structural stability of the electrode. Figure 5b presents the galvanostatical charge/discharge at 200 mA g-1 within the range of 0.01–3.0 V. The first charge and discharge capacities are 1205 and 1500 mAh g-1, respectively. The corresponding coulombic efficiency is 80.3%. The loss of capacity in the first cycle might be due to the irreversible electrochemical reactions, such as the inevitable formation of SEI layer and decomposition of the electrolyte. Despite the initial capacity loss, the charge-discharge profiles are basically invariable without capacity fading from the subsequent cycles. Figure 5c presents the cycling performance at different densities of 0.2, 1, and 1.5 A g-1, respectively. Apparently, the

Figure 4. (a) Optical image of Mn-PBA precursors. (b-d) SEM images and (e-g) Mapping images of the Mn3O4/C sample.

In order to better observe the morphology and architecture of the final Mn3O4/C composite, scanning electron microscopy was applied. Compared to the block shape of Mn-PBA precursor, the shape of Mn3O4/C became microspheres. While the size slightly shrinks ranging from 25 to 5 µm due to the decomposition and volume contraction after calcination (Figures 3b-d). Additionally, element mapping images further 4

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Figure 5. Electrochemical performance of Mn3O4/C electrode measured at room temperature (25 ºC). (a) Cyclic voltammetry tests. (b) Discharge-charge curves of Mn3O4/C at 200 mA g-1. (c) Coulombic efficiency and cycling performance at various current densities of 0.2, 1, and 1.5 A g-1. (d) Nyquist plots of the Mn3O4/C electrode before and after cycles with the proposed equivalent circuit.

process.46,47 Moreover, XRD pattern of the electrode after 500 cycles reveals that most peak positions are similar to that before cycles (Figure S10), indicating structure stability of the electrode during charge/discharge process, which are further confirmed by SEM and TEM images (Figure S11). Meanwhile, N2 adsorption-desorption curve in Figure S12 reveals that the mesoporous feature is still maintained while the BET surface area decreases to 86 m2 g-1, indicating partial pore are blocked during the lithium storage process. Generally speaking, annealing temperature is a crucial factor to get desired materials when using MOFs as precursors. To evaluate the effect of decomposition temperature, comparative experiments were performed by changing the annealing temperature at 600 and 700 ºC. The obtained corresponding porous Mn3O4/C composites were designated as Mn3O4/C-600 and Mn3O4/C-700, respectively. Apparently, both of the PXRD patterns confirm the tetragonal Mn3O4 phase after thermal transformation process (Figure S13), which is similar to that of Mn3O4/C-500. It can be clearly found that the Mn3O4/C-500 electrode displays much better cycling performance than those of the Mn3O4/C-600 and Mn3O4/C-700 electrode (Figure S14). As depicted from the SEM images (Figure S15), some agglomerates are observed at 600 ºC. At a higher temperature of 700 ºC, the initial morphology was found to collapse and non-regular shapes were generated. In addition, Figure 6a displays the rate performance of Mn3O4/C500 electrode under different current densities. Corresponding discharge capacities at 0.2, 0.5, 1 and 2 A g-1 are 1032, 946, 848, and 695 mAh g-1, respectively. Especially, the capacity can recover back to the similar value as the original one when returns to 0.2 A g-1, demonstrating the excellent

capacity of the electrode under all conditions was stabilized and coulombic efficiency maintained almost 100% after initial cycle. Especially, a reversible capacity of 1032 mAh g-1 still retained after 500 cycles at 0.2 A g-1, which confirms the reusability of the Mn3O4/C composite electrode. This observed specific capacity is higher than the theoretical value of Mn3O4 (937 mAh g − 1) and other Mn3O4 hybrid anode materials reported previously (Table S3). Moreover, in comparison with the only example of MOFs-derived porous Mn3O4 materials, our Mn3O4/C electrode shows much higher reversible capacity and more superior rate capacity.45 Additional, we also observe the electrochemical behavior of pure carbon material by using Mn-PBA as precursor. PXRD pattern shows two characteristic peaks at 24o and 43o (Figure S8), indicating the successful synthesis of carbon material. As depicted in Figure S9, the carbon electrode exhibits a discharge capacity of 516 mAh g-1, which is higher than the theoretical capacity of commercial graphite material. We believe that the advantage of structural and compositional features contribute to the improvement of capacity. To get further insight into electrochemical performance, electrochemical impedance spectra (EIS) of Mn3O4/C after 500 cycles were also investigated (Figure 5d). All Nyquist curves consist of a semicircle (high-frequency region) related to the charge-transfer resistance (Rct) and an inclined line (low-frequency) indexed to the diffusion rate of lithium ion. The values of Rct is smaller for Mn3O4/C electrode after the 500th cycle than that before cycling (before cycling: 59.94 Ω; after cycling: 48.98 Ω), indicating that the fast charge transport during the lithium intercalation/deintercalation 5

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*E-mail: [email protected].

electrochemical reversibility. However, Mn3O4/C-600 and Mn3O4/C-700 samples display much lower capacities of only 598 and 482 mAh g-1 at the high current of 2 A g-1. Meanwhile, the capacity of these two samples drops quickly. Besides, the impedance related with the Rct in Mn3O4/C-500 is lower than that of the Mn3O4/C-600 and Mn3O4/C-700 (Figure 6b), demonstrating the superior conductive capability of the electrode.48,49 This results also explain the better electrochemical property of the Mn3O4/C-500 electrode.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grants 21401059, 21471061, and 21671071), Applied Science and Technology Planning Project of Guangdong Province (2017A010104015 and 2015B010135009), Natural Science Foundation of Guangdong Province (2014A030311001), Innovation Team Project of Guangdong Ordinary University (No. 2015KCXTD005), and the Great Scientific Research Project of Guangdong Ordinary University (No. 2016KZDXM023).

REFERENCES 1 2 3 4 5 6

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10 Figure 6. Mn3O4/C-500, Mn3O4/C-600 and Mn3O4/C-700 electrodes. (a) Rate capability at various current densities. The first fifth discharge capacities were omitted. (b) Nyquist plots before cycles.

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4. CONCLUSION In summary, a mesoporous Mn3O4/C microspheres was successfully fabricated via one-step calcination of Mn-PBA as templates under air atmosphere. The resultant Mn3O4/C electrode exhibited high reversible specific capacity (1032 mAh g-1 at 0.2 A g-1) together with stable cycling behavior and good rate performance. The improved lithium storage is supposed to benefit from the unique microspheres and large accessible specific area. The remarkable performances make it a promising anode material for LIBs.

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Supporting Information 20

Details of experimental and characterization, XRD patterns, TGA curve, IR spectra, 1H NMR spectrum, SEM/TEM images, and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author 23

*E-mail: [email protected].

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For Table of Contents Use Only Mesoporous Mn3O4/C Microspheres Fabricated from MOF template as Advanced Lithium-Ion Battery Anode Hai-Jun Peng,† Gui-Xia Hao,‡ Zhao-Hua Chu,‡ Jia Lin,† Xiao-Ming Lin,*,†,§ and Yue-Peng Cai*,†

Mesoporous Mn3O4/C micropheres were obtained by thermolysis of a Mn-PBA MOF as precursor under air atmosphere and used as LIBs anode material. The results display that Mn3O4/C electrode shows a high capacity about 1032 mAh g-1 at 0.2 A g-1 after 500 cycles, together with the stable cycling behavior and good rate performance.

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