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Tuning the Morphologies of MnO/C Hybrids by Space Constraint Assembly of Mn-MOFs for High Performance Li Ion Batteries Dan Sun,† Yougen Tang,† Delai Ye,‡ Jun Yan,† Haoshen Zhou,*,§ and Haiyan Wang*,† †

College of Chemistry and Chemical Engineering, Central South University, 410083 Changsha, P. R. China Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, St. Lucia, QLD 4072, Australia § Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Umezono, Tsukuba 305-8568, Japan ‡

S Supporting Information *

ABSTRACT: Morphology controllable fabrication of electrode materials is of great significance but is still a major challenge for constructing advanced Li ion batteries. Herein, we propose a novel space constraint assembly approach to tune the morphology of Mn(terephthalic acid) (PTA)-MOF, in which benzonic acid was employed as a modulator to adjust the available MOF assembly directions. As a result, Mn(PTA)-MOFs with microquadrangulars, microflakes, and spindle-like microrods morphologies have been achieved. MnO/C hybrids with preserved morphologies were further obtained by selfsacrificial and thermal transformation of Mn(PTA)-MOFs. As anodes for Li ion batteries, these morphologies showed great influence on the electrochemical properties. Owing to the abundant porous structure and unique architecture, the MnO/C spindle-like microrods demonstrated superior electrochemical properties with a high reversible capacity of 1165 mAh g−1 at 0.3 A g−1, excellent rate capability of 580 mAh g−1 at 3 A g−1, and no considerable capacity loss after 200 cycles at 1 A g−1. This strategy could be extended to engineering the morphology of other MOF-derived functional materials in various structure-dependent applications. KEYWORDS: Li ion battery, MnO/C anode, metal−organic frameworks, morphology tuning, space constraint assembly

1. INTRODUCTION Li ion batteries (LIBs) with long cycling life and high reversible capacity are considered as clean, versatile, and promising power sources to meet the rapid development of electric vehicles and portable electronic devices.1,2 With the aim to further upgrade the energy density of current LIBs, new types of anode materials with high capacity and desired stability are essential to substitute for current graphite anode whose theoretical capacity is 372 mAh g−1.3 Transition metal oxides with a conversion reaction mechanism are promising anodes to fulfill the requirements of next-generation LIBs by virtues of their high reversible capacity and low cost.4−6 Among the transition metal oxide anodes for LIBs, MnO has been attracting more and more attention due to its natural abundance, environment benignity, low operation potential (1.032 V, vs Li+/Li), and high theoretical capacity (756 mAh g−1).2,3,7,8 However, the drastic capacity fading from the large volume expansion during the repeated charge/discharge process and the low rate capability resulting from the kinetic limitations have restricted the pure MnO from practical application.2,3,9 To enhance the electrochemical properties of MnO, some effective strategies such as nanocrystallization, carbon coating, advanced carbon matrixes blending, and © 2017 American Chemical Society

hollow/porous micro/nano architecture design have been proposed.3 MnO/C core/shell nanorods,10 hollow porous MnO/C microspheres,2 MnO/graphene,11 and MnO/carbon nanopeapods3 with superior electrochemical properties have been achieved recently. It is well-known that the morphology and architecture of electrode material greatly contribute to the electrochemical properties,12,13 which compel us to construct prospective MnO/C composites by tuning their morphology and architecture to further improve the cycling stability for practical application. Recently, metal−organic frameworks (MOFs) have demonstrated good potentials in a broad range of practical applications including separation, catalyst, chemical sensors, and drug delivery on account of their facile preparation, abundant porosity, and huge surface area.14−17 In addition, there have been growing efforts on using MOFs as sacrificial templates to design nano/micro anode materials with various morphologies for LIBs, such as spindle-like mesoporous α-Fe2O3,18 porous ZnO/ZnFe2O4/C octahedra,19 porous anatase TiO2,20 mesoReceived: November 18, 2016 Accepted: January 19, 2017 Published: January 19, 2017 5254

DOI: 10.1021/acsami.6b14801 ACS Appl. Mater. Interfaces 2017, 9, 5254−5262

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ACS Applied Materials & Interfaces porous nanostructured Co3O4,21 MnO nanoparticles confined in porous carbon framework,22 etc. The porous ZnO/ ZnFe2O4/C octahedral with hollow interiors derived from MOF precursor demonstrated excellent rate performance of 762 mAh g−1 at 10 A g−1.19 However, all these fabrication approaches are based on the direct calcination of some wellreported MOFs precursors. To our best knowledge, tuning the morphology of electrode materials via adjusting the complex structure or assembly process of MOF precursor has never been reported before. In this work, a new space constraint assembly (SCA) strategy was developed to control the morphology of Mn(PTA)-MOF precursor. By adjusting the amount of benzonic acid modulator, three different morphologies of Mn(PTA)-MOFs including microquadrangular, microflake, and spindle-like microrod can be obtained. After thermal treatment, MnO/C hybrids with the corresponding morphologies were fabricated using MOFs as sacrificial templates. Moreover, the as-prepared MnO/C hybrids feature a unique hierarchical architecture in which MnO nanoparticles coated by a thin and uniform carbon layer were well embedded into carbon matrix. As anode in LIBs, MnO/C with various morphologies exhibited different electrochemical properties. And the spindle-like MnO/C microrods achieved the best performance with a high reversible capacity of 1165 mAh g−1 at 0.3 A g−1 and good cycling stability with no considerable capacity loss after 200 cycles.

employing the local functions appproximation (LDA) of Perdew and Wang (PWC).23 The energy of each configuration calculated after geometry optimization, in which the energy tolerance, maximum force, and maximum displacements are 2 × 10−5 Hartree energy, 2 × 10−3 Hartree energy/Å, and 0.05 Å, respectively. 2.3. Electrochemical Measurements. The active material, super P carbon, and poly(vinylidene fluoride) in a weight ratio of 80:10:10 were first mixed using N-methyl-2-pyrrolidone as solvent. The asformed slurry was cast onto Cu foil using the doctor-blade technique. After drying at 110 °C overnight under vacuum, the Cu foil with active material was cut into disk electrodes. The mass density of active material in each electrode was 2.0−2.5 mg cm−2. The CR2016 cointype cells were assembled in an Ar-filled Mikrouna glovebox by using MnO/C electrode as cathode, 1 M LiPF6 in 1:1 v/v dimethyl carbonate/ethylene carbonate (Luoyang Dasheng Materials Technology Co., Ltd.) as electrolyte, and Li metal as counter electrode. After at least 4 h rest, the cells were tested between 0 and 3 V (vs Li+/Li) in a Neware battery testing system at 30 °C. A CHI 660e electrochemical station (Shanghai Chenhua, China) was employed to perform the cyclic voltammetry (CV) test with a sweep rate of 0.3 mV s−1 at room temperature. A Princeton workstation (PARSTAT2273, EG&G, US) was used to record the electrochemical impedance spectroscopy (EIS) over the frequency range from 100 kHz to 10 mH with an amplitude of 5 mV. To confirm the cells have reached a stable state, the cells were first charged to 1.2 V and then held at least 1 h before the test.

3. RESULTS AND DISCUSSION Figure 1 demonstrates the main fabrication process of Mn(PTA)-MOF precursors with different morphologies. PTA

2. EXPERIMENTAL SECTION 2.1. Material Preparation. All the raw materials were analytically pure grade and used directly. 1.354 g of Mn(CH3COO)2·4H2O and a certain amount of benzoic acid (0, 0.674, 1.348, and 2.696 g, marked as solution I, II, III, and IV) were dissolved in 60 mL of dimethylformamide (DMF), respectively, and then stirred overnight. Note that the mole ratio of Mn2+ to benzoic acid is 1:0, 1:1, 1:2, and 1:4 for solution I, II, III, and IV, respectively. Then a certain amount of PTA (2.75, 2.018, 1.834, and 0.917 g) was added to solution I, II, III, and IV, respectively. After that the solution was poured into a 100 mL Teflon lined stainless steel autoclave and reacted at 180 °C under constant stirring for 10 h, followed by a slow cooling down at room temperature. A white precipitate was collected by centrifuge and then thoroughly washed with ethanol and distilled water several times before drying at 80 °C overnight. MnO/C hybrids were finally obtained by sintering the white precipitates at 600 °C for 2 h with a ramping rate of 5 °C/min in a mixed flow of H2/Ar (5:95, v/v). Note the white precipitates are marked as MOF-Mn(PTA)B0, MOFMn(PTA)B1, MOF-Mn(PTA)B2, and MOF-Mn(PTA)B4 from solution I, II, III, and IV, and the as-obtained samples are marked as MnO-B0/C, MnO-B1/C, MnO-B2/C, and MnO-B4/C, accordingly. 2.2. Characterizations. The structure of the samples was performed by X-ray diffractometer (Bruker, D8 Discover 2500) using a Cu Kα1 source. Scanning electron microscope (SEM) images were performed on a Nova NanoSEM 230 SEM. Transmission electron microscope (TEM) and high resolution TEM (HRTEM) images were obtained using a FEI Tecnai G2 F20 S-TWIX TEM. Raman spectra were collected with LabRAM Aramis (HORIBA Jobin Yvon) spectrometer. Thermogravimetric analysis (TGA) was conducted on a STA 449C with a temperature range of 10−800 °C and a heating rate of 10 °C/min. X-ray photoelectron spectroscopy (XPS) measurement was applied on the ESCALAB 250Xi (Thermo Fisher) with Al Kα radiation at 6 mA and 12 kV. The XPS fitting was performed using XPSPEAK software. The Brunauer−Emmett−Teller (BET) surface area was measured by nitrogen adsorption/desorption on a Builder SSA-4200 instrument. The Fourier transform infrared (FTIR) spectra were recorded on Nicolet 6700. The ultraviolet (UV) spectra were recorded on a UV1780 (Shimadzu). Calculations are performed through density functional theory (DFT) implemented in the DMol3 package of Materials Studio,

Figure 1. Schematic diagram of the preparation strategies of Mn(PTA)-MOF precursors with different morphologies: (a) MOFMn(PTA)B0, (b) MOF-Mn(PTA)B1, (c) MOF-Mn(PTA)B2, and (d) MOF-Mn(PTA)B4. The hydrogen atoms are omitted for better observation.

is a common bridge ligand for MOF fabrication with center ions like Mn2+, Fe2+, Gd3+, and so on.18,24,25 Normally, the Mn2+ central metal is six-coordinated with octahedron geometry,26 and the assembly process of Mn(PTA)-MOF in solution was based on the coordination of PTA with Mn2+ from 5255

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ACS Applied Materials & Interfaces six directions (Figure 1a).24 When the benzoic acid modulator was added in advance, the −COOH of benzoic acid would be deprotonated and coordinate with Mn2+ as verified by the UV spectra as shown in Figure S1. Because the benzoic acid has single −COOH and is unable to act as a bridge ligand (no product was obtained when Mn2+ and benzoic acid reacted at the same condition in our previous experiment), the MOF assembly process and the resulting structure will be much different. If the mole ratio of benzoic acid to Mn2+ is 1:1, the benzoic acid prefers to occupy one vertice of the octahedral field (y-axis, for example) and PTA could coordinate with Mn2+ from xy plane and −y direction (Figure 1b). If the ratio of benzoic acid to Mn2+ is 2:1, the benzoic acid trends to occupy the opposite vertex of the octahedral field because of the steric factor, resulting in the assembly of Mn(PTA)-MOF in a plane (Figure 1c). When the ratio of benzoic acid to Mn2+ further increases to 4:1, the benzoic acid will occupy four vertices of plane and the assembly of Mn(PTA)-MOF would be limited to the axis which is normal to the plane (Figure 1d). Thus, the morphologies of Mn(PTA)-MOF are expected to be controlled through such a space constraint assembly approach.27−29 To well estimate the molecular conformations of different ratio of benzoic acid to Mn2+, which is critical for the as-resulted structure of MOF precursor, a simple theoretic calculation was performed. To evaluate the stability of different molecular conformations, a reaction enthalpy (Er) was termed according to formula 1 (Supporting Information), which could be expressed by eq 1 (Supporting Information). If the ratio of benzoic acid to Mn2+ is 2:1, there are two types of possible coordination geography. The calculation results show that trans-Mn(DMF)4B2 in which the benzoic acid occupy diagonal sites has a lower Er (about 16.4 kJ/mol lower) than that of the cis-Mn(DMF)2B4 (Figure 2a). When the ratio of benzoic acid to Mn2+ increases to 4:1, two possible coordination models are also found, and the Er of trans-Mn(DMF)2B4 consisting of four

benzoic acids occupying the plane is 12.5 kJ/mol lower than the other (Figure 2b). The calculation results reveal that the transMn(DMF)4B2 and trans-Mn(DMF)2B4 are much more energy favorable and stable, which demonstrate the feasibility of the formation mechanism proposed in Figure 1. The morphologies of MOF-Mn(PTA)B0, MOF-Mn(PTA)B1, MOF-Mn(PTA)B2, and MOF-Mn(PTA)B4 are investigated by SEM. As shown in Figure 3a, MOF-Mn(PTA)B0 consists of

Figure 3. SEM images of (a) MOF-Mn(PTA)B0, (b) MOFMn(PTA)B1, (c) MOF-Mn(PTA)B2, and (d) MOF-Mn(PTA)B4.

microquadrangular with an average particle size of ∼5 μm and thickness of 1−2 μm. After the addition of benzoic acid, the morphology of Mn-MOF obviously changes. When the ratios of benzoic acid to Mn2+ are 1:1 and 2:1, both Mn-MOFs display microflake-like morphology with a size of 2−3 μm as observed in MOF-Mn(PTA)B1 (Figure 3b) and MOFMn(PTA)B2 (Figure 3c). For MOF-Mn(PTA)B4 with the ratio of 4:1, a microrod-like morphology with average length of 3−5 μm is observed probably due to the suppressed assembly at the xy plane. The SEM results provide good support to the proposed assembly mechanism in Figure 1. Crystal structures of as-prepared MOFs were investigated by XRD. Like previous reported Mn-terephthalic acid MOF, MOF-Mn(PTA)B0, MOF-Mn(PTA)B1, MOF-Mn(PTA)B2, and MOF-Mn(PTA)B4 (Figure 4a) show very similar XRD patterns with high crystallinity.24 The disappearance of benzoic acid in as-obtained MOF may be related to the ligand exchange process during ripening.30 With the increase of benzoic acid, the diffraction peak at ∼9.5° (2θ) shifts obviously to low angle area (as enlarge in Figure 4b) and some other weak peaks (as marked by asterisks) disappear. In addition, the changes in peak intensity and position as marked in red can be also observed, indicating the minor differences of these MOFs in structure. Although the exact structure information on Mn-MOFs is unavailable (we have not obtained the detailed structure information on single crystal Mn-MOFs though many efforts have been tried), their differences in structure and morphology can be clearly identified in this work, which are closely related to different assembly process and show important effect in determining the structure and morphology of the final products. The FTIR spectra (Figure S2) of as-prepared

Figure 2. Possible molecular structures when the ratios of benzoic acid to Mn2+ are 2:1 (a) and 4:1 (b). The DMF and B refer to the solution molecular (dimethylformamide) and deprotonated benzonic acid, respectively. 5256

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Figure 4. XRD patterns of MOF-Mn(PTA)B0, MOF-Mn(PTA)B1, MOF-Mn(PTA)B2, and MOF-Mn(PTA)B4 (a). (b) is the magnified image of the marked part in (a).

MOFs indicate that the chemical compositions of these four samples are basically the same. The corresponding MnO/C composites were obtained by calcining the MOF-Mn(PTA)B0, MOF-Mn(PTA)B1, MOFMn(PTA)B2, and MOF-Mn(PTA)B4 and named as MnO-B0/ C, MnO-B1/C, MnO-B2/C, and MnO-B4/C, respectively. Figure S3 gives the SEM pictures of as-prepared MnO/C. All the MnO/C composites well inherit the morphologies of their precursors after sintering. Compared to the MOF precursor, the MnO/C slightly shrinks due to the volume contraction during the high-temperature calcination. To better investigate the architecture of as-prepared MnO/ C, TEM is also applied (Figure 5). Because of the large size, only the corners of MnO-B0/C (Figure 5a) and MnO-B1/C (Figure 5c) were observed. The MnO-B2/C displays typical microflake morphology with length of ∼3 μm and width of ∼1 μm (Figure 5e). Spindle-like microrod with length of ∼4 μm and diameter of ∼0.3 μm is observed for the MnO-B4/C in Figure 5g and Figure S4. From Figure 5a,c,e,g we can also find that MnO (∼100 nm) are well confined into the carbon matrix, which is further confirmed by the STEM-EDS mapping of MnO-B2/C (Figure S5). The HRTEM images of MnO/C hybrids (Figure 5b,d,f,h) demonstrate obvious lattice fringes with space of 0.22 nm corresponding to the (200) lattice plane of the cubic MnO. Interestingly, MnO nanoparticles are homogeneously coated by a uniform thin carbon layer (∼4 nm). It also reveals that the MnO particles are strongly fixed on the carbon matrix. Such unique architecture is much better than those obtained by conventional loading/coating methods such as physical coprecipitation/mixture of active materials on/with carbon matrix.31 The surface well-coated carbon layer here could not only ensure fast electron transfer on the electrode surface but also retain the electrode integrity, resulting in better electrochemical properties.31 The unique architecture should be ascribed to the MOF precursor, in which the Mn atoms are well coordinated by benzoic acid. TGA was employed to measure carbon contents of MnO/C samples. As demonstrated in Figure S6a−d, the carbon mass percentages for MnO-B0/C, MnO-B1/C, MnO-B2/C, and MnO-B4/C are 27.9%, 29.0%, 27.1%, and 29.1%, respectively. The surface area and porosity of as-prepared MnO/C hybrids are displayed in Figure 6. The surface areas of MnOB0/C, MnO-B1/C, MnO-B2/C, and MnO-B4/C are 216, 220, 219, and 309 m2/g, respectively, which are much larger than

Figure 5. TEM images of as-prepared MnO/C samples. (a), (c), (e), and (g) are the TEM images of MnO-B0/C, MnO-B1/C, MnO-B2/C, and MnO-B4/C, respectively. (b), (d), (f), and (h) are the HRTEM images of MnO-B0/C, MnO-B1/C, MnO-B2/C, and MnO-B4/C, respectively.

those reported, such as porous MnO/C microspheres (76.9 m2/g, carbon content 23.5%),2 MnO/carbon nanopeapods (103 m2/g, carbon content 16.1%),3 amorphous MnOx-carbon nanocomposites (24.2 m2/g, carbon content 39%),9 MnO/ graphene (50 m2/g, graphene content 17.4%),32 and MnOx/ ordered mesoporous carbon nanorod (178 m2/g, carbon content 31.6%).33 The large BET surface area for MnO/C here should be attributed to the abundant porous structure derived from the MOF precursor. As seen from Figure 6 (inset), these four samples exhibit the similar pore size of 10− 20 nm. It is well-known that mesoporous structure could accommodate the strain/stress during the cycling and improve the electrode/electrolyte contact, leading to better electrochemical properties.19 To gain a deep insight into the structure of the carbon layer, Raman spectroscopy was performed (Figure S7). The bands centered at 1341 and 1596 cm−1 are 5257

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Figure 6. N2 adsorption−desorption isotherm of as-prepared MnO/C hybrids: MnO-B0/C (a), MnO-B1/C (b), MnO-B2/C (c), and MnO-B4/C (d). Inset pictures are the related pore size distribution plots.

assigned to the inplane vibrations of disordered amorphous carbon (D band) and crystalline graphic carbon (G band), respectively.2,34 Higher intensity ratio of G-band/D-band value reveals better graphitization for carbon matrix or carbon layers.2 XRD patterns of MnO/C hybrids are presented in Figure 7. All the as-prepared samples demonstrate similar diffraction

XPS of as-prepared MnO-B4/C was further investigated (Figure 8a). The peaks of Mn (2p, 2s, 3s, and 3p), O 1s, and C 1s can be observed. The high-resolution XPS of Mn 2p after fitting is shown in Figure 8b. The main peaks centered at 641.3 and 652.9 eV are assigned to Mn(II) 2p3/2 and 2p1/2,13 and the minor peaks located at 643.4 and 654.0 eV indicate the existence of certain amount of Mn(III),11 in agreement with XRD to suggest the existence of tiny Mn3O4. Peaks at 530.1, 531.6, and 533.7 eV (Figure 8c) are ascribed to the O in Mn− O, C−O, and CO, respectively,8 and the peak at 284.6 eV (Figure 8d) is related to C 1s.38−40 Figure S8 shows the typical CV curves at 0.2 mV s−1. In the first cathodic scan, the peak around 0.70 V corresponds to the irreversible reduction of electrolyte and the formation of a solid electrolyte interphase layer.10 The sharp reduction peak near 0.10 V involves the reduction of Mn2+ to Mn0.41−43 It shifts to 0.25 V in the subsequent cycles, indicating an irreversible phase transformation that leads to the formation of metallic manganese and Li2O.3 The wide oxidation peak at about 1.27 V in the anodic sweep should correspond to the oxidation of Mn0 to Mn2+.11 The small oxidation peak at 2.10 V may originate from the reoxidation of Mn(II) to a higher oxidation state benefiting from the fast Li reaction kinetics and synergistic effects of carbon and MnOx.11,36 The electrochemical properties of MnO/C hybrids for LIB were further investigated. As compared in Figure 9a, the morphologies play a significant role in the electrochemical properties of MnO/C. The microrod-like MnO-B4/C demonstrates the highest reversible capacity of 1165 mAh g−1 at a current density of 0.3 A g−1. Even after 50 cycles, a discharge capacity of 1120 mAh g−1 is still maintained, which is much higher than that of MnO-B0/C (532 mAh g−1), MnO-B0/C (631 mAh g−1), and MnO-B2/C (869 mAh g−1). The charge− discharge curves of MnO-B4/C microrods at 0.3 A g−1 during 0−3 V are shown in Figure 9b. There are two plateaus located at 0.71 and 0.25 V in the first discharge curve, which match well with the CV results. The second cycle displays a big capacity

Figure 7. XRD patterns of as-prepared MnO-B0/C, MnO-B1/C, MnO-B2/C, and MnO-B4/C.

patterns, which can be well indexed to the cubic phase of MnO (JCPDS #07-0230).35 The XRD results also illustrate that the slight differences in crystal structure between the MOF precursors have basically no influence on the final structure of MnO/C hybrids after calcination. Several impurity peaks marked by asterisks are found to be probably the Mn3O4 phase. From the XRD results, the phase ratio of Mn3O4 in the MnOB0/C, MnO-B1/C MnO-B2/C, and MnO-B4/C sample are 4.0%, 7.7%, 8.2%, and 11.2%, respectively. As reported, MnO is easily transformed to Mn3O4 due to the local heating influence or photochemically induced transformation under beam irradiation.36 Considering Mn3O4 also shows good electrochemical performance, such a slight amount of Mn3O4 impurity is acceptable.37 5258

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Figure 8. XPS spectra of MnO-B4/C: survey spectrum and high-solution (a), Mn 2p(b), O 1s (c), and C 1s (d) spectra.

Figure 9. Cycling performance of MnO/C samples at 0.3 A g−1 (a). Charge/discharge curves of MnO-B4/C at 0.3 A g−1 (b). Cycling performance of MnO/C samples at 0.6 A g−1 (c). Rate performance of MnO-B4/C at different current densities (d). Cycling performance and Coulombic efficiency of MnO-B4/C at 1 A g−1 (e). Electrochemical impedance spectroscopy (EIS) of MnO-B0/C, MnO-B1/C, MnO-B2/C, and MnO-B4/C (f). The electrodes were charged to 1.2 V and held 2 h to retain a stable state before testing.

loss caused by the formation of SEI.42,44,45 From the second cycle, the charge/discharge curves display two obvious platforms at 0.5 and 1.2 V, which remain very well during the following cycles, demonstrating the outstanding cycling stability. When the current density increases to 0.6 A g−1, a reversible capacity of 936 mAh g−1 is achieved for MnO-B4/C microrods. When the cycling continues, the discharge capacity gradually increases to the maximum value of 994 mAh g−1 at the 50th cycle and maintains 804 mAh g−1 after 100 cycles, much higher than those of MnO-B0/C (281 mAh g−1), MnOB1/C (554 mAh g−1), and MnO-B2/C (670 mAh g−1). Actually, it is very common to see a higher specific capacity than

theoretical value and capacity rising upon cycling for MnO/C, which may be due to an activation process and the reversible formation of organic polymeric/gel-like films at low potentials.46 Moreover, the reversible oxidation/reduction of certain amount high valence Mn in MnO/C hybrids here may contribute to the high capacity.46 According to previous reports, a high manganese oxidation state can be anticipated at a fast Li+ reaction kinetics.32,47 Considering the superior electrochemical properties of MnO-B4/C, we further investigated its rate capability. As given in Figure 9c, MnO-B4/C microrods deliver the discharge capacities of 1102, 880, and 682 mA h g−1 at 0.3, 1, and 2.5 A g−1. Even at 3 A g−1, the electrode 5259

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Figure 10. SEM images of MnO-B4/C electrode surface after 1 cycle (a) and 50 cycles (b) at 0.3 A g−1. (c) is the SEM image of MnO-B4/C after 50 cycles and soaking in NMP to remove PVDF. (d) represents the charging/discharging mechanism of MnO-B4/C.

still shows a high capacity of 584 mAh g−1. The rate capability of MnO-B4/C microrods is much better than many other reported MnO composites, including MnO@carbon core−shell nanowire (462 mA g−1 at 2 A g−1), porous MnO/C microspheres (234.7 mAh g−1 at 3 A g−1), amorphous MnOx−carbon nanocomposites (500 mAh g−1 at 800 mA g−1), and MnO/reduced graphene oxide composites (160 mAh g−1 at 1.6 A g−1). The MnO-B4/C also exhibits outstanding cycling stability (Figure 9e). It delivers a reversible capacity of 800 mAh g−1 at 1 A g−1 and almost no capacity fading from the second cycle to the 200th cycle. The corresponding Coulombic efficiency is also shown in Figure 9e. It is 56% at the first cycle for MnO-B4/C but increases to above 98% and keeps stable in the subsequent cycles. The EIS results of MnO-B0/C, MnOB1/C, MnO-B2/C, and MnO-B4/C also consistent with their electrochemical performance. The Nyquist plots (Figure 9f) demonstrate two depressed semicircles from medium to high frequency range. Generally, the small intercept represents the solution impedance of the cells (Rs). The high frequency semicircle (Rf) should be ascribed to the interface parameters. The mediate semicircle is due to the charge-transfer resistance (Rct) from a number of factors like electronic conductivity, crystal structure, the electrode surface condition, and the interparticle contacts.39,47 Accordingly, the suppression of Rct was considered as an important factor for the better electrochemical properties in previous literature.40,48,49 The slope line at low frequency are attributed to the Warburg impedance (ZW) which represents the diffusion of Li ions in the solid matrix. As seen in Table S1, the Rct of MnO-B4/C is much smaller than that of the other electrodes, which is consistent with its best electrochemical properties. It has been well accepted that the huge volume expansion is a vital factor for capacity fading of metal oxide anode for Li ion battery. The expansion would lead to the contact loss between active materials and conductive agents and peeling off of active materials from current collector. To better understand the

excellent cycling stability of MnO-B4/C microrods, crystal structure and surface morphology change of MnO-B4/C electrodes after 1 and 50 cycles were investigated. As seen in Figure S9, the crystallinity of MnO decreases drastically, and it converts to amorphous phase after 50 cycles following its conversion reaction mechanism.3,50 Figure 10 is the SEM images of MnO-B4/C after 1 cycle (a) and 50 cycles (b). The surface of MnO-B4/C electrode basically remains after 50 cycles. To better observe the MnO-B4/C microrods after 50 cycles, we washed the PVDF binder away from the electrode in NMP solution. As shown in Figure 10c, the architecture of MnO/C microrods is still well maintained (highlighted by red arrows). It is believed that mesoporous microrod matrix as well as thin and uniform carbon layer in this unique architecture could provide dual protection to MnO despite the huge volume change and crystal pulverization during the cycling, thus guaranteeing the electrode integrity. The amorphous MnOx dispersed in carbon matrix was also reported to demonstrate superior electrochemical properties.49 The superior electrochemical properties of MnO-B4/C should be attributed to its unique microrod-like morphology obtained by this space constrained approach. As illustrated in Figure 10d, the microrod-like MnO/C could allow the fast lithium ion diffusion into the electrode along all directions and provide a good conductive pathway for electron transfer. In addition, the abundant porous structure derived from MOF precursor provides a high BET surface area of 309 m2 g−1, which substantially enhances the electrode/electrolyte contact and well accommodates the volume expansion during the cycling.22,51 Finally, the carbon matrix and the outer thin carbon coating layer on MnO could well maintain the electrode integrity during the cycling, leading to the improvement of cycling stability. 5260

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Research Article

ACS Applied Materials & Interfaces

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4. CONCLUSION In summary, a novel SCA approach has been demonstrated to successfully fabricate Mn(PTA)-MOF precursors with microquadrangulars, microflakes, and microrods morphologies, which were well inherited by MnO/C hybrids via a simple thermal treatment. Among these materials, the MnO/C microrods exhibited the best lithium storage performance with high reversible capacity (1165 mAh g−1 at 0.3A g−1), excellent rate capability (584 mAh g−1 at 3A g−1), and outstanding cycling stability (no obvious capacity fading after 200 cycles). The excellent electrochemical properties were attributed to the high porous structure and unique hierarchical architecture which could enhance the fast Li+/e− diffusion, accommodate the huge volume expansion, and keep the integrity of electrode during cycling. Considering the large family of MOFs and metal oxides, this SCA approach not only provides an efficient method to optimize the morphologies of electrode materials for better LIBs but may also shed a light on the morphology and structure design for other functional materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14801. UV spectra of Mn2+, benzoic acid, and Mn2+ + benzoic acid, IR spectra of as-prepared MOFs, SEM pictures, TGA curves, Raman spectra, CV curves, and impedance parameters of as-prepared MnO/C hybrids, STEM-EDS picture of MnO-B2/C, Coulombic efficiency of MnO-B4/ C and XRD patterns of MnO-B4/C electrodes after different cycles (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Z.). *E-mail: [email protected] (H.W.). ORCID

Jun Yan: 0000-0002-6158-0614 Haiyan Wang: 0000-0002-2242-6534 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (No. 21301193, No. 21271187, and No. 21571189), the Fundamental Research Funds for the Central Universities of Central South University, the Open-End Fund for Valuable and Precision Instruments of Central South University (CSUZC201622), and High Performance Computing Center of CSU, China.

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REFERENCES

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DOI: 10.1021/acsami.6b14801 ACS Appl. Mater. Interfaces 2017, 9, 5254−5262

Research Article

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