MnO Nanoparticles Sandwiched within 3D Graphene-Based

Feb 11, 2019 - MnO Nanoparticles Sandwiched within 3D Graphene-Based Hierarchical Architecture for Efficient Lithium Storage. Fangyan Cheng ...
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MnO Nanoparticles Sandwiched within 3D Graphene-Based Hierarchical Architecture for Efficient Lithium Storage Fangyan Cheng,† Xiangyang Zhou,† Juan Yang, Antao Sun, Hui Wang, and Jingjing Tang* School of Metallurgy and Environment, Central South University, Changsha 410083, China

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S Supporting Information *

ABSTRACT: Manganese monoxide (MnO) has drawn considerable attention as anode candidate for lithium-ion batteries (LIBs) due to its high theoretical capacity of 755.6 mAh g−1 (over twice as much as graphite) and relatively low voltage hysteresis. However, some challenging issues such as poor cyclic performance and inferior rate capability caused by the limited reaction kinetics, severe particle agglomeration of MnO, and large volume expansions during cycling still hampered its commercial implementation. Herein, we developed a rational design, in which MnO nanoparticles are sandwiched within 3D graphene-based N-doped carbon (NC) networks, which is denoted as NC/MnO/rGO. When investigated as anode for LIBs, the well-designed NC/MnO/rGO nanohybrid demonstrates high reversible capacity (1360 mAh g−1 at 0.2 A g−1 over 150 cycles), excellent rate capability, and good cyclability (648 mAh g−1 at 2 A g−1 without fading over 600 cycles). In addition, the mechanism of electrochemical reaction for the NC/MnO/rGO anode is further investigated by conducting cyclic voltammetry under different cutoff voltage ranges to explain the capacity increasing phenomenon upon cycling.

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INTRODUCTION The huge amount of fossil fuels combustion has threatened energy sustainability and brought unprecedented affluence to human society. To ensure social development, extensive efforts are being made to develop new energy storage devices and conversion systems. Owing to their high energy density, lack of memory effect, environmental benignity, and long lifespan, lithium-ion batteries (LIBs) have been dominated the field of portable electronics and hybrid electrical vehicles.1−4 As one of the key components, anode material plays a crucial role in the whole performance of LIBs. However, a commercial graphite anode could not meet the current demands for the development of advanced LIBs because of its limited capacity.5 Thus, tremendous efforts have been devoted to searching for a potential substitute with high specific capacity as well as good cyclability for LIBs.6 Among various anode materials, transition metal oxides (TMOs) (e.g., MnOx,7 Fe2O3,8 Co3O4,9 and NiO10) have attracted great attention owing to their relatively high theoretical capacities, affordable cost, abundant resources, and environmental benignity.11,12 Particularly, MnO is of great significance as anode for LIBs owing to its narrow voltage hysteresis, cost effectiveness, eco-friendliness, and lower operating potential compared with other metal oxides.13,14 However, the poor cyclability of a MnO electrode aroused by severe volume changes during conversion reaction and the reduced electrochemical reactivity ascribing to its low electronic conductivity, hindering its successful practical © XXXX American Chemical Society

implementation. Consequently, it is essential to enhance the mechanical strength and improve the conductivity of MnObased anode materials. So far, extensive efforts have been paid to addressing these issues to enhance the electrochemical performance of the MnO electrode. One appealing approach is constructing nanostructured MnO, such as MnO nanoparticles, nanotubes, nanospheres, and nanowires.15−19 In general, the nanostructured materials have large surface area to make adequate contact for electrode/electrolyte, and can shorten the path of Li-ion diffusion, therefore can significantly enhance the electrochemical performance to some degree.20 However, the preparations of nanostructured materials are usually complicated and costly, making it difficult to realize commercial applications. Another effective approach to obtain an enhanced electrochemical performance anode is hybridizing MnO with carbonaceous materials (e.g., graphene, porous carbon, carbon nanotubes, etc.), which act as buffering and conductive matrix for MnO.21,22 In addition, heteroatom-doped carbonaceous materials not only can improve the electric conductivity but also can increase reaction sites and reduce energy barriers for ion transport, thus improving the electrochemical performance of MnO.13,20,23,24 For instance, Dou et al.25 reported a nitrogen-doped graphene ribbon assembled core-sheath MnO@graphene scrolls (MnO@N-GSC/GR) composite Received: December 5, 2018

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DOI: 10.1021/acs.inorgchem.8b03390 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry anode, which could deliver a high capacity of 1202 mAh g−1 over 100 cycles at 0.2 A g−1. This capacity was much higher than that of MnO/rGO-based materials reported previously. Nevertheless, due to the loose combination of MnO and carbonaceous material, MnO particles tend to detach from the conductive substrate during the repeated lithiation/delithiation, resulting in unexpected lithium consumption and poor cycling stability of the electrode. Hence, a rational design should be proposed to realize the well combination of high capacity MnO and good conductive substrate for preventing the detaching of active MnO from the substrate and the aggregation of MnO. Herein, we synthesized MnO nanoparticles sandwiched within 3D graphene-based carbon networks (NC/MnO/rGO) through a straightforward wet chemical process and subsequent annealing treatment. The elaborately designed NC/MnO/rGO nanohybrid demonstrates superb electrochemical performances as anode for LIBs, including high reversible capacity of 1360 mAh g−1 at 0.2 A g−1 over 150 cycles, excellent rate capability, and stable cyclability (648 mAh g−1 at 2 A g−1 over 600 cycles). The capacity increasing upon cycling feature is further studied by conducting cyclic voltammetry under different cutoff voltage ranges. The impressive results should be ascribed to the well combination of MnO nanoparticles with a highly conductive graphene matrix and additional N-doped carbon protection layer. And the increased capacity shall be attributed to the multivalence states of Mn ions during cycling.

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Scheme 1. Schematic Illustration of the Synthetic Process of NC/MnO/rGO Composites

Subsequently, the obtained slurry was uniformly cast onto a copper foil, and then dried in a vacuum oven overnight. The mass loading on each electrode was about 1.0 mg cm−2. Electrochemical behaviors of the electrodes were performed with CR2025 coin cells assembled in a glovebox filled with an Ar atmosphere. Li metal foil was employed as the counter electrode, and the separator was microporous polypropylene film. The LiPF6 (1 M) dissolved in a mixture of dimethyl carbonate, diethyl carbonate, and ethylene carbonate (1:1:1 vol %) served as the electrolyte. The cyclic voltammetry (CV) plots were recorded within a voltage varied from 0.01 to 3.0 V, and the electrochemical impedance spectra (EIS) measurements were collected on a CHI 660D electrochemical workstation with the frequency range from 0.1 to 100 kHz.

EXPERIMENTAL SECTION

Preparation of NC/MnO/rGO. Graphene Oxide (GO) was prepared through Hummers’ method.26 Tris-GO/PDA (PDA stands for polydopamine) was synthesized according to our previous report.27 First, the as-synthesized Tris-rGO/PDA (0.18 g) was dispersed into deionized (DI) water (100 mL) via ultrasound to obtain solution A. And then, 0.169 g of KMnO4 was dissolved in solution A under vigorously stirring. After 30 min, the prepared precipitation was collected by washing with deionized water thoroughly, and further drying at 60 °C in an oven; the resultant sample was marked as MnO2/rGO/PDA. To obtain NC/MnO/rGO nanohybrids, the as-synthesized MnO2/rGO/PDA sample was thermally treated at 700 °C for 2 h under a H2/Ar (95:5, vol %) atmosphere. For comparison, the MnO/rGO composite was synthesized under the same manner without adding dopamine, and pure MnO was synthesized by annealing the reaction product of KMnO4 and Tris. The detailed synthetic process of NC/MnO/rGO is displayed in Scheme 1. Materials Characterization. The structures and morphologies of the composites were examined by a scanning electron microscope (SEM, Japan). Transmission electron microscopy (TEM, Japan) images, high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) images were observed on a JEM-2100F instrument. The elemental distribution was collected on an energy dispersive spectrometry (EDS) attached to the TEM instrument. Xray diffraction (XRD, Cu Kα) analysis of the as-prepared samples was determined by employing a Bruker D8 diffractometer. X-ray photoelectron spectroscopy (XPS, K-Alpha 1063) was applied to acquire information on the surface element composition and bonding state of the NC/MnO/rGO hybrid. Raman spectrum was conducted by a Raman spectrometer at room temperature. Thermogravimetric analysis (TGA, SDQ 600) of these samples was measured under air flowing. Electrochemical Measurements. The working electrodes were prepared for the battery experiments by dissolving active materials, acetylene black, and poly(vinylidene fluoride) binder (8:1:1 wt %) into N-methyl-2-pyrrolidone to form a homogeneous slurry.

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RESULTS AND DISCUSSION Synthesis and Characterization. The typical fabrication process for the NC/MnO/rGO nanohybrid is schematically illustrated in Scheme 1. First, the MnO2/rGO/PDA precursor was prepared by a wet chemical reaction. The XRD patterns showed (Figure S1) that all the diffraction peaks of the prepared MnO2/rGO/PDA precursors can be well ascribed to the δ-type MnO2 (JCPDS No. 18-0802). More details for MnO2/rGO/PDA are described in our previous study.27 The NC/MnO/rGO nanohybrids are finally obtained by annealing the precursors at 700 °C under a H2/Ar atmosphere for 2 h. As can be seen from Figure 1a, the precursors were completely transformed into cubic MnO (JCPDS No. 07-0230) in the reductive atmosphere. Additionally, no other characteristic peaks can be observed, which implied the high purity of these samples. The obtained XRD patterns for rGO and NC/rGO confirmed that GO was thermally reduced under a reductive atmosphere. Raman spectroscopy was conducted to further survey the graphitic degree of as-synthesized composites. As presented in Figure 1b, all these hybrids demonstrated two obvious peaks located at about 1346 and 1586 cm−1, which can be indexed to the D-band and G-band, respectively.28,29 The ID/IG ratio of rGO, NC/rGO, MnO/rGO, and NC/MnO/rGO nanohybrids are estimated to be 0.98, 1.04, 0.97, and 1.02, respectively. The higher value of ID/IG suggests that the defects increased after N doping, which can help provide more insertion sites for lithium storage and facilitate the Li+ B

DOI: 10.1021/acs.inorgchem.8b03390 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) XRD patterns and (b) Raman spectra of NC/MnO/rGO, MnO@rGO, NC/rGO, and rGO.

Figure 2. Characterizations of NC/MnO/rGO nanohybrids. (a) SEM, (b) TEM, (c) HRTEM, (d) SAED, and (e) EDS mapping images of NC/MnO/rGO.

transportation, thus improving electrochemical performance.14 In addition, the peak located at 650 cm−1 assigned to the MnO vibration proved the presence of MnO in the NC/MnO/rGO nanohybrid.30 The microstructure and typical morphology were observed by SEM and TEM. As shown in Figure S2a,b, the as-obtained Tris-GO/PDA displays a sheet-like structure. The morphologies of MnO2/rGO/PDA precursor shown in Figure S2c,d indicated that MnO2 is uniformly distributed on the rGO/PDA substrate. After the annealing treatment under a redox atmosphere, the as-prepared NC/MnO/rGO nanohybrid (Figure 2a) exhibits a similar sheet-like structure with many wrinkles and ripples, and a large amount of MnO nanoparticles are anchored on the graphene matrix. More detailed observation from the TEM image (Figure 2b) of the NC/MnO/rGO nanohybrid further demonstrates that the MnO particles with a size about 40 nm are uniformly sandwiched within the graphene-based carbon matrix. As depicted in the HRTEM image (Figure 2c), the well-defined lattice fringes with a lattice spacing of 0.26 nm correspond to the (1 1 1) plane of cubic MnO, which is consistent with the

XRD results. Furthermore, it can be observed that MnO nanoparticles are completely wrapped by a uniform amorphous carbon layer with a thickness about 2 nm, and then anchored on the graphene matrix. The SAED pattern (Figure 2d) reveals the crystallinity feature of MnO nanoparticles. The TEM elemental mapping (Figure 2e) demonstrates the distribution of C, Mn, O, and N in the NC/MnO/rGO hybrid. Moreover, the uniform distribution of C and N elements verified that the polydopamine is evenly coated on the surface of graphene oxide. The detailed chemical composition and surface oxidation state of the NC/MnO/rGO hybrid is investigated by XPS. From the XPS survey spectrum displayed in Figure 3a, the elements of C, Mn, O, and N are detected in the hybrid. Figure 3b exhibits the high-resolution Mn 2p spectrum, in which two apparent peaks located at 642.04 and 653.59 eV can be assigned to the Mn 2p3/2 and Mn 2p1/2, respectively; combined with the XRD and Raman analysis mentioned above, the presence of MnO can be confirmed.20,25 As exhibited in Figure 3c, the deconvolutions of the N 1s spectrum can be fitted into three peaks centered at 398.39, 400.19, and 404.64 eV, which C

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Figure 3. XPS survey spectrum of (a) NC/MnO/rGO, high-resolution of (b) Mn 2p, (c) N 1s spectra. (d) Representative N2 adsorption/ desorption isotherms, and inset is the pore size distribution curve of NC/MnO/rGO.

subsequent CV curves basically overlapped, indicating the good reversibility and capacity retention of the NC/MnO/rGO hybrid. Figure 4b demonstrates the representative galvanostatic discharge−charge curves of NC/MnO/rGO at 200 mA g−1. The electrode delivers high initial discharge and charge capacities of 1286 mAh g−1 and 851 mAh g−1, respectively, with a corresponding initial Coulombic efficiency (CE) of 66.2%. The initial capacity loss is mainly ascribed to the generation of solid electrolyte interphase (SEI) layers and electrolyte decomposition, which is corresponding to the CV analysis.43 The cyclic performance of the NC/MnO/rGO nanohybrid at 200 mA g−1 is investigated. As can be seen in Figure 4c, it delivered reversible capacities of 864, 926, 1146, 1393, and 1360 mAh g−1 over 10, 20, 50, 100, and 150 cycles. The capacity is gradually increased from the 10th to 100th cycle; after about 100 cycles, the capacity gradually tends to stabilize. This behavior of increasing capacity is probably due to the generation of higher oxidation state (Mnn+, n > 2) products and the enhanced transfer kinetics during cycling, which can contribute capacity.34,43 This is commonly detected in conversion electrode materials, and is mainly ascribed to the activation process and the improved reaction kinetics.13,17,43,44To further clarify the effects of the dual carbon matrix on the superior performance of the NC/MnO/rGO hybrids, MnO/rGO, NC/rGO and pure MnO samples were also synthesized and tested as anode materials. As revealed in Figure 4c, the MnO/rGO shows a similar growth trend as NC/MnO/rGO, but its growth rate and growth amplitude are not as high as NC/MnO/rGO, and it shows an apparent lower capacity. This suggests that NC plays a crucial role in improving the capacity and the cyclic performance of

correspond to pyridinic N, pyrrolic N, and graphitic N types of nitrogen doped in the hybrid, respectively.31−33 N-doping has a positive effect on enhancing the electronic conductivity of carbon-based material and hence improving the lithium storage performance.21,34,35 The elemental contents of N in the NC/MnO/rGO hybrid is 1.37 wt % based by the XPS analysis, which is originated from dopamine.36 The porous feature of the as-synthesized NC/MnO/rGO hybrid is further investigated by N2 adsorption/desorption isotherms, as shown in Figure 3d. The BET surface area is determined to be about 125.84 m2 g−1. The pore size distribution of NC/MnO/rGO nanoparticles is centered at 3.8 and 7.8 nm, and the average pore size is about 7.53 nm. Such a porous structure will be favor for alleviating the volume change of electrode and enhancing the diffusion of electrolyte and Li ion during cycling.37 Electrochemical Performance. The electrochemical properties of the as-synthesized NC/MnO/rGO hybrid are investigated as anode for LIBs. The CV profiles for the first five consecutive cycles are showed in Figure 4a. During the initial cathodic process, the reduction peak located at about 1.4 V can be observed, which could be attributed to the partial reduction of the oxidized Mn2+ on the surface of NC/MnO/rGO.21,38 This peak did not appear in the subsequent cycles, revealing that this reaction is irreversible. The peak close to 0.18 V is ascribed to the reduction of MnO to metallic Mn (MnO + 2Li+ + 2e− → Mn + Li2O) and the generation of solid electrolyte interface (SEI) layer.39,40 The peak then shifts to about 0.25 V at the subsequent cycles, higher than that in the first cycle, which is due to the enhanced reaction kinetics.41,42 During the first anodic process, the peak appearing at about 1.35 V is assigned to the formation of MnO and Li2O decomposition (Mn + Li2O → MnO + 2Li+ + 2e−).16,40 Moreover, the D

DOI: 10.1021/acs.inorgchem.8b03390 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Electrochemical performance of the NC/MnO/rGO electrode. (a) Cyclic voltammetry curves at a scan rate of 0.2 mV s−1. (b) Discharge−charge voltage profiles at 200 mA g−1. (c) Cycling performance at 200 mA g−1. (d) Rate capabilities at different current densities. (e) Long-term cycling performance at 2 A g−1.

Figure 5. (a) The galvanostatic voltage profiles and (b) the corresponding specific capacities of the NC/MnO/rGO anode measured at different cutoff voltages at 200 mA g−1.

E

DOI: 10.1021/acs.inorgchem.8b03390 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) The charge profiles at the voltage window of 0−2 V and 0−3 V, (b, c) CV curves (0−2 V) and (d, e) CV curves (0−3 V) of cycled cells.

cyclic performance and the corresponding CE of NC/MnO/rGO at a high density of 2 A g−1 after activation for the initial three cycles at 0.5 A g−1. It can be seen that the NC/MnO/rGO presents a high reversible capacity of 648 mAh g−1 within 600 cycles with a CE of over 97%. Apart from the excellent cyclic and rate performance, the capacity recoverability at different cutoff voltages should also be considered.46−48 In this respect, we evaluated the NC/MnO/rGO anode with varied cutoff voltages from 3 to 1.5 V at 200 mA g−1; the charge profiles at each cutoff voltage are displayed in Figure 5a, the good overlapping of these profiles suggesting that it has small memory effect.48 Figure 5b exhibits the corresponding specific capacities of the NC/MnO/rGO anode explored at different cutoff voltages at 200 mA g−1, demonstrating its stable cycling performance at each cutoff voltage. The different capacities exhibited at each cutoff voltage reflect the lithium intercalation extent at corresponding cutoff voltages. In addition, the NC/MnO/rGO anode exhibits similar capacities when cycling at the same cutoff voltage. For instance, when cycling at 0−2.5 V, the average capacity at 10th −20th cycles and at 50th −60th are both about 800 mAh g−1 after suffering the cutoff voltage of 0−

NC/MnO/rGO. The TGA curves shown in Figure S3 exhibit that the total weight loss in MnO/rGO and NC/MnO/rGO is estimated to be 10.4 and 15.2 wt %, respectively, which is ascribed to the collective effects of the weight loss results from combustion of carbon to CO2, and the weight gain arises from oxidation of MnO to Mn2O3.14,45 On the basis of the theoretical value (11.27 wt %) of weight gain from MnO to Mn2O3, the carbon contents of MnO/rGO and NC/MnO/rGO are calculated to be about 19.45 and 23.76 wt %, respectively. Furthermore, the rate capability of the NC/MnO/rGO electrode exhibited in Figure 4d is further explored at different current densities ranging from 0.1 to 2 A g−1. The capacity declines regularly with the current density increasing, and the average capacity of NC/MnO/rGO at 0.1 A g−1 is about 950 mAh g−1. Even when reaching at relatively high rates of 0.2, 0.5, 1.0, and 2.0 A g−1, the NC/MnO/rGO electrode can still deliver reversible capacities of 880, 810, 760, and 650 mAh g−1, respectively. When the current density is turned back to 0.1 A g−1 after the superfast charge/discharge at 2 A g−1, a capacity of 1020 mAh g−1 is recovered, suggesting a good lithium storage reversibility. Figure 4e demonstrates the long-term F

DOI: 10.1021/acs.inorgchem.8b03390 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) EIS curves (inset is the corresponding fitted equivalent circuit model). (b) The corresponding Z′−ω−1/2 patterns of pure MnO, MnO/rGO, and NC/MnO/rGO electrodes.

DLi + = (R2T 2)/(2A2 n 4F 4C 2σ 2)

2 V and 0−1.5 V. Namely, the lithium storage is recoverable after experiencing different cutoff voltages, indicating the good capacity recoverability and small memory effect. The excellent lithium storage properties make it have great potential for LIBs to realize practical application. Mechanism Analysis. From the above results, it can be found that the NC/MnO/rGO hybrid exhibits an apparent capacity increasing over a hundred subsequent cycles, reaching up to 1393 mAh g−1 over 100 cycles at 0.2 A g−1, and then tends to stabilize. To investigate the reason for the capacity increasing phenomenon of NC/MnO/rGO, the cycling performance under different voltage ranges and the CV of cycled cells were studied. Figure 6a exhibits the reversible capacity of the NC/MnO/rGO anode at cutoff voltages of 0−3 V and 0−2 V. When cycled at the voltage window of 0−2 V, the capacity decreases upon cycling and stabilizes at ∼460 mAh g−1 after 20 cycles. From the CV curves (0−2 V) before cycling (Figure 6b) and after 20 cycles (Figure 6c), there is no obvious change in the redox peaks; this is owing to the fact that the cutoff voltage of charge/discharge is lower than the oxidation potential (about 2.1 V) for Mn2+ to Mnn+ (n > 2).17 In the CV curves (Figure 4a and 6d) at the voltage of 0−3 V appeared the peak (at about 2.1 V) corresponding to the transformation of Mn2+ to Mnn+ (n > 2), and the intensity of this peak becomes stronger as the number of cycles increases (Figure 6e). Therefore, it can be speculated that the formation of Mnn+ (n > 2) content is increased upon cycling under a voltage range of 0−3 V, giving rise to the continuous rise in capacity. When a dynamic equilibrium is reached between Mn2+ and Mnn+ (n > 2), the capacity will not rise again, as shown in Figure 4c. This result conformed to the previous MnO-based researches.21,43,49,50 To better understand the mechanism leading to the outstanding performance of the NC/MnO/rGO hybrid, EIS measurements are conducted. For comparison, the EIS of the pure MnO and MnO/rGO are also measured, as shown in Figure 7a. All of these samples exhibit an inclined line in the low frequency region and a depressed semicircle in the high frequency region, which represented Li+ diffusion resistance and interfacial resistance, respectively. From the fitting results exhibited in Table S1, it is clearly shown that the NC/MnO/rGO hybrid has the smallest interfacial resistance among the three samples, indicating a more enhanced electrochemical kinetics for NC/MnO/rGO. The Li+ diffusion coefficient (DLi+) of these three samples could be quantitatively evaluated through the following equation (eq 1)32,51

(1)

where R, T, and F represent the gas constant, the absolute temperature, and the Faraday constant, respectively. A is the geometric area of the electrode. n and C are the number of electrons per molecule for the redox couple and the concentration of Li+, respectively. σ is the Warburg factor, which is determined by the slope of the lines (the relationship between Z′ and ω−1/2) in Figure 7b according to the following equation (eq 2): Z′ = R s + R ct + σω−1/2 +

(2) +

Therefore, the Li diffusion coefficient (DLi ) for pure MnO, MnO/rGO, and NC/MnO/rGO hybrid were calculated to be 5.96 × 10−14, 1.28 × 10−13, and 2.04 × 10−13 m2 s−1, respectively, reflecting the unique structure of NC/MnO/rGO facilitated the fast Li+ diffusion. The structure stability is essential for enhancing the electrochemical performance. To further confirm the structure stability of the NC/MnO/rGO hybrid, the morphology of the pristine and cycled electrodes is characterized by SEM. Figure S4a,b shows the images of the pristine electrode; Figure S4c,d shows the morphology of the electrode after 50 cycles. It can be found that the integrity of active materials is still maintained in the electrode without obvious aggregation after cycling, suggesting the structure stability of the NC/MnO/rGO hybrid.52

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CONCLUSIONS In summary, we developed a rational construction of sandwiching MnO nanoparticles within graphene and Ndoped carbon networks (NC/MnO/rGO). The sandwiched MnO nanoparticles possess multiple benefits, including highly conductive networks from graphene and N-doped carbon layer, the structural protection, and inhibition of active materials loss from the graphene matrix. Consequently, the as-prepared NC/MnO/rGO anode exhibits excellent rate capability, superior cyclability, and outstanding capacity recoverability. Remarkably, the as-prepared NC/MnO/rGO hybrid delivered a high capacity of 648 mAh g−1 over 600 cycles at the high rate of 2 A g−1. Such superb lithium storage performance could be ascribed to the effectiveness of this unique structure in facilitating the volume expansion, enhancing conductivity, and preventing aggregation of MnO nanoparticles. All of these merits enable the NC/MnO/rGO hybrid a promising substitute as anode toward high energy G

DOI: 10.1021/acs.inorgchem.8b03390 Inorg. Chem. XXXX, XXX, XXX−XXX

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(11) Zheng, M.; Tang, H.; Li, L.; Hu, Q.; Zhang, L.; Xue, H.; Pang, H. Hierarchically Nanostructured Transition Metal Oxides for Lithium-Ion Batteries. Adv. Sci. 2018, 5, 1700592. (12) Zhang, G.; Wu, H. B.; Hoster, H. E.; Lou, X. W. Strongly Coupled Carbon Nanofiber-Metal Oxide Coaxial Nanocables with Enhanced Lithium Storage Properties. Energy Environ. Sci. 2014, 7, 302−305. (13) Chen, W. M.; Qie, L.; Shen, Y.; Sun, Y. M.; Yuan, L. X.; Hu, X. L.; Zhang, W. X.; Huang, Y. H. Superior Lithium Storage Performance in Nanoscaled MnO Promoted by N-Doped Carbon Webs. Nano Energy 2013, 2, 412−418. (14) Jiang, X.; Yu, W.; Wang, H.; Xu, H.; Liu, X.; Ding, Y. Enhancing the Performance of MnO by Double Carbon Modification for Advanced Lithium-Ion Battery Anodes. J. Mater. Chem. A 2016, 4, 920−925. (15) Dai, S.; Liu, Z.; Zhao, B.; Zeng, J.; Hu, H.; Zhang, Q.; Chen, D.; Qu, C.; Dang, D.; Liu, M. A High-Performance Supercapacitor Electrode Based on N-Doped Porous Graphene. J. Power Sources 2018, 387, 43−48. (16) Jiang, H.; Hu, Y.; Guo, S.; Yan, C.; Lee, P. S.; Li, C. Rational Design of MnO/Carbon Nanopeapods with Internal Void Space for High-Rate and Long-Life Li-Ion Batteries. ACS Nano 2014, 8, 6038. (17) Wang, J.; Zhang, C.; Jin, D.; Xie, K.; Wei, B. Synthesis of Ultralong MnO/C Coaxial Nanowires as Freestanding Anodes for High-Performance Lithium Ion Batteries. J. Mater. Chem. A 2015, 3, 13699−13705. (18) Chen, L.-F.; Ma, S.-X.; Lu, S.; Feng, Y.; Zhang, J.; Xin, S.; Yu, S.-H. Biotemplated Synthesis of Three-Dimensional Porous MnO/CN Nanocomposites from Renewable Rapeseed Pollen: An Anode Material for Lithium-Ion Batteries. Nano Res. 2017, 10, 1−11. (19) Xiao, Y.; Cao, M. Carbon-Anchored MnO Nanosheets as an Anode for High-Rate and Long-Life Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 12840−12849. (20) Xiao, Y.; Wang, X.; Wang, W.; Zhao, D.; Cao, M. Engineering Hybrid Between MnO and N-Doped Carbon to Achieve Exceptionally High Capacity for Lithium-Ion Battery Anode. ACS Appl. Mater. Interfaces 2014, 6, 2051−2058. (21) Chu, Y.; Guo, L.; Xi, B.; Feng, Z.; Wu, F.; Lin, Y.; Liu, J.; Sun, D.; Feng, J.; Qian, Y.; Xiong, S. Embedding MnO@Mn 3 O 4 Nanoparticles in an N-Doped-Carbon Framework Derived from Mn-Organic Clusters for Efficient Lithium Storage. Adv. Mater. 2018, 30, 1704244. (22) Liu, D. S.; Liu, D. H.; Hou, B. H.; Wang, Y. Y.; Guo, J. Z.; Ning, Q. L.; Wu, X. L. 1D Porous MnO@N-Doped Carbon Nanotubes with Improved Li-Storage Properties as Advanced Anode Material for Lithium-ion Batteries. Electrochim. Acta 2018, 264, 292−300. (23) Xie, D.; Xia, X.; Zhong, Y.; Wang, Y.; Wang, D.; Wang, X.; Tu, J. Exploring Advanced Sandwiched Arrays by Vertical Graphene and N-Doped Carbon for Enhanced Sodium Storage. Adv. Energy Mater. 2017, 7, 1601804. (24) Ding, Y.; Chen, L.; Pan, P.; Du, J.; Fu, Z.; Qin, C.; Wang, F. Nitrogen-Doped Carbon Coated MnO Nanopeapods as Superior Anode Materials for Lithium Ion Batteries. Appl. Surf. Sci. 2017, 422, 1113−1119. (25) Zhang, Y.; Chen, P.; Gao, X.; Wang, B.; Liu, H.; Wu, H.; Liu, H.; Dou, S. Nitrogen-Doped Graphene Ribbon Assembled CoreSheath MnO@Graphene Scrolls as Hierarchically Ordered 3D Porous Electrodes for Fast and Durable Lithium Storage. Adv. Funct. Mater. 2016, 26, 7754−7765. (26) Hummers, W. S.; Offeman, R. E. Offerman, Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (27) Zhou, X.; Cheng, F.; Tang, J.; Sun, A.; Bai, T.; Yu, Y.; Yang, J. Embedding MnO2 Ultrafine Nanoparticles within Graphene-Based Hybrid Elastomer as an Anode for Enhanced Lithium Storage. ChemElectroChem 2018, 5, 2310−2315. (28) Petnikota, S.; Srikanth, V. V. S. S.; Nithyadharseni, P.; Reddy, M. V.; Adams, S.; Chowdari, B. V. R. Sustainable Graphenothermal Reduction Chemistry to Obtain MnO Nanonetwork Supported

density for enhanced lithium storage. Moreover, the reported research strategies and synthesis method can be extended to prepare other battery materials which also have the drawbacks of severe volume fluctuation and poor conductivity.

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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/acs.inorgchem.8b03390.

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Additional XRD, SEM, TEM, and TGA (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Juan Yang: 0000-0003-1090-7135 Jingjing Tang: 0000-0002-6383-469X Author Contributions †

These authors contributed equally.

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 (Grant No. 51802354).

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DOI: 10.1021/acs.inorgchem.8b03390 Inorg. Chem. XXXX, XXX, XXX−XXX