Efficient Production of Coaxial Core–Shell MnO@Carbon Nanopipes

Research Article pubs.acs.org/journal/ascecg

Efficient Production of Coaxial Core−Shell MnO@Carbon Nanopipes for Sustainable Electrochemical Energy Storage Applications Linpo Li,†,§ Jianhui Zhu,‡ Yanli Niu,†,§ Zhaoyang Chen,†,§ Yani Liu,†,§ Siyuan Liu,†,§ Maowen Xu,†,§ Chang Ming Li,*,†,§ and Jian Jiang*,†,§ †

Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, No. 2 Tiansheng Road, BeiBei District, Chongqing 400715, P.R. China ‡ School of Physical Science and Technology, Southwest University, No. 2 Tiansheng Road, BeiBei District, Chongqing 400715, P.R. China § Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, No. 2 Tiansheng Road, BeiBei District, Chongqing 400715, P.R. China S Supporting Information *

ABSTRACT: Adverse structural changes and poor intrinsic electrical conductivity as well as the lack of an environmentally benign synthesis for MnO species are major factors to limit their further progress on electrochemical energy storage applications. To overcome the above constraints, the development of reliable and scalable techniques to confine MnO within a conductive matrix is highly desired. We herein propose an efficient and reliable way to fabricate coaxial core−shell hybrids of MnO@carbon nanopipes merely via simple ultrasonication and calcination treatments. The evolved MnO nanowires disconnected/confined in pipe-like carbon nanoreactors show great promise in sustainable supercapacitors (SCs) and Li-ion battery (LIB) applications. When used in SCs, such core−shell MnO@carbon configurations exhibit outstanding positive and negative capacitive behaviors in distinct aqueous electrolyte systems. This hybrid can also function as a prominent LIB electrode, demonstrating a high reversible capacity, excellent rate capability, long lifespan, and stable battery operation. The present work may shed light on effective and scalable production of Mn-based hybrids for practical applications, not merely for energy storage but also in other broad fields such as catalysts and biosensors. KEYWORDS: Efficient production, MnO@carbon nanopipes, Coaxial core−shell configurations, Sustainable energy storage applications

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INTRODUCTION Depletion of current mineral deposits and ever-increasing environmental issues have prompted development of clean energy conversion systems and other alternative materials for sustainable energy storage usage. Mn-based oxides (MnOx) have long attracted tremendous interest in electrochemical energy storage applications due to their great abundance in nature, environmental friendliness, low cost/toxicity, and foremost large theoretical capacities for power systems such as commonly used primary cells (e.g., Zn-MnO2 cell, etc.), supercapacitors (SCs), and Li-ion batteries (LIBs).1−8 With MnO as an example, its theoretical capacity for LIB anode reaches as high as ∼756 mAh g−1, more than twice that of commercial graphite (∼372 mAh g−1).9,10 The above intriguing merits enable MnOx to be promising candidates for sustainable active electrodes in next-generation energy-storage devices. However, their further progress is severely impeded because nearly all MnOx materials share the same shortcomings, unfortunately, comprising: (i) low ionic/electrical conductivity (e.g., ∼10−7−10−8 S cm−1 for Mn3O4) and (ii) inevitable © 2017 American Chemical Society

phase/volume changes during repeated cell charge/discharge procedures.11 Such insulating properties and structural variations are adverse to electrode kinetics in terms of electron transfer, electrochemical activity, and utilization ratio of actives, leading to electrode pulverization, irreversible side reactions, and inferior cyclic performance.11−15 To overcome the above constraints, the smart combination of nanostructured MnOx with conductive carbon matrix is a common and applicable way to improve the energy-storage performance. For example, Kim et al. purposely designed MnO@CNTs nanocomposites for LIBs because CNTs with superb electrical conductivity can greatly enhance the electrode properties.16 Jiang et al. once reported the construction of onedimensional (1D) core−shell architecture made of Mn3O4 nanowires and highly graphitic/mesoporous carbons. Such unique hybrid products delivered large specific capacitances of Received: April 23, 2017 Revised: May 30, 2017 Published: June 11, 2017 6288

DOI: 10.1021/acssuschemeng.7b01256 ACS Sustainable Chem. Eng. 2017, 5, 6288−6296

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) General schematics displaying the entire evolution of MnO@C nanopipes. SEM observations toward samples at distinct evolution stages: (b and c) MnO2 NWs, (d and e) MnO2@PDA, and (f and g) MnO@C nanopipes.

266 F g−1 at 1 A g−1 and 150 F g−1 at 60 A g−1, exhibiting high energy and power densities up to 20.8 W h kg−1 and 30 kW kg−1, respectively.17 Though great advances have been made on the functionalized construction of MnOx@carbon (C) systems, they can rarely be utilized for practical industry because these hybrid fabrications need to be aided by the vast use of hazardous KMnO4 as major a Mn resource or oxidizer (often within a concentrated HCl solution). Such chemical conditions are rather oxidative and extremely corrosive, which is detrimental to humans and inconvenient for scalable production and further applications. Thereby, avoiding the use of risky KMnO4 and seeking for other rational and green synthetic alternatives for LIBs and SCs is currently highly pursued.18−25 Herein, we put forward a proper way to fabricate coaxial core−shell hybrids of MnO@C nanopipes. Unlike pioneering literature, MnSO4 and (NH4)2S2O8 are successively used as Mn resources and a reliable oxidant, enabling massive synthesis of long MnO2 nanowires (NWs) in a mild acid condition.26−28 Another highlight is that our total fabrication is facile and more efficient; MnO2 NWs precursors merely suffer from a quick ultrasonication treatment in a dopamine (DA)-contained Tris buffer solution and a calcination process, leading to the massive formation of core−shell MnO@C nanopipe products. The choice of DA rather than other alternative organic carbon sources, is given that DA is widespread, low-cost, and nontoxic to the environment. It can self-polymerize at mild alkaline conditions and particularly shows excellent affinity for almost all organic/inorganic surfaces. Thus, the evolved polydopamine (PDA) molecules can deposit on each MnO2 NW to form an intact and conformal layer with a robust and intimate contact property. Also note that the inner MnO NWs are highly

discrete and encapsulated within C “nanoreactors”; the pipelike C outer shell can function as good electron-transfer routes, while the internal void space yielded in between adjacent NWs may facilitate ionic diffusions and provide ample buffer places against adverse volume expansions in electrochemical energy storage applications. When serving as the anode of LIBs, the resultant hybrids are capable of exhibiting much more stable electrode behaviors, including high electrochemical activity and capacity (∼680 mAh g−1 at 0.4 A g−1), outstanding cyclability (nearly 90% capacitance retention after 600 cycles), and excellent rate capabilities. The SCs made of MnO@C nanopipes also exhibit large capacitances of 299.3 F g−1 (at ∼0.75 A g−1) in neutral 0.5 M Na2SO4 electrolyte and ∼364.5 F g−1 (at ∼0.75 A g−1) in 3 M KOH solution (by using Mn valence state variation) with very negligible capacity degradation upon a long-term cyclic period. The above prominent energy-storage performances and the efficiency for our synthesis may privide hybrid MnO@C nanopipes as a sustainable electrode material for LIBs and SCs.

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EXPERIMENTAL SECTION

Synthesis of Ultralong MnO2 NWs. MnO2 NWs were prepared beforehand and directly used as the initiating materials. First, 1.12 g of MnSO4·H2O, 2.85 g of (NH4)2SO4, and 1.51 g of (NH4)2S2O8 were dispersed into deionized water (30 mL) by ultrasonication for 30 min. The obtained homogeneous solution was then transferred into a Teflon-lined stainless autoclave, heated at 200 °C for 12 h and cooled to the ambient temperature naturally. The precipitate products were collected by vacuum filtration, washed with deionized water several times, and dried in a vacuum oven at 60 °C. Synthesis of MnO@C Nanoreactors. MnO2 NWs (0.5 g) and DA (0.15 g) were successively dispersed into a 300 mL Tris buffer (10 mM, pH 8.5) solution by ultrasonication. After a 30 min ultra6289

DOI: 10.1021/acssuschemeng.7b01256 ACS Sustainable Chem. Eng. 2017, 5, 6288−6296

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ACS Sustainable Chemistry & Engineering

Figure 2. (a) XRD patterns of MnO2 NWs precursors and final products of MnO@C. (b−d) TEM observations, (e) EDS detecting, and (f−i) elemental mappings on the hybrid of MnO@C nanopipes. The inset in panel e shows TEM images of acid-treated samples. sonication treatment, the mud-like MnO2@ PDA intermediates were collected by vacuum filtration and washed with deionized water several times. The ultimate MnO@C nanopipe products were made by annealing the intermediates at 800 °C (heating rate: 10 °C min−1) under Ar atmosphere for 1 h. Characterizations and Electrochemical Measurements. The samples’ morphology and crystalline structures were characterized by JEOL JSM-7800F field emission scanning electron microscope (FESEM) with energy dispersive X-ray spectroscopy (EDS) and a JEM 2010F transmission electron microscope (TEM). X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS; Thermo Electron, VG ESCALAB 250 spectrometer) was also used to characterize the products. Thermogravimetric analysis (TGA) was performed on a SDT600 apparatus under a heating rate of ∼5 K min−1 in O2 atmosphere. The mass of electrode materials was measured on a microbalance with an accuracy of ∼0.01 mg (A&D Company N92). The working electrodes were fabricated by the conventional slurry-coating method. The active MnO@C hybrid powders, poly(vinylidene fluoride) (PVDF) binder, and carbon black were mixed in a mass ratio of 80:10:10 and dispersed and homogenized in N-methyl-2-pyrrolidone (NMP) to form slurries. Then, the homogeneous slurry was pasted onto a Cu film and dried at 120 °C for 12 h under a vacuum condition. For LIB anodes, the mass loading on each current collector was controlled at the level of ∼1.5 mg cm−2. The electrode testing was performed using CR-2032 coin-type cells in a voltage range of 0.005− 3.0 V (vs Li/Li+). Cells were assembled in an Ar-filled glovebox (H2O < 0.1 ppm, O2 < 0.1 ppm) by using Li foil as the counter and reference electrode. One molar LiPF6 dissolved in a 1:1 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte. Galvanostatic charge/discharge tests were conducted by a professional battery tester (NEWARE, Shenzhen), while the electrochemical impedance spectroscopy and cyclic voltammetry (CV) scans were measured on an electrochemical workstation (CorrTest CS310). Prior to electrochemical testing, all cells were aged for 6 h. For SCs, the as-formed slurry was uniformly pasted onto nickel foam and then heated at 120 °C under vacuum condition. The mass loading for active materials was measured to be ∼3−4 mg cm−2. To fully understand and

evaluate the capacitive behaviors of MnO@C hybrids, the electrochemical test was conducted in a three-electrode system in 0.5 M Na2SO4 and 3 M KOH aqueous electrolytes, respectively, with a Pt wire as the counter electrode and an Ag/AgCl as the reference electrode. Before testing, all electrodes were activated by continuous CV scans at 50 mV s−1 for 200 cycles. The specific capacities (C), energy density (E), and power density (P) based on the total mass of actives on electrodes were calculated according to the formulas below: C=

I×t m × ΔV

E=

∫0

(1)

t

P = E /t

IV (t )dt

(2) (3)

wherein I, t, ΔV, and m successively represent the discharge current (A), discharging time (s), testing voltage window (V), and total active mass on electrode (g); V is the discharging voltage (V), and dt is the time differential (s).

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RESULTS AND DISCUSSIONS The fabrication flow is schematically illustrated in Figure 1a. There are two main procedures involved during the total evolution process. In step I, a conventional hydrothermal method through the oxidation of Mn2+ by S2O82− was employed for the massive production of MnO2 NWs precursors. These MnO2 NWs are then transferred into a DA-involved Tris buffer solution and simply treated by ultrasonication for 30 min, resulting in the formation of MnO2 NWs@PDA core−shell intermediate products. Step II is a calcination treatment toward such intermediates, enabling them to change into the ultimate hybrid of MnO@C nanopipes. It is a highlight that our synthesis is green, efficient, and scalable when compared to previous approaches using chemically active KMnO4 as the major Mn source or oxidizer in a highly acid condition. In our case, there merely involves the 6290

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Figure 3. (a) Raman spectrum and (b−d) XPS analysis of MnO@C nanopipes.

treatments, all of MnO2 species are chemically in situ reduced into MnO. The XRD pattern (black) presents the sharp peaks at 2θ of 34.93, 40.57, 58.71, 70.12, and 73.79°, all of which are well indexed to spinel MnO crystals (JCPDS no. 070230).29−31 To make clear their inner structures and crystalline information, TEM has been utilized to characterize the samples. Figures 2b and c uncover the coaxial core−shell hybrid constructions of evolved MnO@C nanopipes. It is noteworthy that their inner core is made up of numerous discrete 1D nanounits rather than the pristine continuous NWs. This may be associated with the high-temperature reduction treatment on MnO2 NWs, enabling them to break and fuse into disconnected MnO NWs or even nanoparticles (Figure 2c). In spite of this, it is noted that all of nanosized MnO species are perfectly packaged in a C matrix. Figure 2d displays the high-resolution TEM (HRTEM) image performed on the specific region in Figure 2b. Two distinct lattice fringes with interplanar spacing of 0.25 and 0.22 nm are observed, which is highly in accordance with the (111) and (200) planes of MnO, respectively, and matches well to our former XRD result.8,30 The EDS spectrum (Figure 2f) further confirms the presence of C, O, and Mn in MnO@C hybirds. To ensure the successful generation of C nanoshells, the inner MnO core in hybrid systems is purposely removed by concentrated HCl. The inset TEM observations reveal such acid-treated samples possess an appealing continuous and pipe-like integrated architecture, which would be very beneficial for electron transfer, structural buffering during ions insertion, and electrode operation stability. The EDS elemental mappings (Figures 2e−i) on the designated area (see SEM image in Figure 2f) illustrate the uniform distribution of Mn, C, and O. Such MnO components are firmly confined in C layers and uniformly dispersed (rather than gathering in local

commonly used ammonium/Mn-based salts and DA; no other hazardous or toxic agents are utilized in the fabrication flow. Also note the fact that the hydrothermal end-products of (NH4)2SO4 can be fully recycled and reused to synthesize MnO2 NWs precursors or utilized for other usage (e.g., making fertilizers, etc.). To get insights into the morphological evolution, the overall synthesis is purposely monitored by scanning electron microscopy (SEM) (Figures 1b−g). Figures 1b and c show the precursors of ultralong MnO2 NWs (single NW diameter: ∼30 nm) are closely intertwined with each other. After a short ultrasonication treatment, MnO2 NWs still maintain the pristine interwoven architecture, but their surface evidently becomes rough (Figures 1d and e). This suggests that the encapsulation of PDA onto MnO2 NWs can be realized via the self-polymerization reaction of DA molecules in a weak alkaline solution, leading to the generation of intriguing coaxial MnO2@PDA core−shell hybrid nanostructures. Figures 1f and g show the morphological observations on the ultimate hybrid products. As noted, MnO@C nanopipes still preserve an intriguing 1D integrated hybrid configurations even after a heating process; their surface turns smooth again, but the diameter for individual NWs increases up to a value of ∼60 nm, which indicates the formation of C nanoshells (layer thickness: ∼10−15 nm). To further identify the intrinsic nature of these nanomaterials, XRD characterizations toward samples at different evolution stages were performed (Figure 2a). In the initial stage, there are notable diffraction peaks present at a 2θ of 12.73, 18.06, 28.71, 37.62, 49.89, and 60.19° (green pattern), which correspond well to the (110), (200), (310), (121), (411), and (521) facets of MnO2 (JCPDS card no. 72-1982), respectively. After the subsequent ultrasonication and thermal 6291

DOI: 10.1021/acssuschemeng.7b01256 ACS Sustainable Chem. Eng. 2017, 5, 6288−6296

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Figure 4. (a) CV plots, (b) charge−discharge curves, and (c) cyclic performance of MnO@C nanopipes in 0.5 M Na2SO4 at a potential range of −0.1−0.9 V (vs Ag/AgCl). (d−f) Identical electrochemical tests of MnO@C nanopipes in 3 M KOH at a potential range of −0.7−0.2 V (vs Ag/ AgCl).

cathodic and anodic peaks appear on each CV plot, indicative of reversible faradic behavior for Mn species due to reversible adsorptions or intercalations of Na+ and good pseudocapacitive properties of such hybrids when serving as positive electrodes in asymmetric SCs devices.32 Galvanostatic charge/discharge measurements toward the electrode of MnO@C nanopipes were carried out as well. The charge/discharge profiles at ∼0.75, ∼1.5, ∼3, ∼6, ∼12, and ∼24 A g−1 are displayed in Figure 4b. The symmetrical features strongly suggest the high Coulombic efficiency of MnO@C nanopipes. According to the above records, the stored capacitances can be successively calculated to be ∼299.32, ∼268.95, ∼220.74, ∼194.35, ∼160.44, and ∼115.2 F g−1. The relationship between the specific capacitance and current rate is plotted in the inset of Figure 4b. Even when tested at a current as large as ∼24 A g−1, a notable capacitance near ∼115 F g−1 is still retained for this hybrid electrode. In addition to capacitive behaviors, the cyclic endurance is another important parameter for SC applications (Figure 4c). Under a constant current rate of ∼1.5 A g−1, the capacitance retention of MnO@C hybrids always maintains at the level of ∼100% after 3000 cycles and remains above ∼90.0% even after 5000 cycles, which evidences their outstanding electrochemical stability and endurance in a longtime cyclic period. It is interesting to note that core−shell MnO@C hybrids exhibit a continual increase in capacitance over the initial 1600 cycles. This activation process may be ascribed to the slow electrolyte infiltration into well-packaged interior space, delayed MnO oxidation, and redox reactions (Mn3+/Mn4+) in KOH solution; the outer protective C layers on MnO would not only postpone the electrolytic ions permeation to inner electrode structures but also retard the electrochemical oxidations of MnO and reversible conversion reactions. In parallel, the potential utilization of MnO@C nanopipes as negative electrodes is also examined in 3 M KOH electrolyte. Figure 4d displays CV curves of such core−shell

regions), which may help to avoid negative aggregations of nanoactives during the inevitable phase/volume changes. Raman spectroscopy has been used to examine the graphitization degree of C shells (Figure 3a). Clearly, there is a sharp peak present at the position of ∼640 cm−1. This peak signal corresponds well to the fingerprint vibration model of MnO, whereas the other two peaks located at ∼1349 and ∼1579 cm−1 originate from the defective (sp3) and graphitic (sp2) C atoms, respectively. The ratio of aforementioned peaks (IG/ID) is calculated to be ∼1.17, implying that the PDAevolved nanopipes are highly carbonized and have a good electrical conducting property.22,23 XPS measurements are further conducted to detect the chemical valence state of ultimate samples (Figures 3b and d). The higher-resolution Mn 2p spectrum (Figure 3b) fitted into two peaks at the large binding energies (BE) of ∼641.8 and ∼653.8 eV, which are assigned to Mn 2p3/2 and Mn 2p1/2, respectively, and approach the literature data for MnO.20,21 The high-resolution C 1s spectrum (Figure 3c) can be divided to three peak signals, corresponding to the bonds of CC (∼284.6 eV), C−O and CN (∼285.8 eV), and CO and C−N (∼288.7 eV), respectively,32 indicating other heteroatoms (such as N) may be contained within our hybrid systems. The typical N 1s spectrum was thus detected and analyzed, as shown in Figure 3d. The fitting peaks at ∼398.3, ∼400.1, and ∼401.2 eV are in good agreement with pyridinic-, pyrrolic-, and graphitic-type N, respectively. We believe that these doped heteroatoms might be all derived from the N-rich PDA molecules.12,31,32 Inspired by previous works on energy-storage devices using reversible Mn valence state changes, we evaluated MnO@C nanopipes as the positive and negative electrodes for SC applications. Figure 4a shows CV curves of MnO@C nanopipes tested at varied scan rates in a three-electrode system over a potential range of −0.1−0.9 V (vs Ag/AgCl) using 0.5 M Na2SO4 solution as the electrolyte. Clearly, a pair of symmetric 6292

DOI: 10.1021/acssuschemeng.7b01256 ACS Sustainable Chem. Eng. 2017, 5, 6288−6296

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Figure 5. (a) Schematic showing the SC device of α-Co(OH)2 NWs (−)//MnO@C nanopipes (+). (b) Comparative CV curves of α-Co(OH)2 NW anode and MnO@C nanopipe cathode at a scan rate of 10 mV s−1 in a three-electrode system. (c) CV plot, (d) discharge profiles at various current densities, (e) rate behaviors, (f) cyclic performance, and (g) Ragone plot of as-assembled SC devices.

KOH aqueous electrolyte (the configuration details for MnO@ C nanopipes (−)//α-Co(OH)2 NWs (+) device are schematically displayed in Figure 5a. The choice of α-Co(OH)2 NWs as the counter electrode was due to their superb electrochemically Faradaic performance (e.g., ultrahigh theoretical capacity of ∼3460 F g−1 due to multielectron redox reactions of the elemental Co).33−35 The mass ratio of MnO@C and Co(OH)2 NWs in assembled SCs is determined by referring to capacitive behaviors (e.g., integrated area in CV plots) of each single electrode (Figure 5b). Figure 5c presents the CV curve of asassembled SCs within a potential window of 0−1.25 V under a scan rate of 10 mV s−1. The overall CV profile is quite symmetric and definitely inherits the electrochemical characteristics of both anode and cathode, exhibiting the good reversibility and well-defined pseudocapacitive properties of SC devices. Such a large integrated area in CV plot unquestionably implies the notable stored capacitance. The cell performance is further measured by galvanostatic tests. Corresponding discharge profiles at district current densities are shown in Figure 5d. According to the above measurements, the cell capacitances are calculated and plotted as a function of current rate (see Figure 5e). When operated at a slow rate of ∼0.5 A g−1, this SC device can deliver a high specific capacitance of ∼187.5 F g−1. Even at a current rate as large as 16 A g−1 (cells discharged in few seconds), it still remains a substantial capacitance of 113 F g−1, capable of retaining ∼60.3% of the maximum value. This result highly reflects the fast reacting kinetics and superior rate capabilities of MnO@C nanopipe (−)//α-Co(OH)2 NWs (+) devices. The cyclic endurance of full-cell SCs was also examined at a current density of ∼2 A g−1 (Figure 5f). The preactivated SC devices exhibit a stable cyclic performance. The capacity retention always approaches a high level of ∼100% within the prime 3500 cycles and still maintains above ∼90.5% at the end of the cyclic period. To prove the potential in practical use, two SC devices

hybrids at varied scan rates. After electrode activations, a couple of symmetric cathodic and anodic peaks emerge on each CV plot in a potential window of −0.7 to 0.2 V (vs Ag/AgCl), which might be attributed to reversible Faradaic conversions of Mn2+/Mn3+ redox couples. All involved redox reactions are simplified and expressed as follows:21,32 Mn 2 + ⇄ Mn 3 + + e−(negative; KOH electrolyte)

(4)

Mn 3 + ⇄ Mn 4 + + e−(positive; Na 2SO4 solution)

(5)

As known, the peak potential shifts are common phenomena and tightly associated with electrode reaction kinetics. Upon the gradual increase of scan rates from 10 to 90 mV s−1, there are no significant changes in either position or shape for redox peaks, implying our hybrid configurations are highly applicable to SCs devices. Figure 4e shows galvanostatic charge/discharge curves of MnO@C nanopipes at varied current densities from ∼0.75 to ∼24 A g−1 (the calculated specific capacitance is shown in the inset). A maximum capacitance is able to achieve ∼364.5 F g−1 underneath a slow current of 0.75 A g−1. The electrode capacitance stepwise decreases along with the rise of current rates, which should be related to ionic diffusion limitations in the charge/discharge processes. Nevertheless, when the current rises up to 24 A g−1, it can still retain a high capacitance of ∼143.22 F g−1. The cyclic performance is further evaluated by repeated charge/discharge testing. The stored capacitance initially keeps increasing until the retention value reaches ∼110% (at the 500th cycle), which is mainly due to the retarded electrolyte infiltration and slow electrode activation processes. In subsequent charge/discharge procedures, the delivered capacitance maintains at a high level of ∼98% over 5000 cycles, further verifying the prominent structural stability of MnO@C nanopipes. A full-cell SC device was then assembled by coupling MnO@ C nanopipes anode with α-Co(OH)2 NWs cathode using a 3 M 6293

DOI: 10.1021/acssuschemeng.7b01256 ACS Sustainable Chem. Eng. 2017, 5, 6288−6296

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Figure 6. (a) CV plots of MnO@C hybrid electrodes at a scan rate of 0.5 mV s−1 in a potential window of 0.005−3 V (vs Li/Li+). (b) Cyclic performance, (c and d) charge/discharge profiles, (f) EIS result, (e and g) rate behaviors, and (h) specific capacity vs current density plot of MnO@ C nanopipes.

5st to 20th cycle). This may result from the delayed electrolyte infiltration into a well-capsulated hybrid nanostructures, leading to the gradual electrochemical activation of MnO. At the 20th cycle, the CV shape remains unchanged, illustrating an electrochemical equilibrium state for Li storage has been built. Within the above cycles, there are no apparent changes on integral areas of CV curves, suggesting the excellent electrochemical reversibility and stability of MnO@C hybrid electrodes.18 Figure 6b shows their cyclic performance at a current density of 0.5 A g−1. The MnO@C nanopipes exhibit a high discharge capacity of ∼686 mAh g−1. Strikingly, there is no obvious capacity degradation phenomenon present for continuous 600 cycles. Moreover, the Coulombic efficiency always retains close to ∼100%, indicative of their outstanding electrochemical reversibility. EDS, ex situ SEM, and TEM observations toward cycled MnO@C hybrid electrodes were performed (see Figures S1 and S2) to uncover the structural changes before and after 600 times of cyclic testing. It is a highlight that the cycled electrodes can still preserve the hybrid composition and 1D pipe-like hollow nanoarchitectures, though the constructions for inner MnO core (nanoparticle size: ∼30 nm) varies into tiny nanodots (