Bottom-Up Construction of Reduced-Graphene-Oxide-Anchored MnO

Oct 17, 2018 - Hou, H. S.; Banks, C. E.; Jing, M. J.; Zhang, Y.; Ji, X. B. Carbon Quantum Dots and Their Derivative 3D Porous Carbon Frameworks for ...
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Bottom-Up Construction of Reduced-Graphene-Oxide-Anchored MnO with an Nitrogen-Doped Carbon Coating for Synergistically Improving Lithium-Ion Storage Yujie Wang,†,‡ Hao Wu,*,† Zhifang Liu,§ Hang Zhao,† Heng Liu,† and Yun Zhang*,† †

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/19/18. For personal use only.

Department of Advanced Energy Materials, College of Materials Science and Engineering, Sichuan University, Chengdu 610064, P. R. China ‡ Research Institute of Natural Gas Technology, Petrochina Southwest Oil & Gas Field Company, Chengdu 610213, P. R. China § Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *

ABSTRACT: Designing an advanced architecture to overcome the innate issue of MnO-based anode materials in terms of low electrical conductivity and severe volume change during cycling is still a challenge toward which more effort needs devoted. Here, an intriguing hybrid involving the architecture of reduced graphene oxide (RGO)-anchored MnO within an nitrogen-codoped carbon coating (RGOMnO@NC) is reported via a simple and facile approach and regarded as a promising lithium-ion (Li+) anode material with high rate capacity, large specific capacity, and a long cycle lifespan simultaneously. The resulting porous conductive carbon layer could not only promote the electron/ion transfer but also alleviate the volume variation for retaining a relatively stable solid electrolyte interphase and prevent MnO from direct contact with the electrolyte to reduce unexpected lithium consumption. The existing internal voids offer the space to accommodate volume expansion in the lithiation/delithiation processes, and RGO could build a large conductive network for better electron transfer. Consequently, the RGO-MnO@NC electrode presents high Li+ storage capacity (699 mAh g−1 at 0.1 A g−1), excellent cycling performance (607 mAh g−1 at 1 A g−1 over 550 cycles), and a remarkable rate performance. Through kinetic analysis, it is revealed that RGO-MnO@NC exhibits an enhanced capacitive contribution for Li+ storage, showing a typical faradaic surface pseudocapacitive mechanism. This work proposes a new strategy to ameliorate the deficiency of the electrode material toward the conductivity and volume change for enhanced Li+ storage.

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INTRODUCTION Currently, among the usage of sustainable energies, rechargeable lithium-ion (Li+) batteries (LIBs) have been deemed as a promising power source to be widely probed and applied for liberating the worsening energy plight and environmental crisis because of their long cycle lifespan and high energy density, compared with unstable solar, wind, and tide energy.1−4 Facing the burgeoning electrochemical performance requirements for large-scale applications, including electric vehicles, portable electronic devices, and smart electric grids, the traditional commercial graphite anode for LIBs with a low theoretical capacity of 372 mAh g−1 encounters insurmountable breakthrough in terms of capacity.5 Therefore, tremendous efforts have been devoted to seeking advanced anode alternatives with higher specific capacity as well as a long cycle lifetime to satisfy the existing demands, such as intercalation-based hard carbon,6,7 alloy-type IVA group materials,8−10 and conversion−mechansim materials involving transition-metal oxides (TMOs),11−14 chalcogenides,15,16 and nitrides and phosphides.17,18 Together with © XXXX American Chemical Society

the noteworthy merits of narrower voltage hysteresis, harmfulless, and nature abundance,19−21 MnO, possessing a theoretical specific capacity of 756 mAh g−1, sticks out from other TMOs and attracts much attention as one of the most potential anode candidates. Nonetheless, two inherent drawbacks of unfavorable electrochemical kinetics due to the inferior electronic conductivity and dramatic volume variation owing to the repeated lithiation/delithiation process, slowing ion/electron-transport kinetics to impair rate capability, and pulverizing active materials to shorten cyclability are of pivotal hindrance to be concerned with for advancing MnO LIB anodes toward successful practical applications.22,23 To this end, the deliberately controllable assembly of MnO with carbonaceous materials, involving reduced graphene oxide (RGO),24,25 carbon nanotubes,26,27 and amorphous carbon,28,29 is regarded as the most efficient and low-cost route to be explored intensively for enhancing the performance of Received: August 10, 2018

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DOI: 10.1021/acs.inorgchem.8b02270 Inorg. Chem. XXXX, XXX, XXX−XXX

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the same procedure except for no addition of RGO and undergoing the coating process of dopamine. Material Characterizations. Field-emission scanning electron microscopy (SEM; FEI Inspect F50) was utilized to acquire the SEM images. The microstructural features were surveyed by transmission electron microscopy [TEM; aberration-corrected FEI Titan G2 60300, coupled with an energy-dispersive X-ray (EDX) spectrometer]. The crystalline phase composition and structure were inspected by Xray diffraction (XRD) using a Bruker D8 ADVANCE A25X diffractometer with Cu Kα radiation. Raman spectra were recorded with an Andor SR-500i Raman system using a 532 nm excitation laser. N2 adsorption/desorption isotherms were obtained with a Micromeritics ASAP 2460 instrument. ζ-potential tests were conducted via a Zetasizer 3600 (Malvern Instruments). X-ray photoelectron spectroscopy (XPS) was performed on an ESCALab 250Xi spectrometer. Thermogravimetric analysis (TGA) was conducted by a Netzsch STA 449 F3 analyzer with the range of room temperature to 800 °C in air flow at a heating rate of 10 °C min−1. Electrochemical Characterizations. The electrochemical behaviors of samples were inspected by the assembled CR2032 cointype cells in the highly pure argon-filled glovebox using pure lithium as the counter/reference electrode, a solution of LiPF6 (1 M) in ethylene carbonate, dimethyl carbonate, and diethyl carbonate (1:1:1, v/v/v) as the electrolyte, and a Celgard 2400 membrane as the separator. For fabrication of the working electrode, a homogeneous slurry composed of active materials, carbon black, and a styrene− butadiene rubber/carboxymethylcellulose binder at a weight ratio of 80:10:10 was coated on copper foil. After vacuum drying at 100 °C for 2 h, the obtained copper foil was punched into disks at a diameter of 12 mm. The active material loading of each electrode was controlled at about 1.0 mg cm−2. The calculated gravimetric specific capacity is based on all active materials. The galvanostatic charge/ discharge measurements were conducted on a Neware battery test station (BTS-5 V 10 mA) under different current densities within the potential window of 0.01−3.0 V (vs Li+/Li) at room temperature. The cyclic voltammetry (CV) and electrochemical impedance spectrometry (EIS) tests were carried out on a PARSTAT PMC1000 electrochemistry workstation (Princeton Applied Research, USA). With the potential range from 0.01 to 3.0 V, the CV curves were tested under different scan rates. EIS was surveyed with a frequency range of 0.05−100000 Hz at an amplitude of 5 mV.

LIB anodes, which can upgrade the electrical conductivity and preserve the structure integrity of the electrode because of the introduction of carbon. Additionally, benefiting from the surging development of nanotechnology, a paradigm shift has been able to offer the opportunity of designing delicate architecture, desirable composition, and fascinating morphology for overcoming the aforementioned challenging issues, such as porous structure facilitating Li+ transport,30 elastic shell coating relieving mechanical stress,31−33 internal void space accommodating volumetric expansion,34 heterogeneous interface providing extra capacity,35 and wire shape with volume change nearly along the radial direction.36 Despite a few attempts at achieving significant progress to tackle the above issues, a facile and rational strategy still needs to be elaborately explored to gain unexpected performances of the MnO anode for LIBs. Herein, an ingenious approach of fabricating RGO-anchored MnO with an N-doped carbon (NC) coating structure (RGOMnO@NC) is proposed that incorporates a solvothermal reaction, coating of dopamine, and a one-step carbonizing and doping process, sequentially. In this architecture, depending on the advantages of outstanding electric conductivity and flexible nature, RGO can not only offer enhanced rate performance but also alleviate structure collapse of the electrode during discharge/charge for better capacity retention. Moreover, a porous carbon coating confers a shorter transport pathway for Li+, thus accelerating the electrochemical reaction kinetics at a high rate. Meanwhile, the dopamine-derived carbon coating can further boost the rate capability because of N-atom doping for increased electrode conductivity, effectively avert the aggregation of MnO particles over cycling, and avoid direct contact between the active material and electrolyte to reduce the unwanted side reactions for ameliorated capacity fading. Furthermore, the void space inside the MnO microsphere, arising from the escape of CO2 gas in the pyrolysis, could endow additional space to uphold lithium insertion for shunning fracture of the solid electrolyte interphase (SEI) and provide an enlarged specific surface area to enhance the pseudocapacitance for LIBs. As anticipated, by virtue of the synergetic effects of conductive-flexible RGO, porous NC coating, and void space within the MnO microsphere, the RGO-MnO@NC anode exhibited preeminent Li+ storage for LIBs.

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RESULTS AND DISCUSSION The overall synthesis process of the RGO-MnO@NC hybrid with internal void space is schematically illustrated in Figure 1.

EXPERIMENTAL SECTION

Synthesis of a RGO-MnO@NC Hybrid. Typically, RGO sheets (20 mg) were first homodispersed in ethylene glycol (35 mL) by ultrasound for 1 h to obtain suspension A. Then, Mn(CH3COO)2· 4H2O (0.25 g) was dissolved in suspension A under magnetic stirring for 1 h, followed by the dropping addition of 0.15 g of urea with a further strong stirring treatment for 10 min. After thorough mixing, the above mixture was treated under solvothermal conditions at 180 °C for 15 h. The synthesized precipitation was collected via centrifugation, washed with ethanol and water, and finally added to 110 mL of a Tris-buffer solution (pH = ∼8.5) under magnetic stirring to form suspension B. Subsequently, 50 mg of dopamine was added to suspension B under stirring and held at 30 °C for 24 h. After centrifugation, washing with deionized water and ethanol, and drying in an oven at 80 °C, the resulting RGO-MnCO3@polydopamines were annealed at 400 °C for 2 h with a heating rate of 1 °C min−1 under a pure argon flow and then at 800 °C for 2 h at a heating rate of 3 °C min−1 to obtain a RGO-MnO@NC hybrid. Additionally, RGOMnO was synthesized via the same routine without the coating process of dopamine, and a pure MnO sample was obtained through

Figure 1. Schematic illustration of the synthesis process for the RGOMnO@NC architecture.

First, Mn2+ was used in close combination with the electronegative RGO because of incomplete reduction via electrostatic force (Figure S1) and then as an active site for preparing a RGO-trapped MnCO3 microsphere through a modified solvothermal method.37 After that, a facile polymerization and coating process of dopamine was adopted to wrap MnCO3 within a polydopamine layer, followed by thermal treatment in argon to synchronously realize an internal void space and a porous NC coating, consequently obtaining the desirable RGO-MnO@NC composites. SEM and TEM were utilized to inspect the morphologies and microstructures of the synthesized RGO-MnO@NC B

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S4),39 playing a critical role in facilitating ion access and alleviating volume expansion for sustaining the SEI film. Such a fascinating structure could be beneficial to effectively improving rate and cycle performances. The HRTEM image (Figure 3c) clearly exhibits the existence of a cubic MnO phase, as evidenced by lattice fringes of 0.22 nm coinciding with its (200) crystal plane. Acquired from the high-angle annular dark-field scanning TEM (HAADF-STEM) image (Figure 3d), the EDX elemental mapping images (Figure 3e− i) show that Mn, O, N, and C elements are uniformly distributed in RGO-MnO@NC, in which Mn and O have an even distribution across the whole microsphere. Particularly, N mainly emerges in the microsphere area to confirm the entire coating of NC toward the MnO microsphere. The crystalline phase composition and structure of the RGO-MnO@NC hybrid were inspected by powder XRD. Undergoing calcination of polydopamine-coated RGOMnCO3 (Figure S5), the typical XRD pattern of as-obtained RGO-MnO@NC (Figure 4a) displays the (111), (200), (220), (311), and (222) diffraction peaks indexed to cubic MnO (JCPDS 07-0230),40 while a hunchback-like peak around 25° indicates the formation of NC.41 The carbon content in RGOMnO@NC was evaluated with the TGA results (Figure S6), revealing that the weight fraction of carbon is about 45%, by which the RGO-MnO@NC anode can be expected to perform superior electrochemical energy storage because of the advanced electroconductivity and more stable configuration of the electrode. As presented in Figure 4b, Raman spectroscopy was employed to survey the graphitic quality of RGO-MnO@NC. Two Raman peaks emerging at about 1347 and 1589 cm−1 are indexed to the D and G bands from partially graphitized carbon, respectively.42 The corresponding intensity ratio of D band to G band (ID/IG = 1.01) demonstrates the copious defects and disordered structure of carbon after nitrogen doping, affording a favor to boost the electrical conductivity with regard to a remarkable rate capability. In addition, a worth-noting Raman shift at about 649 cm−1 associated with the Mn−O vibration manifests the existence of MnO in RGO-MnO@NC, in good agreement with the XRD results. XPS was implemented to further investigate the surface chemical composition and bonding state of the as-synthesized RGO-MnO@NC hybrid. Figure 4c represents the full-surveyscan XPS spectrum, showing the presence of Mn, O, C, and N elements and no detection of impurities. As observed from the high-resolution Mn 2p spectrum (Figure 4d), two remarkable peaks appear at 641.0 eV (Mn 2p1/2) and 653.1 eV (Mn 2p3/2) with an energy difference of 11.7 eV, proving the formation of MnO.43 As depicted in Figure 4e, the fitted C 1s XPS spectrum of RGO-MnO@NC could be deconvoluted into four energy bands, existing at 284.6 eV (CC), 284.9 eV (C−N), 286.4 eV (C−O−C), and 289.0 eV (OCO).44 The N 1s XPS spectrum illustrates the existence of three typical N atoms (located at 398.2 eV for pyridinic N, 400.5 eV for pyrrolic N, and 403.5 eV for graphitic N) in Figure 4f, which could boost the electrical conductivity, Li+ intercalation, and pseudocapacitive effect of the hybrid, thus improving the lithium-storage performance.45,46 N2 adsorption/desorption isotherm measurement (Figure 4g) was used to study the pore characteristic of RGO-MnO@NC, indicating this composite due to a high Brunauer−Emmett−Teller specific surface area of 85.8 m2 g−1 with sufficient mesopores centered at about 4 nm in light of the Barrett−Joyner−Halenda pore-size distribution (inset in

materials. As shown from the SEM images of RGO-MnO@NC (Figure 2a,b), a MnO microsphere anchored on RGO (marked

Figure 2. SEM images of RGO-MnO@NC (a and b) and RGO-MnO (c and d) hybrids.

with yellow dotted circles) is well encapsulated by a thin carbon layer originating from polydopamine calcination compared with those of RGO-MnO (Figure 2c,d) attained through direct annealing of RGO-MnCO3, indicating that the MnO microsphere is composed of numerous nanoparticles with an average diameter of about 100 nm, well inherits the quasi-spherical shape of MnCO3 (Figure S2), and holds a nonuniform particle size distribution. Additionally, the void space inside the MnO microsphere, as a consequence of CO2 gas escape rooting from the decomposition of MnCO3 during calcination, could obviously be discovered in Figure S3 (marked with yellow dotted circles). Further details with respect to the RGO-MnO@NC structure were clarified by TEM and high-resolution TEM (HRTEM). Observation of the TEM images (Figure 3a,b) definitely reveals that highly electroconductive RGO successfully captures active substances of the MnO microsphere to build a sea−islandlike configuration, suggesting superior electron transport.38 In addition to bonding with RGO, the MnO microsphere with internal void space is well confined within the porous NC coating (Figure

Figure 3. TEM (a and b) and HRTEM (c) images of the RGOMnO@NC composite (the inset shows the corresponding fastFourier-transformed lattice fringe image). HAADF-STEM (d) and correlating elemental mapping (e−i) images of RGO-MnO@NC. C

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Figure 4. (a) XRD pattern of the RGO-MnO@NC hybrid. (b) Raman and XPS spectra of the RGO-MnO@NC. (c) Survey spectra. Highresolution spectra: (d) Mn 2p, (e) C 1s, and (f) N 1s, respectively. (g) N2 adsorption/desorption isotherms (inset: corresponding pore-size distribution) of RGO-MnO@NC.

the synergistic effect of RGO and NC.24,49 Furthermore, starting from the third cycle, the CV curves of RGO-MnO@ NC are quite well-overlapped, implying good electrochemical reversibility. Figure 5b reveals the characteristic discharge/ charge curves of the RGO-MnO@NC electrode for the first three cycles at a current of 0.025 A g−1 with a potential window of 0.01−3.0 V (vs Li/Li+). The first cycle offers a high discharge capacity of 1069 mAh g−1 and a charge capacity of 740 mAh g−1, obtaining an initial Coulombic efficiency (CE) of 69%, of which the paramount capacity-loss is assigned to the irreversible loss of lithium as a consequence of SEI film formation, inadequate metallic Mn reaction with Li2O in the charge process, as well as irreversible Li+ insertion in the defects of RGO and NC.36 In line with the above CV discoveries, the flat lithiation plateaus at around 0.65 V for the discharge curve and the corresponding sloped delithiation plateaus between 1.2 and 1.6 V for the charge curve are exhibited in the discharge/charge curves after the first cycle. Additionally, two interesting voltage plateaus emerge at ca. 2.1 V (region 2) and 1.1 V (region 1) in the typical discharge/ charge curves (Figure 5c), attributed to the further oxidation of Mn2+ to Mn3+ and the corresponding reduction of Mn3+ to Mn2+, which is beneficial to upgrading the performance of the RGO-MnO@NC electrode because of such a redox reaction imparting extra capacity after the 50th cycle along with everupsurging over the ensuing cycles, as demonstrated in Figure S7 (dQ/dV curves for regions 1 and 2 in Figure 5c).24,37

Figure 4g), which is helpful toward electrolyte penetration, Li+ diffusion, and alleviation of volume change during cycling. The virtues stemming from the intriguing architecture of RGO-MnO@NC inspired us to explore the electrochemical performance of this hybrid as an anode for LIBs within the voltage window from 0.01 to 3.0 V (vs Li/Li+). Figure 5a shows the representative CV curves of RGO-MnO@NC for the initial four successive cycles at a scan rate of 0.1 mV s−1. The cathodic peaks that emerged below 1.0 V in the first cathodic process are ascribed to generating the SEI layer and initially reducing MnO to metallic Mn (MnO + 2Li+ + 2e− → Mn + Li2O).29,47 Undergoing the first cathodic scan, the generated Li2O and metal Mn could change the microstructure of the RGO-MnO@NC electrode, which is advantageous to promoting the reaction kinetics and extreme utilization of active materials, thus resulting in the reduction peak at 0.25 V moving to ca. 0.45 V during following cycles.36 During the first anodic process, a broad oxidation peak appearing at 1.27 V is assigned to the reversible oxidation of metallic Mn to MnO (Mn + Li2O → MnO + 2Li+ + 2e−) as well as Li2O decomposition, shifting to 1.3 V from the second cycle.48 It is noteworthy that a pair of weak oxidation peaks located at about 1.07 and 2.07 V, respectively, remain along with cycling, suggesting a lower overpotential between Mn2+ and Mn3+ owing to the enhanced electrochemical kinetics arising from the refining of MnO nanoparticles with the proceeding of the reaction and the improved electrical conductivity deriving from D

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Figure 5. Electrochemical performance of the RGO-MnO@NC electrode: (a) CV curves of the initial four cycles with a scan rate of 0.1 mV s−1, (b) discharge/charge curves in the first three cycles at a current of 0.025 A g−1, and (c) typical discharge/charge curves at a current of 1 A g−1. (d) Comparative cycling performance of RGO-MnO@NC, RGO-MnO@NC-80, and RGO-MnO@NC-30 at a current of 5 A g−1 for 200 cycles. Rate capabilities (e) and Nyquist plots before cycling (f) of RGO-MnO@NC, RGO-MnO, and MnO electrodes. In the inset of the equivalent circuit model, Rs and Rct represent the internal resistance of the solution and electrodes and the charge-transfer resistance, CPEsei and CPEdl indicate the SEI film capacitance and the double-layer capacitance, and ZW refers to the Warburg impedance.

chemical kinetics. In particular, at the low current densities of 0.1 and 0.2 A g−1, no obvious difference is presented between RGO-MnO@NC and RGO-MnO, which might be ascribed to the synergetic effect of the negligible overpotential difference especially existing in the case of the low current density and relative stabilization of the SEI film during the initial cycles. Capacity degradation along with increasing rate is a pervasive phenomenon with respect to the mass-diffusion-limiting process, with the result that RGO-MnO@NC delivers excellent discharge capacities of 599, 530, 455, and 331 mAh g−1 with high current densities of 0.5, 1.0, 2.0, and 5.0 A g−1, respectively, surpassing the performance of RGO-MnO by virtue of better conductivity occupying the predominant role in accelerating electron transport at higher current density. Even though the current density is returned to 0.1 A g−1 from high rate measurements, RGO-MnO@NC still shows almost the same discharge capacity of 733 mAh g−1, verifying the excellent recovery performance for Li+ insertion/extraction. To further survey the eminent rate performance of the RGO-MnO@NC electrode, EIS was conducted, as shown in Figure 5f. The charge-transfer resistance (Rct) stemming from the electrolyte/ electrode interface shows a compressed semicircle of Nyquist plots from high to medium frequency, displaying that the Rct value of RGO-MnO@NC (106 Ω) is lower than those of RGO-MnO (136 Ω) and MnO (179 Ω) for superior electrical conductivity, matching well with rate analysis. As stated, designing the rational structure of RGO-MnO@NC coupled with the structural merits of previously reported anodes could provide exceptional performance as expected. As discovered in other metal-oxide-based electrodes, the excellent rate capability of RGO-MnO@NC may be a result of the enhanced pseudocapacitive behavior, strongly depending

To better understand the NC-layer-dependent electrochemical performance, RGO-MnO@NC hybrids with different thicknesses of NC layers were synthesized by tuning the adding content of dopamine during the coating process (a number behind RGO-MnO@NC means the adding content of dopamine with units of milligrams). Figure 5d presents the comparative cycling performances of RGO-MnO@NC, RGOMnO@NC-80, and RGO-MnO@NC-30 at a current of 5 A g−1 for 200 cycles. Notwithstanding that RGO-MnO@NC-80 with a thicker NC layer can favor better integrality of the electrode for remarkable cyclability, the increased Li+-transport path with thickening of the electrode/electrolyte interface is detrimental to the electrochemical performance toward specific capacity as a result of the relatively lower content of MnO. Compared to RGO-MnO@NC (discharge capacity of 421 mAh g−1 after the 200th cycle) and RGO-MnO@NC-80 (discharge capacity of 329 mAh g−1 after the 200th cycle), RGO-MnO@NC-30 possessing a thinner NC layer could not effectively advance the electrode conductivity and sustain stable SEI films on the electrode surface, performing capacity decay after the 100th cycle (discharge capacity of 224 mAh g−1 after the 200th cycle). Therefore, the optimal thickness of the NC layer plays a significant role in improving Li+ storage for the RGO-MnO@NC electrode. The rate capabilities of the RGO-MnO@NC, RGO-MnO, and MnO electrodes were further evaluated in Figure 5e. First, the assembled cells were measured with a current density of 0.025 A g−1 in the initiate three cycles and subsequently at different current densities from 0.1 to 5 A g−1. In sharp contrast with MnO, RGO-MnO@NC and RGO-MnO show an outstanding rate performance because of RGO and NC enduing superb electrical conductivity to elevate the electroE

DOI: 10.1021/acs.inorgchem.8b02270 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Kinetic analysis of the lithium-storage performance for the RGO-MnO@NC electrode: (a) scan-rate-dependent CVs from 0.2 to 1.0 mV s−1, (b) log(i) versus log(v) plots of cathodic/anodic peaks, (c) capacitive contribution (green lines) separated with the CV profile with a scan rate of 0.1 mV s−1, and (d) contribution ratio of the capacitive effect (green) and diffusion-controlled (red) mechanisms for the fresh cell under different scan rates.

Figure 7. (a) Cycling performances of RGO-MnO@NC, RGO-MnO, and MnO at a current density of 1.0 A g−1. (b) Schematic illustration of the behaviors of RGO-MnO@NC during lithiation/delithiation processes.

on the improved electrical conductivity for diminishing potential energy barrier and the high specific surface area for more copious active sites.50 The electrochemical kinetic analysis was applied to further elucidate the capative-like behavior of the RGO-MnO@NC electrode, as shown in Figure

6. CV curves (Figure 6a) at various scan rates display a pair of broad cathodic/anodic peaks retaining a well shape with increasing scan rate, featuring a typical pseudocapacitive material. More insight of the charge-storage mechanism could be gained through a related quantitative analysis based F

DOI: 10.1021/acs.inorgchem.8b02270 Inorg. Chem. XXXX, XXX, XXX−XXX

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CONCLUSION In summary, here a facile strategy has been developed to elaborately synthesize RGO-anchored MnO with an N-doped porous carbon coating through successive solvothermal, dopamine self-polymerization, and carbonization methods. The dopamine-derived N-doped porous carbon coating, like a cap, uniformly encapsulates the RGO-supported MnO, not only providing plenty of surface defects, pores, and active sites to enhance the electron/ion-transfer kinetics but also serving as a buffer and separation layer to accommodate the volume expansion of MnO during the discharge process and reduce the unpredicted consumption of lithium due to direct contact between the active materials and electrolyte, respectively. Moreover, the RGO sheet could enable the large conductive network for accelerating electron transfer, meanwhile mitigating the volume change of MnO during cycling due to its remarkable toughness. Last but not least, the internal void construction can further improve the structural stability of the electrode for better cycling performance. Therefore, together with the above-stated advantages, the RGO-MnO@NC anode manifested a prominent reversibility (almost 100% CE after the first cycle at 1.0 A g−1), cycling stability (607 mAh g−1 after 550 cycles at 1.0 A g−1), and rate performance (331 mAh g−1 at 5.0 A g−1). Overall, such a rational architecture may supply a feasible and simple route for preparing hybrid electrode materials with excellent Li+ storage.

on the power-law relationship of the current (i) and scan rate (v), i = avb, where an estimated b value could infer the chargestorage model of the electrode material: b with a value of 0.5 represents a diffusion-controlled nature, and b having a value of 1.0 refers to a capacitive nature.29 As revealed in Figure 6b, the b values of peaks 1 and 2 are 0.68 and 0.80, respectively, thus suggesting that both charge-storage mechanisms occur synchronously, which is beneficial to improving the electrochemical performance due to faster lithium storage of surfacecontrolled pseudocapacitive behavior, especially at high rate. The pseudocapacitive contribution ratio was further quantized by using the method reported by Dunn,51 dividing the current (i) into two parts of the pseudocapacitive behavior (k1v) and diffusion-controlled process (k2v1/2) under a defined potential V: i(V) = k1v + k2v1/2. With a scan rate of 0.1 mV s−1 (Figure 6c), about 18.9% of the total capacity arises from the capacitive contribution for the fresh RGO-MnO@NC electrode. Figure 6d clearly shows the capacitive contribution going up to 42.4% when the scan rate increases to 1.0 mV s−1, also confirming that this hybrid holds the advantage of capacitive behavior toward improved Li+ storage. The long-term cycling stability of RGO-MnO@NC was also evaluated at a current density of 1.0 A g−1 after the initial three activating cycles at 0.025 A g−1 with the range of 0.01−3.0 V versus Li/Li+ (Figure 7a), in addition to the samples of RGOMnO and MnO. Subjected to a gentle decline over the first 20 cycles, the RGO-MnO@NC electrode still delivers a high reversible capacity of 607 mAh g−1 after 550 cycles under a current of 1.0 A g−1, obviously outperforming the RGO-MnO (258 mAh g−1) and MnO (164 mAh g−1) electrodes. In the rear of the first discharge/charge process at 1.0 A g−1, RGOMnO@NC shows nearly 100% CE within the whole cycle period, indicating distinguished reversibility for lithium storage. To further clarify the close relationship between the admirable cycling stability and favorable architectural features, the behaviors of RGO-MnO@NC during lithiation/delithiation processes were illustrated, as shown in Figure 7b. The porous carbon coating can not only shorten the Li+ transport path and serve as an isolated layer between MnO and the electrolyte to reduce the unexpected consumption of lithium but also efficiently alleviate the volume variation of MnO during the cycling process. Moreover, RGO regarding the very attractive electroconductivity matrix can afford the larger conductive network to boost electron transport. Additionally, the internal voids of spherelike MnO also play an important role in accommodating volume expansion to maintain the integrity of the electrode. More evidence about the different structural evolutions of the RGO-MnO@NC electrode was revealed by TEM characterizations (Figure S8). No obvious morphology variations could be found for the electrode after the first charge, the first discharge, and the 250th discharge, implying overall structural integrity after extended cycling. Owing to the impact of carbon black and binder in the assembly process with the internal voids clearly distinguished, the microstructures of marked areas in Figure S8a−c were clarified by the high-magnification TEM images (Figure S8d− f), respectively. More internal voids could be apparently recognized after the first charge, compared with the 1st and 250th discharges. Furthermore, the relatively narrowing bright regions also suggest the existence of internal voids even after undergoing the 250th discharge, indicating the significant role of internal voids in enhancing the cycling stability of the RGOMnO@NC electrode for improving Li+ storage.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02270. Additional ζ potential, SEM, TEM, XRD, and TGA (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hao Wu: 0000-0002-8366-3648 Heng Liu: 0000-0003-2458-5915 Yun Zhang: 0000-0001-7505-1097 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support by the Sichuan Province Science and Technology Support Program (2017GZ0132).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b02270 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02270 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02270 Inorg. Chem. XXXX, XXX, XXX−XXX