Hierarchically Porous N,S-Codoped Carbon-Embedded Dual Phase

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Hierarchically Porous N,S-Codoped Carbon-Embedded Dual Phase MnO/MnS Nanoparticles for Efficient Lithium Ion Storage Yujie Wang,†,‡,§ Hao Wu,†,§ Ling Huang,† Hang Zhao,† Zhifang Liu,† Xianchun Chen,∥ Heng Liu,† and Yun Zhang*,† †

Department of Advanced Energy Materials, College of Materials Science and Engineering, Sichuan University, Chengdu 610064, P. R. China ∥ Department of Inorganic Materials Engineering, 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 S Supporting Information *

ABSTRACT: An intriguingly nanostructured composite comprising of MnO/MnS nanoparticles embedded in an N,S-codoped carbon frame (MnO/MnS@C) is designed here and employed as a promising Li-ion storage electrode material to address the challenge of inferior conductivity and large volume change toward manganese chalcogenide-based anode. Combining with the merits of coherent MnO/MnS, elaborately hierarchical-porous architecture and N,S-codoped carbon frame, this composite exhibits high lithium-ion storage capacity (591 mAh g−1 at 0.1 A g−1) and remarkable cyclic performance (628 mAh g−1 at 1 A g−1 over 330 cycles). It is revealed via quantitative analysis that capacitive effect is also responsible for Li+ storage except ordinary diffusion-controlled mechanism, which consists of faradaic surface pseudocapacitance rooting from further oxidation of Mn2+ and nonfaradaic interfacial double-layer capacitance stemming from the charge separation at the MnO/MnS phase boundary. As a dynamic equilibrium for diffusion-controlled lithium storage, such capacitive contribution leads to ever-increasing Li-ion storage. The delicate construction endows an improved ion/electron transport kinetics, increased electrode/electrolyte contact area and plentifully heterogeneous interface, accounting for the high capacity and long-cycle stability.

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INTRODUCTION Because of the burgeoning demand of traditionally exhaustible fossil fuels, lithium ion batteries (LIBs) have become one of the considerably promising power sources to handle the urgencies of energy dilemma and serious greenhouse effect because of their reversibility and absence of polluting gas emissions.1−3 Compared to ubiquitous graphite anode for LIBs holding a low theoretical capacity (372 mAh g−1), constructing advanced anode alternatives with high energy density and long cycle lifespan, such as alloy-type materials concerning Si4 and conversion-type transition-metal compounds involving iron oxides,5,6 are highly desirable to fulfill the ever-growing needs for portable electronics and electric vehicles.7−10 Plenty of efforts have been dedicated to developing transition-metal oxides (TMOs) as anodes for LIBs because of their high theoretical capacity and low cost as well as low toxicity.11−13 Compared with other TMOs, manganese oxides are of attractive anode materials because of their abundant reserves, affordable cost, low overpotential, and environmental kindness.14−16 Notwithstanding the rich redox chemistry based on multiple electron transfer contributing high specific capacity, © XXXX American Chemical Society

there are some generally encountered obstacles for MnO, to be potential anode candidates, such as sluggish ion/electron transport kinetics, volumetric expansion and contraction, as well as low surface area of the bulks, leading to degraded rate performance, rapid capacity diminution and low capacitive contribution.17,18 Currently, various strategies have been deliberately conceived for the sake of dealing with these tackles, including coating process, nanostructuring, and morphology controlling.19,20 With the advantages of improved electronic conductivity via reducing the charge transfer resistance and mitigated volume change to evade the pulverization, amorphous carbon are extensively utilized as a passivation coating-shell to benefit the cycling stability and rate performance.21−25 In addition, nanosized materials could shorten the diffusion distance of Li-ion within the electrodes, increase the contact area between electrode and electrolyte, and reduce the areal current density, thus improving the reaction kinetics to better rate capaReceived: April 27, 2018

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DOI: 10.1021/acs.inorgchem.8b01156 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Schematic illustration of the synthesis for the MnO/MnS@C core−shell composite architecture. approach.38 Then, 100 mg of MnO2 nanowires was dispersed in 75 mL of ethylene glycol under ultrasonication for 1 h, followed by the addition of 370 mg of thiourea with further ultrasonic treatment for 10 min. The above mixture was transferred to a 100 mL Teflon-lined autoclave and hold at 200 °C in an oven for 15 h. The as-resulted precipitates were gathered through centrifugation, washed with ethanol, and finally dispersed in a 110 mL of Tris-buffer solution (pH ∼8.5) by magnetic stirring to form a suspension. Subsequently, 200 mg of dopamine was added into the mixture under stirring and kept at 30 °C for 24 h. After they were centrifuged, washed with distilled water and ethanol, and dried, the resulting MnO/MnS@ polydopamines were heated at 400 °C for 2 h with a rate of 1 °C min−1 under a Ar atmosphere, and then at 800 °C for 2 h with a heating rate of 3 °C min−1 to harvest MnO/MnS@C-15 nanoparticles. Additional MnO/MnS@C-1 and MnO/MnS@C-24 samples were also obtained via changing the solvothermal time. Material Characterization and Analysis. SEM images were obtained using a field-emission scanning electron microscopy (FESEM, FEI Inspect F50). The microstructural properties were studied by TEM (aberration-corrected FEI Titan G2 60−300, coupled with an EDX spectrometer). Powder X-ray diffraction (XRD) was implemented on a Bruker D2 PHASER diffractometer using Cu Kα radiation. Raman spectra were recorded with a 532 nm excitation laser on a Renishaw in Via Reflex Raman system. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALab250Xi spectrometer. N2 adsorption/desorption isotherms were acquired with a Micromeritics ASAP 2460 instrument. Thermogravimetric analysis (TGA) was performed by a NetzschSTA 449 F3 from room temperature up to 1000 °C under air with a heating rate of 10 °C min−1. Electrochemical Measurements. The electrochemical behavior of the MnO/MnS@C was measured using CR2032 coin-type cells with a pure lithium disk as the counter/reference electrode and a Celgard 2400 membrane as the separator. The working electrodes were fabricated by mixing the active materials, carbon black and SBR/ CMC binder with a weight ratio of 80:10:10 and coated on copper foil. Subsequently drying at 100 °C for 2 h in a vacuum oven, the resulting Cu foil was punched into disks with a diameter of 1.2 cm. The mass loading of active materials was controlled at about 1.0 mg cm−2. The electrolyte composes of a solution of 1 M LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (1:1:1 in volume). All the Coin cells were assembled in the high-purity Ar-filled glovebox. Galvanostatic charge−discharge tests were performed on a Neware battery test station (BTS-5 V10 mA) at various current densities with the potential range of 0.01−3.0 V (vs Li+/Li) at room temperature. The CV and EIS measurements were conducted on an electrochemistry workstation of Princeton Applied Research (USA). The CV curves were tested between 0.01 and 3.0 V at different scan rates. The EIS was investigated in a frequency range of 0.01−100000 Hz with an amplitude of 5 mV.

bility.26−28 Furthermore, a few endeavors have been tried to take the merits of diversified architectures, such as wire, sheet, and sphere, to offer excellent electrochemical active materials for satisfying the practical needs of high performance batteries.29−33 Unfortunately, in spite of aforementioned approaches achieving some alternative electrodes, superior anode materials remain to be a great concern because of the instinctive drawbacks. Recently, heterogeneous iron oxidesbased composites have exhibited an intriguingly synergistic improvement of LIBs performance by virtue of introducing inactive Fe3C phase.34−36 However, MnO inheriting the features of TMOs, a high theoretical capacity of 755 mAh g−1, has not yet been rationally designed as a new anode of carbon embedded binary cooperative complementary active nanomaterials for highly efficient lithium ion storage. Herein, we present, for the first time, a strategy to fabricate a unique porous N,S-codoped carbon-embedded manganese chalcogenide anode configuration for LIBs, which consists of irregular N,S-codoped carbon frame and dual phase MnO/MnS core (MnO/MnS@C). Synthesizing such a nanohybrid architecture relates to sulfuration of MnO2 nanowires, polymerization and coating of dopamine, concurrent with in situ carbonization and subsequent doping. Therefore, combining the advantages of doped carbon coating, nanocrystallization and heterogeneous synergy, this composite is anticipated to well address several important concerns for LIBs: (1) Porous diffusion channels can promote electron/ion access, allowing quicker reaction kinetics. (2) Heteroatoms doping tailors the electronic structure of carbon, hence increasing the electrode conductivity and chemical activity to further enhance the rate capability.37 (3) Carbon frame featured with a high hardness not only maintains the structural integrity of electrode upon lithiation/delithiation, meanwhile, also inhibits the active species directly contacting with the electrolyte, thereby reducing the side reaction to improve the capacity retention. (4) Nanostructure shortens the diffusion pathway of electron/ ion in the crystal and endows more reaction sites, facilitating a higher interface lithium-ion flux. (5) In view of the conductivity difference, MnS phase should construct a heterogeneous interface with MnO to form space charge layers, permitting additional lithium ion storage. Consequently, the as-synthesized MnO/MnS@C nanohybrid demonstrates a long cycling stability (628 mAh g−1 remaining after 330 cycles at 1.0 A g−1) and superior rate performance (315 mAh g−1 at 2.0 A g−1; 144 mAh g−1 at 5.0 A g−1) and, accordingly, can be of greatly intriguing anode material for LIBs.

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RESULTS AND DISCUSSION The MnO/MnS@C nanocomposites were fabricated through a three-step synthetic process in which sulfurated MnO 2

EXPERIMENTAL SECTION

Synthesis of MnO/MnS@C Materials. Typically, the Ultralong MnO2 nanowires were first prepared by a reported hydrothermal B

DOI: 10.1021/acs.inorgchem.8b01156 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

results demonstrate that a rational construct with the neighboring dual phase MnO/MnS particles embedded in a carbon frame is designed successfully. This intriguing structure is further verified by energy dispersive X-ray (EDX) elemental mappings (Figure 3e−i) attained from the high-angle annular dark-field scanning TEM (HAADF-STEM) image (Figure 3d). It is visible that Mn, O, S, and C have a uniform distribution in MnO/MnS@C-15, indicating the adjoining MnO/MnS nanoparticles offering heterogeneously bonded interfaces appear to exist with well wrapped by carbon. Notably, the N element derived from dopamine is distributed continuously, implying a homogeneous doping of N atom in carbon frame. Figure S2 acquired from HAADF-STEM technique also confirms the structure formation of well carbon-embedded dual-phase MnO/MnS nanoparticles. X-ray powder diffraction (XRD) was utilized to identify the phase purity and crystal structure of MnO/MnS@C. The XRD patterns of samples (Figure 4a) exhibit both the typical (111), (200), (220), (311), and (222) peaks assigned to cubic MnO (JCPDS No. 07-0230)38,41 and (111), (200), (220), (311), (222), and (400) peaks attributed to cubic MnS (JCPDS No. 06-0518). An additional diffraction peak at ∼26° is detected, which can be indexed to the codoping of N and S atom in carbon.42,43 The desirable phase ratio of MnO to MnS can be deliberate to tune by altering solvothermal reaction time, thus expecting more efficient storage of lithium ion owing to the appropriate electro-conductibility and induction of more interfaces. TGA was adopted to evaluate the content of carbon, MnO and MnS in MnO/MnS@C-15 composite (Figure S3), showing that the weight fraction is about 52%, 13%, and 35%, respectively. Such considerable carbon content is favorable to confer the MnO/MnS@C material high electrical conductivity and alleviative configuration collapse during discharge/charge process. The graphitic quality of carbon frame in MnO/MnS@ C-15 sample was investigated by Raman spectroscopy, as shown in Figure 4b. Two typical Raman shifts at around 1358 and 1584 cm−1 are identified, corresponding to sp3-defect disordered carbon and sp2-hybridized graphitic carbon, respectively.44,45 The reduced ratio of the D to G band (ID/ IG = 0.79) suggests plentiful defects and disordered carbon structure after N,S-codoping, benefiting the improvement of electrical conductivity toward an outstanding rate performance. Additionally, a remarkable peak at about 642 cm−1 representing Mn−O vibrational band confirms the presence of MnO in MnO/MnS@C-15, while two weak intensity peaks appearing at 307 and 355 cm−1 reveal the presence of MnS, in line with XRD results. To survey the surface chemical composition and valence state of MnO/MnS@C-15composite, XPS was employed. Figure 4c shows the existence of Mn 2p, Mn 3p, Mn 3s, O 1s, C 1s, S 2s, S 2p, and N 1s, without observing other impurities. Two notable peaks located at 641.0 eV for Mn 2p1/2 and 653.1 eV for Mn 2p3/2 are displayed in the high-resolution Mn 2p spectrum (Figure 4d). The Mn 2p 3/2 peak is deconvoluted into three peaks located at 640.6, 641.6, and 642.8 eV, corresponding with the Mn2+, Mn3+ and Mn4+ species, respectively. Considering no high valence manganese species were recognized by Raman spectroscopy, the XPS Mn3+ and Mn4+ species might be resulted from the oxidation of surface layer on contacting with dissolved oxygen during the synthetic process, which could increase the capacity of the composite due to their higher theoretical capacity. The highresolution C 1s spectra (Figure 4e) can be deconvoluted into

nanowire precursors are coated with dopamine-derived porous carbon (Figure 1). First, the ultralong MnO2 nanowires with an average diameter of ∼100 nm and lengths of hundreds of micrometers were prepared via a hydrothermal reaction (Figure S1).38 Then, a solvothermal process was utilized to sulfurate the as-formed MnO2 nanowire precursors with a sulfur source of thiourea in an ethylene glycol system, resulting in the formation of electrochemically active dual phase MnO/MnS (a followed number represents the solvothermal reaction time with unit of hour). These MnO/MnS nanoparticles were further wrapped with a polydopamine layer, followed by annealing at high temperature in argon to transform into the desired composites.39 During the annealing process, a porous N, Scodoped carbon frame was obtained. The morphologies and microstructures of the resultant MnO/MnS@C-15 nanohybrids were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 2a−d show the characteristic

Figure 2. SEM images of (a, c) MnO/MnS and (b, d) nO/MnS@C15 nanoparticles.

SEM images of MnO/MnS and MnO/MnS@C-15 nanoparticles, unshrouding randomly aggregative and anomalous appearances. It clearly displays that MnO/MnS@C-15 nanoparticles possess rough surfaces compared to MnO/MnS nanoparticles, due to the carbon coating from poly dopamine pyrolysis under an Ar atmosphere. The aggregation of both nanoparticles emerges because of annealing treatment. Figure 2d exhibits MnO/MnS@C-15 nanoparticles have uneven diameters ranging from about 30 to 100 nm. More structural details of the MnO/MnS@C-15 nanoparticles were further elucidated by TEM and high-resolution TEM (HRTEM). High-magnification TEM images (Figure 3a and b) distinctly show that the material has a variable core-particle size distribution and in which the MnO/MnS particles have been entirely and uniformly encapsulated by carbon frame, establishing a sea-like highly electronically conductive matrix and islandlike active substance domain to ensure better electrochemical performance possibly.40 Observation from the HRTEM image (Figure 3c) reveals high crystallinity with distinct lattice fringes adjacently. The corresponding fast Fourier transformation images (insets of Figure 3c) show the cubic structure of MnO (111) lattice fringes with a d-spacing of 0.257 nm and MnS (111) lattice fringes of 0.302 nm. Combined with the white arrow and dashed line indications of carbon matrix, these C

DOI: 10.1021/acs.inorgchem.8b01156 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a, b) EM and (c) HRTEM images of MnO/MnS@C-15 nanoparticles. (The inset shows the correlating fast Fourier transformed lattice fringe images.) (d) HAADF-STEM image and (e−i) corresponding elemental mapping images of MnO/MnS@C-15.

metallic Mn (MnO + 2Li+ + 2e− → Mn + Li2O, MnS + 2Li+ + 2e− → Mn + Li2S) and the formation of a solid−electrolyte interface (SEI) layer.50,51 During subsequent cycles, owing to the enhanced kinetics and variations in the microstructure of the electrode stemmed from the generating of Li2O, Li2S, and metal Mn after the first lithiation process, the peak at 0.05 V shifts to about 0.35 V from the second cycle.52 In the first anodic process or in succeeding cycles, one broad peak located at around 1.2 V can be viewed in consequence of the oxidation of metallic Mn (Mn + Li2O →MnO + 2Li+ + 2e−, Mn + Li2S → MnS + 2Li+ + 2e−) as well as decomposition of Li2O and Li2S.53 It is worth noting that two additional weak oxidation peaks centered at around 2.07 and 2.5 V remain as cycling proceeds, which can be attributed to the formation of higher oxidation state manganese (Mn3+ or Mn4+) resulted from the reduced over potential of Mn2+ to Mn3+ or Mn4+ and elevated electro-conductibility on account of heteroatom-doped carbon, facilitating to endow extra capacity of the MnO/MnS@C-15 electrode within the following cycles.53−55After the second cycle, the well-overlapped CV curves denotes a comparatively stable discharge/charge process with superior electrochemical reversibility. Figure 5b shows representative discharge/charge voltage profiles of MnO/MnS@C-15 electrode for the initial three cycles at a current density of 0.025 A g−1 and the subsequent cyclesat a current density of 1.0 A g−1 between 0.01 and 3.0 V (vs Li/Li+). The first cycle exhibits a discharge and charge capacity of 1085 mAh g−1 and 705 mAhg−1, respectively, with an initial Coulombic efficiency (CE) of 65%. The initial irreversible capacity is dominantly ascribed to the formation of an SEI layer and irreversible insertion of Li+ into the defects of carbon frame.52 Corresponding with the above CV results, the

five peaks. Typically, the peak centered at 283.8 eV is assigned to C−S bonds.46 The binding energy peak at 284.7 eV is ascribed to C−N bonds, while the other peaks located at 284.2, 286.8, and 288.3 eV coincide with C−C, C−O, and CO bonds, respectively. The high-resolution spectra of N 1s (Figure 4f) reveals the presence of pyridinic N (397.9 eV), pyrrolic N (398.5 eV), and graphitic N (401.3 eV), enhancing the surfaceinduced pseudocapacitance, lithium-ion diffusion kinetics and electrical conductivity of the composite, therefore upgrading the electrochemical performance.47,48 Figure 4g exhibits the deconvolution of high resolution peaks of S 2p, in which three deconvoluted peaks correlate with C−S−C (163.2 eV for S 2p3/2, 164.4 eV for S 2p1/2) and C−SOχ-C (168.2 eV) species, demonstrating the sulfur doping in carbon frame.49 The pore feature of MnO/MnS@C-15 composite was further characterized by N2 adsorption−desorption isothermal measurement (Figure 4h), revealing a high Brunauer−Emmett−Teller (BET) surface area of 93.7 m2g−1 with abundant mesoporous channels of about 4 nm on the grounds of the Barrett−Joyner−Halenda (BJH) pore size distribution (inset in Figure 4h), which is advantageous to electrolyte permeation, lithium ion entrance and mitigating volume expansion while cycling. The LIB performance of the as-synthesized hierarchical porous N,S-codoped MnO/MnS@C-15 was evaluated as an anode to shed light on the vital role of this unique morphology and microstructure. Typical cyclic voltammetry (CV) measurements (Figure 5a) of the MnO/MnS@C-15 electrode were carried out at a scan rate of 0.1 mV s−1 within a potential range from 0.01 to 3.0 V (vs Li/Li+) for the first four consecutive cycles. During the first cathodic scan, two peaks occurred below 0.5 V correspond to the initial reduction of MnO and MnS to D

DOI: 10.1021/acs.inorgchem.8b01156 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. (a) XRD patterns of MnO/MnS@C nanoparticles. (b) Raman spectra of MnO/MnS@C-15. (c−-g) XPS spectra of MnO/MnS@C-15: (c) survey and high-resolution spectra of (d) Mn 2p, (e) C 1s, (f) N 1s, and (g) S 2p, respectively. (h) N2 adsorption/desorption isotherms (inset shows corresponding pore size distribution) of MnO/MnS@C-15.

higher than those of MnO/MnS@C-1 (541 mAh g−1) and MnO/MnS@C-24 (551 mAh g−1), which might be induced by a synergetic effect of the electrical conductivity tuned by the proportion of MnS, high theoretical capacity adjusted by the content of MnO and capacitive charge storage rooting from faradic surface redox reactions and nonfaradic interfacial storage capacity locating at nanoscale MnO/MnS heterojunctions regarded as space charge layers.35,36 Even at relatively high current densities of 0.2, 0.5, 1.0, 2.0, and 5.0 A g−1, MnO/ MnS@C-15 still owns superior discharge capacities of 547, 474, 405, 315, and 144 mAh g−1, respectively. Moreover, the capacity of MnO/MnS@C-15 can achieve 573 mAh g−1 as the current density is switched back to 0.1 A g−1 after high rate tests, sticking out an excellent reversibility of Li+ storage. A pervasive scene of capacity deterioration with the increasing rate is observed toward mass transport limiting process. Impressively, such MnO/MnS@C electrodes almost deliver an equal capacity at the current density of 5.0 A g−1 owing to inferior intrinsic conductivity of metal oxides and sulfides, since conductive capability of material would become a dominant

well-defined and flat discharge plateaus of lithiation process in the range from 0.2 to 0.55 V, as well as the corresponding sloped charge plateaus of delithiation process between 1.2 and 1.6 V, are delivered in the initial discharge/charge profile. Furthermore, a slope over the voltage range of 1.8−2.2 V arising out of the further oxidation of Mn2+ to a higher oxidation state is observed in the charge process.54 It is well demonstrated that the capacity originating from the extra electrochemical oxidation of Mn2+ could be evidently examined after 50 cycles along with ever-increasing over the ensuing cycles, as clearly shown in Figures 5b and S4 (corresponding to dQ/dV curves at charge region 1 and discharge region 2 in Figure 5b).56 The rate capabilities of the MnO/MnS@C electrodes were further probed in Figure 5c. The assembled cells were first tested at the current density of 0.025 A g−1 for the initiate 3 cycles, and thereafter at various current densities from 0.1 to 5 A g−1. Along with current density increasing, the reversible capacity of MnO/MnS@C declines gently. When the current density of charge/discharge at 0.1 A g−1, the MnO/MnS@C-15 electrode delivers a stable discharge capacity of 591 mAh g−1, E

DOI: 10.1021/acs.inorgchem.8b01156 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. Electrochemical performance of the MnO/MnS@C-15 electrode: (a) CVs of the first four cycles with a scan rate of 0.1 mV s−1; (b) typical discharge−charge curves; (c) rate capabilities; (d) cyclic performances at a current of 1 A g−1; and (e) Nyquist plots of MnO/MnS@C electrodes before cycling. Within the inserted equivalent circuit model, RS, Rct, CPEsei, CPEdl and Zw refer to the internal resistance of solution and electrodes, the charge transfer resistance, SEI film capacitance, the double layer capacitance, and the Warburg impedance, respectively.

leads to Rct of about 132 Ω for MnO/MnS@C-15 pristine electrode, lower than 213 Ω for MnO/MnS@C-1 and higher than 101 Ω for MnO/MnS@C-24 pristine electrodes. Moreover, the variation of Zw also shows MnS content-dependent characterization, in good with Rct results. In our case, this phenomenon of ever-rising capacity along with cycling, which has been reported in many metal-oxidebased electrode materials,57,58 could be ascribe to two possible charge-storage mechanisms (the faradaic contribution of pseudocapacitance from surface redox reactions and the nonfaradaic contribution of interfacial storage capability from double-layer capacitance),59 offsetting the capacity fading from conversion reaction with the diffusion-controlled faradaic contribution. To further shed light on the capacitive-like behavior of MnO/MnS@C-15 electrode, CV measurements (Figure 6a) were performed at different scan rates from 0.2 to 1.0 mV s−1, showing a well-retained shape notwithstanding the increment of scan rate. Depending on the power-law relationship between current (i) and scan rate (v), i = avb, the degree of capacitive effect can be quantified upon the CV curves.50 Generally, the diffusion-controlled electrochemical process has a value of b approaching 0.5, while the surface controlled capacitance possesses a value of b near 1.0. Therefore, when the b value is between 0.5 and 1.0, it implies that the double mechanisms emerge simultaneously during the charge-storage process. The calculated b values (Figure 6b) for peaks 1, 2, 3, and 4, are 0.83, 0.84, 0.68, and 0.92, respectively, indicating a favored pseudocapacitive effect of MnO/MnS@C15. The ratio of pseudocapacitive contribution, employing the

influence for extremely fast electron transfer, especially at higher current density. In addition to the attractive rate capability, the novel hierarchically porous architecture also confers the MnO/ MnS@C-15 with distinguished cycling stability. Figure 5d presents the cycling performance and corresponding CE data of the MnO/MnS@C-15 electrode at 1.0 A g−1 after the first three activation processes at 0.025 A g−1 in the range of 0.01−3.0 V versus Li/Li+, as well for the contrast samples of MnO/MnS@ C-1 and MnO/MnS@C-24. The initial reversible capacity of MnO/MnS@ C-15 electrode is about 405 mAh g−1 at a current of 1.0 A g−1. Suffering from moderate decline over the first 30 cycles, the discharge capacity gradually climbs to capacity values as high as 628 mAh g−1 after 330 cycles, significantly surpassing that of MnO/MnS@C-1 (195 mAh g−1) and MnO/MnS@C24 (211 mAh g−1) electrodes within the same testing conditions. In the rear of the first cycle of MnO/MnS@C-15 at 1.0 A g−1, closely 100% CE is acquired throughout the whole cycle time, indicating advanced lithium-storage ability. To more clarify the admirable electrochemical performance of MnO/ MnS@C-15 electrode, the electrochemical impedance spectra were further applied, as displayed in Figure 5e. The depressed semicircle of Nyquist plots with a range from high to medium frequencies is related to the charge-transfer resistance (Rct) at the electrolyte/electrode interface, and the slope line in the low frequency region is correlated with Warburg resistance (Zw) on behalf of Li+ diffusion inside the electrodes. In view of metal sulfides exhibiting higher electrical conductivity than their corresponding metal oxides, the different proportion of MnS F

DOI: 10.1021/acs.inorgchem.8b01156 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Kinetics of lithium-storage behavior for MnO/MnS@C-15 electrode: (a) scan rate-relevant CVs from 0.1 to 1.0 mV s−1, (b) log i vs log v plots for different cathodic/anodic peaks, (c) capacitive contribution (green lines) divided with CV at a scan rate of 0.1 mV s−1, and (d) contribution ratio of capacitive (green) and diffusion-controlled (red) capacities for the fresh cell under various scan rates. (e) Schematic illustration of the interfacial lithium storage mechanism emerging at the two-phase boundary.

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approach reported by Dunn,60 is calculated through separating the current (i) at a fixed potential V into surface controlled pseudocapacitive effects (k1v) and diffusion-controlled electrochemical reactions (k2v1/2): i(V) = k1v + k2v1/2. Figure 6c reveals that about 24.6% of the total lithium charge derives from capacitive contribution toward the fresh MnO/MnS@C15 electrode at a scan rate of 0.1 mV s−1. As shown in Figure 6d, the capacitance contribution increases gradually to 50.1% while the scan raterises to 1.0 mV s−1. Together with the climbing tide of reversible capacity (Figure 5d) and increment of electrode resistance (Rct + RSEI) during the discharge−charge process (Figure S5), it implies that the interfacial nonfaradaic capacitive effect generating in the metal oxide/sulfide heterogeneous structure could lead to boosting the lithiumion storage performance of the hierarchically porous nanoparticle anode (Figure 6e).

CONCLUSION

In summary, through the coating strategy of dopamine selfpolymerization followed with high temperature annealing toward hydrothermally treated MnO nanowires, an ingenious hierarchically porous MnO/MnS@C anode was elaborately fabricated featuring neighboring dual phase MnO/MnS nanoparticles delicately embedded in an N,S-codoped carbon frame. It is vital that the heteroatom-doped carbon frame not only offers sufficient surface defects, nanopores and active sites, but also buffers the volume change during discharge/charge cycling. An appropriate fraction of MnS was introduced to form heterostructure, resulting in a possible interfacial nonfaradaic capacitive effect for enhancing capacity, meanwhile improving electro-conductivity of the whole electrode and maintaining high theoretical capacity compared to the nonactive phase inducing. Combining above stated advantages, the as-fabricated MnO/MnS@C-15 anode demonstrated an outstanding reverG

DOI: 10.1021/acs.inorgchem.8b01156 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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sibility (closely 100% CE over the whole cycle period), cycling stability (628 mAh g−1 after 330 cycles at 1.0 A g−1), and rate capability (315 mAh g−1 at 2.0 A g−1). This novel structure design route would inspire the conception of other hybrid materials with advanced performance for LIBs.

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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.8b01156. Additional SEM, HAADF-STEM, TGA, and EIS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Heng Liu: 0000-0003-2458-5915 Yun Zhang: 0000-0001-7505-1097 Author Contributions §

Y.W. and H.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Sichuan Province Science and Technology Support Program (No. 2017GZ0132).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01156 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01156 Inorg. Chem. XXXX, XXX, XXX−XXX