Electrospinning Synthesis of Mesoporous MnCoNiOx@Double

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Electrospinning Synthesis of Mesoporous MnCoNiOx@DoubleCarbon Nanofibers for Sodium Ion Battery Anode with Pseudocapacitive Behavior and Long Cycle Life Lijun Wu, Junwei Lang, Rutao Wang, Ruisheng Guo, and Xingbin Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11238 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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ACS Applied Materials & Interfaces

Electrospinning Synthesis of Mesoporous MnCoNiOx@Double-Carbon Nanofibers for Sodium Ion Battery Anode with Pseudocapacitive Behavior and Long Cycle Life Lijun Wua,b, Junwei Langa, Rutao Wang,a Ruisheng Guoa, Xingbin Yana,* a

Laboratory of Clean Energy Chemistry and Materials, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Lanzhou 730000, P. R. China.

b

University of Chinese Academy of Sciences, Beijing, 100039, P.R. China.

*Corresponding author. Tel.: +86-931-4968055; Fax: +86-931-4968055. E-mail address: [email protected] ABSTRACT In this work, MnCoNiOx (denoted as MCNO) nanocrystals (with the size of less than 30 nm) finely encapsulated in double-carbon (denoted as DC, including reduced graphene oxide and amorphous carbon derived by polymer) composite nanofibers (denoted as MCNO@DC) are successfully synthesized via an electrospinning method followed by a sintering treatment. The as-obtained MCNO@DC nanofibers present the superior sodium storage performance, and especially retain a high specific capacity of 230 mAh g−1 with a large capacity retention of about 96% at 0.1 A g−1 after 500 cycles and a specific capacity of 107 mAh g−1 with capacity retention of about 89% at 1 A g−1 after 6500 cycles. The outstanding cycle characteristic is mainly due to the tiny MCNO nanoparticles which could shorten the ion migration distance, and the three-dimensional (3D) DC framework which would remarkably promote the electronic transfer and efficiently limit the volume expansion during the progress of insertion and extraction of Na+ ions. Moreover, nitrogen-doped in carbon is able to improve the electrochemical capability as well. Finally, kinetic analysis of the redox reactions is used to verify the pseudocapacitive mechanism in charge storage and the feasibility of using MCNO@DC composite nanofibers as an anode for SIBs with above mentioned behavior. KEYWORDS: sodium ion battery, anode, transition metal oxide, carbon-encapsulating,

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pseudocapacitive behavior 1. INTRODUCTION Compared with lithium ion batteries (LIBs) with limited lithium resources and the increasing price in future, sodium ion batteries (SIBs) become an alternative to LIBs because of its low cost and abundant reserves.1,2 However, because the size of Na+ (0.98 Å) is larger than Li+ (0.68 Å), many conventional materials do not have enough interstitial spaces for the reversibly insertion and extraction of Na+ ions.3 Furthermore, the redox potential of Na/Na+ vs SHE is -2.71 V, which is higher than -3.04 V of Li/Li+.4 Therefore, most materials used as electrode materials for SIBs have lower energy density and poor cycle stability than that of LIBs.5 Thereby, further enhancing the electrochemical property of electrode materials for SIBs is still an important goal for researchers.6-8 However, although exciting progress has been made for the cathode materials of SIBs, there are a lot of problems still waiting to be solved before the large-scale applications of anode materials, such as low specific capacity, short cycle life, and poor rate capability.9-13 Among the anode materials used for SIBs, benefited from the chemical stability and capability of storing considerable amount of Na+ ions via a conversion reaction mechanism, transition metal oxides (TMxOy) are considered to be one of the promising anode materials for SIBs.14 But the large volume expansion involved during the insertion and extraction of Na+ ions, would lead to the collapsing of the TMxOy anodes, resulting in rapid capacity fade and poor cycling performance.15-18 In order to address these limitations, constructing stable nanostructured anodes to provide enough space for the large volume expansion was considered as one of effective strategies.19 For example, some researcher designed special nanostructures to relieve the mechanical stress caused by large volume variation during insertion and extraction of Na+ ions


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and shorten the ion/electron diffusion pathways.20-25 Especially, tailoring of a carbon matrix with nanostructured TMxOy finely encapsulated has attracted widely attention of many researchers in the past few years.26-28 The carbon materials in composite play an important role in improving the electronic conductivity and avoiding materials crushing during cycling.29,30 The interconnected porous framework purposely devised in composite not only can provide more spaces for Na+ storage than the counterparts without this framework, but also can increase the contact area between the electrolyte and electrode materials.26,31-33 Thus, many methods of preparing carbon-containing composites have been reported,29-35 and among them, electrospinning technique with many advantages of controllable morphology of products, feasible process and low-cost was usually considered as an effective method for the preparation of various carbon-based composites.36-38 In particular, this method was usually used to prepare a variety of composite materials with small nanoparticles and porous carbon-skeleton structure that can effectively improve the electrochemical property of host materials as anode for LIBs and SIBs.32-35 Recently， doping nitrogen is also reported to be a feasible method for enhancing the sodium storage characteristics of carbon electrodes for SIBs, and various kinds of nitrogen-doped (N-doped) carbon materials such as N-doped carbon nanofibers,39 nanosheets40 and nanofoams,41 have present better sodium storage performance when were in contrast to their corresponding undoped materials. In this work, the different N species were also attempted to dope into the TMxOy and carbon composites to improve electrochemical performances of them. According to recent reports, due to the mutual cooperation between the heterogeneous metallic elements, heterogeneous metal doping can obviously enhance the cyclic stability and rate capability of metal oxides during the process of Li or Na insertion or extraction, and was received



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greatly attention.42,43 Especially, it is reported that cobalt, nickel, and manganese ions in Mn-Co-Ni ternary oxide (MCNO) could play a mutual complementary role during the process of redox reactions.43,44 Besides, due to the similar atom radii of Mn, Ni, and Co, the introduction of Ni atoms into MnCo2O4 partly replacing Co atoms would obtain a Mn-Co-Ni ternary oxide (MCNO) without a distinct crystal structure change.43-46,51 Therefore, the MCNO has been widely studied as catalyst,47,48 negative temperature coefficient thermistor,49,50 electrode materials for Li ion batteries43 and asymmetric energy storage devices44-46,51 and so on,52,53 Nevertheless, this ternary oxide as anode material for SIBs was rarely reported. Despite the electrochemical property of supercapacitors and LIBs with this ternary oxide as electrode material have been greatly improved, the conductivity is still too poor to promote rapid electron transport to the property of high rate .44-47 However, improving the conductivity and relieving the volume expansion of MCNO as anode for SIB are not reported. As well known, because graphene has various advantages such as large surface area, light weight, good structural flexibility and high conductivity and so on, it is usually used as additive or matrix of electrode materials for LIBs and SIBs.29,31,54 Recently, some studies have shown that the volume expansion of composites consisted of TMxOy and reduced graphene oxide (rGO) can be effectively prevented, the conductivity can be also improved, and the lithium and sodium storage characteristic of these composites can be also obviously promoted.29,31,55-57 Therefore, the present research aims at improving the cyclic stability of the MCNO as anode for SIBs through the synergistic effects of tiny nanoparticles, three-dimentional (3D) double-carbon (DC) framework, N-doped in carbon. Herein, we report the synthesis of MnCoNiOx (denoted as MCNO) nanocrystals (with the size of less than 30 nm) finely encapsulated in mesoporous double-carbon (denoted as DC,


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including rGO and carbon derived from PVP, respectively) via an electrospinning method followed by a calcination treatment and their application for SIBs. The synthesized composite nanofibers were denoted as MCNO@DC. When employed as anode for SIBs, the as-obtained MCNO@DC nanofibers exhibit a superior sodium storage performance. That is, MCNO@DC nanofibers present a high specific capacity of 230 mAh g−1 (capacity retention ratio is about 96%) at 0.1 A g-1 after 500 cycles and 107 mAh g−1 (capacity retention ratio is 89%) at 1 A g-1 after 6500 cycles. The outstanding electrochemical property is mainly due to tiny MCNO nanoparticles which would shorten the ion migration distance, and the 3D DC framework which would remarkably enhance the electronic transmission and efficiently limit collapsing of MCNO nanoparticles caused by the volume fluctuation during the progress of charge and discharge. 2. EXPERIMENTAL SECTION 2.1 Synthesis of MCNO@DC nanofibers Graphene oxide (GO) was synthesized from natural graphite powders (99.99% 325 mesh Qingdao Huatai Tech. Co., Ltd., China) through a modified Hummers’ method.29 All metal salts were used after grinding in a mortar. GO powder was firstly dispersed into a solvent of 2 mL absolute alcohol and 8 mL N, N-dimethylformamide (DMF) through ultrasonic dispersion. A typical synthesis of MCNO@DC nanofibers was conducted as follows: 2 g poly(vinyl pyrrolidone) (PVP, Mw=130000) was dissolved in the above mentioned mixed solvent of GO under vigorous stirring to form a uniform nonaqueous mixed solution. 2 mmol Mn(AC)2·4H2O, Ni(AC)2·4H2O and Co(NO3)2·6H2O with a Mn/Co/Ni molar ratio of 1:1:1 were slowly poured into the solution, and subsequently stirred for more than 24 h at room temperature. Finally the coffee gel-like solution was obtained. The concentration of PVP, Mn(AC)2·4H2O, Ni(AC)2·4H2O and



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Co(NO3)2·6H2O was fixed, the addition of GO was adjusted by controlling the mass ratio of GO/MCNO from 1:20 to 1:10 and 1:5. The as-prepared gel-like solution was electrospun into precursor fibers at a flow rate of 2 mL/h, under a voltage of 15 kV and with a distance of 15 cm between the needle tip and the aluminum collector. Then, the as-spun fibers membrane was first stabilized in laboratory air at 200 °C for 6 h, and followed at 600 °C for 2 h with a heating rate of 2 °C/min in Ar. Finally, the membrane was sintered at 600 °C for less than 10 min in air to obtain the products with pure crystal structures, these products are named as MCNO@LGO-DC, MCNO@DC and MCNO@HGO-DC corresponding to the different mass ratio of GO/MCNO with 1:20, 1:10 and 1:5, respectively. 2.2 Structural Characterization The powder X-ray diffraction (XRD) was performed by using Philips X'Pert Pro Super diffractometer with Cu Ka radiation (λ=1.54016 Å), and Raman spectroscopy was carried out on JY-HR800 with the excitation wavelength of 532 nm. The morphology of the prepared samples was observed by using Field Emission Scanning Electronic Microscopy (FESEM, JSM-6701F). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed by using HITACHI-H7650 operated at 200 keV. The X-ray photoelectron spectroscopy (XPS) spectra were collected on Perkin-Elmer PHI-5702 photoelectronic spectrometer with Al-Kα excitation. The TGA measurements were conducted on TGA-2050 at a heating rate of 10 °C/min between 40 and 800 °C with an N2 flow-rate of 10 mL/min. 2.3 Electrochemical Characterization For electrochemical tests, the working electrodes were consisted of 80 wt% of active material, 10 wt% of conductivity agent (acetylene black, Super-P), and 10 wt% of polyvinylidenefluoride


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(PVDF). After stayed at 110 °C for 16 h in vacuum, the working electrodes were pressed on the pressure machine at 60 MPa, and used as positive electrodes for SIBs. Bulk Sodium was firstly rolled into thin slice with uniform thickness of 0.2 mm, and then the slice was cut into sodium discs with the diameter of 14 mm, these sodium discs were used as the counter electrode. Two-electrode button-type cells were assembled (size: 2032) in an argon-filled glove box with the moisture and oxygen keeping less than 10 ppm. The coating quality of the active material of these electrodes was 0.8-1.2 mg. The electrolyte used in the cells was 1 M LiPF6 dissolved in a 1:1 (v/v) mixture of ethylene carbonate/dimethyl carbonate (EC/DMC) for Li-ion batteries, 1 M NaClO4 dissolved in a 1:1 (v/v) mixture of ethylene carbonate/dimethyl carbonate (EC/DMC) for Na-ion batteries. Cyclic voltammetry (CV) was performed on PGSTAT 302N (Metrohm AG., Swiss) at different scan rates, electrochemical impedance spectroscopy (EIS) were carried out by applying PGSTAT 302N (Metrohm AG., Swiss) in the frequency range from 100 KHz to 10 MHz. The cells were cycled between 0.01 V and 3.0 V with a Land CT2001A battery tester system (Wuhan Land Electronics. Ltd.). 3. RESULTS AND DISCUSSION



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Figure 1 The illustrated sketch of the preparation process for MCNO@DC nanofibers.

3.1 Morphological and Structural Analysis

Figure 2 (a) XRD pattern and (b) Raman spectra of MCNO@DC nanofibers.

(c and d) Low and high

magnification FESEM images of MCNO@DC nanofibers. (e and f) The low (inset, selected area electron

diffraction) and high magnification (inset Figure 2f) bright-field TEM images of MCNO@DC nanofibers. (g) The

annular dark-field TEM image of MCNO@DC nanofibers, and (h)-(m) the corresponding EDS elemental

mappings: Ni, Co, Mn, O, C and N.

The preparation procedure for MCNO@DC nanofibers is shown in Figure 1. Figure S1a,b show the morphologies of GO used in this research. Figure S2a and 2b show the morphology of the precursor of MCNO@DC nanofibers observed by the low and high magnification FESEM. For comparison, MCNO@carbon (denoted as MCNO@C) derived by PVP, MCNO@LGO-DC, MCNO@HGO-DC and GO@carbon (denoted as GO@C) derived from PVP were also fabricated


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by altering the mass ratio of GO and MCNO, the SEM images of precursors of the four nanofibers are shown in Figure S2c-S2f. Due to the added formation of GO is powder, so the viscosity of spinning solution increases with the increasing amount of GO. When the mass ratio of GO/MCNO is 1:10, the viscosity of spinning solution is optimum, and the uniform distribution of the average diameter of MCNO@DC is obtained. XRD pattern of MCNO@DC nanofibers is presented in Figure 2a. The characteristic peaks are consistent with the cubic spinel-type MnCo2O4 (JCPDS card No. 23-1237), which belongs to Fd3m space group (a = 8.50 Å).57 There is an intensive peak observed at about 15°, belonging to the characteristic peak of carbon. Compared with the patterns of MCNO@LGO-DC and MCNO@HGO-DC in Figure S5a, MCNO@DC has higher crystallinity, which is consisted with the result of Raman Spectra in Figure 2b and S5b. Figure 2c and 2d presents FESEM images of MCNO@DC nanofibers. The diameters of MCNO@DC nanofibers are 500~600 nm and uniform, and the surface becomes rougher than the precursor nanofibers of MCNO@DC (Figure S2a and S2b). As can be seen from Figure 2c and 2d, there are almost no rGO sheets dispersed on the surface of MCNO@DC nanofibers, which indicates the GO sheets added are uniformly mixed inside the nanofibers. The Raman spectra of these MCNO@DC nanofibers were also studied, as shown in Figure 2b, S3b. There are two broad peaks located at 1344 and 1587 cm−1 in Figure 2b, which are respectively corresponds to the defect-induced mode (D-band) and graphenic-induced band (G-band) of amorphous carbon and reduced GO (rGO).35,58 According to the published report, the disorder degree or relative amount of defects in the carbon structure can be expressed by the relative intensity ratio (ID/IG) of the D peak to the G peak.58 In our work, the ID/IG values of MCNO@LGO-DC, MCNO@DC and MCNO@HGO-DC are 1.08, 1.48 and 1.33, respectively,



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indicating that disorder degree in DC structure roughly increases along with the increasing addition amount of GO, which indicates that there are abundant defects and vacancies in MCNO@DC nanofibers.58 The existence of these defects and vacancies greatly improve the diffusion of Na+ and provide more spaces for Na-storage.35 In addition, there is a broad peak at about 550 cm−1, indexing to MCNO vibrations. The carbon contents of MCNO@LGO-DC, MCNO@DC and MCNO@HGO-DC nanofibers were also analyzed by thermogravimetric analysis, their values are 43%, 50% and 62%, respectively, as can be obtained from Figure S4a. TEM (Figure 2e) and HRTEM (Figure 2f) images further verify that all the MCNO nanoparticles are homogeneously encapsulated in the fibrous carbon matrix. The result measured by TEM analysis software indicates that the MCNO grains are nanoparticles with an average size of less 30 nm, as presented in Figure 2f, S5b and S5j. The relative distributing bar graphs of particle diameter of the as-prepared MCNO@LGO-DC, MCNO@DC and MCNO@HGO-DC composites are shown in Figure S4b-d, with the increasing of the additive amount of GO, the average size of MCNO nanoparticles slightly decreases. The electron diffraction pattern of MCNO@DC is shown in the upper right corner of Figure 2e, the inset of Figure 2f shows lattice fringes of a nanorod with regular spacing of 2.49Å, which are consistent with the interplanar distance of (311) plane of MnCo2O4. To further figure out the distribution of element of the MCNO@DC nanofibers, the dark-field TEM and the EDS elemental mappings of MCNO@DC nanofibers were further performed (Figure 2g-2m). The result indicates that Ni, Co, Mn, O, C and N elements uniformly distributed in MCNO@DC nanofibers, which further shows that the nitrogen element is uniformly doped. By contrast, the TEM of MCNO@LGO-DC and MCNO@HGO-DC nanofibers were also performed. As shown in Figure S5, these two kinds of


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materials can also form the same structure as well as MCNO@DC nanofibers in spite of the uneven distribution of nanofiber diameter.

Figure 3 Porosity and N-doped characteristics of MCNO@DC nanofibers: (a) N2 absorption-desorption isotherm curves and the distribution curve of diameters of pores (inset), (b) Survey spectrum of XPS of MCNO@DC

nanofibers, The fine and fitting XPS spectra of (c) N 1s and (d) C 1s.

Table 1 The content percentages of the corresponding three N types of MCNO@DC nanofibers.

N types

pyridinic

pyrrolic

graphitic

N (at.%)

Content (wt%)

43.3

45

11.7

3.58

The BET measurement of MCNO@DC nanofibers was performed to determine the specific surface area and pores size distribution. Figure 3a shows the N2 absorption-desorption isotherm and corresponding distribution of pore size curves (inset Figure 3a) of MCNO@DC nanofibers. The curves of N2 absorption-desorption isotherm exhibit an obvious hysteresis loop between 0.4 and 1 of relative pressure (P/P0), which belongs to a typical type-IV behavior. This implies that the



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MCNO@DC nanofibers contain a large number of mesopores.59 The BET surface area of the nanofibers is 67.58 m2 g-1. The diameters of mesopores was calculated by the BJH theoretical models clearly illustrates that they are mostly between 4 and 6 nm, and less than 30 nm. These mesopores can obviously promote the migration of ions and provide more spaces for Na+ storage. Therefore, the electrochemical property of MCNO@DC mesoporous nanofibers as anode for SIBs is improved. The XPS survey spectrum displayed in Figure 2b indicates the existence of Ni 2p, Co 2p, Mn 2p, O 1s, C 1s, and N 1s in MCNO@DC mesoporous nanofibers. The fine spectra of Co 2p, Mn 2p, Ni 2p suggest the co-existence of Ni2+/Ni3+, Mn2+/Mn3+ and Co2+/Co3+ cations in MCNO@DC nanofibers (Figure S6a-6c). The fine O 1s spectrum was deconvoluted into of a metal-oxygen bond at 529.48 eV and a bond formed by the absorbed oxygen in defect sites and on the surface of nanoparticles at 531 eV, as displayed in Figure S6d.23 Figure 2c shows the fine XPS spectrum of C1s, which was deconvoluted into a graphitic (sp2 hybridized) carbon (C-C) at 284.7 eV, C-N and C-O band at 286.1 eV and C=O bond at 287.6 eV, respectively. It was previously reported that the improvement of Na+ ions storage performance was due to these C-O and C=O groups reversibly inserting and extracting Na+ ions on the surface of carbon material.59 Therefore, pseudocapacitance of MCNO@DC was partly contributed by the carbon-oxygen single and double bonds. N 1s XPS spectrum was deconvoluted into three characteristic peaks including pyridinic-N at 398 eV, pyrrolic-N at 400 eV, and graphitic-N at 401 eV, respectively, as shown in Figure 2d, the ratios of these three kinds of nitrogen were 43.6%, 45% and 11.7%, respectively, as shown in Table 1. Furthermore, MCNO@DC nanofibers exhibited a high N content of 3.5 at.%, which could produce extra storage sites for Na+ ions and therefore improving the electrochemical property of MCNO@DC nanofibers electrode.58


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3.2 Sodium Ion Storage Characteristics

Figure 4 Electrochemical property of MCNO@DC nanofibers: (a) CV curves vs Na/Na+ at a scan rate of 0.1 mV s−1 at 0.01-3.0 V; (b) Charge-discharge specific capacities at current densities of 0.1, 0.2, 0.4, 0.8, 1.6, 3.2 and 5 A g-1, respectively; (c) Cycling property at 1 A g-1 directly carried out after the test of charge-diascharge cycles at

various current density; (d) The fitting b-value of the peak cathodic and anodic currents.

The CV study for the MCNO@DC nanofibers vs Na/Na+ was performed at a scan rate of 0.1 mv s-1 (Figure 4a) and different scan rates (Figure S7a and S7b) between 0.01 and 3.0 V. As displayed in Figure 4a, there are three reductive peaks at 0.50 V, 1.25 V and 1.6 V, respectively, which indicates that the relationship among Mn, Co and Ni is synergistic. The synergistic effect is: when the nickel ion is reduced at 1.60 V, the manganese and cobalt oxides do not react, playing a role in buffering the volume change of nickel oxide; when the cobalt ion is reduced at 1.25 V, the manganese and nickel oxides play the same roles; when the cobalt ion is reduced at 0.50 V, manganese and nickel oxides are the same principle, as shown in Figure 4a (black curve). Interestingly, the area of CV curves of the MCNO@DC nanofibers electrode at low scan rates



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from 0.1 to 0.6 mV s-1 at 2.9-3.0 V is very small (in Figure S7a), which suggests that the capacity is very small, which is according to the formulation for calculating the specific capacity of CV UI

curves, C = (U is the voltage, I is the value of current, ν is the scan rate, m is the mass of mν m

active material, U×I is the area of CV curve).59 However, the area of CV curves gradually increased with the increase of scan rate (Figure S7b), which suggests the capacity is also increased at 2.9-3.0 V. Based on the above results, in the purpose of the improvement of the electrochemical property of the MCNO@DC nanofibers, the galvanostatic charge-discharge, rate performance and cyclic stability tests were performed at 0.01-2.85 V in the first tenth charge-discharge cycles, and then at 0.01-3.0 V. The charge-discharge capacity at various current densities and cycling property of the as-prepared MCNO@DC nanofibers were present in Figure 4b and 4c. During the initial 10 cycles, the MCNO@DC electrode delivers a specific capacity of 364 mAh g−1 at a current density of 0.1 A g−1. Then, the current density is gradually increased to 0.2, 0.4, 0.8, 1.6, 3.2, and 5 A g−1 after keeping ten cycles per rate. The according capacities are 260, 213, 183, 151, 117 and 90 mAh g−1, respectively. Surprisingly, when the rate is returned to 0.1 A g−1, the reversible specific capacity still remains 248 mAh g−1, which shows the MCNO@DC composite nanofibers are not only completely activated through Na+ inserting/extracting at different current density, but also beneficial for swiftness of the above process. By contrast, the reversible specific capacity of MCNO@LGO-DC is 260 mAh g-1, which is higher than MCNO@DC at the second discharge. But it decreases rapidly to 180 mAh g-1 with the increase of cycles, which may be caused by its lower rGO content than MCNO@DC. While MCNO@HGO-DC electrode delivers a lower second discharge specific capacity of 240 mAh g−1 at 0.1 A g−1 (Figure S8a), and shows an inferior


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specific capacity (only 180 mAh g−1 after 500 cycles and 90 mAh g−1 at 1 A g−1, Figure S8c). The cycling stability at 0.1 A g−1 of the MCNO@DC electrode was also investigated. The reversible capacity can retain 230 mAh g-1 after 500 cycles at 0.1 A g−1, which is the largest among these synthesized DC composite nanofibers (Figure S8a). While the cycling stability of all the materials at 1 A g-1 was directly carried out after the testing of rate capability at different current densities. According to the literature 35, the previously charge/discharge rates test can be also seen as an activation process. It is important to note that the activation process in this work is conducive to improve the cycle stability of material under subsequently high current density. As shown in Figure 4c, the specific capacity of MCNO@DC electrode still retains 107 mAh g−1 (capacity retention of 89%) at 1 A g-1 even after 6500 cycles. Remarkably, the Coulomb Efficiency (CE) approaches 100% (>99.6%), which is beneficial for the long cyclic life.

Figure 5 CV of MCNO@DC nanofibers tested vs Na+/Na at scan rates of (a) 0.6, (b) 1, (c) 1.4, (d) 1.8, and (e) 2



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mV s−1, respectively. The shadowed areas represent the capacity of capacitive contribution. (f) Separation of

capacity of diffusion-controlled and capacitive at different scan rates.

Because the intercalation pseudocapacitance is also an important factor of improving the sodium storage characteristics of MCNO@DC mesoporous nanofibers, here it was further analyzed. A fitting plot of log(i) vs log(ν) for scan rates between 0.4 and 50 mV s−1 of cathodic and anodic peaks of MCNO@DC electrode was displayed in Figure 4d. Usually the current and scan rate obey a power-law 60-62 i=avb

(1)

where a and b are appropriate values. According to the report by Dunn and Wang et al., when b-value is 0.5, the reaction kinetics of the electrode was controlled by the diffusion of ion during the charge-discharge process; when b-value is equal to 1, the reversible redox reaction was a capacitive behavior taking place on a surface or near-surface.60,61 In our work, for scan rates ranging from 0.4 to 10 mV s-1, the b-value of the cathodic peaks is 0.57, which is very close to 0.5, indicating that the reaction kinetics of MCNO@DC electrode is mainly controlled by the diffusion of Na+ ions within MCNO@DC nanofibers. However, the value is a little more than 0.5, indicating that partial capacity is derived from surface or near surface reversible redox reaction, and thus the reaction kinetics of MCNO@DC nanofibers electrode is faster than the diffusion of Na+. Therefore, MCNO@DC nanofibers electrode present an excellent cycling stability at 1 A g-1 when employed as anode for SIBs. Finally, we also used a method employed by Dunn et al to further distinguish the capacity of surface or near-surface-controlled contribution from that of Na+ diffusion-controlled contribution in electrode.61,62 Accordingly, the current response at a fixed voltage can be considered as the combination of two distinct mechanisms of


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pseudocapacitive effects (k1ν) and diffusion-controlled Na+ insertion/ extraction (k2ν1/2):61,62 i(V) = k1ν+ k2ν1/2

(2)

For convenience of analysis, both sides of Equation (2) were divided by ν1/2, and then Equation (3) was obtained: i(V)/ ν1/2 = k1ν1/2 + k2

(3)

On the basis of the linear fitting of voltammetric currents at each potential, we can determine the coefficients k1 and k2, and calculate the k1ν and k2ν1/2. Then the charge contributed from the capacitive current or diffusion-controlled current can be distinguished.61 As shown in Figure 5a-e, the shadowed regions stand for the contribution from the surface capacitive current, while the nonprinting areas represent the diffusion-controlled current contribution. The results show that the capacitive-controlled capacity takes up about 16% of the total charge storage at 0.1 mV s−1 (Figure. 5a), whereas this value increases to about 25.9% at 2 mV s−1 (Figure. 5e), which indicates pseudocapacitive charge storage exactly alleviates the fundamental kinetic barriers associated with solid-state diffusion processes and fast charge transfer, especially at high scan rates.

Figure 6 (a) Nyquist dots of the MCNO@DC, MCNO@LGO-DC and MCNO@HGO-DC nanofibers. (b) The plot

of real parts of the impedance (Z′) vs the reciprocal of square root of angular frequency (ω) in the low frequency

region of the above three samples.

Simultaneously, to better elucidate the enhanced electrochemical performance of



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MCNO@DC nanofibers, EIS was carried out to analyze the change of impedance of MCNO@DC electrodes before and after 500 charge-discharge cycles at 0.1 A g-1. The Nyquist plots of MCNO@DC electrodes are presented in Figure S9a and S9c. Before the test of cyclic property, the Nyquist plot can be fitted by employing Randles circuit, as present in Figure S9b (the fitting parameters before and after 500 cycles are summarized in Table S1). In this circuit,43,63,64 Rs corresponds to the total electrical resistance of the internal of SIB system. Rp stands for the transfer impedance of Na+ ions in electrode and is ascribed to semicircle in medium and high frequency region. W stands for the Warburg impedance of the diffusion of Na+ ions at the interface of electrode-electrolyte and within the bulk of MCNO@DC nanofibers. Warburg impedance expressed by a straight line with a certain slope in low frequency region. As displayed in Figure S9a, MCNO@DC nanofibers electrode shows a flattened semicircle in high frequency region and the impedance of 190 ohm (Table S1), which indicates that the Warburg impedance of the diffusion of Na+ ions at the interface of electrode-electrolyte and within the body of MCNO@DC nanofibers is low before charge-discharge cycles. This will be beneficial for the improvement of the electrochemical property of MCNO@DC electrode. After 500 charge-discharge cycles at 0.1 A g-1, the EIS of MCNO@DC nanofibers electrodes is presented in Figure S9c, the corresponding circuit fitted is presented in Figure S9d. As can be seen from Figure S9d, there are a C standing for double-layer capacitance, a CPE standing for constant phase element and two Rp in medium and high frequency region standing for the impedance of MCNO@DC nanofibers electrode. The appearance of two Rp in Randles circuit of mesoporous MCNO@DC nanofibers electrodes after 600 cycles of charge-discharge indicates that a solid electrolyte interphase (SEI) film was formed, that is, two additional interfaces including the interface of electrode-electrolyte and SEI


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layer-electrode material during the process of charge-discharge were formed. Similar results have been reported in other literatures.63,64 Rp paralleling to C stands for the resistance of the SEI film (RSEI), and the C stands for its relative double-layer capacitance (CSEI). RSEI and CSEI are usually expressed by the flattened semicircle in high frequency region. Rp paralleling to constant phase element (CPE) stands for the impedance of an interface between SEI layer and electrode material, Rp and CPE are routinely expressed by the flattened semicircle in medium frequency region. CPE standing for capacitance, n is the coefficient of CPE, and the fitted data of CPE is positively correlated with the value of n.65 As can be obtained from Table S1, the value of n increased from 0.508 to 0.708, which indicates the capacity of capacitive contribution increases with the process of charging and discharging. This may also lead to remarkably enhancing cyclic stability of MCNO@DC mesoporous nanofibers. As we all know, EIS can be employed to calculate and compare the diffusion coefficient of Na+ (DNa) within the electrode:35,43,64 D

Na

= 0.5 (

γRT An 2 F 2 C σ w

Z' = Rs + Rct + σw ω−0.5

)2

(4) (5)

In eq (4), R is the gas constant, T is the absolute temperature, A is the coating area of electrode material, n is the number of electron transport per mole of active substance occurring redox reaction, F is the Faraday constant, C is the mole volume concentration of sodium ion in system, σw is the Warburg factor, Z' is the real of impedance. σw relates to Z' through eq (4) and its value is the slope of the plot obtained from Z' and ω−1/2 at the lower angular frequencies as shown in Figure 6b. As can be calculated, the value of MCNO@DC is the smallest (Table S1), reflecting the largest DNa.



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To further clarify that how much the capacity of rGO@C accounts for the total capacity of MCNO@DC nanofibers, the capacity of rGO@C composites (carbon-derived from PVP) with the different addition of rGO and MCNO@C nanofibers are also compared. Firstly, SEM images of precursor nanofibers and three rGO@C samples were presented in Figure S10. As can be seen from Figure S10a-c, compared with the diameter of the precursor with 1:10 of the mass ratio of GO/MCNO, that of the precursors with 1:20 and 1:5 of the mass ration of GO/MCNO are non-uniform, which indicates that the dosage of GO exactly affects the morphology of the spun fibers. However, after the heat treatment under the same calcination conditions as MCNO@DC, the morphologies of these fibers could not be maintained, as shown in Figure S10d-f. Nonetheless, to further demonstrate that the contribution of rGO@C (such as tiny MCNO nanoparticles, 3D DC framework and N-doped in carbon) and combined effects on capability promotion and cyclic stability. The electrochemical properties of three kinds of rGO@C with different amounts of rGO and MCNO@C nanofibers were also investigated. As shown in Figure S12a, compared with the specific capacity values (at 0.1 A g-1) of MCNO@LGO-DC, MCNO@DC and MCNO@HGO-DC, those of rGO@C composite materials with the various amounts of rGO nanofibers were lower and less than 100 mAh g-1. Compared with the discharge capacity (230 mAh g-1, at 0.1 A g-1) of MCNO@DC nanofibers after 500 cycles, that of MCNO@C nanofibers (Figure S11, TEM image) only remains 180 mAh g-1 after 400 cycles. And the capacity retention ratio of MCNO@C only remains 37.9%, which was far lower than that (96%) of MCNO@DC nanofibers, as shown in Figure S12b and S8a. All the above results further verify that the combined effects of tiny MCNO nanoparticles and 3D DC framework improve cyclic stability of MCNO@DC nanofibers.


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Figure 7 (a) FESEM, (b) TEM and (c-h) EDS mapping images of a typical MCNO@DC nanofiber used as current collector-free anode for SIB and taken from the electrode charged at 3.0 V after 500 cycles.

The morphological and structural changes of MCNO@DC used as current collector-free anode for SIB after 500 cycles at 0.1 A g−1 were investigated by using SEM and TEM. As displayed in Figure 7a and 7b, except some particles on the surface of MCNO@DC nanofibers aggregated, MCNO@DC still retains its original appearance and structure, which indicates that this tailored unique nanostructure of MCNO@DC has effectively depressed the pulverization and aggregation of the active materials caused by volume change. This also accounts for the excellent cyclic stability of MCNO@DC electrode. Figure 7c-7h indicates that the distribution of Ni, Co, Mn, O, C and N elements of MCNO@DC nanofibers is still uniform after 500 cycles at 0.1 A g−1, which is also conducive to improve cyclic stability of MCNO@DC nanofibers. 3.3 Lithium Ion Storage Characteristics



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Figure 8 (a) The fitting plot of b-value of the cathodic peak of MCNO@DC nanofibers electrode vs Li/Li+ at scan rates of 0.1-7 mV s-1 within 0.01-3.0 V; (b) Rate property of MCNO@DC nanofibers vs Li/Li+; (c) Cycling performance vs Li/Li+ for MCNO@DC nanofibers at 5 A g-1 directly carried out after the testing of discharge capacities at different current densities; (d) The Nyquist plot of MCNO@DC nanofibers electrode vs Li/Li+ at 5 A g-1 after 3000 cycles (blue points: the test result, the red line: the fitting result).

To further expand the application of this material, we had also investigated the electrochemical property of MCNO@DC nanofibers as an anode for LIBs. Figure 8a shows the variation of peak current of the CV curves of the MCNO@DC nanofibers electrode vs Li/Li+ from scan rates of 0.1 to 7 mV s-1. This linear relationship between peak current and scan rate suggests that the reaction kinetic of MCNO@DC nanofibers vs Li/Li+ was also partly controlled by a surface or near-surface pseudocapacitive characteristic.43,61 Figure 8c and 8d show the discharge capacity at different current densities and cyclic property of MCNO@DC nanofibers vs Li/Li+. The discharge capacities can reach to 340 mAh g-1 at 5 A g-1 (Figure 8c) after 3000 cycles between 0.01-3.0 V, indicating that MCNO@DC nanofibers exhibit a good rate capability and cyclic stability for LIBs.


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The excellent cyclic stability at 1 A g-1 of MCNO@DC nanofibers is mainly attributed to the following aspects: (i) the mesoporous structure facilitate the electrolyte to infiltrate in the active materials, (ii) the 3D conductive network interlinked by N-doped DC nanofibers provides rapid electron and ion transmission channels for MCNO nanocrystals, and effectively depresses the pulverization and aggregation of MCNO nanoparticles caused by volume change, (iii) the uniform N-doped carbon nanofibers can remarkably improve the electrochemical capability. 4. CONCLUSIONS In summary, mesoporous MCNO@DC composite nanofibers as an anode material for SIBs were successfully synthesized via a feasible electrospinning and a subsequently calcinating process. The obtained nanofibers exhibited outstanding electrochemical performance, especially long-term cyclic stability, and this was mainly attributed to the unique nanostructure of MCNO@DC. Tiny-sized MCNO nanoparticles as well as the double-carbon assistance not only provide short ion diffusion path and facilitate the insertion/extraction of Na+ ions, but also effectively inhibit the volume expansion of MCNO and avoid MCNO exfoliating from rGO surface during the insertion/extraction of Na+ ions. Thereby, mesoporous MCNO@DC nanofibers vs Na/Na+ exhibit an obvious pseudocapacitive behavior and display a favorable specific capacity and superior cyclic stability at the large current density, which allows this material to be a potential anode material for SIBs. ASSOCIATED CONTENT Supporting Information Low magnification FESEM and TEM images of graphene oxide, FESEM images of the precursors of MCNO@LGO-DC, MCNO@DC and MCNO@HGO-DC nanofibers synthesized with different



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ratios of GO and metal salts, XRD patterns and the Raman spectra of MCNO@LGO-DC and MCNO@HGO-DC nanofibers. TGA curves of the as-prepared MCNO@LGO-DC, MCNO@DC and MCNO@HGO-DC, The low (inset, Selected area electron diffraction) and high magnification TEM images of MCNO@LGO-DC and MCNO@HGO-DC nanofibers, XPS survey scan, high resolution and the peak fit for Mn 2p, Co 2p, Ni 2p and O 1s regions, Cyclic voltammetry curves of MCNO@DC nanofibers electrode vs Na/Na+ between scan rates of 0.1 and 10 mV s-1 at 0.01-3.0 V, Cyclic stability of MCNO@LGO-DC and MCNO@HGO-DC electrode vs Na/Na+ at 0.1 A g-1 for 500 cycles and at 1 A g-1 for 2500 cycles; The Nyquist plot and the fitted Randles circuit of MCNO@DC nanofibers electrodes vs Na/Na+ before charge/discharge process; SEM imges of the precursors of GO@C with different contents of GO; SEM imges of rGO@C composites with different amounts of rGO after heat treatment under the same calcination conditions as MCNO@DC; Cyclic stabilities of rGO@C composites with different amounts of rGO vs Na/Na+ at 0.1 A g-1 and 0.01-3.0 V; Cyclic stability of MCNO@C nanofibers vs Na/Na+ at 0.1 A g-1 and 0.01-3.0 V for 400 cycles. This material can be freely available through the Internet at http:// pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Prof. Xingbin Yan Fax: +86-931-4968055. E-mail:[email protected] CONFLICT OF INTEREST The authors declare no competing financial conflict of interest. ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundations of China (51501208,


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Electrospinning Synthesis of Mesoporous MnCoNiOx@Double

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