Accepted Manuscript Title: MnO-carbon hybrid nanofiber composites as superior anode materials for lithium-ion batteries Author: Jian-Gan Wang Ying Yang Zheng-Hong Huang Feiyu Kang PII: DOI: Reference:
S0013-4686(15)01090-7 http://dx.doi.org/doi:10.1016/j.electacta.2015.04.157 EA 24911
To appear in:
Received date: Revised date: Accepted date:
4-3-2015 22-4-2015 27-4-2015
Please cite this article as: Jian-Gan Wang, Ying Yang, Zheng-Hong Huang, Feiyu Kang, MnO-carbon hybrid nanofiber composites as superior anode materials for lithium-ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.04.157 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
MnO-carbon hybrid nanofiber composites as superior anode materials for lithium-ion batteries Jian-Gan Wanga,b,c∗, Ying Yangd, Zheng-Hong Huangb, Feiyu Kangb,c a
Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern
Polytechnical University, Xi’an 710072, PR China b
School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
Institute of Advanced Materials Research, Graduate School at Shenzhen, Tsinghua University,
Shenzhen 518055, China d
Department of Electrical Engineering, Tsinghua University, Beijing 100084, China Corresponding author. Tel. and fax: +86 029 8846 0361
Email address: [email protected]
Abstract MnO-carbon hybrid nanofiber composites are fabricated by electrospinning polyimide/manganese acetylacetonate precursor and a subsequent carbonization process. The composition, phase structure and morphology of the composites are characterized by scanning and transmission electron microscopy, X-ray diffraction and thermogravimetric analysis. The results indicate that the composites exhibit good nanofibrous morphology with MnO nanoparticles uniformly encapsulated by carbon nanofibers. The hybrid nanofiber composites are used directly as freestanding anodes for lithium-ion batteries to evaluate their electrochemical properties. It is found that the optimized MnO-carbon nanofiber composite can deliver a high reversible capacity of 663 mAh g-1, along with excellent cycling stability and good rate capability. The superior performance enables the composites to be promising candidates as an anode alternative for high-performance lithium-ion batteries. Keywords: Manganese oxide; Carbon nanofiber; Nanocomposite; Anode; Lithium-ion battery
1. Introduction Electrochemical energy storage systems have received remarkable attention in the past decades owing to their ability to deliver high energy/power densities for large-scale applications in powering electric vehicles, portable electronic devices and smart utility grids [1, 2]. Until now, rechargeable Li-ion batteries (LIBs) still dominate the market. However, the currently LIBs can not meet the future requirements with high level performance of higher energy/power densities, excellent rate capability, long cycle life, environmental benignity and low cost [3, 4]. Electrode material is considered to be an important factor to determine the electrochemical properties of LIBs, however, it remains a big challenging issue to explore high-performance alternatives. It is well-known that commercial graphite anode suffers from low theoretical specific capacity (372 mAh g-1) and poor rate capability . To solve these problems, substantial efforts have been made to develop new anode materials for the next-generation LIBs, such as the metal oxide nanomaterials (including SnOx, MnOx, FeOx, NiOx, CoOx, CuO, etc.) [6-13]. Among these alternatives, MnOx (MnO2, Mn4O3, Mn2O3, MnO) has received particular interest because of its high specific capacity, low environmental footprints both in synthesis and applications, abundant resources, and low cost . However, there are several hurdles when using MnOx as anode materials: (i) low electronic conductivity (10-6-10-8 S cm-1) and (ii) large volume change (>170%) during the lithiation/delithiation processes, which result in poor rate capability and fast capacity decay . To circumvent these critical issues, there are two effective strategies: (i) reducing MnOx particle size down to nanoscale range because the nanostructures can shorten the electronic/ionic distance for improved electrode reaction kinetics; (ii) compositing MnOx with carbon materials because carbon can act as a buffering barrier to accommodate the volume change of MnOx and can
increase the electrical conductivity to enhance the electron transfer rate. By combining the two strategies, various kinds of MnOx/carbon (e.g., carbon coating, carbon nanotube (CNT), graphene, carbon nanofiber (CNF), mesoporous carbon) nanocomposites have been developed and showed improved capacity and cycling performance [7, 8, 14-25]. It is noted that most of these studies are related to powder-like nanomaterials, which require extra binders, conductive agents and current collector to constitute an electrode. In addition, the conventional slurry-casting procedure during the electrode fabrication is complex and time-consuming. Electrospinning is a versatile technique to fabricate freestanding fabrics consisting of interconnected nanofibers. This technique is employed to fabricate one-dimensional (1D) MnOx/CNF coaxial nanofiber fabrics by using polyacrylonitrile (PAN) as carbon precursors and Mn(CH3COO)2 as Mn sources [8, 23, 24]. The resulting products can be directly used as freestanding anode for LIBs. In the hybrid structure, the CNFs provide large specific surface area, good electrical conductivity and adequate void space for the nanosized MnOx inclusions, aiming to facilitate easy access of Li-ion to the active sites, decrease Li-ion diffusion distance, boost electron transfer rate, accommodate huge volume change of MnOx, and prevent the electrode pulverization. It is believed that carbon precursors and Mn sources have a prominent influence on the morphology of the composites and thus the lithium storage performance. With respect to the PAN/Mn(CH3COO)2 system, the Mn2+ disassociated from Mn(CH3COO)2 salt would give rise to electrospinning instability of the polymer precursor due to the charge disturbance, which leads to the formation of nonuniform nanofibers with short segments and diameter inconformity . Additionally, the PAN has a low carbon yield (40-50%) during the carbonization process, and accordingly, the PAN-derived CNFs show inferior mechanical strength . In a sharp contrast,
polyimide (PI) precursor has a relatively higher carbon yield (70%), better mechanical strength and enhanced electrical conductivity than those reported polyacrylonitrile (PAN) . These are desirable attributes for loading high-capacity metal oxides and retaining structure stability. Our previous study demonstrated that the PI-derived porous CNFs can preserve good nanofiber structure for a stable cycling of LIBs . In this work, we report MnO/CNF hybrid nanofiber fabrics by a facile co-electrospinning of PI/manganese acetylacetonate (Mn(acac)2) and a subsequent carbonization process. The PI polymer precursor containing organic salt of Mn(acac)2 possesses a stable electrospinning capability, which generates a robust fabric containing uniform nanofibers. MnO nanoparticles are encapsulated by the CNFs within a mass loading level. The as-prepared freestanding MnO/CNF hybrid nanofiber fabric with a porous network structure, when used as a binder-, conductive additive- and current collector-free anode, shows good electrochemical performance in terms of reversible capacity, cycling stability and rate capability. The influence of MnO distribution on the electrochemical properties is also investigated.
2. Experimental 2.1 Materials preparation Electrospinning technique was employed to fabricate CNF and MnO/CNF hybrid nanofibers. In a typical procedure, 2.2 g of pyromellitic dianhydride (PMDA) and 2 g of 4,4-oxydianilline (ODA) were dissolved in 30.8 g of N,N-dimethyacetamide (DMAc) to form a homogeneous polymer solution of polyamic acid (PAA), which was firstly used to prepare the bare CNF. Then different amounts of manganese acetylacetonate (Mn(acac)2 15, 25, 30, 40 and 50 wt.% relative to PAA) were added into the above polymer solution under magnetic stirring for 12 h. The mixture was used
as a precursor for electrospinning hybrid nanofibers. In a typical electrospinning procedure, the polymer precursor was loaded into a syringe pump and a high voltage of 25 kV was provided by a high-voltage power supply. A flow rate of 1 ml h-1 and a needle-to-collector distance of 25 cm were employed to ensure a stable electrospinning. The as-electrospun nanofiber fabrics were peeled off the Al foil collector. Subsequently, the as-electrospun fabrics were treated by a typical imidization process to form PI nanofiber  and finally carbonized at 600 °C under argon atmosphere for 2 h (heating rate: 5 °C min-1). The final black products were obtained, and the composites from 15, 25, 30, 40 and 50 wt.% of Mn(acac)2 were designated as M15C, M25C, M30C, M40C and M50C, respectively. 2.2 Materials characterization and electrochemical properties The morphology and structure of the products were examined using a field-emission scanning electron microscopy (FE-SEM, LEO-1530) equipped with a energy dispersive spectroscopy (EDS) and a high-resolution transmission electron microscopy (TEM, JEOL-2010). Selected area electron diffraction (SAED) pattern were taken on the JEOL-2010 TEM equipment. The crystal structure of the hybrid nanofibers was characterized by powder X-ray diffraction (XRD, Rigaku D/Max 2500PC). Thermo-gravimetric analysis (TGA) was used to determine the mass loading of MnO in the composites at a temperature ramp of 10 °C min-1 from room temperature to 800 °C in an air environment (TA instruments, SDT-Q600). The oxidation state variation of Mn element in the M30C electrode after discharge to 0.005 V and re-charge to 2.5 V was investigated using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific). Electrochemical performance was evaluated using 2032 button coin cells with metallic lithium as the counter electrode and Celgard 2400 as the separator. The as-obtained fabrics were tailored
into freestanding discs with 10 mm in diameter, which were directly used as the working electrode without any binders, conductive additives or current collectors (i.e. copper foil). The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC=1:1:1, v/v). The cells were assembled in an argon-filled glove box. CV measurement of the cell was performed using a Princeton electrochemical workstation. Galvanostatic charge/discharge tests were carried out on Land Battery Testing system at different current densities between cut-off potentials of 0.01 and 2.50 V vs. Li/Li+.
3. Results and discussion The phase structure of the as-prepared products was investigated by XRD. As shown in Fig. 1, the broad diffraction peak with 2θ at around 24° can be assigned to (002) plane of amorphous carbon in electrospun CNF with turbostatic structure . Additionally, the new diffraction peaks appeared in the composites at 2θ=34.9°, 40.7°, 58.8°, 70.3° and 73.9° can be well-indexed to (111), (200), (220), (311) and (222) planes of face-centered cubic phase of MnO (JCPDS 07-0230) [20-22, 25]. All peaks from the MnO phase exhibit relatively broad features, indicating the MnO is in the form of nanocrystals. The low intensity of the diffraction peaks in the composites with low contents of Mn(acac)2 in the starting precursor is related to a lower amount of MnO. The growth of the MnO nanocrystals in CNFs is further confirmed by the following morphological observation. Fig. 2 exhibits the SEM images of the hybrid nanofibers having different loading amounts of MnO. The bare electrospun CNF (Fig. 2(a)) is composed of uniform nanofibers with smooth surfaces and average diameter of ca. 200 nm. Our experiments also demonstrate that the precursors containing Mn(acac)2 have excellent electrospinning capability, which can be evidenced by the well-preserved morphology of interwoven nanofibers (Fig. 2(b-f)). In contrast, these hybrid
nanofibers have a relatively larger diameter in the range of 200-400 nm, probably owing to the increased viscosity of the mixture precursor. When the loading amount of MnO is low (M15C), there is no obvious morphology change on the SEM image, indicating the MnO nanoparticles are encapsulated completely by the CNF. As for the Mn25C (Fig. 2(c)) and Mn30C (Fig. 2(d)) hybrids, a large majority of MnO nanoparticles are encapsulated by the CNF except for a few scattered on the exterior surfaces. The main signals of Mn, O and C elements in the EDS spectrum reveals the presence of MnO and the absence of other impurities. As the MnO amount further increases (M40C and M50C in Figs. 2(e-f)), MnO nanoparticles are observed to crystallize on the external surfaces of the CNF and their diameters increase to tens of nanometers. This suggests that a higher content of Mn(acac)2 leads to the formation and precipitation of MnO phases outside the CNF body, which may not be favorable for LIB applications. More structural details of the MnO/CNF composites were further characterized by TEM imaging. From the low magnification TEM images (Fig. 3 (a-c)), numerous MnO tiny particles are uniformly dispersed throughout the CNF. The increasing loading amount of MnO leads to an increase of the average size of the nanoparticles from several nanometers to tens of nanometers. At a relatively low mass loading of MnO (M15C and M30C), the nanoparticles are well enwrapped by CNF, which is in good agreement with the SEM results. As the mass loading further increases (M50C), the MnO grow into bigger square-like nanoparticles with 30-50 nm in size. The SAED pattern of the MnO nanoparticle in Fig. 3(c) consists of some discrete spots, elucidating the nanocrystalline nature of the MnO phase. The lattice fringes can be clearly resolved to be ca. 0.25 nm in corresponding HRTEM image (Fig. 3(d)), agreeing well with the interplanar spacing of (111) planes of the cubic MnO phase.
TGA was carried out to determine the mass loading of MnO in MnO/CNF hybrid composites. The typical TGA behavior of the samples is shown in Fig. 4. The initial small weight loss below 100 °C corresponds to the removal of physically absorbed water in the samples. The subsequent predominant weight loss in the temperature range of 250-550 °C is ascribed to the oxidation of carbon component. It is observed that the carbon-oxidation temperature decreases rapidly with the increasing content of MnO, which can result from the strong catalytic function of Mn species in the composites . According to the TGA results, the mass loading of the MnO component is estimated to be about 9.1 wt.%, 15.6 wt.%, 19.4 wt.%, 26.2 wt.% and 32.7 wt.% for M15C, M25C, M30C, M40C and M50C, respectively. To evaluate the Li-ion storage capacity of the MnO/CNF composites, the as-prepared hybrid nanofiber fabrics were directly used as anodes without using any polymeric binders or conductive additives. The specific capacity is measured using galvanostatic charge/discharge tests at a current density of 50 mA g-1, and Fig. 5(a) shows the specific capacity and cycling performance of the freestanding electrodes of pure CNF, M15C, M25C, M30C, M40C and M50C at a current density of 50 mA g-1. As shown in Fig. 5(a), the M30C electrode exhibits the highest specific capacity among the electrodes. The typical voltage profiles of the M30C electrode in the 1st, 2nd, 3rd, 10th, 25th and 50th cycles are exhibited in Fig 5(b). The first discharge and charge capacities are 1021 and 663 mAh g-1, respectively, corresponding to an initial Coulombic efficiency of 65%. The Coulombic efficiency increases rapidly to 97% in the second cycle and remains close to 100% afterwards. The irreversible capacity loss in the first cycle can be mainly attributed to the formation of solid-electrolyte-interphase (SEI) layer on the surface of electrode, the reductive decomposition of the electrolyte as well as some Li+ confined in the CNF, which is supported by the disappearance
of the voltage plateau at 0.75-1.0 V in the second and forward cycles [8, 28, 31-33]. In addition, the M30C displays good cycling stability with a reversible capacity of approximately 557 mAh g-1 being retained after 50 cycles. The good capacity retention (84%) is superior to that of the reported MnOx/PAN-derived CNFs (76%) [8, 24, 34] and porous PI-derived CNFs (61%) . By contrast, the counterparts of CNF, M15C, M25C, M40C and M50C electrodes deliver a lower reversible capacities of 221, 457, 501, 436 and 195 mAh g-1, respectively. Overall, the MnO/CNF electrodes with low MnO loading (i.e., CNF, M15C, M25C and M30C) show relatively good capacity retention, whereas the M40C and M50C electrode suffers from considerable capacity degradation (i.e., 42.4% and 72.1%). The different cycling performance can be rationalized by the MnO distribution in the composite. As discussed previously, the MnO nanoparticles in the M15C, M25C and M30C composites are well-encapsulated by the CNF. Consequently, the CNF can worked as an elastic matrix to effectively accommodate the large volume change caused by the MnO phases during the charge/discharge processes. As for the M40C and the M50C composites, the MnO nanoparticles are exposed on the surface of the CNF, and thereby lose the strain buffering protection offered by the CNF. The lithiation/delithiation cycling would result in pulverization of MnO, then a loss of electrical connection to CNF, and finally a serious capacity degradation. To gain a better insight into the reason for the cycling stability, the microstructure of the M30C and M50C nanocomposite after 50 cycles was characterized by SEM. As shown in Fig. 5(c), the M30C nanocomposite preserves the original interwoven network structure with each individual nanofiber enwrapped by a uniform thin SEI film, confirming its good structural integrity during the cycling tests. By contrast, the nanofibers in the M50C nanocomposite (Fig. 6(d)) are covered by thick and nonuniform SEI films, which may presumably be caused by the pulverization of the exposed MnO
nanoparticles. The electrochemical behavior of the M30C composite was further investigated by CV method. Fig. 6(a) shows the resulting CV curves of the M30C composite in the initial three cycles. In the first cycle, a broad hump centered around 0.65 V in the cathodic curve is related to the formation of the SEI film on the surface of the electrode and the decomposition reactions of the electrolyte [31-33]. The peak at around 0.15 V corresponds to the initial reduction of MnO to metallic Mn (MnO + 2Li+ + 2e- → Mn + Li2O). In the subsequent anodic scan, the exclusive peak observed at around 1.25 V represents the regeneration of MnO and Li2O decomposition through a reverse delithiation reaction: Mn + Li2O → MnO + 2Li+ + 2e-. From the second and the onward cycles, a pair of peaks at 0.5 and 1.3 V corresponds to the reversible lithiation/delithiation reactions of the reduction of MnO and oxidation of Mn, respectively. The shift of the reduction peak from 0.15 to 0.5 V may be partially associated with the improved kinetics of MnO after the first lithiation. [35, 36]. XRD and XPS were performed to confirm the lithium storage mechanism. As shown in Fig. 6(b), the characteristic diffraction peaks of MnO disappear when the electrode was discharged (lithiation) to a potential of 0.005 V, and re-appear after charging (delithiation) to 2.5 V, indicating the decomposition/re-formation of MnO during the lithiation/delithiation processes. The phase variation can be confirmed by XPS analysis, because the oxidation state of Mn can be determined by analyzing the spin energy separation of Mn 3s doublet spectrum . As shown in Fig. 6(c), when the electrode was charged to 2.5 V, the Mn 3s doublet spectrum exhibits a typical spin energy separation of 5.8 eV, corresponding to a Mn valence of +2 . After discharging to 0.005 V, the Mn 3s doublet disappears, and a new Li 1s peak with a binding energy at 55.3 eV, attributed to Li2O, appears (inset in Fig. 6(c)), confirming the conversion of MnO to metallic Mn accompanying with
the formation of Li2O during the discharge process . Fig. 7 (a) shows the rate capability of the M30C composite electrode. It delivers an average reversible capacity of 632 mAh g-1 in the first 10 cycles. As the current density increases stepwise to 100, 200 and 500 mA g-1, the average reversible capacities at each of these current densities are 519, 410 and 227 mAh g-1. When the rate returns to the original 50 mA g-1, a capacity of 583 mAh g-1 is still recoverable and sustainable up to the 50th cycle without noticeable loss. To further investigate the long-term cycling stability of Mn30C, another cell was tested at a higher current density of 200 mA g-1 for 200 cycles. As shown in Fig. 7(b), the Mn30C can a reversible capacity of 398 mAh g-1 after 200 cycles, which is still larger than the theoretical value of commercial graphite anode (372 mAh g-1). These results again demonstrate good rate capability and cycling stability of the M30C nanocomposite electrode. Combining the high reversible capacity, reliable cycling stability and good rate capability, it is believed that the superior performance of M30C composite electrode can be explained by its unique hybrid nanostructure that integrates the advantages of MnO and CNF components. First, the monolithic fabric provides a porous network for rapid Li-ion ingress and diffusion throughout the freestanding electrode, while the nanosized fiber shortens the distances of Li-ion transport. At the same time, the high-aspect-ratio CNF offers long-range electrically conducting pathways for fast electron collection and transfer. The enhanced ion/electron transport ability ensures fast reaction kinetics. Second, the Li-ion storage capacity can be stemmed from the active bi-components of CNFs and particularly the high-capacity MnO nanoparticles. The nanostructured hybrid can also provide a large amount of electrode/electrolyte interfaces for better utilization of active materials. Third, the M30C has a relatively high mass loading of MnO nanosized particles with good
encapsulation by the CNF to accommodate the volumetric expansion/contraction of MnO. Finally, the PI-derived CNFs offer good structure stability to maintain the hybrid structure for sustaining the electrode integrity. These characteristics are favorable for enhancing the Li-ion storage capacity, cycling stability and rate capability.
4. Conclusions MnO/CNF hybrid nanofiber composites have been successfully fabricated by a combination of a electrospinning technique and a carbonization treatment. MnO nanoparticles can be well encapsulated by the CNF when the mass loading of MnO is no more than 19.4%. Such a unique hybrid nanostructure provides large electrode/electrolyte contact area for Li-ion storage, short path length for Li-ion transport, enhanced electrical conductivity for fast electron transfer and strong structural integrity for electrode stability. The optimized M30C composite exhibits a high reversible capacity of 663 mAh g-1 along with reliable cycling stability and good rate capability, highlighting the one-dimensional hybrid nanofibers as promising anode candidates for LIB applications.
Acknowledgments The authors acknowledge the financial supports from the National Natural Science Foundation of China (51402236, 51472204 and 51232005), the Fundamental Research Funds for the Central Universities (3102014JCQ01020), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU) (83-TZ-2013), the Programme of Introducing Talents of Discipline to Universities (B08040) and the Guangdong Province Innovation R&D Team Plan.
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Figure captions Fig. 1. XRD patterns of the as-prepared MnO-CNF products. Fig. 2. SEM images of (a) bare CNFs, (b) M15C, (c) M25C, (d) M30C, (e) M40Cand (f) M50C composites. The inset is the corresponding EDS spectrum. Fig. 3. TEM images of the (a) M15C, (b) M30C and (c) M50C hybrid nanofibers. Inset is the corresponding SAED pattern. (d) HRTEM image of the MnO nanocrystal. Fig. 4. TGA curves of the CNF and MnO/CNF products. Fig. 5. (a) Cycling performance of the CNF, M15C, M25C, M30C, M40C and M50C electrodes (solid symbols: discharge capacity, open symbols: charge capacity). (b) Galvanostatic charge/discharge curves of the M30C composite electrode.(c-d) SEM images of (c) M30C and (d) M50C nanocomposites after 50 cycling tests. Fig. 6. (a) Cyclic voltammograms of the M30C composite electrode. (b) XRD patterns and (c) XPS spectra of M30 before and after lithiation (inset is the Li 1s spectrum). Fig. 7. (a) Rate capability of the M30C composite electrode. (b) Cycling stability of the M30C electrode at a current density of 200 mA g-1.
(200) (111) ∇ ∇
(311) (222) ∇ ∇
M50C M40C M30C M25C M15C CNF
2 theta (deg.) Fig. 1
CNF M15C M25C M30C M40C M50C
Weight loss (wt.%)
100 80 60 40
32.7 wt.% 26.2 wt.% 19.4 wt.% 15.6 wt.% 9.1 wt.%
20 0 0
Temperature (°C) Fig. 4
Specific capacity (mAh g )
CNF M15C M25C M30C M40C M50C
discharge discharge discharge discharge discharge discharge
charge charge charge charge charge charge
(a) Current (mA)
1st cycle 2nd cycle 3rd cycle
(b) Intensity (a.u.)
Potential (V) vs. Li/Li
charged to 2.5 V delithiation
discharged to 0.005 V lithiation
(c) Mn 3s
5.8 eV 83.4 eV charged to 2.5 V delithiation Li 1s
discharged to 0.005 V lithiation
Binding energy (eV)
Binding energy (eV) Fig. 6
Specific capacity (mAh g )
(a)1000 800 50 mA g
100 mA g
50 mA g
200 mA g
500 mA g
Cycling number 100
80 600 60 400 Graphite
200 mA g-1 0
Cycle number Fig. 7
Coulombic efficiency (%)
Specific capacity (mAh g-1)