Hybridization and pore engineering for achieving high-performance lithium storage of carbide as anode material

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Hybridization and pore engineering for achieving High-performance lithium storage of carbide as anode material Ying Xiao, Lirong Zheng, Minhua Cao

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Received date: 18 September 2014 Revised date: 1 November 2014 Accepted date: 12 December 2014 Cite this article as: Ying Xiao, Lirong Zheng, Minhua Cao, Hybridization and pore engineering for achieving High-performance lithium storage of carbide as anode material, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2014.12.015 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 galley proof before it is published in its final citable 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.

Hybridization and pore engineering for achieving high-performance lithium storage of carbide as anode material Ying Xiaoa, Lirong Zhengb and Minhua Cao a,* a

Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of

Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China. b

Institute of High Energy Physics, the Chinese Academy of Sciences, Beijing 100049, China.

* Corresponding author Name: Minhua Cao Affiliation: Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: [email protected] TEL: +86-10-68918468

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Abstract: Developing new anode materials to meet the high-energy demands of the next generation of rechargeable lithium-ion batteries (LIBs) is still a challenging work. In this work, hierarchically porous Mo2C-C (HP-Mo2C-C) hybrid has been designed and synthesized by a freeze-dryingassisted route. The resultant HP-Mo2C-C hybrid has a surface area as high as 200.6 m2 g-1. When evaluated as an anode material for LIBs, the HP-Mo2C-C hybrid displays excellent lithium storage performance in terms of specific capacity, cycling stability and rate capability. The pore engineering and hybridization with carbon are believed to be responsible for the significantly improved electrochemical performance. The novel interconnected pore structure allows for easy diffusion of the electrolyte and at the same time can enhance the HP-Mo2C-C/electrolyte contact area, shorten the Li+ diffusion length, and accommodate the strain induced by the volume change during the electrochemical reaction. Moreover, the hybridization with carbon could largely improve the conductivity of the electrode. KEYWORDS: Mo2C; Porous; Freeze-drying; Anode materials; Lithium ion batteries

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Introduction Transition metal carbides (TMCs) have been widely used as catalysts in chemical synthesis, hydrogention and isomerization owing to their affordable price and appealing chemical and physical properties including high electrical and thermal conductivities, excellent mechanical stability and chemical stability, and the resistance against corrosion under reaction conditions [15]. Recently, their applications in energy fields such as oxygen reduction reactions (ORRs), hydrogen production and lithium ion batteries (LIBs) have received intensively investigated [69]. With regards to their promising application in LIBs, Gogotsi’s group reported a series of carbides and opened a door for the investigation of carbides as one new class of potential materials for lithium storage [10-14]. The reported studies combining experiments with theoretical calculations have demonstrated promising lithium storage performance of twodimensional (2D) layered carbides, such as Nb2C, V2C, Cr2C, Ti2C and so on, and a Liintercalation mechanism has also been proposed. Zhou et al. also verified the Li storage capability of Ti3C2 by density functional theory (DFT) computations and the results demonstrated that Ti3C2 is a promising anode material for LIBs because of its exceptional properties, including good electronic conductivity, fast Li diffusion, low operating voltage, and high theoretical Li storage capacity [15]. Although 2D layered carbides have been successfully developed as electrode materials for LIBs, their limited capacities due to intrinsic structure features greatly hinder their practice applications. Therefore, it is highly desirable to develop other non-layered carbide electrode materials with high specific capacity for LIBs and that above reported studies lay a solid foundation for the further investigation of carbides. As well know, materials with nanoscale particle size often exhibit high surface area and relatively high activity. The high surface area of electrode materials can increase their interaction

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with electrolyte, which is regarded to be particularly important for electrode reactions. However, materials with a high activity will usually lead to their vulnerability to oxidation or agglomeration, thus severely degrading their performance [16,17]. Therefore, designing a novel structure that can overcome the aforementioned drawback is of particular importance for developing non-layered carbides with desired lithium storage performance. Recently, porous materials have been widely investigated as electrode materials for LIBs because of their high surface areas, tunable pore sizes, and adjustable framework thickness [18-21]. The porous structure not only is beneficial for the penetration and diffusion of the electrolyte leading to the reduction of transfer path length, but also may act as reservoirs for Li+ storage [22-25]. More importantly, the rigid framework of the porous materials generally exhibits a good mechanical stability, which can provide the mechanical/structural integrity against large changes in volume and crystal structure over extended cycling, thus ensuring much improved cycle performance. Furthermore, compositing nanoscale carbides with carbon is also an efficient approach to alleviate the agglomeration or oxidation of carbides [26,27]. Recently, the core-shell bimetallic carbide/N-doping carbon three-dimensional (3D) hybrid used as an anode material for lithium storage was reported by our group [23]. The experimental results indicate that the 3D porous structure, the hybridization with carbon and the small size of the carbide play a significant role in the final excellent lithium storage performance. Herein, we demonstrate hierarchically porous Mo2C-C (HP-Mo2C-C) hybrid as a highperformance anode material with superior long-term stability in LIBs. The resultant HP-Mo2C-C hybrid has a surface area as high as 200.6 m2 g-1. When evaluated as an anode material for LIBs, the HP-Mo2C-C hybrid displays extremely high capacities of 1196.8 and 873.6 mAh g-1 at current densities of 0.1 and 0.3 A g-1, respectively. Most importantly, the capacity can be up to

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777.7 mAh g-1 even after 1000 cycles at 1 A g-1. The excellent performance can be attributed to its novel structure involving interconnected nanopores and carbon modification as well as smallsize Mo2C nanocrystals. To the best of our knowledge, among all reported TMC-based electrode materials, the current material with this level of performance is the first time. Experimental Synthesis of HP-Mo2C-C hybrid 1.0 g of F127 was mixed with 1.0 g of phosphomolybdic acid (PMA) dispersed in 15 mL of deionized water to form F127-PMA gel via vigorous stirring for 30 min, which then was subjected to freeze-drying for 24 h to yield a precursor. Finally the typical sample (HP-Mo2C-C hybrid) can be obtained by calcining the precursor at 750 °C for 2 h at a N2 atmosphere. The control sample (pure Mo2C) was synthesized at 850 °C for 2 h while keeping other conditions same to the typical sample (denoted as bare Mo2C). Structural characterization X-ray diffraction (XRD) patterns were recorded by a Bruker D8 X-ray diffractormeter at a voltage of 40 kV and a current of 40 mA. Field-emission scanning electron microscopy (FESEM) images and Transmission electron microscopy (TEM) images were taken on a JEOL microscopy and JEOL JEM-2010 microscopes, respectively. Energy dispersive spectrometry (EDS) element mapping images were obtained by using a scanning transmission electron microscope (STEM) (FEI Technai G2 F20). Raman spectra were collected on an Invia Raman spectrometer with the excitation laser wavelength of 633 nm. X-ray spectroscopy (XPS) was performed on the ESCALAB 250 sepctrometer (Perkin-Elmer). Besides, the XPS measurement for the cycled electrode was conducted with sputtering for 360 s with Air-ion beam operating at 3 kV and 3 µA. High-resolution spectra were carried out with the pass energy of 20 eV and the

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energy step of 0.1 eV. For the testing of the materials after cycling, the cell was disassembled and the electrode was taken out. The electrode was washed by dimethyl carbonate (DEC) and then dried at the vacuum condition. The composition of sample was acquired by using inductively coupled plasma (ICP) optical emission spectroscopy (Perkin-Elmer). Nitrogen absorption-desorption measurements were carried out by a Belsorp-max surface area detecting instrument by N2 physisorption at 77 K. The freeze drying was carried out by a Freeze Drier (FD-1C-50). Thermogravimetric and differential scanning calorimetry analysis (TG/DSC) was carried out with a DTG-60AH instrument. Electrochemical measurements The electrochemical tests were performed by using CR2025 coin cells. The working electrodes were prepared by mixing active material, conductive carbon black, and sodium carboxymethyl cellulose binder (CMC) at a mass ratio of 80:10:10. The deionized water was used as the solvent. The as-resultant slurry was uniformly pasted on a Cu foil and dried at 120 °C for 36 h in vacuum oven. The mass loading of the electrode was around 0.96˗1.04 mg. The cell assembly was performed in an Ar-filled glovebox. The used electrolyte was 1M LiPF6 solution in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1, in vol%). A lithium foil was used as counter electrode. Cyclic voltammetry (CV) were measured by a CHI-760E electrochemical workstation with a scan rate of 0.1 mV s-1. The impendence spectra were obtained by applying a sine wave with amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz. Results and discussion The HP-Mo2C-C hybrid was experimentally realized, as shown schematically in Figure 1. The PMA-pluronic F127 gel (denoted as PMA-F127) was first synthesized by stirring an aqueous solution of PMA and F127 at room temperature and a freeze-drying process. Then the as-

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resultant green PMA-F127 precursor with a loose structure was calcined at 750 °C for 2 h under N2 atmosphere, thus leading to the formation of HP-Mo2C-C hybrid. The calcination temperature was determined according to TG/DSC analysis (Figure S1), which was performed on the PMAF127 precursor between 25 °C and 900 °C at N2 atmosphere with a heating rate 10 °C min-1. The XRD measurement was performed to reveal the crystal structure of the calcined samples. For the sample obtained at 750 °C, all diffraction peaks shown in Figure 2a can be well indexed to cubic Mo2C (JCPDS No. 15-0457), while that weak broad peak in the angle range of 20o–30o may be attributed to carbon and the Raman spectrum further confirmed this fact. As shown in Figure 2b, two intense peaks at 1580 and 1360 cm-1 were observed, corresponding to the well-documented G band and D band of carbon [23], respectively, whereas the peaks located at 818.6 and 992.4 cm-1 can be attributed to Mo2C [28]. These results are well consistent with that from above XRD analysis. The ICP-MS combined with a CHN element analysis gives the mass percentage of the amorphous carbon to be 4.177%. If the calcination temperature was increased to 850 °C, bare Mo2C was obtained, which has been confirmed by XRD pattern and Raman spectrum (Figure S2). Figure 2c shows the high resolution Mo 3d XPS spectrum of HP-Mo2C-C, which can be deconvoluted into five peaks. The peaks at 229.0 and 232.9 cm-1 can be ascribed to Mo 3d of Mo2C, while those at 235.9, 231.9 and 230.2 cm-1 are related to MoOx mainly derived from the surface oxidation of Mo2C during the test process [26-30]. The deconvoluted C 1s spectrum was utilized to investigate the surface carbon (Figure 2d). According to the literature, the peak at the low binding energy around 284.3 eV corresponds to carbidic carbon [29]; the peaks at 284.9, 286.3 and 288.9 eV are related to the C-C sp2 bonding, epoxy and hydroxyl groups, and carboxyl groups, respectively. Moreover, the electronic structures of the Mo2C/C hybrid were further determined by XANES analysis. The Mo K edge XANES spectra and the derived differential

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spectra (Figure 2e,f) indicate that the Mo2C/C and the bare Mo2C displayed the pre-edge at 20003.8 and 20003.5 eV, respectively, both of which shift towards higher energy compared to Mo foil (20001.5 eV). The possible reason may be ascribed to the negative charge transfer from molybdenum to carbon [31]. Besides, the higher peak intensity of the HP-Mo2C-C hybrid compared with the bare Mo2C (Figure 2f) suggests a lower electron density at the Mo site in the HP-Mo2C-C sample, further confirming the interaction between Mo2C and C [32]. The morphology of the sample was investigated by FE-SEM and TEM measurements. From Figure S3, it can be observed that the HP-Mo2C-C hybrid is composed of interconnected small particles. Figure 3a–c present the typical TEM images of the as-prepared hybrid, which reveal an interconnected pore network structure with an obvious texture contrast. Numerous white dots (Figure 3c, yellow cycles) represent nanopores generated by the decomposition of the precursor, which is more obvious in the magnified TEM image. The pore wall consists of ultrafine Mo2C nanoparticles with diameters in the range of about 4–7 nm (blue cycles). Besides, no single nanoparticles can be observed, suggesting that the HP-Mo2C-C hybrid exhibits a 3D interconnected porous architecture. The HRTEM image indicates that Mo2C nanoparticles bind with each other and the spacing of the adjacent lattice planes is 0.238 nm, corresponding to the (111) planes of Mo2C (Figure 3d). The concentric diffraction rings in the selected area electron diffraction (SAED) pattern (inset in Figure 3d) confirms the polycrystalline nature of Mo2C. Furthermore, the STEM image and the element mapping images combining with EDS spectrum of the HP-Mo2C-C hybrid disclose the existence and homogeneous distribution of Mo, C, and O elements in this hybrid (Figure 3e–i). The porous nature of this hybrid is further investigated by the nitrogen adsorption/desorption analysis. As shown in Figure 3j, the HP-Mo2C-C hybrid displays a combined type I/IV sorption isotherms accompanied by a hysteresis loop in the

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relative pressure range of 0.3–0.8 (inset in Figure 3j) along with a sharp rise of sorption capacity in the low pressures, thus indicating this sample exhibits a hierarchically mesoporousmicroporous structure. The pore size calculated based on the NLDFT model also verified this conclusion (Figure 3k), and the mesopore size mainly centers at 2.6 nm, which is well consistent with the observations from above TEM images. Besides, it is worthy of noticing that this kind of hierarchically porous structure gives the BET surface area as high as 200.6 m2 g-1, which is far higher than those of bare Mo2C (see in Figure S2e) and Mo2C materials reported previously [5,7,33]. Such a high surface area may provide more surface active sites, thus inevitably leading to an enhancement of the electrochemical performance. In view of novel structure and composition of the resultant HP-Mo2C-C hybrid, its lithium storage performance was evaluated as an anode material for LIBs. Figure 4a shows the galvanostatic discharge and charge voltage profiles generated by the HP-Mo2C-C electrode at a current density of 0.1 A g-1 in a potential window of 0.01–3 V (vs. Li+/Li) for 1st, 2nd, 20th and 100th cycles. The initial discharge and charge specific capacities are 1054.3 and 925.5 mAh g-1, respectively, corresponding to a Coulombic efficiency of 87.8%. Interesting, the correlative plateau regions can also be observed in the discharge-charge profiles, indicating the likely existence of conversion reaction. Moreover, the HP-Mo2C-C hybrid exhibits excellent cycling performance as displayed in Figure 4b. After an initial discharge capacity of 1054.3 mAh g-1 at 0.1 A g-1, the HP-Mo2C-C electrode exhibits a capacity decrease in the first several cycles, and subsequently the capacity started to steadily rise, and finally a capacity as high as 1196.8 mAh g1

can be retained after 100 cycles. The capacity decrease in the first several cycles is related to

the formation of solid electrolyte interface (SEI) films on the surface of the electrode, which is a common phenomenon for most anode materials;[34,35] subsequent steady increase of the

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capacity is considered to originate from the gradual access of more electrolyte into the meso- and micro-pores of the active material during the cycling process and the progressive generation of electro-chemistry active polymeric gel-like films or the interfacial Li storage, which result in an increased accommodation behavior for lithium.[36-40] The average Coulombic efficiency was calculated to be above 98%. Even at 0.3 A g-1, a capacity of 873.6 mAh g-1 can be delivered after 100 cycles. On the contrary, the reference cells based on the pure carbon and the bare Mo2C showed lower capacities, 310.2 and 207.6 mAh g-1 after 100 cycles, respectively (Figure S4 shows the related characterizations for the pure carbon). To better understand the lithium storage performance of the HP-Mo2C-C hybrid in LIBs, the robustness of the HP-Mo2C-C hybrid was evaluated by cycling the corresponding cell at various current densities (Figure 4c). Obviously, compared with the pure carbon, the hybrid displays higher capacity. Even at a high current density of 10 A g-1, a capacity of 317 mAh g-1 can still be achieved. When it switched abruptly from 10 A g-1 to 0.1 A g-1 again, the capacity can be recovered to or surpass the original value, indicating the good reversibility and robustness of the electrode material. Most importantly, after 1000 cycles at 1 A g-1 (Figure 4e), the HP-Mo2C-C hybrid can maintain a high reversible capacity of 777.7 mAh g-1 and the Coulombic efficiencies can retain above 99.0% after the second cycle, representing its excellent long-term cycling stability and good capability at high rates. The excellent rate capability of the HP-Mo2C-C hybrid should be closely related to its fast kinetics of the redox reactions involving lithium ion and electron diffusion to/from the electrolyte/particle interface. To confirm this fact, electrochemical impedance spectroscopy (EIS) measurements were conducted on the HP-Mo2CC electrode after 100 cycles at 0.1 A g-1. As displayed in Figure 4d, the intercept on Zreal axis in the high frequency region corresponds to the resistance of electrolyte (Rs); the semicircle at high

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frequency reflects the SEI resistance (RSEI) and the high-medium frequency represents the charge transfer resistance (Rct) on the electrode/electrolyte interface; the incline line at the low frequency corresponds to the Warburg impedance (Zw) related to the diffusion of lithium ions in the electrode materials [2,41,42]. Obviously, the semicircle diameter of the HP-Mo2C-C electrode in the high-medium frequency is far smaller than that of the bare Mo2C, implying its improved conductivity. Additionally, an equivalent circuit was used to fit the Nyquist plots, as depicted in Figure 4d (the inset). Based on the fitted results, the bare Mo2C gives Rct of 176.2 Ω, much larger than that of the HP-Mo2C-C hybrid (85.5 Ω). We believe that the high conductivity of the HP-Mo2C-C hybrid may result from the incorporation of carbon, which can greatly enhance the conductivity of Mo2C, leading to significant improved electrochemical performance. To disclose the structure stability of the HP-Mo2C-C hybrid after cycling, TEM analysis was performed. As shown in Figure 5, the HP-Mo2C-C electrodes after 100 cycles in the discharged state at 0.1 A g-1 (a,b) and 1000 cycles in the discharged state at 1 A g-1 (c,d) both retained the pristine morphology, which is of great significance for keeping long-term stability of the electrodes. The Mo2C nanoparticles (blue cycles) still possess small sizes and have no evident aggregation at all. In addition, nanopores (black arrows) between the nanoparticles can still be detected, which is considered to make significant contributions not only to increasing the contact area between the electrode and the electrolyte but also to shortening the diffusion lengths for both electrons and ion transport [2,43]. These evidences demonstrate the excellent structural stability of the current electrode, implying a perfect mechanical stability of the HP-Mo2C-C hybrid, which can provide the mechanical/structural integrity against large changes in volume and crystal structure over extended cycling. In detailed, the small-size Mo2C nanoparticles can reduce the transfer path of Li+ and is favorable for adsorption of more Li+ during the cycling

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process, while the presence of carbon can enhance the conductivity of Mo2C and effectively stabilize the as-formed SEI films as well as provide more active sites for Li+ insertion and extraction. Furthermore, the interconnected pores between the Mo2C particles provide a continuous pathway for electron transport and Li+ diffusion and effectively withstand the volume change and restrict the aggregation of Mo2C [43,44]. All of these aspects contributed to the longterm cycling stability and excellent rate capability of the HP-Mo2C-C hybrid. In order to explore the lithium storage mechanism of the HP-Mo2C-C hybrid, the cyclic voltammetry (CV) curves of the first five cycles in the voltage range of 0.01–3 V were presented (Figure 6a). During the first lithiation cycle, a broad peak at around 0.30 V was observed, and from the third cycle, it disappeared. This irreversible peak may result from the formation of SEI films. In the subsequent cycles, two peaks at 1.18 and 1.51 V were observed during the lithiation and delithiation process, which are similar to those of MoO2-based electrode materials [45-48]. Thus it can be tentatively deduced that these two peaks may correspond to conversion reactions between Mo2C and Mo. Besides, the CV curves of the bare Mo2C exhibit similar behavior to those of the HP-Mo2C-C hybrid (Figure S5), indicating the Mo2C in the HP-Mo2C-C hybrid severing as mainly active materials for lithium storage. In order to further examine the lithium storage mechanism between Mo2C and lithium, XRD and Mo XPS measurements of the HPMo2C-C after charged to 3 V and discharged to 0.01 V for 10 cycles were provided (Figure 6b,c). It should be noted that those three very sharp peaks detected in the XRD patterns correspond to the Cu foil. In both cases, the composition and crystal structure of the HP-Mo2C-C almost do not change. However, from the Li 1s XPS spectra with lithiated and delithiated states (Figure 6d), a peak belonging to LiC (ca. 57.0 eV) was detected in the former spectrum and disappeared in the latter one, indicating the existence of transformation reaction during cycling process [48]. This fact was further confirmed when the cell was proceeded to 100 cycles. Figure 6e,f show XRD pattern and HRTEM image of the HP-Mo2C-C electrode after 100 cycles. The XRD pattern (Figure 6e) discloses the formation of LiC, but, no obvious peaks belonging to Mo were detected, probably due to its deep embedding or amorphous nature. The corresponding HRTEM image confirmed the existence of Mo2C (d = 0.205 nm) and LiC (d = 0.272 nm). All these results indicate the presence of conversion between Mo2C and Mo, and the electrochemical reaction equation between Mo2C and Li can be described as: Mo 2 C + Li + + e− 2Mo + LiC . Additionally, based on recently reported studies [19,23,49], Li can adsorb on the surface of nanoparticles and the interspersed pores to achieve Li-storage. Thus, in our case, the conversion of Mo2C, the hierarchical micro-/meso-pores, the small-size Mo2C nanoparticles, and 3D carbon conductive network contributed to the excellent Li-storage performance of the HP-Mo2C-C hybrid.

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Conclusions In summary, we have reported for the first time the successful fabrication of hierarchically porous Mo2C-C hybrid via a simple freeze-drying following annealing in a N2 atmosphere. The resultant product exhibits a hierarchically porous structure with a surface area as high as 200.6 m2 g-1. When evaluated as an anode material for LIBs, the hierarchically porous Mo2C-C hybrid shows superior lithium storage performance in terms of high capacity, excellent rate capability, and long-term stability upon extended cycling. The excellent electrochemical performance may arise from the synergistic effects induced by its novel structure including the small-size Mo2C nanoparticles, the presence of carbon, the high surface area and the interconnected nanopores. Insight gained from this work may be helpful in exploring other carbides or alloy systems for LIBs.

Acknowledgement This work was supported by the National Natural Science Foundation of China (21471016 and 21271023) and the 111 Project (B07012).

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Figure capations Figure 1 The schematic illustration of the synthesis procedure for the HP-Mo2C-C hybrid. Figure 2 The composition characterization of the HP-Mo2C-C hybrid: (a) XRD pattern, (b) Raman spectrum, (c) Mo and (d) C 1s of XPS spectra. (e) XANES spectra and (f) the corresponding derived differential spectra of the HP-Mo2C-C hybrid, Mo2C and Mo foil.

Figure 3 (a-c) TEM images of the HP-Mo2C-C hybrid with different magnifications. (d) HRTEM image and SAED pattern of the HP-Mo2C-C hybrid (the inset). (e-i) The STEM image, the element mapping images, and the EDS spectrum of the HP-Mo2C-C hybrid. (j) Nitrogen adsorption-desorption isotherms of the HP-Mo2C-C hybrid and (k) the corresponding pore size distribution.

Figure 4 (a) The discharge/charge voltage profiles of the typical HP-Mo2C-C hybrid electrode. (b) The cycling performance of the typical HP-Mo2C-C electrode at 0.1 A g-1 and 0.3 A g-1 as well as the pure carbon and bare Mo2C cycled at 0.1 A g-1. The corresponding Coulombic efficiency at 0.1 A g-1. (c) Rate performance of the typical HP-Mo2C-C hybrid and the pure

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carbon from 0.1 A g-1 to 10 A g-1. (d) The Niquist plots of the typical HP-Mo2C-C hybrid and bare Mo2C after 100 cycles at 0.1 A g-1. The inset is the corresponding equivalent circuit model. (e) The cycling performance and the corresponding Columbic efficiency of the HP-Mo2C-C hybrid at 1 A g-1 for 1000 cycles.

Figure 5 TEM images of the HP-Mo2C-C electrode after 100 cycles at 0.1 A g-1 (a,b) and after 1000 cycles at 1 A g-1 (c,d). The arrows and circles represent nanopores and nanoparticles, respectively.

Figure 6 (a) CV curves of the HP-Mo2C-C hybrid. (b–d) XRD patterns and XPS spectra of the HP-Mo2C-C electrode after 10 discharges and 10 charges. (e) XRD pattern and (f) HRTEM image of the HP-Mo2C-C electrode after 100 discharges at discharged state.

Figure 1

Freeze-drying

H3PMo12O40+H2O

Calcining

HP-Mo2C-C

H3PMo12O40-F127 gel Mesopores

F127 H3PMo12O40

Mo2C-C Micropores

18

30

40

50

60

70

80

2θ (degree)

284.9

286.3 288.9

Normalized absorption

Intensity (a.u.)

284.3

292 290 288 286 284 282 280 278

Bind energy (eV)

600

900 1200 1500 1800 -1 )

Raman shift (cm

(d)

C 1s

300 1.2

(e)

Mo K edge 1.0 0.8 0.6

Mo2C/C

0.4

Mo foil

Mo2C

0.2 0.0 19980

20010

20040

20070

Energy (eV)

Intensity (a.u.)

1358.6 1596.5

235.9

231.9 229.0 230.2

240

237

234

231

228

225

Bind energy (eV)

)

20

(311)

818.6 992.4

(c)

232.9 Mo 3d

-1

(200) (220)

(b)

Dfferential energy (eV D

10

Mo2C (JCPDS No:15-0457) (a) (111)

Intensity (a.u.)

Intensity (a.u.)

Figure 2

0.21

(f)

0.18

HP-Mo2C-C

0.15

Mo2C

0.12

Mo foil

0.09 0.06 0.03 0.00

-0.03

19980

20010

20040

Energy (eV)

20070

19

Figure 3

(a) a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

C

O

Mo

(j)

(k)

)

0.06

-1

80

dV/dD (cm g nm

80

60

-3 -1

(i)

Quantity absorbed (cm3g-1 )

100

40 70

20

0.02

0 -20 0.0

0.00

60 0.3

0.2

0.4

NLDFT method

0.04

0.6

0.6

0.8

Relative pressure (p/p0)

1.0

0

5

10

Pore size (nm)

15

20

20

Figure 4

1500 -1

Capacity (mAh g )

Voltage (V)

2.5 2.0 1st 2nd 20th 100th

1.5 1.0 0.5

1200

(0.3 A g )

Pure C

300

(c)

1000 800

HP-Mo2C-C

250

C 2 2.5

600

3

0

20

40

310.2

-1

207.6

40 60 Cycle number

80

20

0 100

200

4

5

150

HP-Mo2C-C (Fitted)

100

Bare Mo2C (Fitted) HP-Mo2C-C

8 10

400

-1

(0.1 A g )

(d)

0.1

-Z''/ohm

1200

60

Bare Mo2C (0.1 A g )

0

400 600 800 1000 1200 Capacity (mAh g-1)

0.1 0.3 0.6 0.9 1.2 1.5

-1

Capacity (mAh g )

200

1196.8 80 873.6

-1

600

Bare Mo2C

50

200

0

0

20

1500 -1

-1

HP-Mo2C-C (0.1 A g )

0

0

100

Coulombic efficiency (%)

900

0.0

Capacity (mAh g )

(b)

Coulombic efficiency (%)

(a)

40

60 80 100 120 140 Cycle number

0

50

100 150 Z'/ohm

200

250

100

(e)

1200

Coulombic efficiency

80 -1

1Ag

900

60

600

40

300

20

0

0

200

400

600 Cycle number

800

0 1000

Coulcombic efficiency (%)

3.0

21

Figure 5

(a)

(b)

(c)

(d)

22

Figure 6

1.51

tion thia Deli

0.0

-0.1

1.18

iatio Lith

1st 2nd 3rd 4th 5th

n

-0.2 -0.3 0.0

0.5

1.0

Intensity (a.u) 60

2.0

Voltage (V)

(d) Li 1s

1.5

2.5

56.3

20

Discahrged to 0.01 V

56

54

52

40

Cu

(e)

Charged to 3 V

Bind energy (eV)

30

(c)

50

60

70

Charged to 3 V

Mo 3d

Discharged to 0.01 V

80 240

2θ (degree)

55.4

58

Cu

Discharger to 0.01 V

10

3.0

Cu

Charged to 3 V

55.3

56.4 57.0

Cu

(b)

Intensity (a.u.)

(a)

Intensity (a.u.)

0.1

Intensity (a.u)

Current (mA)

0.2

237

234

231

228

225

Bind energy (eV)

Cu

Cu

(f)

Discharged to 0.01 V -1 after cyled at 0.1 A g

50 10

LiC

20

30

40

50

60

2 θ (degree)

70

80

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Biographies Ying Xiao received her B.S. and M.S. degree from Henan Normal University in 2008 and 2011, respectively. She is now a Ph.D. candidate at Beijing Insinuate of Technology. Her current research interests focus on the design and synthesis of inorganic nanomaterials and their property for lithium ion batteries.

Min-Hua Cao is a Professor at Beijing Institute of Technology, China. She received her Ph. D. degree in inorganic chemistry from the Northeast Normal University, China (2005). In 2006 she was awarded a Humboldt Research Fellowship and joined the Max Planck Institute of Colloids and Interface, Germany (2006–2007). Her current research interests focus on synthesis and property study of nanostructural materials including metal oxides, metal sulphides, graphenebased composites and so on.

Li-Rong Zheng is an associate research fellow at Institute of High Energy Physics, Chinese Academy of Sciences, China. He received his Ph. D. degree in Condensed Matter Physics and graduated from the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (2009). From 2009 to now, He has been working at XAFS beamline of Beijing Synchrotron Radiation Facility in IHEP. His current research interests focus on the Methodology of Synchrotron radiation for nanostructural materials .

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Highlights • Hierarchically porous Mo2C-C hybrid was synthesized by a freeze-drying-assisted route. • The porous Mo2C-C hybrid was first used as an anode material for lithium ion battery. • The porous Mo2C-C hybrid exhibited high capacity and excellent cycling stability and rate performance.

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The table of contents entry

Hybridization and pore engineering for achieving high-performance lithium storage of carbide as anode material

Ying Xiao, Lirong Zheng and Minhua Cao*

100

1200

HP-Mo2C-C

80

900

60

600

40

300

20

0 0

10

20

30

40

50

60

70

80

90

Coulombic efficiency (%)

-1

Capacity (mAh g )

1500

0 100

Cycle number

Hierarchically porous Mo2C-C hybrid anode with long-term cycling stability has been successfully synthesized through a simple route. The synergistic effects resulting from its novel structure induced the effects of short transfer path of Li+, high conductivity of the electrode, large electrode/electrolyte contact area and porous structural configuration, leading to the performance enhancement of the hierarchically porous Mo2C-C electrode.

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