- Email: [email protected]
N-doped carbon nanowires as anode materials for sodium-ion batteries
Accepted Manuscript Mo2C/N-Doped Carbon Nanowires as Anode Materials for Sodium-Ion Batteries Xiang Li, Mengdie Deng, Wenbiao Zhang, Qingsheng Gao, Hui Wang, Bin Yuan, Lichun Yang, Min Zhu PII: DOI: Reference:
S0167-577X(17)30197-0 http://dx.doi.org/10.1016/j.matlet.2017.02.015 MLBLUE 22117
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
21 October 2016 26 January 2017 6 February 2017
Please cite this article as: X. Li, M. Deng, W. Zhang, Q. Gao, H. Wang, B. Yuan, L. Yang, M. Zhu, Mo2C/N-Doped Carbon Nanowires as Anode Materials for Sodium-Ion Batteries, Materials Letters (2017), doi: http://dx.doi.org/ 10.1016/j.matlet.2017.02.015
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 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.
Mo2C/N-Doped Carbon Nanowires as Anode Materials for Sodium-Ion Batteries Xiang Li,a Mengdie Deng,b Wenbiao Zhang,b Qingsheng Gao,b * Hui Wang,a Bin Yuan,a Lichun Yang,a* Min Zhua a
School of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Advanced
Energy Storage Materials, South China University of Technology, Guangzhou, 510641, China. E-mail: [email protected] b
Department of Chemistry, Jinan University, Guangzhou, 510632, China.
E-mail: [email protected] Abstract Herein we report Mo2C/N-doped carbon hierarchical nanowires (Mo2C/N-C HNWs) as anode materials for sodium-ion batteries. In the Mo2C/N-C HNWs, Mo2C nanocrystallites are uniformly distributed in N-doped carbon matrix. The nanocrystallites of Mo2C offer short paths for Na+ diffusion, the mesoporous structure facilitates the diffusion of electrolyte, and the N-doped carbon matrix accelerates the electron transfer. As anode materials for sodium-ion batteries, the Mo2C/N-C HNWs exhibited reversible capacities of 381 and 308 mAh g-1 at current densities of 50 and 200 mA g-1, respectively. Our results demonstrate the enhanced Na+ storage activity of Mo2C after structure tailoring, elucidating the potential of transition-metal carbides as promising anode materials for sodium-ion batteries. Keywords: Mo2C; N-doped carbon; nanocomposite; sodium-ion battery; energy storage and conversion 1. Introduction Sodium-ion batteries (SIBs) have attracted wide attention as alternative energy storage technologies to lithium-ion batteries in large scale applications, [1] owing to more abundant resources and lower cost of sodium. Transition metal carbides (TMCs) emerge as promising host materials for alkali metal ions
1
recently, [2-5] because they exhibit metallic conductivity. This is a great advantage compared to transition metal oxides which are almost semiconductors. As anode materials for SIBs, layered TMCs, e. g., Ti2CTx and Ti3C2 Tx (T = OH, F, O) exhibit reversible capacities of 233 mAh g-1 and 150 mAh g-1, [6, 7] respectively. Non-layered TMCs, such as N-doped MoC (MoCxNy) [8] and Co3ZnC [9] show even higher capacity (> 300 mAh g-1), which inspire us to investigate the Na+ storage performance of non-layered TMCs. Herein, we reported for the first time on the Na+ storage performance of Mo2C/N-doped carbon hierarchical nanowires (Mo2C/N-C HNWs). As anode materials for SIBs, they exhibit reversible capacities of 381 and 308 mAh g-1 at 50 and 200 mA g-1, respectively, which is much improved compared with the counterpart of microparticles. Our results demonstrate the capability of Mo2C in the reversible storage of Na+, elucidating the potential application of TMC-based materials for SIBs. 2. Experimental Mo3O10(C6H8N)2·2H2O nanowires prepared according to our previous report [10] were calcined at 775 o
C for 5h under a Ar flow to obtain Mo2C/N-C HNWs. The electrochemical behavior was investigated
using coin cells (CR 2016) in which Na sheets were used as the counter and reference electrodes, glass fiber membranes (Whatman GF/D) were used as separators, and 1 M NaClO4 solution in propylene carbonate/ethylene carbonate (PC/EC) (1:1 in volume) was used as electrolyte. 3. Results and Discussion
2
Fig. 1 (a) XRD patterns of the Mo2C/N-C HNWs and the Mo2C MPs. (b) Raman spectrum, (c) XPS survey, and (d) high resolution XPS spectra for N1s of the Mo2C/N-C HNWs. The XRD patterns (Fig. 1a) correspond to hexagonal β-Mo2C (JCPDS: 35-0787). And the diffraction peaks of the Mo2C/N-C HNWs are broader than those of commercial Mo2C microparticles (MPs, Fig. S1), indicating smaller crystallites in the Mo2C/N-C HNWs. The Raman spectrum of the Mo2C/N-C HNWs (Fig. 1b) presents the characteristic D- and G-bands of carbon at ~1360 and 1600 cm-1, respectively, confirming the existence of free carbon. And the survey XPS spectrum (Fig. 1c) identifies the presence of N besides Mo, C, and O. The high resolution profile of N 1s (Fig. 1d) can be deconvoluted into three peaks. The peaks at 398.4 and 400.2 eV are assigned to pyridinic and pyrrolic N, respectively, indicating the N-doping in the carbon. [11] The peak at 397.8 eV is associated with N bonding to Mo, suggesting a small amount of MoNx may be formed during the pyrolysis. Based on CHN elemental analysis and ICP-AES measurement, the contents of free carbon and nitrogen can be further determined as 5.8 and 0.5 wt%, respectively.
3
Fig. 2 (a) SEM, (b) TEM and (c) HR-TEM images of the Mo2C/N-C HNWs; (d) TEM image of the product obtained after removing Mo2C from Mo2C/N-C HNWs. Insets of panels b and d are the corresponding SAED pattern of the nanowires.
The SEM image (Fig. 2a) shows that the Mo2C/N-C HNWs maintain the nanowire morphology of the Mo3O10(C6 H8N)2·2H2O precursor (Fig. S2). The TEM image reveals mesoporous structure of the Mo2C/N-C HNWs which is stacked by nanoparticles (Fig. 2b). The selected area electron diffraction (SAED) pattern (inset of Fig. 2b) and lattice fringes in the HR-TEM (Fig. 2c) are attributed to hexagonal β-Mo2C nanocrystallites. The elemental mapping (Fig. S3) indicates uniform distribution of Mo2C and N-doped carbon. Mo2C can be removed by H2O2, leaving amorphous carbon nanowires (Fig. 2f), which demonstrate Mo2C nanocrystallites are embedded in the continuous N-doped carbon matrix.
Fig. 3 CV curves of (a) the Mo2C/N-C HNWs and (b) the Mo2C MPs tested at a scan rate of 0.1 mV s-1. The electrochemical performance of the Mo2C/N-C HNWs was evaluated as anode materials for SIBs. 4
In the CV profiles of Mo2C/N-C HNWs (Fig. 3a), a wide reduction peak located ~2.0 V appears in the first cycle and disappears in subsequent cycles, which may be related to the decomposition of the electrolyte and the formation of a solid electrolyte interphase (SEI) film. By contrast, the SEI formation peak of the Mo2C MPs is located at 1.05 V (Fig. 3b), which is lower than that of the Mo2C/N-C HNWs. It may be due to the higher catalytic ability of the Mo2C nanocrytallites in the composite. For the Mo2C/N-C HNWs, two pair of redox peaks at 0.8/0.66 V and 0.22/0.38 V can be observed, which are similar to the Na+ storage/release behavior of Mo2N. [12] As for the Mo2C MPs, the redox peaks on the CV profiles (Fig. 3b) are inconspicuous, indicating much lower activity for the reversible Na+ storage. So far, the Na+ storage mechanism for Mo2C is not yet clear. According to the previous report, Mo2C is considered to store Li+ through conversion reaction. [3, 13] Due to the similarity of Li+ and Na+ in the electrochemical properties, the storage of Na+ in Mo2C may be through insertion and the conversion reaction (Mo2C+xNa++xe- ↔ NaxC+ 2Mo). However, in our preliminary XRD investigation, neither NaxC nor Mo can be detected in the fully discharged state (Fig. S4). And further investigation is ongoing to reveal the mechanisms for the reversible Na+ storage in TMCs.
Fig. 4 Electrochemical performance of the Mo2C/N-C HNWs and the Mo2C MPs. (a) First charge/discharge curves, (b) cycling performance at 200 mA g-1, (c) rate capability and (d) Nyquist plots.
5
The Mo2C/N-C HNWs and the Mo2C MPs are discharged/charged in the range of 0.01 V- 3.0 V vs. Na/Na+ at 200 mA g-1 (Fig. 4a). The initial discharge and charge capacities of the Mo2C/N-C HNWs are 441 and 308 mAh g-1, respectively, with a coulombic efficiency of 70%. And the amorphous carbon nanowire matrix only exhibits an initial reversible capacity of 61.1 mAh g-1 (Fig. S5). Considering the low capacity and low content, the capacity contribution of the carbon matrix is negligible. The Mo2C MPs presents discharge and charge capacities of 212 and 86 mAh g-1, respectively, with a coulombic efficiency of 40%. After 100 cycles, the Mo2C/N-C HNWs maintain a reversible capacity of 166 mAh g-1, while the Mo2C MPs only deliver a reversible capacity of 60 mAh g-1 (Fig. 4b). The rate performance of the two samples is shown in Fig. 4c. The Mo2C/N-C HNWs deliver a reversible capacity of 381.5 mAh g-1 at 50 mA g-1, maintain 167.5 mAh g-1 at 500 mA g-1, and regain 242.4 mAh g-1 when the current density reduces to 50 mA g-1. By contrast, the Mo2C MPs deliver 108.8 mAh g-1 at 50 mA g-1, and only retain 26.1 mAh g-1 at 500 mA g-1, which is much lower than that of the Mo2C/N-C HNWs. The galvanostatic charge/discharge results demonstrate the greatly enhanced Na+ storage activity of the Mo2C/N-C HNWs compared with the Mo2C MPs, and the improvement is highly related with the nanostructure. In the Mo2C/N-C HNWs, the mesopores enable easy penetration of electrolyte, and the Mo2C nanoparticles offer decreased paths and more active sites for Na+ diffusion. Moreover, the continuous N-doped carbon matrix connects the nanoparticles, which facilitates the charge transfer. As compared in Fig. 4d, the diameter in the Nyquist plot of the Mo2C/N-C HNWs is obviously smaller than that of the Mo2C MPs. The fitted results (Fig. S6 and Table S1) demonstrate lower charge transfer resistance in the Mo2C/N-C HNWs. The enhanced kinetics for Na+ diffusion and charge transfer result in the higher capacity delivery of the Mo2C/N-C HNWs. The Na+ storage capacity of the Mo2C/N-C HNWs is comparable with those of explored TMCs (e.g. 6
Ti2CTx (T = OH, F, O) and Ti3C2Tx[6, 7]) and transition metal nitrides (e.g. Mo2N nanobelts, [12] VN/graphene hybrid, [14] Cu3N [15] and Ni3N [16]), which are also interstitial alloys. And the cyclic stability for reversible Na+ storage of the Mo2C/N-C HNWs needs further improvement, compared with the reported N-doped MoC (MoCxNy) [8] and Co3ZnC [9] based materials. Our work shows potential application of Mo2C as an anode material for SIBs. With future investigation into the mechanism for Na+ storage and capacity fading, Mo2C-based materials with improved cyclic performance can be further developed for SIBs. 4. Conclusions In summary, we have reported an anode material of Mo2C/N-C HNWs for SIBs. The structure features such as nanocrystallites, mesopores and continuous N-doped carbon matrix enhance kinetics for Na+ diffusion and charge transfer. As anode materials for SIBs, they exhibit reversible capacities of 381 and 308 mAh g-1 at 50 and 200 mA g-1, respectively. And non-layered TMCs with appropriate structure design show great promise as anode materials for SIBs. ACKNOWLEDGMENT We acknowledge financial support from the National Natural Science Foundation of China (51671089, 21373102), Natural Science Foundation of Guangdong Province (2016A030312011).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at XXX References [1] H.L. Pan, Y.S. Hu, L.Q. Chen, Energy Environ. Sci. 6 (2013) 2338-2360. [2] M. Naguib, J. Halim, J. Lu, K.M. Cook, L. Hultman, Y. Gogotsi, M.W. Barsoum, J. Am. Chem. Soc. 135 (2013) 15966-15969.
7
[3] Y. Xiao, L. Zheng, M. Cao, Nano Energy 12 (2015) 152-160. [4] L. Yang, X. Li, S. He, G. Du, X. Yu, J. Liu, Q. Gao, R. Hu, M. Zhu, J. Mater. Chem. A 4 (2016) 10842-10849. [5] L. Su, Z. Zhou, P. Shen, Electrochimica Acta 87 (2013) 180-185. [6] X. Wang, S. Kajiyama, H. Iinuma, E. Hosono, S. Oro, I. Moriguchi, M. Okubo, A. Yamada, Nature communications 6 (2015) 6544. [7] S. Kajiyama, L. Szabova, K. Sodeyama, H. Iinuma, R. Morita, K. Gotoh, Y. Tateyama, M. Okubo, A. Yamada, ACS Nano 10 (2016) 3334-3341. [8] J. Qiu, Z. Yang, Q. Li, Y. Li, X. Wu, C. Qi, Q. Qiao, J. Mater. Chem. A 4 (2016) 13296-13306. [9] T. Chen, B. Cheng, R. Chen, Y. Hu, H. Lv, G. Zhu, Y. Wang, L. Ma, J. Liang, Z. Tie, Z. Jin, J. Liu, Acs Applied Materials & Interfaces 8 (2016) 26834-26841. [10] Q.S. Gao, C.X. Zhang, S.H. Xie, W.M. Hua, Y.H. Zhang, N. Ren, H.L. Xu, Y. Tang, Chem. Mater. 21 (2009) 5560-5562. [11] P.F. Zhang, Y.T. Gong, H.R. Li, Z.R. Chen, Y. Wang, Nat. Commun. 4 (2013) 1593. [12] S.L. Liu, J.Y. Huang, J. Liu, M. Lei, J. Min, S.T. Li, G. Liu, Mater. Lett. 172 (2016) 56-59. [13] X. Wang, X. Shen, Y. Gao, Z. Wang, R. Yu, L. Chen, J. Am. Chem. Soc. 137 (2015) 2715-2721. [14] L. Wang, J. Sun, R. Song, S. Yang, H. Song, Adv. Energy Mater. 6 (2016) 1502067-1502067. [15] X. Li, A.L. Hector, J.R. Owen, J. Phys. Chem. C 118 (2014) 29568-29573. [16] L. Xianji, M.M. Hasan, A.L. Hector, J.R. Owen, J. Mater. Chem. A 1 (2013) 6441-6445.
8
Highlights: 1.
Firstly applying Mo2C/N-C HNWs in sodium ion batteries (SIBs).
2.
The hybrid nanostructure enables good performance of Na+ storage.
3.
Such material will be a new promising anode material for SIBs.
Graaphical Absttract
S0167-577X(17)30197-0 http://dx.doi.org/10.1016/j.matlet.2017.02.015 MLBLUE 22117
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
21 October 2016 26 January 2017 6 February 2017
Please cite this article as: X. Li, M. Deng, W. Zhang, Q. Gao, H. Wang, B. Yuan, L. Yang, M. Zhu, Mo2C/N-Doped Carbon Nanowires as Anode Materials for Sodium-Ion Batteries, Materials Letters (2017), doi: http://dx.doi.org/ 10.1016/j.matlet.2017.02.015
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 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.
Mo2C/N-Doped Carbon Nanowires as Anode Materials for Sodium-Ion Batteries Xiang Li,a Mengdie Deng,b Wenbiao Zhang,b Qingsheng Gao,b * Hui Wang,a Bin Yuan,a Lichun Yang,a* Min Zhua a
School of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Advanced
Energy Storage Materials, South China University of Technology, Guangzhou, 510641, China. E-mail: [email protected] b
Department of Chemistry, Jinan University, Guangzhou, 510632, China.
E-mail: [email protected] Abstract Herein we report Mo2C/N-doped carbon hierarchical nanowires (Mo2C/N-C HNWs) as anode materials for sodium-ion batteries. In the Mo2C/N-C HNWs, Mo2C nanocrystallites are uniformly distributed in N-doped carbon matrix. The nanocrystallites of Mo2C offer short paths for Na+ diffusion, the mesoporous structure facilitates the diffusion of electrolyte, and the N-doped carbon matrix accelerates the electron transfer. As anode materials for sodium-ion batteries, the Mo2C/N-C HNWs exhibited reversible capacities of 381 and 308 mAh g-1 at current densities of 50 and 200 mA g-1, respectively. Our results demonstrate the enhanced Na+ storage activity of Mo2C after structure tailoring, elucidating the potential of transition-metal carbides as promising anode materials for sodium-ion batteries. Keywords: Mo2C; N-doped carbon; nanocomposite; sodium-ion battery; energy storage and conversion 1. Introduction Sodium-ion batteries (SIBs) have attracted wide attention as alternative energy storage technologies to lithium-ion batteries in large scale applications, [1] owing to more abundant resources and lower cost of sodium. Transition metal carbides (TMCs) emerge as promising host materials for alkali metal ions
1
recently, [2-5] because they exhibit metallic conductivity. This is a great advantage compared to transition metal oxides which are almost semiconductors. As anode materials for SIBs, layered TMCs, e. g., Ti2CTx and Ti3C2 Tx (T = OH, F, O) exhibit reversible capacities of 233 mAh g-1 and 150 mAh g-1, [6, 7] respectively. Non-layered TMCs, such as N-doped MoC (MoCxNy) [8] and Co3ZnC [9] show even higher capacity (> 300 mAh g-1), which inspire us to investigate the Na+ storage performance of non-layered TMCs. Herein, we reported for the first time on the Na+ storage performance of Mo2C/N-doped carbon hierarchical nanowires (Mo2C/N-C HNWs). As anode materials for SIBs, they exhibit reversible capacities of 381 and 308 mAh g-1 at 50 and 200 mA g-1, respectively, which is much improved compared with the counterpart of microparticles. Our results demonstrate the capability of Mo2C in the reversible storage of Na+, elucidating the potential application of TMC-based materials for SIBs. 2. Experimental Mo3O10(C6H8N)2·2H2O nanowires prepared according to our previous report [10] were calcined at 775 o
C for 5h under a Ar flow to obtain Mo2C/N-C HNWs. The electrochemical behavior was investigated
using coin cells (CR 2016) in which Na sheets were used as the counter and reference electrodes, glass fiber membranes (Whatman GF/D) were used as separators, and 1 M NaClO4 solution in propylene carbonate/ethylene carbonate (PC/EC) (1:1 in volume) was used as electrolyte. 3. Results and Discussion
2
Fig. 1 (a) XRD patterns of the Mo2C/N-C HNWs and the Mo2C MPs. (b) Raman spectrum, (c) XPS survey, and (d) high resolution XPS spectra for N1s of the Mo2C/N-C HNWs. The XRD patterns (Fig. 1a) correspond to hexagonal β-Mo2C (JCPDS: 35-0787). And the diffraction peaks of the Mo2C/N-C HNWs are broader than those of commercial Mo2C microparticles (MPs, Fig. S1), indicating smaller crystallites in the Mo2C/N-C HNWs. The Raman spectrum of the Mo2C/N-C HNWs (Fig. 1b) presents the characteristic D- and G-bands of carbon at ~1360 and 1600 cm-1, respectively, confirming the existence of free carbon. And the survey XPS spectrum (Fig. 1c) identifies the presence of N besides Mo, C, and O. The high resolution profile of N 1s (Fig. 1d) can be deconvoluted into three peaks. The peaks at 398.4 and 400.2 eV are assigned to pyridinic and pyrrolic N, respectively, indicating the N-doping in the carbon. [11] The peak at 397.8 eV is associated with N bonding to Mo, suggesting a small amount of MoNx may be formed during the pyrolysis. Based on CHN elemental analysis and ICP-AES measurement, the contents of free carbon and nitrogen can be further determined as 5.8 and 0.5 wt%, respectively.
3
Fig. 2 (a) SEM, (b) TEM and (c) HR-TEM images of the Mo2C/N-C HNWs; (d) TEM image of the product obtained after removing Mo2C from Mo2C/N-C HNWs. Insets of panels b and d are the corresponding SAED pattern of the nanowires.
The SEM image (Fig. 2a) shows that the Mo2C/N-C HNWs maintain the nanowire morphology of the Mo3O10(C6 H8N)2·2H2O precursor (Fig. S2). The TEM image reveals mesoporous structure of the Mo2C/N-C HNWs which is stacked by nanoparticles (Fig. 2b). The selected area electron diffraction (SAED) pattern (inset of Fig. 2b) and lattice fringes in the HR-TEM (Fig. 2c) are attributed to hexagonal β-Mo2C nanocrystallites. The elemental mapping (Fig. S3) indicates uniform distribution of Mo2C and N-doped carbon. Mo2C can be removed by H2O2, leaving amorphous carbon nanowires (Fig. 2f), which demonstrate Mo2C nanocrystallites are embedded in the continuous N-doped carbon matrix.
Fig. 3 CV curves of (a) the Mo2C/N-C HNWs and (b) the Mo2C MPs tested at a scan rate of 0.1 mV s-1. The electrochemical performance of the Mo2C/N-C HNWs was evaluated as anode materials for SIBs. 4
In the CV profiles of Mo2C/N-C HNWs (Fig. 3a), a wide reduction peak located ~2.0 V appears in the first cycle and disappears in subsequent cycles, which may be related to the decomposition of the electrolyte and the formation of a solid electrolyte interphase (SEI) film. By contrast, the SEI formation peak of the Mo2C MPs is located at 1.05 V (Fig. 3b), which is lower than that of the Mo2C/N-C HNWs. It may be due to the higher catalytic ability of the Mo2C nanocrytallites in the composite. For the Mo2C/N-C HNWs, two pair of redox peaks at 0.8/0.66 V and 0.22/0.38 V can be observed, which are similar to the Na+ storage/release behavior of Mo2N. [12] As for the Mo2C MPs, the redox peaks on the CV profiles (Fig. 3b) are inconspicuous, indicating much lower activity for the reversible Na+ storage. So far, the Na+ storage mechanism for Mo2C is not yet clear. According to the previous report, Mo2C is considered to store Li+ through conversion reaction. [3, 13] Due to the similarity of Li+ and Na+ in the electrochemical properties, the storage of Na+ in Mo2C may be through insertion and the conversion reaction (Mo2C+xNa++xe- ↔ NaxC+ 2Mo). However, in our preliminary XRD investigation, neither NaxC nor Mo can be detected in the fully discharged state (Fig. S4). And further investigation is ongoing to reveal the mechanisms for the reversible Na+ storage in TMCs.
Fig. 4 Electrochemical performance of the Mo2C/N-C HNWs and the Mo2C MPs. (a) First charge/discharge curves, (b) cycling performance at 200 mA g-1, (c) rate capability and (d) Nyquist plots.
5
The Mo2C/N-C HNWs and the Mo2C MPs are discharged/charged in the range of 0.01 V- 3.0 V vs. Na/Na+ at 200 mA g-1 (Fig. 4a). The initial discharge and charge capacities of the Mo2C/N-C HNWs are 441 and 308 mAh g-1, respectively, with a coulombic efficiency of 70%. And the amorphous carbon nanowire matrix only exhibits an initial reversible capacity of 61.1 mAh g-1 (Fig. S5). Considering the low capacity and low content, the capacity contribution of the carbon matrix is negligible. The Mo2C MPs presents discharge and charge capacities of 212 and 86 mAh g-1, respectively, with a coulombic efficiency of 40%. After 100 cycles, the Mo2C/N-C HNWs maintain a reversible capacity of 166 mAh g-1, while the Mo2C MPs only deliver a reversible capacity of 60 mAh g-1 (Fig. 4b). The rate performance of the two samples is shown in Fig. 4c. The Mo2C/N-C HNWs deliver a reversible capacity of 381.5 mAh g-1 at 50 mA g-1, maintain 167.5 mAh g-1 at 500 mA g-1, and regain 242.4 mAh g-1 when the current density reduces to 50 mA g-1. By contrast, the Mo2C MPs deliver 108.8 mAh g-1 at 50 mA g-1, and only retain 26.1 mAh g-1 at 500 mA g-1, which is much lower than that of the Mo2C/N-C HNWs. The galvanostatic charge/discharge results demonstrate the greatly enhanced Na+ storage activity of the Mo2C/N-C HNWs compared with the Mo2C MPs, and the improvement is highly related with the nanostructure. In the Mo2C/N-C HNWs, the mesopores enable easy penetration of electrolyte, and the Mo2C nanoparticles offer decreased paths and more active sites for Na+ diffusion. Moreover, the continuous N-doped carbon matrix connects the nanoparticles, which facilitates the charge transfer. As compared in Fig. 4d, the diameter in the Nyquist plot of the Mo2C/N-C HNWs is obviously smaller than that of the Mo2C MPs. The fitted results (Fig. S6 and Table S1) demonstrate lower charge transfer resistance in the Mo2C/N-C HNWs. The enhanced kinetics for Na+ diffusion and charge transfer result in the higher capacity delivery of the Mo2C/N-C HNWs. The Na+ storage capacity of the Mo2C/N-C HNWs is comparable with those of explored TMCs (e.g. 6
Ti2CTx (T = OH, F, O) and Ti3C2Tx[6, 7]) and transition metal nitrides (e.g. Mo2N nanobelts, [12] VN/graphene hybrid, [14] Cu3N [15] and Ni3N [16]), which are also interstitial alloys. And the cyclic stability for reversible Na+ storage of the Mo2C/N-C HNWs needs further improvement, compared with the reported N-doped MoC (MoCxNy) [8] and Co3ZnC [9] based materials. Our work shows potential application of Mo2C as an anode material for SIBs. With future investigation into the mechanism for Na+ storage and capacity fading, Mo2C-based materials with improved cyclic performance can be further developed for SIBs. 4. Conclusions In summary, we have reported an anode material of Mo2C/N-C HNWs for SIBs. The structure features such as nanocrystallites, mesopores and continuous N-doped carbon matrix enhance kinetics for Na+ diffusion and charge transfer. As anode materials for SIBs, they exhibit reversible capacities of 381 and 308 mAh g-1 at 50 and 200 mA g-1, respectively. And non-layered TMCs with appropriate structure design show great promise as anode materials for SIBs. ACKNOWLEDGMENT We acknowledge financial support from the National Natural Science Foundation of China (51671089, 21373102), Natural Science Foundation of Guangdong Province (2016A030312011).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at XXX References [1] H.L. Pan, Y.S. Hu, L.Q. Chen, Energy Environ. Sci. 6 (2013) 2338-2360. [2] M. Naguib, J. Halim, J. Lu, K.M. Cook, L. Hultman, Y. Gogotsi, M.W. Barsoum, J. Am. Chem. Soc. 135 (2013) 15966-15969.
7
[3] Y. Xiao, L. Zheng, M. Cao, Nano Energy 12 (2015) 152-160. [4] L. Yang, X. Li, S. He, G. Du, X. Yu, J. Liu, Q. Gao, R. Hu, M. Zhu, J. Mater. Chem. A 4 (2016) 10842-10849. [5] L. Su, Z. Zhou, P. Shen, Electrochimica Acta 87 (2013) 180-185. [6] X. Wang, S. Kajiyama, H. Iinuma, E. Hosono, S. Oro, I. Moriguchi, M. Okubo, A. Yamada, Nature communications 6 (2015) 6544. [7] S. Kajiyama, L. Szabova, K. Sodeyama, H. Iinuma, R. Morita, K. Gotoh, Y. Tateyama, M. Okubo, A. Yamada, ACS Nano 10 (2016) 3334-3341. [8] J. Qiu, Z. Yang, Q. Li, Y. Li, X. Wu, C. Qi, Q. Qiao, J. Mater. Chem. A 4 (2016) 13296-13306. [9] T. Chen, B. Cheng, R. Chen, Y. Hu, H. Lv, G. Zhu, Y. Wang, L. Ma, J. Liang, Z. Tie, Z. Jin, J. Liu, Acs Applied Materials & Interfaces 8 (2016) 26834-26841. [10] Q.S. Gao, C.X. Zhang, S.H. Xie, W.M. Hua, Y.H. Zhang, N. Ren, H.L. Xu, Y. Tang, Chem. Mater. 21 (2009) 5560-5562. [11] P.F. Zhang, Y.T. Gong, H.R. Li, Z.R. Chen, Y. Wang, Nat. Commun. 4 (2013) 1593. [12] S.L. Liu, J.Y. Huang, J. Liu, M. Lei, J. Min, S.T. Li, G. Liu, Mater. Lett. 172 (2016) 56-59. [13] X. Wang, X. Shen, Y. Gao, Z. Wang, R. Yu, L. Chen, J. Am. Chem. Soc. 137 (2015) 2715-2721. [14] L. Wang, J. Sun, R. Song, S. Yang, H. Song, Adv. Energy Mater. 6 (2016) 1502067-1502067. [15] X. Li, A.L. Hector, J.R. Owen, J. Phys. Chem. C 118 (2014) 29568-29573. [16] L. Xianji, M.M. Hasan, A.L. Hector, J.R. Owen, J. Mater. Chem. A 1 (2013) 6441-6445.
8
Highlights: 1.
Firstly applying Mo2C/N-C HNWs in sodium ion batteries (SIBs).
2.
The hybrid nanostructure enables good performance of Na+ storage.
3.
Such material will be a new promising anode material for SIBs.
Graaphical Absttract