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Partially sulfurized MoO2 film for durable lithium storage
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Materials Research Bulletin xxx (2016) xxx–xxx
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Partially sulfurized MoO2 film for durable lithium storage Yu Jianga , Haichen Lianga , S.V. Savilovb , Jiangfeng Nia,* , Liang Lia,* a b
College of Physics, Optoelectronics and Energy, Center for Energy Conversion Materials & Physics (CECMP), Soochow University, Suzhou 215006, PR China Chemistry Department, M.V. Lomonosov Moscow State University, Moscow 119991, Russia
A R T I C L E I N F O
Article history: Received 6 March 2017 Received in revised form 23 March 2017 Accepted 24 March 2017 Available online xxx Keywords: Lithium ion battery Molybdenum dioxide Hybrid Binder-free electrode
A B S T R A C T
In this work we present a simple and scalable approach to fabricate highly-active, self-supported molybdenum dioxide (MoO2) film for lithium storage. The fabrication is based on a thermal oxidation of Mo foil under air atmosphere and a subsequent sulfurization treatment. A hybrid film composed of partially sulfurized MoO2 can be produced by controlling the sulfurization conditions. This hybrid film can be directly adapted as binder-free electrode for lithium storage, displaying a reversible capacity of 899 mAh g1 and a high initial Coulombic efficiency (CE) of 75%. More importantly, the hybrid film sustains 250 successive cycles at a high rate of 10C without evident capacity decay, thereby suggesting its potential for durable battery application. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Electrode materials play a key role in designing advanced battery systems [1–7]. Molybdenum oxide is a structurally unique material and has been widely exploited as electrode material in a broad spectrum of battery applications [8]. Depending on the arrangement and the relative ratio of Mo and O atoms, numerous molybdenum oxides such as MoO3, Mo17O47, and MoO2 have been identified, which are all attractive electrode materials for rechargeable lithium batteries [9–14]. In particular, rutile MoO2 has captured most imagination owing to the large capacity of 838 mAh g1, high conductivity (104 S cm1), and rapid ion transport property. Previous works have shown that Li storage in MoO2 generally consists of two sequential processes with distinct mechanisms, i.e. insertion or conversion. The latter process that reduces MoO2 to metallic Mo enables a large capacity but also results in substantial volume expansion and breaking off of conducting pathways, leading to a poor cycling and rate capability [15,16]. This is a fundamental issue in conventional twodimensional (2D) thin film cells. Three-dimensional (3D) ordered nanostructures have attracted intensive interest due to their fundamental importance and potential wide-ranging applications. 3D configuration electrodes can maximize power and energy density yet maintain short ion transport distances. This offers an efficient way that battery design does not trade off between available energy per unit area and the
* Corresponding authors. E-mail addresses: [email protected] (J. Ni), [email protected] (L. Li).
ability to release this energy [17–23]. Previously, our group has designed and fabricated 3D nanoarray through an electrochemical anodization method, and achieved ultrastable and robust battery performance in several metal oxides [24–27]. Generally, preparation of array structure needs complex procedure or elaborate facility, while practical application calls for some innovative routes that are more affordable and producible. However, significant gaps in our knowledge still remain. In this work, we propose a facile route to prepare 3D hybrid array structure based on thermal oxidation of Mo foil and subsequent mild sulfurization. The mild treatment is well controlled so that only partial conversion of molybdenum oxides to sulfide, leading to a hybrid film of MoO2 and MoS2 (designated as S-MoO2). With respect to other structures, the hybrid structure possesses a large reaction area, a short ion diffusion path, and intrinsic advantages of heterostructure [28]. When used as anode for Li storage, the S-MoO2 hybrid exhibits superior electrochemical performance, affording a reversible capacity of 899 mAh g1. At an extremely high rate of 10C, it still affords a capacity of 630 mAh g1 with favourable stability, outstripping most reported molybdenum dioxide materials. 2. Experimental 2.1. Material preparation The self-supported S-MoO2 hybrid film was fabricated using a simple two-step approach. Firstly, Mo foil (99%, 50 mm in thickness) was ultrasonically cleaned in hydrochloric acid, acetone, deionized water, respectively, and dried in vacuum. The Mo foil
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Please cite this article in press as: Y. Jiang, et al., Partially sulfurized MoO2 film for durable lithium storage, Mater. Res. Bull. (2017), http://dx.doi. org/10.1016/j.materresbull.2017.03.053
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Fig. 1. (a) Schematic illustrating the fabrication procedure of S-MoO2 film on Mo substrate. Optical images of (b) Mo foil, (c) MoO3 film, and (d) S-MoO2 film on Mo foil.
was then thermally oxidized in air at 450 C for 6 h with a ramping rate of 1 C min1. The oxidized Mo foil was transferred to a tube furnace, in which S powder (99.9%, Sinopharm) was placed on the upstream side of the furnace at carefully adjusted locations to set the temperature. The sulfurization was conducted at 400 C for a duration of 4 h, with Ar as the S vapor carrier. 2.2. Material characterization Crystallographic structure of the S-MoO2 hybrids was identified by X-ray diffraction (XRD) using a Rigaku Dmax-2400 automatic diffractometer equipped with Cu Ka radiation Particle shape and morphology of the products were observed by scanning electron
microscopy (SEM, Hitachi S-8010). As for electrochemical evaluation, a piece of S-MoO2 film on Mo foil with cutting size of 10 10 mm2 was used as the working electrode. One side of the foil was polished with abrasive paper to used as current collect. 2032type coin cells were assembled in an Ar-filled glove box (Mikrouna), in which both water and oxygen concentrations are below 0.1 ppm. The counter and reference electrode were Li metal foil; the electrolyte was 1 M LiClO4 solution in propylene carbonate (100% by volume). Cyclic voltammetry (CV) was performed on an Autolab electrochemical workstation. Charge and discharge tests were performed on a LAND battery test system at room temperature.
Fig. 2. SEM image of (a) MoO3 film and (b, c) S-MoO2 hybrid film sulfurized for 4 h. (b) Top view and (c) side view. (d) XRD pattern of S-MoO2 hybrid film, revealing a major phase of MoO2 and a minor phase of MoS2.
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3. Results and discussion As schematically present in Fig. 1a, 3D ordered S-MoO2 hybrid film can be produced on Mo substrate via a simple two-step approach. During the first step, an ordered MoO3 film grows on the surface of Mo foil when the substrate is subject to thermal oxidization at 450 C in air. The MoO3 film is then heat treated in S vapor under Ar flow, resulting in the generation of MoO2 and its further conversion to MoS2. The photographs of Mo substrate, MO3 film, and S-MoO2 film are presented in Figure b-d, and their difference in color can be clearly distinguished. The conversion is a kinetically controlled process, and thus the final film composite is determined by the sulfurization condition. Fig. 2a shows the SEM of as-oxidized MoO3 film, which is built of intersecting blocks 800 nm in length of and 400 nm in width. Clearly, oxygen uptake results in the expansion and breaking of the surface layer of Mo substrate, leading to a 3D porous architecture. When the MoO3 film is further sulfurized, the 3D porous architecture is well maintained. The building blocks become much smaller and the their surface is furred, which is beneficial for the transport of electron and ion (Fig. 2b). The cross-section SEM image reveals that the film thickness is about 2 mm (Fig. 2c). This
3
3D architecture is an ideal structure for battery application, as it presents several distinct advantages. First, the film consists of intersecting nanoscale blocks tightly attached on the Mo substrate, which ensure good electronic connection with the current collector. In addition, the porous architecture allows the storage and free penetration of electrolyte, leading to increased ion diffusion kinetics. Moreover, the pores provide enough space to accommodate the volume expansion upon Li uptake. The crystal structure of S-MoO2 film was identified by XRD (Fig. 2d). The strong peaks at 26.0, 37.0, 53.1, 53.5, and 66.7 might be assigned to the (111), (211), (220), (312) and (402) planes of rutile MoO2 phase (PDF#32-0671), while weak peaks at 14.4, 32.7, and 39.5 can be indexed into MoS2 phase (PDF#371492). The additional two peaks located at 58.6 and 73.7 come from the Mo substrate. No diffraction peaks due to MoO3 are observed, indicating that it has been completely depleted. On the basis of structure analysis, the mass ration of MoO2 against MoS2 is about 6:1. The sulfurization consists of two successive processes. First, MoO3 is reduced to MoO2 with gaseous S: 2MoO3 ðsÞ þ SðgÞ ¼ 2MoO2 ðsÞ þ SO2 ðgÞ
ð1Þ
Fig. 3. Electrochemical lithium storage in S-MoO2 film sulfurized for 4 h. CV curves at (a) a fixed rate of 0.2 mV s1 and (b) various scan rates ranging from 0.2 to 1.8 mV s1, and (c) initial charge and discharge curves at a rate of 0.2 C. (d) Rate cycling performance of S-MoO2 films sulfurized for different durations of 2, 4, and 6 h. (e) Electrochemical lithium cycling of S-MoO2 film at a rate of 10C for 250 cycles.
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4. Conclusion
Then, the MoO2 further reacts with S to generate MoS2: MoO2 ðsÞ þ 3SðgÞ ¼ MoS2 ðsÞ þ SO2 ðgÞ
ð2Þ
The S-MoO2 hybrid film is directly used as the working electrode to examine its electrochemical Li-storage performance. Fig. 3 shows the electrochemical results of the S-MoO2 hybrid film on CV and galvanostatic tests. Fig. 3a shows the initial CV curves. The CV profiles during the first cycle are rather different from following cycles, suggesting a possible activation step. In the subsequent CV cycles, three main redox peaks can be confirmed. These redox peaks are associated with the phase transition from monoclinic to orthorhombic for MoO2 and 2H to 1T for MoS2. Due to the overlapping of these processes, it is hard to fully separate their contribution. An intensive peak at 0.29 V represents the conversion of lithiated MoO2 and MoS2 to metallic Mo. When the scan rate increases from 0.2 to 1.8 mV s1, the basic CV profiles are well preserved, signifying a favourable reaction kinetic (Fig. 3b). The initial charge and discharge curves of the S-MoO2 hybrid film at a rate of 0.2C (1C is arbitrarily taken as 670 mA g1) are presented in Fig. 3c. The discharge and charge capacities are 1198 and 899 mAh g1, respectively, giving rise to a relatively high Coulombic efficiency (CE) of 75%. The irreversible loss could be attributed to the formation of solid electrolyte interphase (SEI) and trapping of residue Li+ ions. The charge capacity increases in the subsequent cycles, leading to a CE approaching 100%. In addition, the S-MoO2 film exhibits excellent rate capability, as shown in Fig. 3d. It demonstrates capacities of 891, 829, 778, 699 mAh g1 at rates of 0.5, 1, 2, and 5C, respectively. At a much higher rate of 10C, the hybrid film still affords a capacity of 630 mAh g1. More interesting, after deep cycling at various rates, the film retains a capacity 1000 mAh g1 when the rate restores to 0.2C, inferring an amazing reversibility. The electrochemical results are comparable or superior to the values for most MoO2 based electrodes reported in literature [11,15,16,29–31], strongly suggesting the efficacy of our self-supported electrode design. Electrochemical tests confirm an optimal sulfurization duration of 4 h for S-MoO2 film. This establishes a relationship between the degree of sulfurization and electrochemical property. The S-MoO2 electrode was further cycled at a high rate of 10C for Li storage (Fig. 3e). No evident capacity fading is noted, again indicating its superior durability. This superior stability is correlated with the structural integrity of the S-MoO2 film. As illustrated in Fig. 4, the 3D structure of the S-MoO2 film electrode remains intact after electrochemical Li cycling. It is believed that the porous architectures play a key role in maintaining the structure upon Li uptake and release.
In conclusion, we described a facile route to fabricate partially sulfurized MoO2 film directly grown on Mo substrate for highly active Li storage. which enhanced lithium storage performance as anode materials for LIBs. The S-MoO2 hybrid film affords a high reversible capacity of 899 mAh g1 and a superior rate capability of 630 mAh g1 at 10C with good stability. The approach is simple and producible in terms of cost and efficiency, and will find more applications in the preparation of electrode materials. Acknowledgments We are grateful for the financial support of the National Natural Science Foundation of China (51672182, 51422206, 51302181, 51372159), 333 High-Level Talents Project in Jiangsu Province, the Thousand Young Talents Plan, the Jiangsu Natural Science Foundation (BK20151219, BK20140009), Six Talent Peaks Project in Jiangsu Province, and of the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
M.S. Whittingham, Chem. Rev. 114 (2014) 11414–11443. L. Croguennec, M.R. Palacin, J. Am. Chem. Soc. 137 (2015) 3140–3156. J. Ni, L. Zhang, S. Fu, S. Savilov, S. Aldoshin, L. Lu, Carbon 92 (2015) 15–25. J. Ni, Y. Li, Adv. Energy Mater. 6 (2016) 1600278. J. Ni, X. Bi, Y. Jiang, L. Li, J. Lu, Nano Energy 33 (2017), doi:http://dx.doi.org/ 10.1016/j.nanoen.2017.1002.1041. X. Zhao, Z. Zhao, M. Yang, H. Xia, T. Yu, X. Shen, ACS Appl. Mater. Interfaces 9 (2017) 2535–2540. W. Wang, H. Liang, L. Zhang, S.V. Savilov, J. Ni, L. Li, Nano Res. 10 (2017) 229– 237. X. Hu, W. Zhang, X. Liu, Y. Mei, Y. Huang, Chem. Soc. Rev. 44 (2015) 2376–2404. Y. Shi, B. Guo, S.A. Corr, Q. Shi, Y.-S. Hu, K.R. Heier, L. Chen, R. Seshadri, G.D. Stucky, Nano Lett. 9 (2009) 4215–4220. Y. Sun, X. Hu, W. Luo, Y. Huang, ACS Nano 5 (2011) 7100–7107. W. Tang, C.X. Peng, C.T. Nai, J. Su, Y.P. Liu, M.V. Reddy, M. Lin, K.P. Loh, Small 11 (2015) 2446–2453. B. Guo, X. Fang, B. Li, Y. Shi, C. Ouyang, Y.-S. Hu, Z. Wang, G.D. Stucky, L. Chen, Chem. Mater. 24 (2012) 457–463. G. Wang, J. Ni, H. Wang, L. Gao, J. Mater. Chem. A 1 (2013) 4112–4118. J. Ni, G. Wang, J. Yang, D. Gao, J. Chen, L. Gao, Y. Li, J. Power Sources 247 (2014) 90–94. J. Ni, Y. Zhao, L. Li, L. Mai, Nano Energy 11 (2015) 129–135. H.J. Zhang, K.X. Wang, X.Y. Wu, Y.M. Jiang, Y.B. Zhai, C. Wang, X. Wei, J.S. Chen, Adv. Funct. Mater. 24 (2014) 3399–3404. X.H. Xia, C.R. Zhu, J.S. Luo, Z.Y. Zeng, C. Guan, C.F. Ng, H. Zhang, H.J. Fan, Small 10 (2014) 766–773. D. Xie, X. Xia, Y. Zhong, Y. Wang, D. Wang, X. Wang, J. Tu, Adv. Energy Mater. 7 (2017) 1601804. Y. Zhong, X. Xia, J. Zhan, Y. Wang, X. Wang, J. Tu, J. Mater. Chem. A 4 (2016) 18717–18722. S. Liu, J. Feng, X. Bian, J. Liu, H. Xu, Energy Environ. Sci. 9 (2016) 1229–1236. S. Liu, J. Feng, X. Bian, Y. Qian, J. Liu, H. Xu, Nano Energy 13 (2015) 651–657.
Fig. 4. SEM images of cycled S-MoO2 film at (a) low and (b) high magnification.
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[27] J. Ni, W. Wang, C. Wu, H. Liang, J. Maier, Y. Yu, L. Li, Adv. Mater. 29 (2017) 1605607. [28] T. Liu, Y. Zhao, L. Gao, J. Ni, Sci. Rep. 5 (2015) 9307. [29] K.H. Seng, G.D. Du, L. Li, Z.X. Chen, H.K. Liu, Z.P. Guo, J. Mater. Chem. 22 (2012) 16072. [30] X. Zhao, M. Cao, B. Liu, Y. Tian, C. Hu, J. Mater. Chem. 22 (2012) 13334. [31] Y. Sun, X. Hu, W. Luo, Y. Huang, J. Mater. Chem. 22 (2012) 425.
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Materials Research Bulletin xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
Partially sulfurized MoO2 film for durable lithium storage Yu Jianga , Haichen Lianga , S.V. Savilovb , Jiangfeng Nia,* , Liang Lia,* a b
College of Physics, Optoelectronics and Energy, Center for Energy Conversion Materials & Physics (CECMP), Soochow University, Suzhou 215006, PR China Chemistry Department, M.V. Lomonosov Moscow State University, Moscow 119991, Russia
A R T I C L E I N F O
Article history: Received 6 March 2017 Received in revised form 23 March 2017 Accepted 24 March 2017 Available online xxx Keywords: Lithium ion battery Molybdenum dioxide Hybrid Binder-free electrode
A B S T R A C T
In this work we present a simple and scalable approach to fabricate highly-active, self-supported molybdenum dioxide (MoO2) film for lithium storage. The fabrication is based on a thermal oxidation of Mo foil under air atmosphere and a subsequent sulfurization treatment. A hybrid film composed of partially sulfurized MoO2 can be produced by controlling the sulfurization conditions. This hybrid film can be directly adapted as binder-free electrode for lithium storage, displaying a reversible capacity of 899 mAh g1 and a high initial Coulombic efficiency (CE) of 75%. More importantly, the hybrid film sustains 250 successive cycles at a high rate of 10C without evident capacity decay, thereby suggesting its potential for durable battery application. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Electrode materials play a key role in designing advanced battery systems [1–7]. Molybdenum oxide is a structurally unique material and has been widely exploited as electrode material in a broad spectrum of battery applications [8]. Depending on the arrangement and the relative ratio of Mo and O atoms, numerous molybdenum oxides such as MoO3, Mo17O47, and MoO2 have been identified, which are all attractive electrode materials for rechargeable lithium batteries [9–14]. In particular, rutile MoO2 has captured most imagination owing to the large capacity of 838 mAh g1, high conductivity (104 S cm1), and rapid ion transport property. Previous works have shown that Li storage in MoO2 generally consists of two sequential processes with distinct mechanisms, i.e. insertion or conversion. The latter process that reduces MoO2 to metallic Mo enables a large capacity but also results in substantial volume expansion and breaking off of conducting pathways, leading to a poor cycling and rate capability [15,16]. This is a fundamental issue in conventional twodimensional (2D) thin film cells. Three-dimensional (3D) ordered nanostructures have attracted intensive interest due to their fundamental importance and potential wide-ranging applications. 3D configuration electrodes can maximize power and energy density yet maintain short ion transport distances. This offers an efficient way that battery design does not trade off between available energy per unit area and the
* Corresponding authors. E-mail addresses: [email protected] (J. Ni), [email protected] (L. Li).
ability to release this energy [17–23]. Previously, our group has designed and fabricated 3D nanoarray through an electrochemical anodization method, and achieved ultrastable and robust battery performance in several metal oxides [24–27]. Generally, preparation of array structure needs complex procedure or elaborate facility, while practical application calls for some innovative routes that are more affordable and producible. However, significant gaps in our knowledge still remain. In this work, we propose a facile route to prepare 3D hybrid array structure based on thermal oxidation of Mo foil and subsequent mild sulfurization. The mild treatment is well controlled so that only partial conversion of molybdenum oxides to sulfide, leading to a hybrid film of MoO2 and MoS2 (designated as S-MoO2). With respect to other structures, the hybrid structure possesses a large reaction area, a short ion diffusion path, and intrinsic advantages of heterostructure [28]. When used as anode for Li storage, the S-MoO2 hybrid exhibits superior electrochemical performance, affording a reversible capacity of 899 mAh g1. At an extremely high rate of 10C, it still affords a capacity of 630 mAh g1 with favourable stability, outstripping most reported molybdenum dioxide materials. 2. Experimental 2.1. Material preparation The self-supported S-MoO2 hybrid film was fabricated using a simple two-step approach. Firstly, Mo foil (99%, 50 mm in thickness) was ultrasonically cleaned in hydrochloric acid, acetone, deionized water, respectively, and dried in vacuum. The Mo foil
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Fig. 1. (a) Schematic illustrating the fabrication procedure of S-MoO2 film on Mo substrate. Optical images of (b) Mo foil, (c) MoO3 film, and (d) S-MoO2 film on Mo foil.
was then thermally oxidized in air at 450 C for 6 h with a ramping rate of 1 C min1. The oxidized Mo foil was transferred to a tube furnace, in which S powder (99.9%, Sinopharm) was placed on the upstream side of the furnace at carefully adjusted locations to set the temperature. The sulfurization was conducted at 400 C for a duration of 4 h, with Ar as the S vapor carrier. 2.2. Material characterization Crystallographic structure of the S-MoO2 hybrids was identified by X-ray diffraction (XRD) using a Rigaku Dmax-2400 automatic diffractometer equipped with Cu Ka radiation Particle shape and morphology of the products were observed by scanning electron
microscopy (SEM, Hitachi S-8010). As for electrochemical evaluation, a piece of S-MoO2 film on Mo foil with cutting size of 10 10 mm2 was used as the working electrode. One side of the foil was polished with abrasive paper to used as current collect. 2032type coin cells were assembled in an Ar-filled glove box (Mikrouna), in which both water and oxygen concentrations are below 0.1 ppm. The counter and reference electrode were Li metal foil; the electrolyte was 1 M LiClO4 solution in propylene carbonate (100% by volume). Cyclic voltammetry (CV) was performed on an Autolab electrochemical workstation. Charge and discharge tests were performed on a LAND battery test system at room temperature.
Fig. 2. SEM image of (a) MoO3 film and (b, c) S-MoO2 hybrid film sulfurized for 4 h. (b) Top view and (c) side view. (d) XRD pattern of S-MoO2 hybrid film, revealing a major phase of MoO2 and a minor phase of MoS2.
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3. Results and discussion As schematically present in Fig. 1a, 3D ordered S-MoO2 hybrid film can be produced on Mo substrate via a simple two-step approach. During the first step, an ordered MoO3 film grows on the surface of Mo foil when the substrate is subject to thermal oxidization at 450 C in air. The MoO3 film is then heat treated in S vapor under Ar flow, resulting in the generation of MoO2 and its further conversion to MoS2. The photographs of Mo substrate, MO3 film, and S-MoO2 film are presented in Figure b-d, and their difference in color can be clearly distinguished. The conversion is a kinetically controlled process, and thus the final film composite is determined by the sulfurization condition. Fig. 2a shows the SEM of as-oxidized MoO3 film, which is built of intersecting blocks 800 nm in length of and 400 nm in width. Clearly, oxygen uptake results in the expansion and breaking of the surface layer of Mo substrate, leading to a 3D porous architecture. When the MoO3 film is further sulfurized, the 3D porous architecture is well maintained. The building blocks become much smaller and the their surface is furred, which is beneficial for the transport of electron and ion (Fig. 2b). The cross-section SEM image reveals that the film thickness is about 2 mm (Fig. 2c). This
3
3D architecture is an ideal structure for battery application, as it presents several distinct advantages. First, the film consists of intersecting nanoscale blocks tightly attached on the Mo substrate, which ensure good electronic connection with the current collector. In addition, the porous architecture allows the storage and free penetration of electrolyte, leading to increased ion diffusion kinetics. Moreover, the pores provide enough space to accommodate the volume expansion upon Li uptake. The crystal structure of S-MoO2 film was identified by XRD (Fig. 2d). The strong peaks at 26.0, 37.0, 53.1, 53.5, and 66.7 might be assigned to the (111), (211), (220), (312) and (402) planes of rutile MoO2 phase (PDF#32-0671), while weak peaks at 14.4, 32.7, and 39.5 can be indexed into MoS2 phase (PDF#371492). The additional two peaks located at 58.6 and 73.7 come from the Mo substrate. No diffraction peaks due to MoO3 are observed, indicating that it has been completely depleted. On the basis of structure analysis, the mass ration of MoO2 against MoS2 is about 6:1. The sulfurization consists of two successive processes. First, MoO3 is reduced to MoO2 with gaseous S: 2MoO3 ðsÞ þ SðgÞ ¼ 2MoO2 ðsÞ þ SO2 ðgÞ
ð1Þ
Fig. 3. Electrochemical lithium storage in S-MoO2 film sulfurized for 4 h. CV curves at (a) a fixed rate of 0.2 mV s1 and (b) various scan rates ranging from 0.2 to 1.8 mV s1, and (c) initial charge and discharge curves at a rate of 0.2 C. (d) Rate cycling performance of S-MoO2 films sulfurized for different durations of 2, 4, and 6 h. (e) Electrochemical lithium cycling of S-MoO2 film at a rate of 10C for 250 cycles.
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4. Conclusion
Then, the MoO2 further reacts with S to generate MoS2: MoO2 ðsÞ þ 3SðgÞ ¼ MoS2 ðsÞ þ SO2 ðgÞ
ð2Þ
The S-MoO2 hybrid film is directly used as the working electrode to examine its electrochemical Li-storage performance. Fig. 3 shows the electrochemical results of the S-MoO2 hybrid film on CV and galvanostatic tests. Fig. 3a shows the initial CV curves. The CV profiles during the first cycle are rather different from following cycles, suggesting a possible activation step. In the subsequent CV cycles, three main redox peaks can be confirmed. These redox peaks are associated with the phase transition from monoclinic to orthorhombic for MoO2 and 2H to 1T for MoS2. Due to the overlapping of these processes, it is hard to fully separate their contribution. An intensive peak at 0.29 V represents the conversion of lithiated MoO2 and MoS2 to metallic Mo. When the scan rate increases from 0.2 to 1.8 mV s1, the basic CV profiles are well preserved, signifying a favourable reaction kinetic (Fig. 3b). The initial charge and discharge curves of the S-MoO2 hybrid film at a rate of 0.2C (1C is arbitrarily taken as 670 mA g1) are presented in Fig. 3c. The discharge and charge capacities are 1198 and 899 mAh g1, respectively, giving rise to a relatively high Coulombic efficiency (CE) of 75%. The irreversible loss could be attributed to the formation of solid electrolyte interphase (SEI) and trapping of residue Li+ ions. The charge capacity increases in the subsequent cycles, leading to a CE approaching 100%. In addition, the S-MoO2 film exhibits excellent rate capability, as shown in Fig. 3d. It demonstrates capacities of 891, 829, 778, 699 mAh g1 at rates of 0.5, 1, 2, and 5C, respectively. At a much higher rate of 10C, the hybrid film still affords a capacity of 630 mAh g1. More interesting, after deep cycling at various rates, the film retains a capacity 1000 mAh g1 when the rate restores to 0.2C, inferring an amazing reversibility. The electrochemical results are comparable or superior to the values for most MoO2 based electrodes reported in literature [11,15,16,29–31], strongly suggesting the efficacy of our self-supported electrode design. Electrochemical tests confirm an optimal sulfurization duration of 4 h for S-MoO2 film. This establishes a relationship between the degree of sulfurization and electrochemical property. The S-MoO2 electrode was further cycled at a high rate of 10C for Li storage (Fig. 3e). No evident capacity fading is noted, again indicating its superior durability. This superior stability is correlated with the structural integrity of the S-MoO2 film. As illustrated in Fig. 4, the 3D structure of the S-MoO2 film electrode remains intact after electrochemical Li cycling. It is believed that the porous architectures play a key role in maintaining the structure upon Li uptake and release.
In conclusion, we described a facile route to fabricate partially sulfurized MoO2 film directly grown on Mo substrate for highly active Li storage. which enhanced lithium storage performance as anode materials for LIBs. The S-MoO2 hybrid film affords a high reversible capacity of 899 mAh g1 and a superior rate capability of 630 mAh g1 at 10C with good stability. The approach is simple and producible in terms of cost and efficiency, and will find more applications in the preparation of electrode materials. Acknowledgments We are grateful for the financial support of the National Natural Science Foundation of China (51672182, 51422206, 51302181, 51372159), 333 High-Level Talents Project in Jiangsu Province, the Thousand Young Talents Plan, the Jiangsu Natural Science Foundation (BK20151219, BK20140009), Six Talent Peaks Project in Jiangsu Province, and of the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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Fig. 4. SEM images of cycled S-MoO2 film at (a) low and (b) high magnification.
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Please cite this article in press as: Y. Jiang, et al., Partially sulfurized MoO2 film for durable lithium storage, Mater. Res. Bull. (2017), http://dx.doi. org/10.1016/j.materresbull.2017.03.053