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Exfoliated MoO3 nanosheets for high-capacity lithium storage
Electrochemistry Communications 52 (2015) 67–70
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Exfoliated MoO3 nanosheets for high-capacity lithium storage Huijuan Zhang a, Lijun Gao a,⁎, Yongji Gong b,⁎ a b
School of Energy, College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China Department of Chemistry, Rice University, Houston, TX 77005, USA
a r t i c l e
i n f o
Article history: Received 5 December 2014 Received in revised form 13 January 2015 Accepted 13 January 2015 Available online 22 January 2015 Keywords: Two-dimensional materials MoO3 nanosheets Liquid exfoliation Lithium storage
a b s t r a c t 2D MoO3 nanosheets have been fabricated by liquid exfoliation method using low-boiling point mixture solvents of isopropanol (IPA) and water. The resulting MoO3 nanosheets demonstrate high aspect ratio, thin thickness (2–6 nm) and single-crystalline structure properties. With these features the MoO3 nanosheets are capable of exposing much electrode surfaces accessible by electrolyte, and providing abundant reactive sites for Li-ion storage. Such structured nanosheets can minimize the extent of volume change during Li intercalation/deintercalation. As an anode material of Li-ion battery, the 2D MoO3 nanosheets exhibit high reversible capacities of 1110 mAh g−1 (3.0 to 0.01 V) and 841 mAh g−1 (2.0 to 0.01 V), good rate capability and cycling performance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have become one of the most promising power sources due to their high energy density. Although graphitic carbons have been commercially available as anode materials for LIBs, they are limited by their low theoretical capacity (372 mAh g−1) [1]. Recently, graphene sheets have demonstrated to have twice higher capacity of graphitic carbon, and exhibit some unique features such as superior electrical conductivity, high surface area and chemical stability [2–5]. Inspired by graphene, extensive efforts have been devoted to explore other highcapacity two-dimensional (2D) nanosheets made by metal sulfides and oxides (N1000 mAh g−1 capacity) [6–8]. In this regard, 2D SnS2, MoS2 and WS2 nanosheets have been fabricated by hydrothermal [9–11], chemical vapor deposition (CVD) [12,13] and/or liquid-exfoliation approaches [14–17], and demonstrated enhanced electrochemical properties compared to their bulk counterparts, owing to the large surface areas, finite lateral sizes, and increased open-edges [17,18]. However fabrication of 2D metal oxides by exfoliation remains in its infancy. MoO3 is a typical layer structured material [19], each layer is composed of two sub-layers which are formed by corner-sharing [MoO6] octahedra along the [001] and [100] directions, and the other two sub-layers are stacked together by sharing the edges of octahedra along the [001] direction [20,21]. An alternate stacking of these layers along the [010] direction with van der Waals interaction leads to the formation of layered MoO3 (α-MoO3) which is able to act as a temporary host for Li+ intercalation (theoretical capacity: 1116 mAh g−1) [22]. This kind of structure is similar to those of layered materials including MoS2, WS2, BN, which have been successfully exfoliated to ultrathin ⁎ Corresponding authors. Tel.: +86 512 65229905. E-mail addresses: [email protected] (L. Gao), [email protected] (Y. Gong).
http://dx.doi.org/10.1016/j.elecom.2015.01.014 1388-2481/© 2015 Elsevier B.V. All rights reserved.
nanosheets via the liquid-phase exfoliation approach [23], Moreover, bulk MoO3 is commercially available, widely abundant, and electrochemically active. Therefore, the layer structured bulk α-MoO3 material can be an excellent precursor for the fabrication of MoO3 nanosheets for lithium storage [24,25]. In this work, we developed a versatile strategy to fabricate MoO3 nanosheets exfoliated by using mixed solvents. The synthesis protocol involves using α-MoO3 powder as the starting material and low-boiling point solvents (isopropanol (IPA)/water) as the dispersion media. By choosing solvents with appropriate composition, MoO3 nanosheets with high aspect ratios (80–800), thin thickness (2–6 nm) and singlecrystalline structure can be achieved. The exfoliated MoO3 nanosheets exhibit a very high reversible capacity of 1110 mAh g−1, good rate capability and cycling stability, which indicates that such a 2D MoO3 nanosheet holds a promise as anode material for high-capacity LIBs. 2. Experimental Bulk α-MoO 3 was added in mixture solvents of IPA/water (2:8–8:2, v/v), as well as pure IPA and water to create a dispersion with a concentration of 3 mg mL−1, and sonicated for 10 h. The dispersion was cooled by adding ice at hourly interval. During the above processes, bulk α-MoO3 would be gradually exfoliated into nanosheets via interaction of IPA molecules and MoO3 layers. After sonication, the dispersion was centrifuged at 3000 rpm to remove aggregates, giving rise to a homogenous dispersion of MoO3 nanosheets as the volume ratio of IPA and water was 5:5. To make MoO3 electrode, the suspension was centrifuged at 10,000 rpm for 30 min to collect the exfoliated MoO3 nanosheet material. The morphology and microstructure of the samples were systematically investigated by field emission scanning electron microscopy
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H. Zhang et al. / Electrochemistry Communications 52 (2015) 67–70
(FESEM) (JEOL 6500), transmission electron microscopy (TEM), highresolution TEM (HRTEM) (JEOL 2100), AFM (Digital Instrument Nanoscope IIIA), XRD (Rigaku D/Max), and Raman measurements. The electrochemical performance of as-prepared MoO3 nanosheets for lithium storage was examined. The electrode was prepared by pressing a powder mixture of MoO3 nanosheets, acetylene black, and poly(tetrafluoroethylene) (PTFE) in a weight ratio of 80:10:10 onto a Ni grid. Model button cells were assembled in a glovebox with lithium foil as counter electrode, Celgard 2400 as the separator, and LIB315 (1 mol/L LiPF6 solution in a 1:1:1 mixture of EC, DMC, and DEC) as the electrolyte. The cells were assembled in an argon-filled glovebox with the concentrations of moisture and oxygen below 0.1 ppm. Galvanostatic cycling measurements were performed at various current rates in the voltage range of 0.01 − 3.0 V on a LANDct3.3 battery tester. For comparison, bulk MoO3 was also tested under the same conditions. 3. Results and discussion According to the experience equation of ΔHMix/VMix = 2(δG − δsol)2φ/Tsheet (ΔH is the enthalpy of mixing, δ is the square root of the component surface energy, φ is the nanosheet volume fraction, and T is the thickness of nanosheets) [26], it is known that layered materials can be successfully exfoliated when the enthalpy of mixing is minimized as the surface energies of flake and solvent match. To testify this concept, MoO3 powder was suspended in mixture solvents with a series IPA/water ratio to create various dispersions at the same
concentration of 3 mg mL−1. The dispersions were then sonicated at room temperature to achieve gradual exfoliation of bulk MoO3 into nanosheets. This exfoliation process is similar to those reported for other layered materials, such as BN and MoS2, into nanosheets via the interaction of solvent molecules, significantly influenced by the surface energies of involved solvents [15]. By adjusting the ratio of IPA to water, MoO3 nanosheets could be successfully obtained. In the case of solvents with the content of 50% IPA, highest concentration (0.1 mg mL−1) of MoO3 nanosheets can be achieved. Notably, using low-boiling point solvent mixtures to produce MoO3 nanosheets offers several advantages, including low cost, low toxicity, free of additives and easy disposal. The morphology and structure of MoO3 nanosheets were initially investigated by FESEM and TEM. As presented in Fig. 1a, many nanosheets with laminar morphology similar to scraps of paper can be observed, which is in obvious contrast to that of bulk MoO3 (irregular particles). The lateral size of these nanosheets is typically in the range of 500 nm to several micrometers. Moreover, the as-prepared nanosheets are transparent to electron beams, and easily degraded under strong electron beam owing to their thin nature as shown in the TEM (Fig. 1b,c) and HRTEM images (Fig. 1d). HRTEM image further reveals the welldefined crystalline lattices of MoO3 nanosheets with spacing of 0.39 nm and 0.37 nm, corresponding to the d-spacing of the (100) and (001) planes of the orthorhombic phase of MoO3, respectively. Crosssectional AFM was conducted to further investigate the structural features of MoO3 nanosheets. The typical AFM image and thickness analyses (Fig. 1e) reveal the same morphology as seen in the
Fig. 1. (a) Typical FE-SEM, and (b, c) TEM and (d) HRTEM images and diffraction pattern of exfoliated MoO3 nanosheets; (e) AFM image and e(1, 2) corresponding thickness analysis taken around the black line in (e); (f) XRD patterns, (g) Raman spectra of MoO3 nanosheets and bulk MoO3.
H. Zhang et al. / Electrochemistry Communications 52 (2015) 67–70
observations from SEM and TEM, showing uniform thickness of 2–6 nm. The aspect ratio of the prepared MoO3 nanosheets was estimated to be in the range of 80–800, according to SEM and AFM measurements. The crystal and chemical structure of the MoO3 nanosheets were analyzed by their XRD patterns and Raman spectra in comparison with bulk MoO3 powder. As shown in Fig. 1f, the XRD patterns of the bulk MoO3 are indexed to the typical orthorhombic MoO3 (JCPDS: 050508), which is consistent with our HRTEM analysis in Fig. 2d. In comparison, after exfoliation, the intensity of these (020) (110) (040) (120) peaks significantly decreases, clearly demonstrating that the layered α-MoO3 has been successfully exfoliated as it is expected. Raman spectra of bulk MoO3 and MoO3 nanosheets are shown in Fig. 1g, both of them have three sharp characteristic bands of MoO3, suggesting that the structure of MoO3 nanosheets is well kept during the exfoliation process. The Raman bands at 995 cm− 1 and 817 cm− 1 can be assigned to the asymmetrical and symmetrical stretching vibrations of the terminal Mo_O bonds, while the band at 664 cm−1 is attributed to the asymmetrical stretching vibration of O\Mo\O bonds. Peaks observed in the range of 100–400 cm−1 correspond to various bending modes of α-MoO3 crystal. The electrochemical performance of as-prepared MoO3 nanosheets was studied by galvanostatic charge–discharge measurements at a current density of 74 mA g− 1 between 3.0 and 0.01 V vs. Li+/Li. For
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comparison, the electrochemical properties of bulk MoO3 were also tested under the same conditions. As shown in Fig. 2a and b, the first discharge curves of both MoO3 nanosheets and bulk MoO3 display two small plateaus at 2.7 and 2.2 V and a quite long plateau at 0.43 V. The first two peaks should be assigned to the insertion of Li+ into the inner structure of the Mo–O octahedron layers and the van der Waals gaps between the MoO6 bilayers, respectively. The latter long plateau is related to the Li+ insertion into LixMoO3 by a unique conversion reaction to form disordered Mo metal and Li2O [Eqs. (1) and (2)], and the formation of the solid electrolyte interface (SEI) layer [22], resulting in high discharge capacities of ~1400 mAh g−1. However, from the second cycle, as shown from the discharge curves of MoO3 nanosheets that the potential is changing gradually, which are significantly different from those of bulk MoO3 with a classic plateau of potential at ~ 0.4 V. This should be attributed to the smaller crystallite structure, high surface area and disorganized MoO3 nanosheet stacks [22,27]. þ
−
MoO3 þ xLi þ xe →Lix MoO3 þ
ð1Þ −
Liy MoO3 þ ð6−yÞLi þ ð6 −yÞe ↔4Mo þ 3Li2 O
ð2Þ
The reversible capacity starting from the 2nd cycle is around 1100 mAh g−1. And after 30 cycles, a very high reversible capacity of
Fig. 2. Charge–discharge curves of (a) bulk MoO3 and (b) MoO3 nanosheets (0.01–3.0 V), (c) Charge–discharge curves of MoO3 nanosheets (0.01–2.0 V) (d) and (e) cycle performance, (f) high-rate capabilities and coulombic efficiency of MoO3 nanosheets (0.01–3.0 V).
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H. Zhang et al. / Electrochemistry Communications 52 (2015) 67–70
about 1110 mAh g− 1 is achieved for the MoO3 nanosheets electrode shown in Fig. 2e, close to the theoretical reversible capacity of α-MoO3 (1116 mAh g−1, corresponding to 6 Li per Mo atom) [22]. This value is much higher than that of bulk MoO3 (300 mAh g−1) (Fig. 2e). To the best of our knowledge, this specific capacity value is the highest among other MoO3 electrodes reported to date (500–900 mAh g−1) [19,22, 27–29]. As cycle number increases from the 2nd to the 30th, the capacity is steadily increasing probably because more surfaces of the electrode materials are exposed to the electrolyte. Similar behavior has also been observed for many metal oxides such as CoO, Co3O4, and Fe2O3 for lithium storage [30,31]. As cycles proceed, some previously inactive materials become active. When charged and discharged at a potential range of 0.01–2.0 V, the MoO3 nanosheets exhibit reversible capacity of 841 mAh g−1 in the 2nd cycle and 715 mAh g−1 after 10 cycles as shown in Fig. 1c, its cycle performance is also displayed in Fig. 1d. To further evaluate the high rate performance, charge and discharge tests on MoO3 nanosheets at current densities from 74 to 744 mA g−1 are carried out and the results are shown in Fig. 2f. The MoO3 nanosheets deliver reversible capacities of 1110, 750 and 550 mAh g−1 at the current densities of 74, 372 and 744 mA g− 1, respectively. The Coulombic efficiencies (calculated from the discharge and charge capacities) approach almost 100%. The stable cycle performance of MoO3 nanosheets at high rates indicates the ultra-fast solid-state diffusion of Li+ ions among the nanosheets owing to the short diffusion path length and the stable crystalloid structure. Apparently, such high-rate performance of MoO3 nanosheets is superior to that of most of the previously reported MoO3-based nanomaterials, including nanofibers [29], nanobelts [22,32,33], and nanoparticles [28] obtained under similar testing conditions. It is believed that high accessible surfaces by electrolyte and abundant reactive sites of the layered MoO3 nanosheets are the determining factors enabling such significant amount of lithium storage. 4. Conclusion A versatile and scalable mixture solvent (IPA and water) exfoliation method was successfully developed to fabricate MoO3 nanosheets from bulk counterpart. The exfoliated MoO3 nanosheets show twodimensional morphology (2–6 nm thickness) and single-crystalline structure. These features are favorable for lithium storage, leading to very high reversible capacities of 1110 mAh g− 1 (3.0 to 0.01 V) and 841 mAh g−1 (2.0 to 0.01 V). High-rate capabilities and good cycling performance of the MoO3 nanosheets were also demonstrated. The method of fabricating metal oxide nanosheets using this solvent mixture is simple and facile, it is anticipated that such a mixture solvent exfoliation strategy can be extended to explore various functional metal oxide nanosheets, with possible broad applications in batteries, supercapacitors, fuel cells, catalysis and sensors. Conflict of interest The authors declare no conflict of interest. Acknowledgment Support of this work by the National Natural Science Foundation of China (U1401248) is acknowledged. References [1] B. Wang, X. Li, T. Qiu, B. Luo, J. Ning, J. Li, X. Zhang, M. Liang, L. Zhi, High volumetric capacity silicon-based lithium battery anodes by nanoscale system engineering, Nano Lett. 13 (2013) 5578–5584. [2] P. Guo, H.H. Song, X.H. Chen, Electrochemical performance of graphene nanosheets as anode material for lithium-ion batteries, Electrochem. Commun. 11 (2009) 1320–1324.
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Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Exfoliated MoO3 nanosheets for high-capacity lithium storage Huijuan Zhang a, Lijun Gao a,⁎, Yongji Gong b,⁎ a b
School of Energy, College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China Department of Chemistry, Rice University, Houston, TX 77005, USA
a r t i c l e
i n f o
Article history: Received 5 December 2014 Received in revised form 13 January 2015 Accepted 13 January 2015 Available online 22 January 2015 Keywords: Two-dimensional materials MoO3 nanosheets Liquid exfoliation Lithium storage
a b s t r a c t 2D MoO3 nanosheets have been fabricated by liquid exfoliation method using low-boiling point mixture solvents of isopropanol (IPA) and water. The resulting MoO3 nanosheets demonstrate high aspect ratio, thin thickness (2–6 nm) and single-crystalline structure properties. With these features the MoO3 nanosheets are capable of exposing much electrode surfaces accessible by electrolyte, and providing abundant reactive sites for Li-ion storage. Such structured nanosheets can minimize the extent of volume change during Li intercalation/deintercalation. As an anode material of Li-ion battery, the 2D MoO3 nanosheets exhibit high reversible capacities of 1110 mAh g−1 (3.0 to 0.01 V) and 841 mAh g−1 (2.0 to 0.01 V), good rate capability and cycling performance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have become one of the most promising power sources due to their high energy density. Although graphitic carbons have been commercially available as anode materials for LIBs, they are limited by their low theoretical capacity (372 mAh g−1) [1]. Recently, graphene sheets have demonstrated to have twice higher capacity of graphitic carbon, and exhibit some unique features such as superior electrical conductivity, high surface area and chemical stability [2–5]. Inspired by graphene, extensive efforts have been devoted to explore other highcapacity two-dimensional (2D) nanosheets made by metal sulfides and oxides (N1000 mAh g−1 capacity) [6–8]. In this regard, 2D SnS2, MoS2 and WS2 nanosheets have been fabricated by hydrothermal [9–11], chemical vapor deposition (CVD) [12,13] and/or liquid-exfoliation approaches [14–17], and demonstrated enhanced electrochemical properties compared to their bulk counterparts, owing to the large surface areas, finite lateral sizes, and increased open-edges [17,18]. However fabrication of 2D metal oxides by exfoliation remains in its infancy. MoO3 is a typical layer structured material [19], each layer is composed of two sub-layers which are formed by corner-sharing [MoO6] octahedra along the [001] and [100] directions, and the other two sub-layers are stacked together by sharing the edges of octahedra along the [001] direction [20,21]. An alternate stacking of these layers along the [010] direction with van der Waals interaction leads to the formation of layered MoO3 (α-MoO3) which is able to act as a temporary host for Li+ intercalation (theoretical capacity: 1116 mAh g−1) [22]. This kind of structure is similar to those of layered materials including MoS2, WS2, BN, which have been successfully exfoliated to ultrathin ⁎ Corresponding authors. Tel.: +86 512 65229905. E-mail addresses: [email protected] (L. Gao), [email protected] (Y. Gong).
http://dx.doi.org/10.1016/j.elecom.2015.01.014 1388-2481/© 2015 Elsevier B.V. All rights reserved.
nanosheets via the liquid-phase exfoliation approach [23], Moreover, bulk MoO3 is commercially available, widely abundant, and electrochemically active. Therefore, the layer structured bulk α-MoO3 material can be an excellent precursor for the fabrication of MoO3 nanosheets for lithium storage [24,25]. In this work, we developed a versatile strategy to fabricate MoO3 nanosheets exfoliated by using mixed solvents. The synthesis protocol involves using α-MoO3 powder as the starting material and low-boiling point solvents (isopropanol (IPA)/water) as the dispersion media. By choosing solvents with appropriate composition, MoO3 nanosheets with high aspect ratios (80–800), thin thickness (2–6 nm) and singlecrystalline structure can be achieved. The exfoliated MoO3 nanosheets exhibit a very high reversible capacity of 1110 mAh g−1, good rate capability and cycling stability, which indicates that such a 2D MoO3 nanosheet holds a promise as anode material for high-capacity LIBs. 2. Experimental Bulk α-MoO 3 was added in mixture solvents of IPA/water (2:8–8:2, v/v), as well as pure IPA and water to create a dispersion with a concentration of 3 mg mL−1, and sonicated for 10 h. The dispersion was cooled by adding ice at hourly interval. During the above processes, bulk α-MoO3 would be gradually exfoliated into nanosheets via interaction of IPA molecules and MoO3 layers. After sonication, the dispersion was centrifuged at 3000 rpm to remove aggregates, giving rise to a homogenous dispersion of MoO3 nanosheets as the volume ratio of IPA and water was 5:5. To make MoO3 electrode, the suspension was centrifuged at 10,000 rpm for 30 min to collect the exfoliated MoO3 nanosheet material. The morphology and microstructure of the samples were systematically investigated by field emission scanning electron microscopy
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(FESEM) (JEOL 6500), transmission electron microscopy (TEM), highresolution TEM (HRTEM) (JEOL 2100), AFM (Digital Instrument Nanoscope IIIA), XRD (Rigaku D/Max), and Raman measurements. The electrochemical performance of as-prepared MoO3 nanosheets for lithium storage was examined. The electrode was prepared by pressing a powder mixture of MoO3 nanosheets, acetylene black, and poly(tetrafluoroethylene) (PTFE) in a weight ratio of 80:10:10 onto a Ni grid. Model button cells were assembled in a glovebox with lithium foil as counter electrode, Celgard 2400 as the separator, and LIB315 (1 mol/L LiPF6 solution in a 1:1:1 mixture of EC, DMC, and DEC) as the electrolyte. The cells were assembled in an argon-filled glovebox with the concentrations of moisture and oxygen below 0.1 ppm. Galvanostatic cycling measurements were performed at various current rates in the voltage range of 0.01 − 3.0 V on a LANDct3.3 battery tester. For comparison, bulk MoO3 was also tested under the same conditions. 3. Results and discussion According to the experience equation of ΔHMix/VMix = 2(δG − δsol)2φ/Tsheet (ΔH is the enthalpy of mixing, δ is the square root of the component surface energy, φ is the nanosheet volume fraction, and T is the thickness of nanosheets) [26], it is known that layered materials can be successfully exfoliated when the enthalpy of mixing is minimized as the surface energies of flake and solvent match. To testify this concept, MoO3 powder was suspended in mixture solvents with a series IPA/water ratio to create various dispersions at the same
concentration of 3 mg mL−1. The dispersions were then sonicated at room temperature to achieve gradual exfoliation of bulk MoO3 into nanosheets. This exfoliation process is similar to those reported for other layered materials, such as BN and MoS2, into nanosheets via the interaction of solvent molecules, significantly influenced by the surface energies of involved solvents [15]. By adjusting the ratio of IPA to water, MoO3 nanosheets could be successfully obtained. In the case of solvents with the content of 50% IPA, highest concentration (0.1 mg mL−1) of MoO3 nanosheets can be achieved. Notably, using low-boiling point solvent mixtures to produce MoO3 nanosheets offers several advantages, including low cost, low toxicity, free of additives and easy disposal. The morphology and structure of MoO3 nanosheets were initially investigated by FESEM and TEM. As presented in Fig. 1a, many nanosheets with laminar morphology similar to scraps of paper can be observed, which is in obvious contrast to that of bulk MoO3 (irregular particles). The lateral size of these nanosheets is typically in the range of 500 nm to several micrometers. Moreover, the as-prepared nanosheets are transparent to electron beams, and easily degraded under strong electron beam owing to their thin nature as shown in the TEM (Fig. 1b,c) and HRTEM images (Fig. 1d). HRTEM image further reveals the welldefined crystalline lattices of MoO3 nanosheets with spacing of 0.39 nm and 0.37 nm, corresponding to the d-spacing of the (100) and (001) planes of the orthorhombic phase of MoO3, respectively. Crosssectional AFM was conducted to further investigate the structural features of MoO3 nanosheets. The typical AFM image and thickness analyses (Fig. 1e) reveal the same morphology as seen in the
Fig. 1. (a) Typical FE-SEM, and (b, c) TEM and (d) HRTEM images and diffraction pattern of exfoliated MoO3 nanosheets; (e) AFM image and e(1, 2) corresponding thickness analysis taken around the black line in (e); (f) XRD patterns, (g) Raman spectra of MoO3 nanosheets and bulk MoO3.
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observations from SEM and TEM, showing uniform thickness of 2–6 nm. The aspect ratio of the prepared MoO3 nanosheets was estimated to be in the range of 80–800, according to SEM and AFM measurements. The crystal and chemical structure of the MoO3 nanosheets were analyzed by their XRD patterns and Raman spectra in comparison with bulk MoO3 powder. As shown in Fig. 1f, the XRD patterns of the bulk MoO3 are indexed to the typical orthorhombic MoO3 (JCPDS: 050508), which is consistent with our HRTEM analysis in Fig. 2d. In comparison, after exfoliation, the intensity of these (020) (110) (040) (120) peaks significantly decreases, clearly demonstrating that the layered α-MoO3 has been successfully exfoliated as it is expected. Raman spectra of bulk MoO3 and MoO3 nanosheets are shown in Fig. 1g, both of them have three sharp characteristic bands of MoO3, suggesting that the structure of MoO3 nanosheets is well kept during the exfoliation process. The Raman bands at 995 cm− 1 and 817 cm− 1 can be assigned to the asymmetrical and symmetrical stretching vibrations of the terminal Mo_O bonds, while the band at 664 cm−1 is attributed to the asymmetrical stretching vibration of O\Mo\O bonds. Peaks observed in the range of 100–400 cm−1 correspond to various bending modes of α-MoO3 crystal. The electrochemical performance of as-prepared MoO3 nanosheets was studied by galvanostatic charge–discharge measurements at a current density of 74 mA g− 1 between 3.0 and 0.01 V vs. Li+/Li. For
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comparison, the electrochemical properties of bulk MoO3 were also tested under the same conditions. As shown in Fig. 2a and b, the first discharge curves of both MoO3 nanosheets and bulk MoO3 display two small plateaus at 2.7 and 2.2 V and a quite long plateau at 0.43 V. The first two peaks should be assigned to the insertion of Li+ into the inner structure of the Mo–O octahedron layers and the van der Waals gaps between the MoO6 bilayers, respectively. The latter long plateau is related to the Li+ insertion into LixMoO3 by a unique conversion reaction to form disordered Mo metal and Li2O [Eqs. (1) and (2)], and the formation of the solid electrolyte interface (SEI) layer [22], resulting in high discharge capacities of ~1400 mAh g−1. However, from the second cycle, as shown from the discharge curves of MoO3 nanosheets that the potential is changing gradually, which are significantly different from those of bulk MoO3 with a classic plateau of potential at ~ 0.4 V. This should be attributed to the smaller crystallite structure, high surface area and disorganized MoO3 nanosheet stacks [22,27]. þ
−
MoO3 þ xLi þ xe →Lix MoO3 þ
ð1Þ −
Liy MoO3 þ ð6−yÞLi þ ð6 −yÞe ↔4Mo þ 3Li2 O
ð2Þ
The reversible capacity starting from the 2nd cycle is around 1100 mAh g−1. And after 30 cycles, a very high reversible capacity of
Fig. 2. Charge–discharge curves of (a) bulk MoO3 and (b) MoO3 nanosheets (0.01–3.0 V), (c) Charge–discharge curves of MoO3 nanosheets (0.01–2.0 V) (d) and (e) cycle performance, (f) high-rate capabilities and coulombic efficiency of MoO3 nanosheets (0.01–3.0 V).
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about 1110 mAh g− 1 is achieved for the MoO3 nanosheets electrode shown in Fig. 2e, close to the theoretical reversible capacity of α-MoO3 (1116 mAh g−1, corresponding to 6 Li per Mo atom) [22]. This value is much higher than that of bulk MoO3 (300 mAh g−1) (Fig. 2e). To the best of our knowledge, this specific capacity value is the highest among other MoO3 electrodes reported to date (500–900 mAh g−1) [19,22, 27–29]. As cycle number increases from the 2nd to the 30th, the capacity is steadily increasing probably because more surfaces of the electrode materials are exposed to the electrolyte. Similar behavior has also been observed for many metal oxides such as CoO, Co3O4, and Fe2O3 for lithium storage [30,31]. As cycles proceed, some previously inactive materials become active. When charged and discharged at a potential range of 0.01–2.0 V, the MoO3 nanosheets exhibit reversible capacity of 841 mAh g−1 in the 2nd cycle and 715 mAh g−1 after 10 cycles as shown in Fig. 1c, its cycle performance is also displayed in Fig. 1d. To further evaluate the high rate performance, charge and discharge tests on MoO3 nanosheets at current densities from 74 to 744 mA g−1 are carried out and the results are shown in Fig. 2f. The MoO3 nanosheets deliver reversible capacities of 1110, 750 and 550 mAh g−1 at the current densities of 74, 372 and 744 mA g− 1, respectively. The Coulombic efficiencies (calculated from the discharge and charge capacities) approach almost 100%. The stable cycle performance of MoO3 nanosheets at high rates indicates the ultra-fast solid-state diffusion of Li+ ions among the nanosheets owing to the short diffusion path length and the stable crystalloid structure. Apparently, such high-rate performance of MoO3 nanosheets is superior to that of most of the previously reported MoO3-based nanomaterials, including nanofibers [29], nanobelts [22,32,33], and nanoparticles [28] obtained under similar testing conditions. It is believed that high accessible surfaces by electrolyte and abundant reactive sites of the layered MoO3 nanosheets are the determining factors enabling such significant amount of lithium storage. 4. Conclusion A versatile and scalable mixture solvent (IPA and water) exfoliation method was successfully developed to fabricate MoO3 nanosheets from bulk counterpart. The exfoliated MoO3 nanosheets show twodimensional morphology (2–6 nm thickness) and single-crystalline structure. These features are favorable for lithium storage, leading to very high reversible capacities of 1110 mAh g− 1 (3.0 to 0.01 V) and 841 mAh g−1 (2.0 to 0.01 V). High-rate capabilities and good cycling performance of the MoO3 nanosheets were also demonstrated. The method of fabricating metal oxide nanosheets using this solvent mixture is simple and facile, it is anticipated that such a mixture solvent exfoliation strategy can be extended to explore various functional metal oxide nanosheets, with possible broad applications in batteries, supercapacitors, fuel cells, catalysis and sensors. Conflict of interest The authors declare no conflict of interest. Acknowledgment Support of this work by the National Natural Science Foundation of China (U1401248) is acknowledged. References [1] B. Wang, X. Li, T. Qiu, B. Luo, J. Ning, J. Li, X. Zhang, M. Liang, L. Zhi, High volumetric capacity silicon-based lithium battery anodes by nanoscale system engineering, Nano Lett. 13 (2013) 5578–5584. [2] P. Guo, H.H. Song, X.H. Chen, Electrochemical performance of graphene nanosheets as anode material for lithium-ion batteries, Electrochem. Commun. 11 (2009) 1320–1324.
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