Ultrathin Nanoribbons of in Situ Carbon-Coated V3O7Â·H2O for High

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Ultrathin Nanoribbons of in Situ Carbon-Coated V3O7·H2O for HighEnergy and Long-Life Li-Ion Batteries: Synthesis, Electrochemical Performance, and Charge−Discharge Behavior Pengcheng Liu,*,†,‡,# Kan Bian,§,# Kongjun Zhu,*,† Yuan Xu,†,‡,# Yanfeng Gao,∥ Hongjie Luo,∥ Li Lu,⊥ Jing Wang,† Jinsong Liu,†,‡ and Guòan Tai† †

State Key Laboratory of Mechanics and Control of Mechanical Structures and ‡College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China § School of Mechanical and Electric Engineering, Guangzhou University, Guangzhou 510006, China ∥ School of Materials Science and Engineering, Shanghai University, 99 Shangda, Shanghai 200444, China ⊥ Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore S Supporting Information *

ABSTRACT: The ever-growing demands of Li-ion batteries (LIBs) for high-energy and long-life applications, such as electrical vehicles, have prompted great research interest. Herein, by applying an interesting one-step high-temperature mixing method under hydrothermal conditions, ultrathin V3O7·H2O@C nanoribbons with good crystallinity and robust configuration are in situ synthesized as promising cathode materials of high-energy, high-power, and long-life LIBs. Their capacity is up to 319 mA h/g at a current density of 100 mA/g. Moreover, the capacity of 262 mA h/g can be delivered at 500 mA/g, and 94% of capacity can be retained after 100 cycles. Even at a large current density of 3000 mA/g, they can still deliver a high capacity of 165 mA h/g, and 119% of the initial capacity can be kept after 600 cycles. Importantly, their energy density is up to 800 Wh/kg, which is 48−60% higher than those of conventional cathode materials (such as LiCoO2, LiMn2O4, and LiFePO4), and they can maintain an energy density of 355 Wh/kg at a high power density of 8000 W/kg. Furthermore, based on ex situ X-ray diffraction and X-ray photoelectron spectroscopy technology, their exact charge−discharge behavior is reasonably described for the first time. Excitingly, it is found for the first time that the assynthesized V3O7·H2O@C nanoribbons are also great promising cathode materials for Na-ion batteries. KEYWORDS: Li-ion battery, energy density, electrochemical reaction, in situ carbon coating, vanadium oxide specific capacity of 379 mA h/g (corresponding to the 4 Li+ intercalations), which is much larger than those of conventional LiMn2O4 (148 mA h/g),29 LiCoO2 (140 mA h/g),30 and LiFePO4 (170 mA h/g).31 Unfortunately, V3O7·H2O suffers from severe capacity fading (e.g., the capacity fading is up to 50% after only 40 cycles at 50 mA/g).23−27 The bad cycling of V3O7·H2O is possibly attributed to the poor crystallinity caused by the often used conventional hydrothermal method by which there is some formation of unexpected intermediate phases and defects during the temperature rise. To improve the cycling performance of V3O7·H2O, in our previous works32,33 we developed a novel high-temperature mixing method (HTMM) under hydrothermal conditions for the general synthesis of vanadium

1. INTRODUCTION The ever-growing demand for high-energy and long-life Li-ion batteries (LIBs) for electrical vehicles, hybrid electrical vehicles, and other applications has prompted great research interest.1−4 To increase the energy density and cycling life of LIBs, it is more urgent to improve the discharge specific capacity and cycling performance of the cathode materials because the anode materials usually exhibit a higher capacity.5,6 Among the various promising cathode candidates for LIBs, vanadium oxides (e.g., V2O5, VO2, V6O13, etc.) have attracted increasing research interest because of their distinct advantages of large discharge capacity, multiple vanadium oxidation states, abundance, and low cost.7−21 Recent reports have revealed that mixed-valence vanadium oxides, which exhibit larger discharge capacity and energy density, are more promising cathode materials.13,22 Thus, another mixed-valence vanadium oxide, V3O7·H2O,23−27 which was introduced by our recent review,28 has attracted our attention because it can deliver a strikingly large theoretical © 2017 American Chemical Society

Received: January 31, 2017 Accepted: May 1, 2017 Published: May 1, 2017 17002

DOI: 10.1021/acsami.7b01504 ACS Appl. Mater. Interfaces 2017, 9, 17002−17012

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ACS Applied Materials & Interfaces oxides, and synthesized V3O7·H2O nanobelts with high crystallinity. The well-crystallized V3O7·H2O nanobelts delivered a high initial capacity of 245 mA h/g, and 83% of the initial capacity could be retained after 100 cycles at a current density of 100 mA/g, which revealed that the cycling performance was greatly enhanced. However, there remain two major problems for V3O7·H2O that need to be urgently addressed: one is how to further simultaneously improve the discharge capacity and cycling performance; and another is that the charge−discharge behavior during charging/discharging is still unknown, which greatly limits its popularity. To further improve the discharge capacity and cycling performance of V3O7·H2O, coating with carbon and synthesizing nanostructured electrode materials are quite promising methods. Nanomaterials, especially ultrathin one-dimensional (1D) nanomaterials which can display unique properties associated with the bridge between micro- and nano-scale features and possess additional superiority over fascinating 2D nanomaterials,34−36 can improve the discharge capacity and cycling performance due to their large specific surface area and unique dimension.37−40 Moreover, carbon coating can further enhance the cycling and rate properties of nanomaterials because the coated carbon shell can buffer the volume change and increase the electrochemical kinetics.41−43 However, to the best of our knowledge, reports on the synthesis of carboncoated V3O7·H2O for the application of LIBs are rare. Furthermore, most synthesis strategies for carbon-coated nanocomposites involve a time-consuming multiple-step method wherein carbon shells are deposited on the surface of presynthesized products through another separate step.41,42,44 Such strategies can cause severe damage or change the multiple-valence compounds or metastable phase. Thus, it is more desirable to develop a straightforward strategy to in situ synthesize the carbon-coated functional nanomaterials through one step. To understand the charge−discharge behavior of V3O7·H2O, ex situ X-ray diffraction (XRD) technology is an effective, convenient, and useful method.45−49 For example, through ex situ XRD analysis, Manthiram et al.48 investigated the phase transition and electrochemical reaction mechanism of highvoltage LiMn1.5Ni0.5O4 spinel between the 5.0 and 2.0 V. Yamada et al.45 applied ex situ XRD technology to reveal that Ti3C2Tx (MXene), a new kind of 2D material, exhibited a reversible Na-ion intercalation/deintercalation into its interlayer space. They also found that the interlayer distance expanded during the first sodiation, and remained stable in the following cycles. However, there is no report on the charge− discharge behavior investigation of V3O7·H2O. Thus, it remains a huge challenge to investigate the charge−discharge behavior of V3O7·H2O by ex situ XRD technology, especially, combined with ex situ X-ray photoelectron spectroscopy (XPS) technology. Herein, we develop a facile one-step strategy to in situ synthesize 1D ultrathin V3O7·H2O@C nanoribbons with good crystallinity and robust configuration based on the HTMM (Figure 1). Glucose plays the dual roles of reducing reagent and carbon resource. Compared with that of bare V3O7·H2O synthesized by the HTMM and V3O7·H2O synthesized by the conventional hydrothermal method (denoted as V3O7·H2OCH), the V3O7·H2O@C nanoribbons exhibit superior electrochemical performance in terms of large energy density, long life, and high rate as cathode materials for LIBs. Furthermore, we also show the potential applications of V3O7·H2O@C nano-

Figure 1. Scheme of the HTMM synthesis of V3O7·H2O@C nanoribbons.

ribbons as cathode materials for Na-ion batteries (NIBs). Importantly, the charge−discharge behavior of the V3O7·H2O@ C nanoribbons is elucidated for the first time based on ex situ XRD and XPS technology.

2. EXPERIMENTAL SECTION 2.1. Raw Materials. The raw materials, including V2O5 and glucose (H12C6O6), were obtained from Sinopharm Chemical Reagent Co., Ltd., China. All chemical reagents were of analytical grade and used without further purification. 2.2. Synthesis of Ultrathin V3O7·H2O@C Nanoribbons. Ultrathin V3O7·H2O@C nanoribbons were in situ synthesized through the HTMM. The HTMM is a modified hydrothermal method.32,33,50,51 The starting solutions or raw materials for the HTMM are separately heated in a double-chambered autoclave during the temperature rise. When the temperature is increased to the desired reaction temperature, that is, in the temperature preservation period, the raw materials are mixed to start the hydrothermal reaction. This method effectively avoids the formation of intermediate phases and defects during the temperature rise in the conventional hydrothermal method, which can greatly improve the crystallinity of products. Furthermore, the reaction time can be dramatically shortened because the reaction, nucleation, and growth rates are faster, and the transformation from intermediate phases and defects to final products is avoided when the reaction is started at the high and desired temperature. HTMM is also simple method without long time and complex mixing or stirring processes at ambient temperature. In the present synthesis, V2O5 was used as the vanadium source, and glucose was used as the reducing agent and carbon resource. First, 0.182 g of V2O5 and 5 mL of deionized water were placed in chamber I of a Teflon-lined multichamber autoclave, and 5 mL of glucose solution (0.02 M) was poured into chamber II (Figure 1). Then, these solutions were mixed when the temperature was at 240 °C, and this temperature was maintained for 1 h. After cooling to room temperature, the as-formed precipitates were washed several times with deionized water and ethanol. Finally, the 1D ultrathin V3O7· H2O@C nanoribbons were obtained after drying at 80 °C for 10 h under vacuum. 2.3. Synthesis of Bare V3O7·H2O Nanoribbons. Bare V3O7·H2O nanoribbons were synthesized by replacing the glucose solution with 5 mL of oxalic acid solution (0.6 M), and keeping the other procedures unchanged. 2.4. Synthesis of V3O7·H2O-CH Nanoribbons. V3O7·H2O nanoribbons were synthesized by the conventional hydrothermal method and denoted as V3O7·H2O-CH. The synthesis process is as follows: first, V2O5 (5 mmol) was added into 50 mL of an aqueous solution of oxalic acid (0.1 M) and mixed at 50 °C for 30 min; then, the above mixing solution was transferred into a conventional autoclave, and heated at 180 °C for 24 h; finally, the as-formed precipitates were washed several times with deionized water and ethanol, and dried at 80 °C for 10 h under vacuum. 2.5. Material Characterization. The phase structure of the resultant powders was determined by powder XRD (Bruker D8 Advance, Germany) using Cu Kα radiation (λ = 0.15418 nm). The 17003


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Figure 2. Characterization of the V3O7·H2O@C nanoribbons synthesized by the HTMM for 1 h at 240 °C. (a) XRD pattern with an inset of the V3O7·H2O crystal structure along the c axis; (b) XPS spectrum of core level V 2p; (c) XPS spectrum of core level C 1s; (d) Raman spectrum. morphologies of the prepared particles were observed by field emission scanning electron microscopy (FESEM, HITACHI S-4800, Japan) and transmission electron microscopy (TEM; Tecnai G2 F30 S-Twin TEM, FEI). The thickness of the ultrathin single V3O7·H2O nanoribbon was measured by atomic force microscopy (AFM; MICROMERITICS ASAP2020M). The Raman spectra of the samples were collected using a Jobin Yvon Labram HR800 Raman spectroscope at 514 nm with Ar+ lines. XPS measurements were carried out with an ESCALab220i-XL spectrometer. Thermogravimetric analysis (PerkinElmer Pyris 1) was carried out under an air flow with a temperature ramp of 10 °C/min. 2.6. Electrochemical Measurements. The active materials of the as-synthesized V3O7·H2O@C nanoribbons, acetylene black, and PVDF were mixed well into weight ratios of 6:3:1 to obtain a slurry. The obtained slurry was cast on aluminum foil to obtain a laminate. The laminate was dried for 5 min under an incandescent lamp before drying in a vacuum oven at 120 °C for 12 h. The dried laminate was punched into round disks with diameters of 12 mm. The surface density of the active materials was cal. 1.35 mg/cm2. Coin cells (2032) were assembled in an argon-filled glove box with O2 concentrations of less than 1 ppm. A lithium metal foil was used as the counter electrode, and Celgard2400 was used as the separator. The electrolyte solution was 1.0 M LiPF6 in EC/DEC (1:1 v/v). Galvanostatic charge/ discharge measurements were performed by a multichannel battery testing system (LAND CT2001A). Electrochemical impedance spectroscopy (EIS) was measured over the frequency range from 0.1 to 105 Hz by an electrochemical workstation (Princeton PARSTAT MC). All electrochemical measurements were conducted at room temperature. 2.7. Ex Situ XRD Measurements. To understand the charge− discharge behavior of the as-synthesized V3O7·H2O@C nanoribbons, the cells, which were discharged or charged to different voltages at a current density of 50 mA/g during the first cycle, were disassembled. Then, the electrode membrane was separated from the Al substrate by ultrasound treatment in a solution of DEC. The detailed disassembly process of the coin cell for ex situ measurements is shown in Figure S1. Finally, the corresponding XRD patterns were collected from the separated electrode membrane. 2.8. Ex Situ XPS Measurements. To further understand the charge−discharge behavior of the V3O7·H2O@C nanoribbons, ex situ

XPS measurements were conducted to track the valance change during cycling. The measurements were similar to those of the ex situ XRD. The cells, which were discharged or charged to different voltages at 50 mA/g during the first cycle, were disassembled. Then, the electrode membrane was separated from the Al substrate by ultrasound treatment in a solution of DEC. Finally, the separated electrode membranes were used to conduct the XPS measurements.

3. RESULTS AND DISCUSSION 3.1. Synthesis of V3O7·H2O@C Nanoribbons. Figure 2 shows the detailed characterization of the V3O7·H2O@C nanoribbons synthesized by the HTMM. The synthesis of the well-crystallized V3O7·H2O (standard PDF card #85-2401) is confirmed by XRD analysis (Figure 2a). The inset in Figures 2a and S2 clearly show the crystal structures of V3O7·H2O with a large interlayer space and tunnels that provide more intercalation sites and free diffusion paths for Li+. A characteristic broad diffraction peak of carbon between 20 and 35° can be observed, indicating the presence of carbon in the as-synthesized products. In contrast, the characteristic peak of carbon cannot be found in the XRD patterns of the bare V3O7·H2O (Figure S3a) or V3O7·H2O-CH (Figure S4a). It can also be seen that the crystallinity of the products synthesized by the HTMM is higher than that of the materials prepared by the conventional hydrothermal method. Furthermore, the synthesis time of V3O7·H2O prepared by the HTMM is only 1 h, which is much shorter than that by other strategies.23−27 For example, Mai et al.,27 Sediri et al.,52 and Zhou et al.24 synthesized V3O7· H2O for 48, 96, and 168 h by employing the conventional hydrothermal method, respectively. The valence states and bounding types of the elements in the V3O7·H2O@C nanoribbons were identified by XPS. For the V 2p spectrum (Figure 2b), the peaks of both V 2p3/2 and V 2p1/2 can be fitted well and separated into 515.8, 517.3 eV and 523.7, 525.1 eV, respectively, which correspond to V4+ (516.0 and 524.1 eV) and V5+ (517.3 and 525.2 eV).53 These results indicate that V3O7·H2O is a mixed-valence vanadium oxide. 17004


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Figure 3. Morphology of the V3O7·H2O@C nanoribbons. (a) SEM image; (b) TEM image; (c) HRTEM image; (d) elemental mapping (the red color of the background in the C element mapping image is caused by the carbon mesh); (e) TEM image; (f) AFM image with height pattern.

Furthermore, the content ratio of V5+/V4+ can be calculated as 2:1. In the O 1s spectra (Figure S5), two peaks at 529.8 and 532.5 eV can be identified as O−V5+ and O−V4+, respectively. The peak fitting of the core level C 1s spectra is shown in Figure 2c. The peaks at 284.7, 286.2, and 288.6 eV correspond to the C−C, C−O, and C=O bonds,54 respectively, proving the existence of carbon in the final products,55,56 and agreeing well with the XRD results. Figure 2d shows the Raman spectrum of the V3O7·H2O@C nanoribbons. The peaks from 200 to 1000 cm−1 are the typical vibration modes of V3O7·H2O. A band located at the wavenumber of 990 cm−1 is indexed as the stretching mode of vanadyl oxygen. Three bands observed at 872, 690, and 525 cm−1 are associated with the V−O stretching vibrations, whereas the mode at 478 cm−1 is attributed to the threecoordinated O−V(3) bonds. Another two Raman modes can be observed at 404 and 280 cm−1, which originate from the bending vibrations of V−O. Importantly, two modes located at 1355 and 1581 can be indexed as the typical D-bond and Gbond of carbon, which further powerfully confirms the existence of carbon,12 and is consistent with the XRD and XPS results. The morphology observations of the V3O7·H2O@C nanoribbons are shown in Figure 3. The FESEM image (Figure 3a) clearly shows that the morphology of the as-synthesized V3O7· H2O is that of flexible ultrathin nanoribbons. The morphology feature of the flexible nanoribbons can be further observed in the TEM image (Figure 3b). The crystal growth of the nanoribbons can be explained by the oriented attachment mechanism described in our previous works.32,33,50,51,57 Figure 3c shows the HRTEM image of the V3O7·H2O nanoribbons. A uniform and obvious carbon layer with a thickness of ∼1 nm can be clearly observed outside the nanoribbons forming a

core−shell structure. The uniform carbon coating can be further confirmed from elemental mapping (Figure 3d). Furthermore, when the as-synthesized V3O7·H2O@C nanoribbons were annealed under an inert atmosphere, they became VO2, V4O9, and V6O13 with lower valence states (Figure S6) because of the reduction of the carbon layer.58 Thus, the above results adequately prove the realization of a good in situ carbon coating in our HTMM strategy. Glucose is very important in the present synthesis. It simultaneously plays two important roles of reducing reagent and carbon resource to facilitate the in situ formation of a uniform carbon shell through glucose carbonization under hydrothermal conditions.43 To further reveal the carbon content in the V3O7·H2O@C nanoribbons, a TG measurement was conducted in air (Figure S7). The weight loss from 100 to 200 °C and around 300 °C is caused by the volatilization of adsorbed water and the removal of some crystal water, respectively. The weight loss of ∼8.2% between 400 and 600 °C is attributed to the combustion of carbon. From the TEM image (Figure 3e), it can be seen that the nanoribbons are nearly transparent under the exposure of electrons, which shows that the nanoribbons are quite thin. To precisely measure the thickness of a single nanoribbon, AFM measurement was conducted. From Figure 3f, it can be seen that the thickness of such a single nanoribbon is just ∼6.2 nm. So, based on the above results and analysis, flexible ultrathin V3O7·H2O@C nanoribbons with improved crystallinity have been in situ synthesized within a short time by using the HTMM. On the basis of the above results, the present synthesis strategy can provide the following advantages: (1) the reaction time can be dramatically shortened because the reaction, nucleation, and growth rates are faster, and the transformation from intermediate phases and defects to final products is 17005


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Figure 4. Electrochemical measurements of the V3O7·H2O@C nanoribbons. (a) Cycling performance comparison of V3O7·H2O@C at a large current density of 500 mA/g and bare V3O7·H2O at a small current density of 200 mA/g; (b) discharge−charge curves of the V3O7·H2O@C nanoribbons at a current density of 100 mA/g; (c) rate performance of the V3O7·H2O@C nanoribbons; (d) energy density (calculated based on electrode material) comparison of the V3O7·H2O@C nanoribbons and conventional cathode materials of LIBs, the inset Ragone plot illustrates the function relationship of power density (calculated based on electrode material) vs energy density for the V3O7·H2O@C nanoribbons compared with that of typical fuel cells and supercapacitors.

curves. From the inset, it can be seen that the capacity increases gradually to a quite large value of 319 mA h/g (corresponding to the intercalation of 3.3 Li+) after 20 cycles, and then stays stable. The capacity increase can be explained by the activation process caused by the gradual wetting and soaking of electrolytes.59 These results suggest that V3O7·H2O@C nanoribbons possess excellent cycling performance. The rate capability at progressively increased current densities ranging from 50 to 3000 mA/g was measured to evaluate further the advantage of the V3O7·H2O@C nanoribbons (Figure 4c). The rate performance of the V3O7·H2O@C nanoribbons is much better than that of the bare V3O7·H2O nanoribbons in our previous work32 and the V3O7·H2O-CH nanoribbons (Figure S3c,d). The discharge capacities were 320, 284, 262, 243, 215, and 165 when the current densities were 50, 100, 300, 500, 1000, and 2000 mA/g, respectively. Even when subjected to rapid charging−discharging, the cell still shows a stable capacity at each current density. Although the current density increases 60-fold to 3000 mA/g, the cell still delivers a capacity of 146 mA h/g, which is higher than the theoretical capacity of conventional LiCoO2 (140 mA h/g).30 Furthermore, a higher capacity of 265 mA h/g is delivered when the current density is returned to 300 mA/g. These results indicate that the V3O7·H2O@C nanoribbons possess excellent rate performance. Importantly, the superior electrochemical performance of large capacity, stable cycling, and high rate for the V3O7·H2O@C nanoribbons is much better than that of most conventional cathode materials (e.g., LiCoO2, LiMn2O4, LiFePO4, VO2, V2O5, etc.).8,29−31 A detailed comparison is listed in Table S1. More importantly, to further show their advantages, the energy and power density of the V3O7·H2O@C nanoribbons were compared in detail with those of other electrode materials

avoided when the reaction is started at the high and desired temperature; (2) the in situ synthesis of ultrathin carbon-coated nanoribbons is realized, which can buffer large volume changes during cycling, increase the electrode−electrolyte contact area, and provide fast electrochemical kinetics; (3) raw materials are just mixed during the temperature preservation step in the HTMM synthesis, so that intermediate phases and defects can effectively be avoided, which can enhance the crystallinity of V3O7·H2O, and thereby intrinsically improve its electrochemical performance. 3.2. Electrochemical Performance of V3O7·H2O@C Nanoribbons. To evaluate their electrochemical performance, the V3O7·H2O@C nanoribbons were assembled as cathode materials for LIBs. To intuitively and adequately reveal the advantage of the in situ carbon-coated products, V3O7·H2O@C and bare V3O7·H2O were discharged/charged at a large current density of 500 mA/g and a small current density of 200 mA/g (Figure 4a), respectively. The initial discharge specific capacities of V3O7·H2O@C and bare V3O7·H2O were 262 and 259 mA h/ g, respectively. The discharge capacities of V3O7·H2O@C and bare V3O7·H2O became 245 and 184 mA h/g after 100 cycles, respectively, corresponding to 94 and 71% of their initial capacities. This result reveals that the in situ carbon coating method can greatly improve the cycling performance. It should be noted that the capacity retention of 94% for the assynthesized V3O7·H2O@C nanoribbons is much better than that of other reported data for V3O7·H2O prepared by the conventional hydrothermal method, in which only about 50% of the initial capacity can be retained after 40 cycles.24−26 Furthermore, the Coulombic efficiency of the V3O7·H2O@C nanoribbons is close to 100%, except the initial value of 95%. Figure 4b shows that the V3O7·H2O@C nanoribbons have charge−discharge patterns at 100 mA/g with well overlapped 17006


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Figure 5. (a) Cycling performance of the V3O7·H2O@C nanoribbons at a large current density of 3000 mA/g; (b) photograph of 10 blue light emitting diodes (LEDs) powered by one coin cell applying the V3O7·H2O@C nanoribbons as the positive electrode after being cycled 600 times at 3000 mA/g, (c) schematic diagram showing that the carbon shell and flexibility of the V3O7·H2O@C nanoribbons can improve the charge transfer and buffer the volume change during cycling.

and energy storage technologies (Figure 4d). The V3O7·H2O@ C nanoribbons show an energy density of up to 800 Wh/kg, which is 48−60% higher than those of conventional LiCoO2 (540 Wh/kg),30 LiMn2O4 (500 Wh/kg),29 and LiFePO4 (500 Wh/kg).31 Even compared with high-voltage LiNi0.5Mn1.5O4 (650 Wh/kg),13 high-capacity V2O5 (700 Wh/kg),60 and mixed-valence V6O13 (780 Wh/kg),13 the V3O7·H2O@C nanoribbons still deliver a higher energy density. The inset Ragone plot illustrates the function relationship of power density versus energy density for the V3O7·H2O@C nanoribbons. The V3O7·H2O@C nanoribbons possess the merits of both high-energy fuel cells and high-power supercapacitors. For example, when the power density is up to 8000 W/kg, equivalent to that of high-power supercapacitors, a high energy density of 355 Wh/kg, which is much higher than those of reported aqueous system supercapacitors (usually lower than 60 Wh/kg),61,62 can be obtained. The comparison in Figure 4 adequately demonstrates that the as-synthesized V3O7·H2O@C nanoribbons display an outstanding electrochemical performance in terms of large capacity, stable cycling, high rate, and high energy and power densities. To further demonstrate the outstanding electrochemical performance of the V3O7·H2O@C nanoribbons, the long-term cycling was studied at a quite large current density of 3000 mA/ g (Figure 5a). The initial capacity was 122.5 mA h/g. An activity process, accompanied by a gradual capacity increase, can be observed during the following cycles. After 200 cycles, the capacity reaches the highest value of 165 mA h/g. The capacity only slightly decreases and remains stable in the following cycles. For example, when cycled for 500 times, V3O7· H2O@C delivers a capacity of 137.5 mA h/g; even after 600 cycles, a capacity of 133.3 mA h/g with only a capacity loss of 0.042 mA h/g per cycle, which is higher than the initial capacity, can be retained. These results further reveal that the V3O7·H2O@C nanoribbons possess excellent cycling performance. Figure 6 shows the morphology of the V3O7·H2O@C

Figure 6. Morphology of the V3O7·H2O@C nanoribbons after 600 cycles at 3000 mA/g. (a) SEM image; (b) TEM image with low magnification; (c) TEM image with high magnification; (d) HRTEM image.

nanoribbons after 600 cycles. In Figure 6a, it can be seen that the architecture of the nanoribbons is preserved without any obvious deformation or damage. The morphology can be further observed from Figure 6b,c. In the HRTEM image (Figure 6d), a clear lattice fringe corresponding to (200) is observed, and a continuous carbon shell can also be found outside the nanoribbons. Such good integrity of the V3O7· H2O@C nanoribbons can further account for their highcapacity retention. EIS was carried out to evaluate the electrochemical kinetics of the V3O7·H2O@C nanoribbons and bare V3O7·H2O (Figure 17007


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Figure 7. Charge−discharge behavior of the V3O7·H2O@C nanoribbons. (A) Discharge−charge curves for the first cycle at a current density of 50 mA/g between 3.75 and 1.5 V; (B) ex situ XRD patterns that were collected at the points labeled in (A); (C) ex situ XPS patterns during different charge and discharge states that were collected at the points labeled in (A).

S8). The charge transfer resistance (∼48 Ω) of V3O7·H2O@C is much smaller than that of bare V3O7·H2O (∼95 Ω). This reveals that V3O7·H2O@C possesses faster electrochemical kinetics and higher electronic conductivity,13 which also explains its superior rate and cycling performance. Further, the cell, which was cycled 600 times at the large current density of 3000 mA/g, can still easily power 10 blue LEDs in parallel (Figure 5b). Furthermore, we also explored the potential applications of the V3O7·H2O@C nanoribbons as cathode materials for NIBs (Figure S9). When the appropriate electrolyte (1 M NaClO4 dissolved in PC) was used, an initial capacity of 135 mA h/g was obtained at 100 mA/g, which is higher than those of common Na3V2(PO4)3 (102 mA h/g),63 Na4Mn9O18 (110 mA h/g),64 Na1.1V3O7.9 (125 mA h/g),65 and NaFe(SO4)2 (99 mA h/g).66 After 100 cycles, 93% of the initial capacity is maintained, which indicates a good cycling performance for NIBs. 3.3. Charge−Discharge Behavior of the V3O7·H2O@C Nanoribbons. To understand the charge−discharge behavior of the V3O7·H2O@C nanoribbons and their excellent electrochemical performance, ex situ XRD and XPS technologies were applied (Figure 7). First, Figure 7A shows the charge− discharge curve with multiple voltage plateaus during the first cycle between 3.75 and 1.5 V. The ex situ XRD patterns (Figure 7B), collected at the points labeled in Figure 7A, show that no new phase or products can be found during cycling. These results reveal that the electrochemical reaction of V3O7· H2O is a reversible multistep Li+ intercalation and deintercalation process.8 Second, three distinct plateaus denoted as regions I, II, and III appear during discharging, and the corresponding plateaus during charging are denoted as regions I′, II′, and III′ (Figure 7A). The cutoff voltages of regions I, II, and III are 3.0, 2.4, and 1.8 V, respectively, at which the corresponding capacities are 55, 178, and 275 mA h/g (Table 1). Table 2 lists the theoretical capacities when different numbers of Li+ are intercalated into V3O7·H2O. The capacities of regions II and III are very close to the theoretical values caused by the intercalation of 2 and 3 Li+. Although the

Table 2. Theoretical Discharge Specific Capacity Caused by Intercalation of Different Amounts of Li+ into V3O7·H2O theoretical capacity (mA h/g)

region I

region II

region III

3.0 54.4

2.4 177.7

1.8 274.9

2 Li+

3 Li+

94

189

284

capacity of region I is lower than that of 1 Li+ intercalation, which is caused by the open circuit voltage of 3.4 V during the first discharge being lower than the charging cutoff voltage of 3.75 V, it can gradually get closer to the theoretical values by the intercalation of 1 Li+ during the following cycles (Figure 4b). These results indicate that regions I, II, and III correspond to the 1, 2, and 3 Li+ intercalation processes, respectively. Thus, the electrochemical reaction (i.e., a reversible multistep Li+ intercalation and deintercalation process) of V3O7·H2O, which is proposed for the first time, can be concretely described as follows: During discharging from 3.75 to 3.0 V: the first Li+ is intercalated V3O7 ·H 2O + Li+ + e− → LiV3O7 ·H 2O

(1) +

During discharging from 3.0 to 2.4 V: the second Li is intercalated LiV3O7 ·H 2O + Li+ + e− → Li 2V3O7 ·H 2O

During discharging from 2.4 to 1.8 V: the third Li intercalated. Li 2V3O7 ·H 2O + Li+ + e− → Li3V3O7 ·H 2O

(2) +

is (3)

+

Another 0.3 Li is intercalated when discharging from 1.8 to 1.5 V. During charging, Li+ deintercalation is opposite to the intercalation. Lastly, it should also be noted that the shift in peaks shown in Figure 7B is quite small during cycling. This result indicates that the volume change of the V3O7·H2O@C nanoribbons during cycling is very small, and their configuration can be maintained well, which is beneficial for the excellent cycling performance.67 To further understand the charge−discharge process of the V3O7·H2O@C nanoribbons, ex situ XPS measurements were conducted to track the valance state change of the V ions during cycling, and the corresponding results are shown in Figure 7C and Table 3 (showing the content of V ions with different valence states). The V ions in the initial V3O7·H2O@C consist of V5+ ions (65%) and V4+ ions (35%), as shown in Figure 7C(a) and Table 3a. When the first Li ion is intercalated into V3O7·H2O during the initial discharging, the content of V5+

Table 1. Discharge Specific Capacity of the V3O7·H2O@C Nanoribbons Obtained in Different Discharge Plateaus When Discharged at 50 mA/g during the First Cycle

cutoff voltage (V) highest capacity (mA h/g)

1 Li+

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buffer the large volume change and release the consequent mechanical stress during charging/discharging, which can improve the cycling properties; simultaneously, they can improve the electrochemical kinetics, and therefore the rate properties. (2) The ultrathin nanoribbons possess the advantages of a large electrode−electrolyte contact area, good strain accommodation, and short diffusion pathway for Li ions, which helps to deliver the large discharge capacity, good cycling life, and high rate properties, respectively. (3) The flexibility of the core−shell nanoribbons@C also provides a buffering effect to maintain cycling stability (Figure 5c). (4) The ex situ XRD and XPS analysis shows that the volume change during cycling is quite small, which also suggests that the V3O7·H2O@C nanoribbons possess a robust configuration to maintain cycling stability. (5) Figure 8 shows clearly the advantages of the HTMM for synthesizing well-crystallized products. The desired chemical reactions of the hydrothermal method are designed to occur during the temperature preservation step. However, actually, some unexpected reactions occurred during the temperature rise in the conventional hydrothermal method. Therefore, this usually results in the formation of some unexpected intermediate phases and defects, which are extremely disadvantageous for the synthesis of compounds including multiple-valence states or transition metal elements. These intermediate phases and defects possibly remain in the final products although a long-time transformation is conducted during the temperature preservation step, which results in their poor crystallinity, and negatively affects their performance. In the HTMM synthesis, the intermediate phases and defects are effectively avoided because the raw materials are separated during the temperature rise, which results in enhanced crystallinity of the products, and therefore their electrochemical properties are intrinsically improved.

Table 3. Content of V Ions with Different Valence States Calculated Based on the XPS Results (Figure 7C) item

V5+ content (%)

V4+ content (%)

V3+ content (%)

(a) (b) (c) (d) (g) (h)

65 33 30 27 31 60

35 43 41 38 45 40

0 24 29 35 24 0

ions is decreased, and the content of V4+ ions and V3+ ions is increased (Figure 7C(b) and Table 3b), which reveals that the V5+ ions are reduced into V4+ ions and V3+ ions with the first Liion intercalation. Accompanying the intercalation of another two Li ions during the following discharging, the content of V5+ ions and V4+ ions decreases, whereas the content of V3+ ions continually increases (Figure 7C(c,d) and Table 3c,d), which means that the V5+ ions and V4+ ions are further reduced to V3+ ions with the following Li-ion intercalation. In contrast, during charging (i.e., the process of Li-ion deintercalation), V3+ ions are continually oxidized to V5+ ions and V4+ ions with the content of V3+ ions decreasing, but the content of V5+ ions and V4+ ions increasing (Figure 7C(g) and Table 3g). When all Li ions are deintercalated (Figure 7C(h) and Table 3h), the whole V3+ ions and partial V4+ ions are oxidized to V5+ ions, and V3O7· H2O returns to its initial state. The above ex situ XPS results not only further reveal the charge−discharge behavior of V3O7· H2O but also confirm the structural stability of the V3O7· H2O@C nanoribbons. 3.4. Discussion on the Excellent Electrochemical Performance of the V3O7·H2O@C Nanoribbons. The superior electrochemical performance of the as-synthesized ultrathin V3O7·H2O@C nanoribbons can be associated with the following unique advantages. (1) The in situ formed carbon shells (Figure 5c) can play dual roles during cycling: they can

Figure 8. Comparison of the Synthesis Process for (a) the Conventional Hydrothermal Method and (b) the HTMM. 17009



4. CONCLUSIONS In summary, we report a facile one-step strategy to in situ synthesize ultrathin V3O7·H2O@C nanoribbons with good crystallinity and robust configuration based on the HTMM. The carbon shell, robust configuration, and flexibility of the assynthesized ultrathin V3O7·H2O@C nanoribbons can synergistically buffer the large volume change during cycling, increase the electrochemical sites, and provide fast electrochemical kinetics. Moreover, the electrochemical performance can be intrinsically improved because the intermediate phases and defects can be effectively avoided in the HTMM synthesis. So, compared with those of the bare V3O7·H2O and V3O7·H2OCH, the V3O7·H2O@C nanoribbons exhibit excellent electrochemical properties in terms of large capacity, long life, high rate, and high energy and power density for cathode materials of LIBs. Furthermore, we also show the potential applications of the V3O7·H2O@C nanoribbons as cathode materials for NIBs. Importantly, the charge−discharge behavior of the V3O7· H2O@C nanoribbons is elucidated for the first time based on the ex situ XRD and XPS technologies, which revealed that the volume change during cycling is quite small, and 3.3 Li ions can be reversibly intercalated/deintercalated during the multistep between 3.75 and 1.5 V. The above results reveal that the assynthesized V3O7·H2O@C nanoribbons can become great promising high-energy and long-life cathode materials for nextgeneration LIBs. Furthermore, such a finding also suggests that the HTMM is a promising way to controllably synthesize wellcrystalized nanomaterials, ultrathin 2D materials, or in situ carbon-coated nanocomposites including multiple-valence states or transition metal elements, and can even become a more rich-functional synthesis method that can make researchers prepare more attractive nanostructures or design more ingenious synthesis strategies.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Nature Science Foundation of China (NSFC Nos. 51672130 and 51372114), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (No. 0514Y01), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Funding of Jiangsu Innovation Program for Graduate Education (No. KYLX_0262), the Fundamental Research Funds for the Central Universities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01504. Additional information and figures including crystal structure of V3O7·H2O; characterization and electrochemical performance of bare V3O7·H2O and V3O7·H2OCH; XPS spectrum of the V3O7·H2O@C nanoribbons; XRD pattern of the product obtained after annealing V3O7·H2O@C; TG pattern of V3O7·H2O@C under air; EIS patterns of V3O7·H2O@C and V3O7·H2O-CH; electrochemical performance of V3O7·H2O@C as cathode material for NIBs; comparison table of electrochemical performances between the V3O7·H2O@C nanoribbons and other electrode materials (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.L.). *E-mail: [email protected] (K.Z.). ORCID

Pengcheng Liu: 0000-0002-9530-9267 Kongjun Zhu: 0000-0003-0804-8044 Author Contributions #

P.L., K.B., and Y.X. contributed equally.

Notes

The authors declare no competing financial interest. 17010


Research Article

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Ultrathin Nanoribbons of in Situ Carbon-Coated V3O7Â·H2O for High

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