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Keywords:

  • energy storage;
  • nanocrystals;
  • selenium;
  • three dimensional electrodes;
  • tin

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

SnSe nanocrystal electrodes on three-dimensional (3D) carbon fabric and Au-coated polyethylene terephthalate (PET) wafer have been prepared by a simple spray-painting process and were further investigated as binder-free active-electrodes for Lithium-ion batteries (LIBs) and flexible stacked all-solid-state supercapacitors. The as-painted SnSe nanocrystals/carbon fabric electrodes exhibit an outstanding capacity of 676 mAh g−1 after 80 cycles at a current density of 200 mA g−1 and a considerable high-rate capability in lithium storage because of the excellent ion transport from the electrolyte to the active materials and the efficient charge transport between current collector and electrode materials. The binder-free electrodes also provide a larger electrochemical active surface compared with electrodes containing binders, which leads to the enhanced capacities of energy-storage devices. A flexible stacked all-solid-state supercapacitor based on the SnSe nanocrystals on Au-coated PET wafers shows high capacitance reversibility with little performance degradation at different current densities after 2200 charge–discharge cycles and even when bent. This allows for many potential applications in facile, cost-effective, spray-paintable, and flexible energy-storage devices. The results indicate that the fabrication of binder-free electrodes by a spray painting process is an interesting direction for the preparation of high-performance energy-storage devices.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Over the past few years, considerable attention has been paid to develop clean and highly efficient energy-storage devices with high power and energy densities because of the ever-increasing concerns about environmental problems and limited global energy supply.15 As two major devices for electric energy storage, lithium-ion batteries (LIBs) and supercapacitors have attracted worldwide attention because they play vital roles in our daily lives as the dominant power sources for a range of applications, from mobile devices to electric vehicles (EVs)/hybrid electric vehicles (HEVs), and even energy-storage systems.613 Advanced electrode materials, such as graphene,1418 metal oxides,1928 and metal sulfide,2933 have been studied extensively for high-performance LIBs and supercapacitors. However, most of the electrodes are commonly binder-enriched prepared by using the traditional slurry-coating technique for electrochemical evaluation, for which a large part of the electroactive surface area is inaccessible and blocked from the electrolyte and thus cannot participate in the reaction for energy storage. Moreover, the binder involved will greatly decrease the conductivity of the electrode materials, thus hindering their potential application in high-performance energy-storage devices.34 Recently, binder-free electrodes on conductive substrates for energy-storage have been explored widely because of their good electrical conductivity and large accessible reaction surface.3439 However, complicated and cost-consuming synthesis processes were essential, which makes these approaches not suitable for the mass production of energy-storage devices. Therefore, it still remains a great challenge to develop high-performance binder-free energy-storage devices through a facile, low-cost, large-scale, and solution-processable process at room temperature.

Usually, the performances of energy-storage devices largely depend on the properties and structures of the electrodes. Tin-based metal oxide and sulfide electrodes have been investigated intensively over the past decade. Among them, tin selenide has two stoichiometric phases, SnSe and SnSe2, of which SnSe has an orthorhombic structure (space group: pnma), an indirect band gap of 0.90 eV, and a direct band gap of 1.30 eV;40 because of these properties it has investigated as an earth-abundant and environmentally benign component of photovoltaic devices such as solar cells.41 Recently, colloidal SnSe2 and its application in LIBs was also reported.42 However, a solution approach to prepare colloidal SnSe at room temperature for energy-storage devices, such as LIBs and supercapacitors, is still limited.

Herein, we successfully demonstrate the preparation of an ink based on SnSe nanocrystals at room temperature that can be painted onto substrates. A spray-paintable 3D binder-free electrode for high-performance LIBs is prepared by painting this ink onto a 3D flexible conductive carbon fabric. The concept of 3D battery electrodes has been used previously to enhance the specific energy.4345 We adopt a 3D textile conductor as replacement of metal current collector because of the facile ion transport from the electrolyte to the active materials and the good charge transport between current collector and electrode materials. As a result, the assembled battery displays a reversible capacity as high as 676 mAh g−1 after 80 discharge–charge cycles at 200 mA g−1, which is an improvement in comparison to SnSe thin films,46 SnSe nanocrystals,47 and SnSe2 nanoplates.42 The SnSe nanocrystal ink can also be painted onto other substrates, such as PET wafers, which was also evaluated as binder-free electrode for flexible all-solid-state supercapacitors. These exhibited a capacitance of 1800 μF cm−2 at a current density of 20 μA cm−2 and high stability even at higher current densities, allowing for potential applications in facile, cost-effective, paintable, and flexible energy-storage devices.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Figure 1 illustrates the fabrication processes of the 3D SnSe nanocrystals/carbon fabric electrode by using the spray painting method. The SnSe nanocrystal ink was prepared through a sol–gel process at room temperature. After painting, the 3D carbon fabric was dried in a vacuum oven and a 3D integrated electrode covered uniformly with SnSe nanocrystals was obtained. The thickness of the active materials can be controlled by repeating the spray-painting process. An advantage of this method is that the active materials are directly attached to the conductive substrate; thus, the poor conductivity of the electroactive material is not a big concern. More importantly, such a design renders other auxiliary components in conventional thin-film electrodes such as conductive agents and organic binders completely unnecessary.

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Figure 1. Illustration of the fabrication 3D SnSe nanocrystals/carbon fabric electrodes through a spray painting process.

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The XRD pattern in Figure 2 aof the as-prepared product can be indexed to pure SnSe with the orthorhombic structure (JCPDS No. 089-0237). The diffraction peaks of carbon could not be detected, indicating that the SnSe nanocrystal coating on the carbon fabric is compact and uniform. Figure 2 b depicts a typical scanning electron microscopy (SEM) image of as-painted SnSe nanocrystals/carbon fabric architectures, exhibiting interesting 3D textile structures that consist of many carbon fibers coated with uniform SnSe nanocrystals. The SEM image and energy-dispersive X-ray (EDX) spectra of the pure carbon fabric are shown for comparison in Figure S1 in the Supporting Information. Figure 2 c and d provide more information about the 3D architectures: each carbon fiber adhered tightly to the SnSe nanocrystals. We found that porous structures were formed because of the piling up of the nanocrystals (Figure S2), which are highly desired for designing high-performance energy-storage devices. An EDX spectrum of the integrated electrode is shown in Figure S3, and the corresponding elemental distribution of Sn, Se, and C in the SnSe nanocrystals/carbon fabric product is shown in Figure S4, suggesting that the SnSe nanocrystals are uniformly wrapped around the carbon fiber, which enables the good dispersion of the SnSe nanocrystals over the whole carbon fabric.

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Figure 2. (a) XRD pattern of SnSe nanocrystals, (b–d) SEM images of the SnSe nanocrystals painted on carbon cloth, and (e) TEM, (f) HRTEM image, and (insert) SAED pattern of SnSe nanocrystals.

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To obtain more information about the quality and the microstructure of as-prepared SnSe nanocrystals, a transmission electron microscope (TEM) image was recorded (Figure 2 e), in which SnSe nanocrystallites with sizes of several tens of nanometers can be clearly seen. The nanocrystals tend to aggregate to form porous structures with different pore sizes, which is beneficial for the transport of ions and reaction pathways. The high-resolution TEM (HRTEM) image of a single SnSe nanocrystal is shown in Figure 2 f. The marked interplanar d-spacing of 0.29 nm matches well with the (111) planes of orthorhombic SnSe, which is consistent with previous reports.48 The corresponding selected area electron diffraction (SAED) pattern taken from the SnSe nanocrystal (Figure 2 f insert) shows a polycrystalline ring pattern, which corresponds with polycrystalline SnSe with no additional crystalline impurities.

The electrochemical performance of the SnSe nanocrystal/carbon cloth electrodes in LIBs was evaluated by preparing coin-type half-cells that use lithium foil as the counter electrode. Figure 3 a shows the first two cycles galvanostatic discharge–charge profiles of the 3D electrode cycled at a current density of 200 mA g−1. A high initial discharge capacity of 1216 mAh g−1 and a reversible discharge capacity of 1100 mAh g−1 for the SnSe nanocrystal electrode can be obtained. The large discharge capacity during the first cycle may be attributed to the irreversible reaction between Li and SnSe, the alloying reaction between Li and Sn, and the formation of a solid electrolyte interface (SEI) layer. Cyclic voltammograms (CVs) of the electrode are shown in Figure S5. Figure 3 b shows a typical cycling performance at a current density of 200 mA g−1 of the integrated electrode up to 80 cycles. The reversible capacity of the SnSe nanocrystal/carbon fabric electrode is 676 mAh g−1 after 80 cycles, which is considerably higher than that of an electrode based on SnSe nanocrystals on copper foil (Figure S6) and superior to many other tin selenide electrodes.42, 46, 47

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Figure 3. Electrochemical performance of half cells: (a) charge–discharge curves and (b) cycling performance at a current density of 200 mA g−1, (c) rate performance at current densities varied from 100 to 2400 mA g−1, and (d) electrochemical impedance spectra over the frequency range from 100 kHz to 0.01 Hz of the SnSe nanocrystal/carbon cloth electrode.

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The SnSe nanocrystal/carbon fabric electrode also exhibits a greatly improved cycling response to a continuously varying current density although tin-based electrodes are generally suffering from sluggish kinetics for lithium storage. Figure 3 c shows the cycling performance of the integrated electrodes cycled at current densities ranging from 100 to 2400 mA g−1. While cycling at high rate of 2400 mA g−1, a capacity of 230 mAh g−1 is retained. The capacity reverst back to its initial value after 50 cycles once the discharge–charge rate was set back to 100 mA g−1, revealing an excellent reversibility and enhanced rate capacity of the SnSe nanocrystal/carbon cloth electrode. To gain insight into the electrochemical behavior of the SnSe/carbon fabric electrodes, AC impedance measurements of the integrated electrode and pure carbon fabric electrode were performed (Figures 3 d and Figure S7). The high-frequency semicircle corresponds to the constant phase element of the SEI film and the contact resistance and the semicircle in the medium-frequency region is assigned to the charge-transfer impedance and constant phase element of the electrode/electrolyte interface. It can be seen that the internal resistance, contact resistance, and charge-transfer resistance of the SnSe/carbon fabric electrode are approximately equal to those of the pure carbon fabric electrode with high electrical conductivity, which confirms that the 3D electrode can preserve the high conductivity of the anodes, thus greatly enhancing the rapid electron transport during electrochemical lithium insertion–extraction, resulting in a significant improvement of the electrochemical performance.

To eliminate the contribution of the carbon-fabric capacity to the total capacity of the integrated electrodes, CVs of the pure carbon fabric was recorded, as shown in Figure S8. Compared with the CVs of pure carbon fabric, the CVs of the integrated electrode do not show the redox peaks of the carbon fabric confirming that intercalation–deintercalation of Li ions into the carbon fabric did not occur. The cycling capacity of 35 mAh g−1 and areal capacity of 0.43 Ah cm−2 of the blank carbon fabric (Figure S9) further confirm that the carbon fabric does not contribute considerably to the capacity of the integrated electrode, illustrating that the SnSe nanocrystals provide the majority of the capacity of the integrated anode.

The morphologies of the SnSe/carbon fabric electrode after 20 charge–discharge cycles at current densities ranging from 800 to 2400 mA g−1 were studied. The shapes of the integrated electrodes are presented: the active anode material is tightly connected with the carbon fibers (Figure 4 a, c, and e). The charge carriers can migrate back and forth effectively and rapidly from the active material to the current collector because of the intimate contact between the active anode material and the conductive carbon fibers, which leads to high specific capacity and rate capability.49 With increase of the current densities, the porous structure initially formed between the SnSe nanocrystals gradually disappear (Figure 4 b, d, and f): a homogeneous film is formed on the surface of the carbon fibers through merging of ultrafine nanoparticles. Thus, the electrical conductivity and chemical stability of the active material is greatly enhanced, which results in the electronic transmission between nanoparticles not to be a limiting factor. The enhanced conductivity results in a significant increase in the utilization efficiency of the integrated electrodes and an improvement in the charge–discharge ability at high current densities. Batteries based on SnSe nanocrystals/carbon fabric anode and LiCoO2 cathode were also fabricated and investigated; the results are shown in Figure S10 in the Supporting Information.

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Figure 4. SEM images of SnSe/carbon fabric electrodes observed from the surface before (a, c, e) and after 20 charge–discharge cycles (b, d, f) at (a, b) 800, (c, d) 1600, and (e, f) 2400 mA g−1.

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Spray painting is a versatile process for the preparation of advanced functional films for energy storage. To demonstrate this, SnSe nanocrystal ink was sprayed onto flexible Au-coated PET wafer and evaluated as electrodes for flexible supercapacitors. The SEM image of the as-painted SnSe film on a PET wafer is shown in Figure S11. A stacked all-solid-state supercapacitor was assembled using by using a polymer-gel [polyvinyl alcohol (PVA)/KOH] electrolyte as both the ionic electrolyte and separator. The as-fabricated stacked supercapacitor with a working area of 10.0 cm2 is illustrated in Figure 5 a. Figure 5 b shows the CV curves of the device in the potential range of −0.3–0.3 V at scan rates of 20 to 200 mV s−1. All CV curves of the electrode are symmetrical, rectangular, and highly reversible at different scan rates, implying a formation of a double-layer capacitance. CV curves of the pure Au-coated PET wafer is shown in Figure S12 for comparison. The galvanostatic charge–discharge measurements at current densities between 20, 40, and 80 μA cm−2 (Figure 5 c) reveal a highly reversible charge–discharge process. Longer charge–discharge processes at low current densities indicate a considerably higher electric storage capacity. The cycling performance of the stacked supercapacitor at increased current densities was also recorded. Figure S13 displays the first five charge–discharge cycles of the device at a current density of 20 μA cm−2. The specific capacitances of the electrode evaluated from the discharge curves were 1800, 1460, 1124, 735, and 653 μF cm−2 at current densities of 20, 40, 60, 80, and 100 μA cm−2 (Figure 5 d), respectively, which is considerably higher than previously reported.50 The specific capacitance decreases with increase of current density, as the charging is limited by the diffusion of the ions.

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Figure 5. (a) Photographs of the flexible stacked supercapacitor, (b) CV curves measured at different scan rates, (c) galvanostatic charge–discharge curves, (d) specific capacitances at various current densities, (e) EIS spectra after a number of charge–discharge cycles and (f) leakage current of the staked all-solid-state supercapacitor.

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Electrochemical impedance spectroscopy (EIS) was also performed to evaluate the electrostability of the stacked all-solid-state supercapacitor during the charge–discharge processes. As can be seen in Figure 5 e, the EIS curves obtained in the frequency range from 0.01 Hz to 100 kHz were unchanged after several charge–discharge cycles, indicating that the capacitor behavior, contact and charge transfer resistance and Warburg impedance of the device degraded only slightly during the charge–discharge processes. The leakage current of the device was also evaluated and is shown in Figure 5 f. A low leakage current of only 2.5 μA reveals its good energy-storage performance. The flexibility of the stacked all-solid-state supercapacitor was also investigated by bending the device (Figure S14). No clear performance degradation was observed, demonstrating its high stability and excellent flexibility.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

We have prepared a 3D electrode for LIBs by spray-painting SnSe nanocrystal-based ink onto conductive carbon fabric that acted as current collector without any additives and binder. Such a current collector–electrode architecture leads to a considerably improved performance in comparison to that on a flat metal substrate, which is attributed to the lower impedance and the improved ion and electron mobility; this is attributed to an improved electroactivity of the material and the contact between current collector and active material. An assembled battery displays a reversible capacity as high as 676 mAh g−1 after 80 charge–discharge cycles at 200 mA g−1, which is superior to many other tin selenide electrodes. A flexible stacked all-solid-state supercapacitor based on the SnSe nanocrystal electrodes exhibits long cycle life and good rate capability during 2200 cycles at different current densities and high stability even when bent, allowing for many potential applications in facile, cost-effective, printable, and flexible energy-storage devices.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Preparation of SnSe nanocrystal-based ink: All solvents and chemicals used in the present work were of analytical grade. Citric acid, tin(II) chloride dihydrate (SnCl22 H2O), polyvinyl alcohol (PVA), NaOH, KOH, and Se powder were purchased from Sinopharm Chemical Reagent Co., Ltd (China). SnSe nanocrystals were synthesized through a sol–gel method performed at room temperature. Typically, NaOH (6 g) and Se powder (0.05 g) were dissolved in distilled water (15 mL) under mild stirring (solution A). Citric acid (10 g) and SnCl22 H2O (0.45 g) were dissolved in distilled water (10 mL) under mild stirring (solution B). Then, solution A was added to solution B and black SnSe nanocrystals were obtained. The residue in the as-prepared solution was removed by fast speed centrifugation, and then an appropriate amount of distilled water was added followed by ultrasonic vibration to obtain the SnSe nanocrystal-based ink.

Characterization: XRD patterns were obtained from a X-ray diffractometer (X’Pert PRO, PANalytical B.V., the Netherlands) with radiation of a Cu target (Kα, λ=0.15406 nm). The morphologies and size distributions of the samples were characterized by field-emission scanning electron microscopy (FESEM, Sirion 200), and TEM (Philips CM 200).

Electrochemical measurements: Electrochemical cycling tests were performed with coin-type half-cells (2032 size). The working electrode was prepared by painting the SnSe nanocrystal-based ink onto the 3D textile carbon fabric [WOS 1002, purchased from Phychemi (Hong Kong) Co. Ltd.] by using a home-made modified injector. Then, the electrode was dried in a vacuum oven at 80 °C overnight. The loading weight was 2–3 mg cm−2. The electrolyte was 1 M LiPF6 with 1:1 mixture of ethylene carbonate/ diethylene carbonate (EC/DEC), and the counter and reference electrodes were made from lithium foil (purchased from Shanghai Energy Lithium Industrial Co. Ltd.). The cycle-life of the cells was tested on LAND test system (CT2011A) at different discharge rates within a fixed voltage window of 3.0–0.01 V. The rate capability was evaluated by varying the discharge rate from 100 to 2400 mA g−1. Cyclic voltammetry was performed on CHI760D to examine the cathodic (reduction) and the anodic reaction (oxidation) in the voltage range of 3.0–0.01 V (versus Li+/Li) and at a sweep rate of 0.5 mV s−1. For the full battery, LiCoO2 was used as the counter electrode, a polymer–aluminum membrane (DNP, Shenzhen Kaiyuan cody Technology Co. Ltd.) was used to seal the batteries, and the voltage window was fixed at 1.0–3.8 V. The electrochemical impedance spectra were measured by using a CHI 760D electrochemical station (CH Instruments, Inc.) at frequency ranging from 100 kHz–100 mHz.

Electrochemical tests of flexible stacked all-solid-state supercapacitors: The PVA/KOH gel electrolyte was prepared as follows. KOH (3 g) was added to deionized water (60 mL), and then PVA powder (6 g) was added. The mixture was heated to 100 °C under stirring until the solution became clear. Fabrication of the working electrode was conducted by painting the SnSe nanocrystal ink onto a PET film purchased from Shenzhen Hongmei Film Co. Ltd., which was first coated with an Au film through electron-beam evaporation (ML-EB900). For the preparation of the stacked supercapacitor, two pieces of the electrodes were immersed in the PVA/KOH gel solution for 5 min and then pressed together after vaporizing excess water. The cyclic voltammogram (CV) and galvanostatic charge–discharge performances were measured with an electrochemical workstation (CHI 760D). CV tests were performed between −0.3 and 0.3 V at scan rates of 10, 20, 50, 100, 150, and 200 mV s−1, respectively.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

We acknowledge the support from the National Natural Science Foundation (61377033, 91123008), the 973 Program of China (2011CB933300) and the Program for New Century Excellent Talents of the University in China (grant no. NCET-11-0179). We thank the Analytical and Testing Center of Huazhong University Science & Technology and the Center of Micro-Fabrication and Characterization (CMFC) of Wuhan National Laboratory for Optoelectronics (WNLO) for sample measurements.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

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