Article pubs.acs.org/crystal
3D Hierarchically Assembled Porous Wrinkled-Paper-like Structure of ZnCo2O4 and Co-ZnO@C as Anode Materials for Lithium-Ion Batteries Arnab Kanti Giri,† Provas Pal,† Ramadoss Ananthakumar,‡ Muthirulandi Jayachandran,§ Sourindra Mahanty,*,⊥ and Asit Baran Panda*,† †
Discipline of Inorganic Materials and Catalysis, and Academy of Scientific and Innovative Research, CSIRCentral Salt and Marine Chemicals Research Institute, G.B. Marg, Bhavnagar 364002, Gujarat, India ‡ Faculty of Applied Energy System, Jeju National University, Jeju City, Jeju 690-756, Republic of Korea § Electrochemical Materials Science Division, CSIRCentral Electrochemical Research Institute, Karaikudi 623006, Tamil Nadu, India ⊥ Fuel Cell and Battery Division, CSIR-Central Glass & Ceramic Research Institute, Kolkata 700032, West Bengal, India S Supporting Information *
ABSTRACT: Three dimensional (3D) hierarchically assembled porous transition metal oxide nanostructures are promising materials for next generation rechargeable Li-ion batteries (LIBs). Here, the controlled synthesis of 3D hierarchically porous ZnCo2O4 “wrinkled-paper-like” structure constructed from two-dimensional (2D) nanosheets (∼20 nm thick) through calcination of corresponding mixed metal carbonate intermediate is presented. The mixed metal hydroxy-carbonate intermediate with wrinkled-paper-like structure has been synthesized by a novel organic surfactant and organic solvent free protocol at reflux condition using an aqueous solution of corresponding metal salt and ammonium carbonate. Active-inactive nanocomposites of Co-ZnO@C with similar wrinkled-paper-like morphology with varying carbon content, have also been synthesized through carbonation of hydroxyl-carbonate intermediate followed by calcination (under reducing environment). Calcination of the carbon coated mixed metal carbonate results in phase separated uniform Co metal and ZnO particles embedded on carbon matrix. The results demonstrate that incorporation of ∼23% carbon in the matrix significantly improves the performance as anode material in LIB by exhibiting high specific capacity and enhanced cycling performance. At a current density of 100 mAg−1, it shows an excellent initial specific capacity of 527 mAhg−1, which is maintained up to 50 cycles. In fact, a slight gradual increase in capacity with cycling has been observed.
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INTRODUCTION Rechargeable lithium-ion batteries (LIBs), being the most promising alternative power sources, are widely used in portable electronic products, electrical vehicles, and hybrid electrical vehicles and so-on.1,2 Consequently, numerous research efforts have been triggered to develop inexpensive and high-quality electrode materials which would possess high energy density and power density, as well as an enhanced cycle life. In recent years, transition metal oxides (e.g., Co3O4, MnO2, Fe3O4, etc.) have shown promise as anode materials for the next generation LIBs, as they exhibit a higher reversible capacity (400−1000 mAhg−1) compared to conventional graphite anode (372 mAhg−1).3−10 In this respect, use of nanoparticles has been found to be advantageous over their respective bulk counterpart, as the issues of electrode pulverization and loss of inter particle connectivity due to repeated volume expansion/ contraction during cycling could be overcome effectively.11,12 Here, the morphology of the nanoparticles plays a crucial role in determining the overall electrochemical performance. For example, particle agglomeration because of the high surface © 2014 American Chemical Society
energy of nanoparticles leads to capacity fade reducing the cycling efficiency.13 Similarly, a low tap density may result in a lower energy density. In this context, hierarchical porous structures, controlled association of nanoparticles toward specific microstructures with coexistence of meso- and micropores, could effectively solve the problem of agglomeration, buffer the volume expansion and also offer desirable mechanical properties, stability and ease for practical electrode fabrication.14−18 These porous nanosized building blocks with specific microstructures, large surface area, and high surface to volume ratio can offer reduced diffusion length, favorable kinetics, and high cycling performance.19−25 Among the various transition metal oxides explored, Co3O4 is one of the most promising candidates as anode material for LIBs for its high theoretical capacity (890 mAhg−1) and excellent cycle life.3−6,19,21,26−28 However, because of the high Received: February 25, 2014 Revised: May 23, 2014 Published: June 2, 2014 3352
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sugar as the carbonating agent followed by calcination under reducing environment. However, during calcination of the synthesized carbonaceous hydroxyl-carbonate intermediate resulted phase separated metallic Co and ZnO nanocrystalline particles impeded in carbon matrix, and afterward we will call them as Co-ZnO@C-x, where x is amount of sugar in precursor solution. Suitability of these Co-ZnO@C active-inactive materials as anode for lithium-ion batteries has also been studied.
cost and toxic nature of cobalt intensive efforts have been made to replace cobalt partially from Co3O4 by a cost-effective and eco-friendly metal without loss of its electrochemical properties. In this respect, cobalt based ternary metal oxide, ZnCo2O4 is very attractive as zinc is cheap, abundant and electrochemically active toward lithium.29−39 Similar to Co3O4, it is a normal spinel, where the Zn2+ occupying the tetrahedral sites and Co3+ occupying the octahedral sites. As the morphology and microstructures of nanostructure materials are crucial for their electrochemical performance, ZnCo2O4 with varying morphology, which includes 1D wire and their 3D assembly, tubes and 2D disk have been developed and their performances as anode material in LIBs have been evaluated.32−42 Qiu et al.,32 Hu et al.,38 and Li et al.33 reported the ethylene glycol directed glycolate intermediate based hydrothermal/solvothermal synthesis of ZnCo2O4 random flake, microsphere, and yolk-shelled ZnCo2O4 microspheres, respectively. Liu et al.36 synthesized ZnCo2O4 wires on carbon cloth under hydrothermal condition using NH4F and urea as precipitating agent. Du et al.39 synthesized ZnCo2O4 nanowires by microemulsion based soft templeting method using cetyltrimethylammonium bromide (CTAB), cyclohexane, and n-pentanol. Luo et al.34 reported the formation of ZnCo2O4 nanotubes by electrospinning of a ethanolic PVP solution of metal salts. All the synthesized ZnCo2O4 showed reasonably good performance as anode materials in LIB’s. However, most of the developed procedure used costly organic substrate as solvent, structure-directing, or precipitating agent and sometimes used sophisticated instruments. Thus, it is essential to develop a simple organicsubstrate- free method for the synthesis of 3D hierarchical assembly of porous ZnCo2O4 nanostructure. Synthesis through carbonate intermediate is an unique and most effective procedure for surfactant free synthesis of porous pure and mixed metal oxide nanostructures with specific morphologies and carbonate also able to give 3D assembled hierarchical structure in absence of any organic structure directing agent.43−47 During calcination of formed carbonate intermediate a large amount of CO2 and H2O is generated, which in turn creates a reasonable amount of pores in the structure. So far, to the best of our knowledge, there is no report for the synthesis of shape-selective ZnCo2O4 nanostructure through carbonate intermediate. Herein, we report the synthesis of 3D hierarchical assembly of porous ZnCo2O4 nanoflakes toward wrinkled-paper-like structure, via a facile structure directing agent-free method just in reflux condition followed by calcination, through the corresponding carbonate intermediate. Because of its high specific surface area, originating from 3D arrangement of nanoparticles toward micrometer-sized ordered flakes, it could increase the penetration efficiency of electrolyte and increase the electrode/electrolyte interface area for electrochemical reaction, and thereby, increase the energy density as well as rate capability. Therefore, suitability of the synthesized ZnCo2O4 nanoflakes toward wrinkled-paper-like structure as anode material for lithium-ion batteries has also been investigated. Also, recent studies revealed that metal oxide nanocomposites based on carbon, an inactive buffer material with soft nature and good electronic conductivity, increased its capacity and cycle stability, and its positive effect in anode material has been well demonstrated.12,13,36,37,48−51 So, attempts have also been made to synthesize active-inactive nanocomposite of ZnCo2O4@C with varying amount of carbon through carbonation of hydroxyl-carbonate intermediate using
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EXPERIMENTAL SECTION
Chemicals. Zinc nitrate and cobalt nitrate were purchased from Rankem (Ranbaxy), India. Ammonium carbonate (NH4HCO3 and NH2CO2NH4, 95.3%), Sugar was purchased from S. D. Fine-Chem. Limited, India. All the chemicals were of analytical grade and used as received without further purification. Water with a resistivity of 18 MΩ cm was used, obtained from a Millipore water purifier. Materials Synthesis. Synthesis of ZnCo2O4 2D Flakes. All syntheses were performed in reflux condition in ambient pressure In a typical synthesis, 2g of Zn(NO3)2 and 4g of Co(NO3)2 were dissolved in 150 mL of water. Separately, 16g of ammonium carbonate was dissolved in 150 mL of water. Then, the metal nitrate solution was added slowly to ammonium carbonate solution with constant stirring (500 rpm), which resulted in a clear solution. The clear solution was refluxed for 24 h in 500 mL of RB using an oil bath with constant stirring (500 rpm) at 110 °C. A gray colored precipitate was obtained. The precipitate was collected through centrifugation and washed thoroughly by water followed by ethanol. The resulting material was dried in an oven at 70 °C for 12 h and calcined at 500 °C for 4 h under air or nitrogen atmosphere. Synthesis of Co-ZnO@C 2D Flakes. The carbon incorporated materials were synthesized by the hydrothermal treatment of corresponding carbonate precursor in the presence of sugar at 190 °C, followed by calcination at 500 °C for 4h in 5%H2 in N2. In a typical synthesis, the synthesized carbonate of mixed Zn−Co carbonate, obtained from 2g of Zn(NO3)2 and 4g of Co(NO3)2 (as described before) dispersed in 100 mL of water and varying amount of sugar (2/4/6 g) was added to it and dispersed properly. Then, 33 mL of the resultant dispersion was poured into a 50 mL autoclave, sealed properly and was transferred to a preheated electric oven at 190 °C for 12 h. After 12 h, the system was cooled down to room temperature. The resultant precipitate was collected through centrifugation and washed thoroughly by water followed by ethanol. Then, the dried powder was calcined at 500 °C under 5%H2 in N2. Material Characterization. A Rigaku MINIFLEX-II (FD 41521) powder diffractometer was used to obtain powder X-ray diffraction patterns in the 2θ range of 10−80° using Cu Kα (λ = 1.54178 Å) radiation and Ni filter. Mettler-Toledo (TGA/SDTA 851e) instrument was used for the thermogravimetric analysis. An ASAP 2010 Micromeritics, USA, surface area measurement instrument was used for the nitrogen adsorption−desorption measurements. A Leo series 1430 VP scanning electron microscope (SEM) equipped with INCA was used to determine the morphology of samples. A JEOL JEM 2100 microscope was used to collect the transmission electron microscopic (TEM) images. X-ray photoelectron spectroscopy (XPS) was performed using a Multilab-2000 (Thermo-scientific UK) spectrometer using a monochromic MgKR X-ray source (1256 eV) with an analyzer pass energy of 10 eV Samples were mounted on SS sample holder with silver paint. Details of all the performed characterization procedures were described in our previous report.44 Raman spectra were obtained using a Renishaw InVia Reflex micro Raman spectrometer with excitation of argon ion (514 nm) lasers. The laser power was kept sufficiently low to avoid heating of the samples and spectra were collected with a resolution of 1 cm−1. Electrochemical Measurements. Electrochemical properties of the prepared composites were evaluated in 2032 type coin cells vs Li/ Li+. A typical electrode was prepared from a slurry consisting of ZnCo2O4 or ZnCo2O4@C nanocomposite powder (75 wt %), 3353
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acetylene black (15 wt %), and PVDF binder (10 wt %) in nmethylpyrrolidinone (NMP) solvent. The slurry was coated onto a 15 μm thick copper foil (current collector) and dried at 110 °C in an oven for 12 h. After the coated foil was pressed at 4.0 ton inch−2, circular disks of 15 mm in diameter were cut and used as electrode. Typical weight of the active material (excluding acetylene black and PVDF) was 3−6 mg. Coin cells were assembled with these electrodes using Li metal as counter as well as reference electrode, LiPF6 in EC/ DMC (1:2 vol %) as electrolyte and Celgard 2300 as separator within an argon filled glovebox (M’BRAUN, Germany) where the moisture and oxygen levels were both kept below 1.0 ppm. Gavanostatic charge−discharge measurements were carried out using an automatic battery tester (model BT2000, Arbin, USA) in the potential window of 0.01−3.0 V with a constant current density of 50 mAg−1 for the initial formation cycle and 100 mAg−1 for subsequent cycles for both discharge and charge. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 10 mHz−1.0 MHz at open circuit potential with an AC amplitude of 10 mV using a Galvanostat-Potentiostat (PGSTAT 302N, Autolab, The Netherlands).
Figure 1. XRD patterns of pure ZnCo2O4 calcined under air (a) and under N2 (b). XRD patterns of carbon incorporated Co-ZnO@C-2 (c), Co-ZnO@C-4 (d), and Co-ZnO@C-6 (e) calcined at 500 °C under N2.
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RESULTS AND DISCUSSION Synthesis and Characterization of ZnCo2O4. The presently developed synthesis methodology is based on the formation of corresponding metal carbonate intermediate, in a simple organic structure directing agent free method just by refluxing the aqueous precursor solution containing metal nitrate and ammonium carbonate at 110 °C, followed by calcination at 500 °C under air/N2. During calcination, a large amount of CO2 and H2O generates from the metal carbonate intermediate formed, which in turn resulted in pores in the metal oxide structure.43−47 The carbon incorporated materials were synthesized via the formation of carbon on the surface of corresponding carbonate intermediate through decomposition of sugar under hydrothermal condition52,53 and finally the carbon incorporated materials were obtained on calcination at 500 °C under reducing (5% H2 in N2) atmosphere. The XRD patterns of the as-synthesized materials reveal the formation of mixed hydroxyl carbonate of Zinc and cobalt under reflux condition (Figure S1, Supporting Information). The XRD pattern of the calcined samples (at 500 °C, irrespective of calcination environment N2/air) can be indexed to cubic ZnCo2O4 with a spinal structure (JCPDS 23−1390). Absence of any diffraction peak of parental carbonate intermediate and phase separated cobalt oxide or ZnO, confirmed the complete decomposition of intermediate and formation of phase pure ZnCo2O4 (Figure 1a, b). The average crystallite sizes of both the samples calcined under air\nitrogen, that is, ZnCo2O4−O2 and ZnCo2O4−N2, calculated from X-ray line broadening using Scherrer’s equation, were found to be almost identical (∼17 nm). However, the XRD patterns of the calcined carbon incorporated samples were not identical with corresponding pristine ZnCo2O4 (synthesized without sugar). The diffraction patterns can be indexed to the mixed metallic Co (Cubic, JCPDS 01-071-4651) and ZnO (Wurtzite, JCPDS 36-1451) phase and indicate the phase separation during calcination (Figure 1c, d, e). Thus, the carbon incorporated samples were termed as Co-ZnO@C-X, where X = 2, 4, 6, that is, amount of sugar in precursor solution. In XRD pattern, the broad peak around 22° can be attributed to the carbon and revealed the presence of carbon in the matrix. It was observed that with gradual increase in the amount of carbon in the matrix, the crystallite size of both the ZnO and metallic Co was decreased. As for example, the crystallite size of Co in Co-ZnO@C-2 is
∼30 nm, whereas the size of Co in Co-ZnO@C-6 is 8 nm. At the same time, the peak intensity at 2θ = 22° was gradually increased, implying the gradual increment of carbon content in the matrix. Figure 2 represents the SEM images of the synthesized carbonate intermediates as well as corresponding calcined pristine and cabon incorporated materials. SEM image of the carbon free mixed ZnCo-carbonate intermediate evidenced the formation of 2D flake and the flakes are assembled toward 3D hierarchical wrinkled-paper-like structure (Figure 2a). The thickness of the individual flakes is in the range of ∼20 nm. SEM images of carbon incorporated as synthesized carbonate intermediate also showed almost identical morphology as pristine materials (Figure 2b, c, d). However, in the SEM image of Co-ZnO@C-6, presence of some phase separated spherical carbon ball was identified due to presence of excess sugar (Figure 2d). Such kind of carbon spheres is known to form sugar in identical reaction conditions.49,50 Upon calcination, the as-synthesized pure and carbon incorporated carbonate intermediate resulted into the corresponding oxides without noticeable effect on morphologies; the resulted flakes being arranged toward wrinkled-paper-like structures similar to the parental carbonate structure (Figure 2e, f). However, on calcination the surface of flakes became porous. Figure 3 shows the TEM images of calcined ZnCo2O4. TEM images confirmed the formation of 2D flakes with porous structure (Figure 3a and b). Pores are irregular and varied in the range of 20−50 nm. Actually, the pores are formed by some irregular shaped nanoparticles with an average particle size of 15 nm. In the HR-TEM image, the observed lattice fringes with interplane spacing of 0.29 and 0.25 nm can be attribute to the (229) and (311) planes of spinal ZnCo2O4 phase (Figure 3c). Further, the obtained diffraction ring patterns, in the selected area electronic diffraction (SAED), can be attributed to the polycrystalline in nature and all the rings can be indexed with the spinel ZnCo2O4 phase (JCPDS 42-1467) (Figure 3d). All the TEM results are in agreement with the XRD result. High angle annular dark field experiments (HADF) and STEM-X-ray energy dispersive spectroscopy (XEDS) mapping of elements (Zn Co and O) (Figure S2, Supporting Information) shows that elements were uniformly distributed, no island of a specific element was observed and confirmed that the adopted 3354
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Figure 2. SEM images of as-synthesized pristine (a) and carbon incorporated (b−d) [ZnCo2O4@C-2 (b), ZnCo2O4@C-4 (c), ZnCo2O4@C-6 (d)] ZnCo-carbonate intermediate, corresponding calcined (N2 at 500 °C/4 h) pristine ZnCo2O4 (e) and ZnCo2O4@C-4 material (f).
Figure 4. TEM and HR-TEM images of the synthesized ZnCo2O4@ C-4 (a, c, e, f), ZnCo2O4@C-2 (b), and ZnCo2O4@C-6 (d).
portion of flake, the size of these crystalline particles also decreased gradually with the increase in the carbon content (Figure 4f and Figure S3, Supporting Information). Inter plane distances of lattice fringes in HR-TEM images confirmed that the crystalline large and small particles are nothing but the phase separated metallic Co and ZnO, respectively (Figure 4e, f). XEDS mapping of elements (Zn, Co, and O) also supports the homogeneous distribution of metal ions in the carbon matrix (Figure S4, Supporting Information). Further, presence of a number of overlapped electron diffraction rings (difficult to index the individual ring) in the selected area electronic diffraction (SAED) pattern confirmed the presence of more than one component and the ring pattern also established the polycrystalline nature(Figure S5, Supporting Information). The TEM results are in well agreement with the XRD result. In the TG curve of the as-synthesized carbon free ZnCocarbonate intermediate (Figure S6a, Supporting Information), the initial ∼4% weight loss in the temperature range of 30−230 °C corresponds to the removal of adsorbed water and trapped carbon dioxide, and ammonia. In the second and final step, ∼21% weight loss in the temperature range of 230−500 °C can
Figure 3. Panels a and b represent the TEM images of ZnCo2O4 synthesized in absence of sugar calcined at 500 °C in air. Panels c and d represent the corresponding HR-TEM image and electron diffraction patterns, respectively. The inset in panel c presents the corresponding FFT pattern.
procedure is appropriate for the compounds with two or more metals. The low resolution TEM images of the calcined carbon incorporated Co-ZnO@C samples confirmed the flake structure (Figure 4a). However, the flakes are made of dark particles and thin sheet. Magnified image depict that the size of the dark particles varied in the range of 7−35 nm with the variation of carbon content in the matrix (Figure 4b−d). With increase in the carbon content, the particles became uniform, and decreased the size and number density, gradually. It is interesting to note that with increasing carbon in the matrix, the thin sheet portion became gradually denser. HR-TEM image confirmed the presence of some very small crystalline particles (2−8 nm) and carbon in the nearly transparent thin sheet 3355
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corresponding pore size distribution graphs (inset Figure 5b− d) revealed that the presence of micro (
