Correlation between Microstructure and Electrochemical Behavior of

Dec 23, 2013 - An ionothermal strategy is developed to synthesize hydrotalcite-like α-Co(OH)2 sheets. The structure and interlayer chemistry of α-Co...
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Correlation between Microstructure and Electrochemical Behavior of the Mesoporous Co3O4 Sheet and Its Ionothermal Synthesized Hydrotalcite-like α‑Co(OH)2 Precursor X. Ge, C. D. Gu,* X. L. Wang, and J. P. Tu State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: An ionothermal strategy is developed to synthesize hydrotalcite-like α-Co(OH)2 sheets. The structure and interlayer chemistry of α-Co(OH)2 are facilely manipulated, thus determining its pseudocapacitive performance. Significantly, a very large space of 11.2 Å in (003) plane induced by various intercalated species is found in the ionothermal synthesized α-Co(OH)2 obtained at 210 °C (210-α-Co(OH)2), which delivers a better pseudocapacitive performance. Annealing the synthesized α-Co(OH)2 generates mesoporous Co3O4 sheets or nanoparticles, which depends on the structure of the precursors. More defects in the α-Co(OH)2 precursor provide larger driving force during the nucleation of Co3O4 nanocrystals, thus reducing the grain size. The architecture of the 210-Co3O4 derived from 210-α-Co(OH)2 is featured as selfsupporting mesoporous nanosheets composed of ∼5 nm sized grains. The fine grains and ultralarge specific surface area of ∼113.5 m2 g−1 in the 210-Co3O4 can trigger the formation of CoO intermediate during lithium ion insertion into the Co3O4. The 210Co3O4 exhibits superior cycling performance as electrode for lithium ion battery and supercapacitor over the other Co3O4 samples due to its nanocrystallite constructed mesoporous sheet structure, which has a higher strain accommodation capability. The underlying mechanism discussed in this work provides guidance on structural design of advanced energy storage materials. often used to generate two-dimensional Co3O4.6,12−14 More interestingly, under suitable conditions, mesoporous structures can be generated during annealing Co(OH)2 to produce Co3O4 because of intrinsic lattice contraction. Therefore, a systematic study on the annealing process to manipulate the structure of Co3O4 would be more promising. Lamellar cobalt hydroxide has found applications in catalysis, adsorbents, and high-performance electrochemical materials.15−17 Cobalt hydroxide can crystallize in two polymorphs, α and β. The β-form is a stoichiometric phase brucite-like structure with Co2+ occupying octahedral sites. For α-phase Co(OH)2, it consists positively charged layers with anions (CO32−, NO3−, Cl−, etc.) and water molecules residing in the gallery to restore charge neutrality.18 In principle, α-Co(OH)2 displays more interesting interlayer chemistry and has higher specific capacity as a supercapacitor electrode.19 However, the α-Co(OH)2 is metastable and is difficult to be synthesized as they age rapidly to β form in strong alkali. Liu reported that for a very dilute (0.01 M) CoCl2·6H2O solution (9:1 mixture of water and ethanol as solvent) α-Co(OH)2 and β-Co(OH)2 can be selectively generated under hexamethylenetetramine hydrolysis with or without NaCl2 respectively.20 The generation of

1. INTRODUCTION Since the rise of graphene, two-dimensional nanosheet-like materials have attracted enormous attention.1 It is generally accepted that proper design of porous structure can facilitate wettability of electrolyte, alleviate volumetric change, and provide fast mass transport pathway, thus improving the electrode performance.2,3 Meanwhile, for heterogeneous polyphase reaction in electrochemical system, decrepitating is a major reason for capacity fading. Theoretical models indicate there is a critical particle size below which fractures will not propagate further.4 Nanostructured Co3O4 has drawn worldwide attention due to its promising application in lithium ion battery and supercapacitor.5 Lou and co-workers have done numerous works which demonstrated how proper design of nanoporous architecture could enhance the electrochemical performance of Co3O4.6−9 In particular, ultrathin mesoporous Co3O4 nanosheets array on Ni foam showed impressive capacitance when used in supercapacitor (2735 F g−1 at 2 A g−1).9 This work demonstrated two-dimensional porous design should be favorable. Fabricating two-dimensional Co3O4 directly is challenging because of its spinel structure.10 Hao developed a strategy to synthesize Co3O4 nanosheets based on etching CoAl alloy.11 But in most cases, Co-based intermediate compounds, like cobalt carbonate (CoCO3), cobalt hydroxide (Co(OH)2), and cobalt−carbonate−hydroxide (Co(CO3)0.5(OH)·0.11H2O), are © 2013 American Chemical Society

Received: December 5, 2013 Revised: December 21, 2013 Published: December 23, 2013 911

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respectively and then dried at 70 °C under vacuum. The dried products will be denoted as 150-α-Co(OH)2, 180-α-Co(OH)2, and 210-α-Co(OH)2, respectively. Further annealing the obtained α-Co(OH)2 at 300 °C can lead the formation of Co3O4. The Co3O4 annealed from different precursors will be denoted as 150-Co3O4, 180-Co3O4, and 210-Co3O4, respectively. 2.3. Characterization. Characterization of Materials: Structure and morphology of the obtained materials were characterized by X-ray diffraction (XRD, RigakuD/Max-3B), scanning electron microscope (SEM, Hitachi S-4800), and transmission electron microscope (TEM, FEI, Tecnai G2 F20 at kV). The surface area of the Co3O4 was determined by BET (Brunauer−Emmett−Teller) measurements using a AUTOSORB-1-C (QUANTACHROME). FTIR was performed on a Bruker spectrometer (TENSOR 27). Electrochemical Characterization of α-Co(OH)2 and Co3O4: The working electrodes were prepared by a slurry-coating procedure. The slurry consisted of 85 wt % α-Co(OH)2 or Co3O4, 10 wt % acetylene black (AB), and 5 wt % poly(vinylidene fluoride) (PVDF) dissolved in N-methylpyrrolidinone (NMP) and was incorporated on nickel foam. To prepare electrode for assembling coin type lithium cell, the nickle foam is a round piece with diameter of 12 mm. To prepare electrode used for supercapacitor, coating area was typically 1 cm × 1 cm. The coating masses are both about 2.0 mg. After being dried at 90 °C for 24 h, the coated nickel foam were pressed under a pressure of 10 MPa. To study lithium storage behavior, coin-type cells (CR 2025) were assembled in an argon-filled glovebox with metallic lithium foil as both the reference and counter electrodes, 1 M LiPF6 in ethylene carbonate (EC)−dimethyl carbonate (DME) (1:1 in volume) as the electrolyte, and a polypropylene (PP) microporous film (Cellgard 2300) as the separator. To test the performance of the as-synthesized α-Co(OH)2 and Co3O4 as supercapacitor active material, a three-electrode electrochemical cell containing 2 M KOH aqueous solution as electrolyte was used. A Pt plate was used as the counter electrode. Hg/HgO immersed in 1 M KOH aqueous solution was connected with a salt bridge as reference electrode. Cyclic voltammetry (CV) and potentiometry were performed using CHI660e electrochemical workstation (Chenhua, Shanghai) at room temperature. The galvanostatic charge−discharge tests were conducted on a LAND battery program-control test system at room temperature ranging from 0.02 to 3 V. Galvanostatic intermittent titration technique (GITT) experiments were also performed on LAND battery program-control test system by charging/discharging the cells for 2 h at a rate of C/20 of the theoretical capacity of Co3O4 (890 mA h g−1) and relaxed for 4 h. Electrochemical impedance spectrum (EIS) measurements were carried out using a CHI660D electrochemical workstation over a frequency range from 100 kHz to 10 mHz. Before collecting the EIS data of lithium ion battery, the cells were run for 10 cycles, discharged, and then rested for 12 h until the open circuit potentions were constant. The impedance data presented in the figure were multiplied with the mass of active material used in the tested cell.

α-Co(OH)2 was attributed to the addition of excess of salts (anions) that delayed the α to β transformation. However, the paper did not point out whether α-Co(OH)2 could still be generated when cobalt ion is in a larger concentration. Xu and coworkers carried out systematic study into the interconversion of α-Co(OH)2 and β-Co(OH)2. They concluded that atmosphere, addition time of cation, and aging time play important roles in controlling the phase of Co(OH)2.21 Though researchers have devoted numerous efforts in developing synthetic routes and elucidating structural characteristics of Co(OH)2, the research into α-phase Co(OH)2 still remains to be a hot topic at both fundamental scientific and application level owing to the special interlayer chemistry.17 Therefore, it leaves us research opportunities to develop a facile process to produce α-Co(OH)2 with designed structure and interlayer chemistry on a large scale. Our previous work developed a versatile ionothermal protocol based on choline chloride (ChCl)/urea (CU) based deep eutectic solvent (DES) for the synthesis of nanostructured nickel compounds as energy storage materials.22 The CU system is a relatively new class of ionic liquid based on eutectic mixtures of choline chloride with a hydrogen bond donor species.23 The ionothermal strategy has demonstrated that the CU system provides unique advantage for generating nanostructured material. The attempt also found that α-type Ni(OH)2 was produced instead of more thermodynamically stable β type. Considering the similarity between nickel and cobalt element as well as more interesting interlayer chemistry of α-Co(OH)2, in this work we have further developed the ionothermal strategy to generate α-Co(OH)2 and its annealing products, i.e., Co3O4 with nanostructures. Significantly, unlike the previous study on α-Ni(OH)2, the morphology and interlayer chemistry of the hydrotalcite-like α-Co(OH)2 can be facilely manipulated via the proposed ionothermal method. A very large plane distance of about 11.2 Å is obtained in the α-Co(OH)2 sheets. Furthermore, it is found that the structure and interlayer chemistry of α-Co(OH)2 play an important role in controlling the microstructure of the mesoporous Co3O4 sheets. As one of the most promising demonstrations of the Co-based compounds,3 we investigated electrochemical performance of the α-Co(OH)2 and Co3O4 nanostructures as energy storage materials in supercapacitors and lithium ion batteries. The underlying mechanism correlating microstructure and electrochemical performance of the Co-based compounds is discussed systematically.

2. EXPERIMENTAL SECTION 2.1. Preparation of Ionic Liquid. ChCl (AR, Aladdin) and urea (AR, Aladdin) were used as received. The ChCl/urea mixture-based DES was formed by stirring the two components in a molar ratio of 1 ChCl: 2 urea at 80 °C, until a homogeneous colorless liquid was formed. The ChCl/urea mixture-based ionic liquid will hereafter be donated as CU for convenience. 2.2. Preparation of α-Co(OH)2 and Co3O4. As-purchased CoCl2·6H2O was dissolved into CU to get a 0.1 M CoCl2:CU solution. The purple CoCl2 immediately turned dark blue, indicating the occurring of dehydration and complexation process. A 50 mL aliquot of 0.1 M CoCl2:CU solution was transferred into a three-neck flask and heated for 40 min at different temperatures (150, 180, and 210 °C) under magnetic stirring. The mouth of the flask was kept open. Afterward, 100 mL of distilled water (kept at about 10 °C) was quickly poured into the hot solution under vigorous stirring. After 60 s, the flask was taken out and cooled in an ice bath. The product was washed and centrifuged with distilled water and methanol

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of α-Co(OH)2. We have noticed that the urea portion in the CU would decompose to release amino complexes above ∼140 °C in an open reaction vessel.24 By controlling the release process of amino complexes, reduced graphene oxide and α-Ni(OH)2 could be obtained.22,25 In this case, the reaction temperature and the water addition 912

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process are artificially controlled as illustrated in the Experimental Section. Interestingly, the product of α-Co(OH)2 is obtained as revealed by XRD patterns in Figure 1.

Figure 1. XRD patterns of α-Co(OH)2 synthesized at 150 (a), 180 (b), and 210 °C (c).

The characteristic asymmetric peak at 2θ of 34° is due to “two-dimensional reflection” of the turbostratically stacked unit cell.26 It indicates that the obtained hydroxides all have hydrotalcite-like α type phases which can be indexed to a triplelayered hexagonal cell according to reported research.27 Compared to brucite β-type Co(OH)2, α-phase Co(OH)2 exhibits higher capacity when used in alkali battery. Previous molecular dynamic simulation and theoretical study indicate that with hydrogen bond interaction urea inserts into the ChCl lattice, and the chloride ion is dislocated.28,29 The CU system should thus provide a chlorine-active environment. We can rationally attribute the formation of hydrotacite-like α-Co(OH)2 to the chloride-active nature of CU and the specially designed ionothermal parameters (short time, open-air atmosphere). The XRD pattern of 210-α-Co(OH)2 has several extra peaks (indicated with blue arrow in Figure 1). Though the crystallinity is not perfect, we can observe two prominent extra peaks. The corresponding d space of the peak at 2θ = 7.9° (d = 11.2 Å) is twice that of the peak at 15.8° (d = 5.6 Å). So we can index them as (003) and (006), respectively (marked with blue fonts in Figure 1c). The expansion of (003) plane of 180-Co(OH)2 to 210-Co(OH)2 is 4 Å (7.2 to 11.2 Å). To identify the intercalated species, FTIR spectra of the α-Co(OH)2 samples are provided in Figure 2a. As highlighted with vertical dashed lines in Figure 2a, all the three α-Co(OH)2 show adsorption bands of ν(O−H) at 3624 cm−1, ν(N−H) at 3400 cm−1, ν(NCO) at 2253 cm−1, δ(H−O−H) at 1632 cm−1, Cl− related adsorption 1350 cm−1, δ(Co−O−H) at 636 cm−1, and ν(Co−O) at 462 cm−1. The Cl− related adsorption band is also observed in Cl− inserted α-Co(OH)2 prepared by chemical precipitation.30 The common adsorption bands shared by all the three α-Co(OH)2 indicate that H2O, Cl−, and NCO− insert into all the three α-Co(OH)2. The FTIR spectra of 150-Co(OH)2 and 180-Co(OH)2 are basically identical, except the shape of ν(O−H) adsorption band at 3624 cm−1. The ν(O−H) at 3624 cm−1 adsorption band shows a red-shift and become more broadened when the reaction temperature becomes higher, indicating the α-Co(OH) 2 synthesized at higher temperature had more hydrogen bonds formed in the interlayers. However, the FTIR spectrum of 210-α-Co(OH)2 has several distinctive features, especially at

Figure 2. (a) FTIR spectra of 150-α-Co(OH)2, 180-α-Co(OH)2, and 210-α-Co(OH)2. (b) Magnified FTIR spectrum of 210-α-Co(OH)2 within 1900−1300 cm−1.

wavenumber ranging from 1900 to 1300 cm−1. A magnified FTIR spectrum of 210-α-Co(OH)2 is presented in Figure 2b. Within this range, the peak at 1605 cm−1 is assigned to deformation vibration of H2O. It also shows a red-shift which is consistent with ν(O−H) at 3624 cm−1. The 210-α-Co(OH)2 sample have several better resolved adsorption peaks compared with 150-αCo(OH)2 and 180-α-Co(OH)2 which only show a sawtooth shape from 1900 to 1300 cm−1 (Figure 2a). They are indexed as ν(CO) at 1746 cm−1, ν(CON) at 1690 cm−1, ν(C−N) at 1477 cm−1, and a CO32− related peak at 1403 cm−1. The CO32− ion should have another adsorption band at around 870 cm−1. It is indeed identified for 210-α-Co(OH)2 (indicated with a blue arrow in Figure 2a). The ν(C−N) at 1477 cm−1 has a higher wavenumber than usual. It might be intermediate between ν(C−N) and ν(CN) which is a partial double bond character.31 Combining the XRD and FTIR analysis, the structure and interlayer chemistry of 150-α-Co(OH)2 and 180-Co(OH)2 are similar, except a little difference in terms of hydrogen bonding. But 210-α-Co(OH)2 is very distinct from them. The water molecule and NCO− decomposed from urea inserted into all ionothermal synthesized α-Co(OH)2. The different XRD pattern of 210-α-Co(OH)2 (Figure 1c) is due to more intercalated water molecule, CO32−, and some other complex species containing CO, OC−N, and C−N bonds. Previous study indicated when heating urea in an open reaction vessel above 190 °C, ammelide, biuret, cyanuric acid, and other compounds might be generated. These species might contribute to the peaks ranging from 1900 to 1300 cm−1 in FTIR (Figure 2a). EDAX (see Figure S1d−f in the Supporting Information) tests also confirm the existence of chlorine and nitrogen elements in all the three α-Co(OH)2. Several models have been proposed to explain the mechanism of intercalation. Among them, “hydroxyl vacancies” 913

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Figure 3. SEM images of 150-α-Co(OH)2 (a), 180-α-Co(OH)2 (b), and 210-α-Co(OH)2 (c).

and “mixed valent state” have shown some validity.32,33 Compared with hydrotalcite (layered double hydroxide) LDH structure which has been verified by Rietveld XRD refinement,34 the debate about interlayer chemistry of α-Co(OH)2 still lasts since the structure of α-Co(OH)2 samples were always of poor crystallinity. Recently, single-crystal hexagonal α-cobalt hydroxide was prepared by homogeneous precipitation.18 It was calculated through Rietveld XRD refinement that neither hydroxyl vacancy model or mixed valency model fit better than a “tetrahedral Co2+ coordination model” proposed by the author.18 However, the crystallinities of α-Co(OH)2 synthesized in our work are not as perfect and the side reactions happening at temperature higher than 150 °C in a CU system are too complicated. We cannot rule out the possibility that mixed-valent cobalt ions or hydroxyl vacancies might exist. Taking 1.7 Å as the radius of a water molecule and 4.6 Å as the interlayer space of brucite-like phase,21,26 we found that the 150-α-Co(OH)2 and 180-αCo(OH)2 show a smaller interlayer (7.2 Å) space as expected (7.6 Å corresponds to the close-packed water molecules and 8 Å to the non-close-packed water molecules).26 The hydroxyl deficiency model predicts a relatively reduced interlayer space due to the direct coordination between the intercalated anions and Co2+ cations. We deduce that the intercalation of 150-αCo(OH)2 and 180-α-Co(OH)2 occur because of hydroxyl vacancies. This deduction is verified by their pink color (see Figures S1a and S1b) because the hydroxyl deficiency model predicts a pink color for the materials due to the presence of exclusively octahedral Co2+ ions.35 The increment of interlayer space from 150-α-Co(OH)2 to 210-α-Co(OH)2 is 4 Å. Meanwhile, the color turned to blue (see Figure S1c). The expansion of 210-Co(OH)2 along the c-axis can be attributed to the intercalation of a second layer of water molecule accompanied by the occurrence of tetrahedral Co2+. However, more

experimental evidence is needed to analyze the exact interlayer structure of the ionothermal synthesized α-Co(OH)2. Figure 3a−c gives the SEM images of as prepared 150-αCo(OH)2, 180-α-Co(OH)2, and 210-α-Co(OH)2, respectively. The α-Co(OH)2 all exhibit nanosheets morphology, and the size of the nanosheets follows 210-α-Co(OH)2 > 180-α-Co(OH)2 > 150-α-Co(OH)2. It seems that the nanosheets tend to bend to some extent. α-Co(OH)2 has a hydrotalcite-like structure. Lots of stacking faults occur among the randomly oriented slabs. The bending might be due to the characteristic of the turbostratic unit cell with randomly oriented slab of α-Co(OH)2. Figure 4a,c,e gives a TEM image of the α-Co(OH)2 synthesized at 150, 180, and 210 °C, respectively. The three kinds of α-Co(OH)2 all comprise of stacked sheets. The corresponding SAED patterns as the insets of Figure 4a,c,e are given in the negative forms to be seen more clearly. The diffused rings indicate the α-Co(OH)2 samples are all polycrystalline with low crystallinity. The most prominent extra XRD peak of α-Co(OH)2 locates at about 2θ = 7.9° (Figure 1c). The corresponding facet should have a space of 11.2 Å, which should produce a ring with very small radius in the SAED pattern. The calculated location of the ring was marked with a red circle in the inset of Figure 4e. The ring was covered by the transmission beam. Furthermore, the low crystallinity make the SAED rings very diffused. We could not observe extra SAED ring for the 210-α-Co(OH)2 sample. Figure 3b,d,f shows the HRTEM images of 150-α-Co(OH)2 180-α-Co(OH)2, and 210-α-Co(OH)2. For all the examined areas, several discrete lattice fringes distribute on the sheets as marked with white color. The HRTEM images confirm the low crystallinity of all the synthesized hydrotalcite-like Co(OH)2. 3.2. Pseudocapacitive Behavior of α-Co(OH)2. Figure 5 shows the CV curves of 210-α-Co(OH)2 (see CV curves of 150-α-Co(OH)2 and 180-α-Co(OH)2 in Figure S2). There are 914

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Figure 4. TEM images of 150-α-Co(OH)2 (a), 180-α-Co(OH)2 (c), and 210-α-Co(OH)2 (e) with SAED pattern in the insets. HRTEM images of 150α-Co(OH)2 (b), 180-α-Co(OH)2 (d), and 210-α-Co(OH)2 (f).

Figure 5. Pseudocapcitive behavior of the synthesized α-Co(OH)2. (a) CV curve of 210-α-Co(OH)2. (b) Cycling performance of α-Co(OH)2 where the specific capacitances are derived from chronopotentiometry curves of Figure S2c−e.

two redox peaks which can be assigned to the following Faradaic reactions of Co(OH)2 in KOH aqueous solution: Co(OH)2 + OH− → CoOOH + H 2O + e− −

CoOOH + OH → CoO2 + H 2O + e

−

peak at 0.08/0.18 V of 210-α-Co(OH)2 (Figure 5a) is larger than that of 150-α-Co(OH)2 (Figure S2a) and 180-α-Co(OH)2 (Figure S2b), indicating a better redox kinetic of eq 1 for 210α-Co(OH)2. We calculated the specific capacitance of the α-Co(OH)2 from the chronopotentiometry curve using the method

(1) (2)

The shapes of the CV curves indicate that the α-Co(OH)2 show pseudocapacitive behavior. The integral area of the redox

C = I Δt /M ΔV 915

(3)

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where C (F g−1) is specific capacitance. I (mA) represents discharge current. M (mg), ΔV (V), and Δt (s) designate the mass of active material, potential drop during discharge, and total discharge time, respectively. According to chronopotentiometry curves of the α-Co(OH)2 (Figure S2c−e), the specific capacitances of 150-Co(OH)2, 180-Co(OH)2, and 210-Co(OH)2 at 2 A g−1 are 175, 202, and 410 F g−1, respectively. The cycling performance shown in Figure 5b also shows that 210-αCo(OH)2 shows better supercapacitor performance. Hu and co-workers synthesized Cl−, NO3−, CH3COO−, or SO42− intercalated α-Co(OH)2. The specific capacitance decreases with the order of Cl−, NO3−, CH3COO−, and SO42−, which is consistent with proton affinity of the intercalated anion.30 However, the morphology and cobalt percentage are very different among the samples. Assuming that the molar ratios of intercalated anions are similar, the formula weight of SO42− intercalated α-Co(OH)2 will be larger, which might decreased the specific capacitance. In this work, the ionothermal synthesized α-Co(OH)2 provide better sample to minimize the influence of morphology and proton affinity of intercalated species. The FTIR pattern (Figure 2) shows that the 210-α-Co(OH)2 have more intercalated anions and water molecule. The additional mass of intercalated anions did not decrease the specific capacitance. The type of additional intercalated species of 210-α-Co(OH)2 (species containing CO, OC−N, and C−N bonds) does not have significantly larger proton affinity than Cl− and OCN−. Meanwhile, the twodimensional morphology of all the three α-Co(OH)2 should all provide fast mass transport path way. So we believe the larger space between positively charged cobalt layers should be the major reason for the enhanced specific capacitance. 3.3. Structure and Morphology of Co3O4. The structure of the α-Co(OH)2 plays a key role in determining the structure of its annealed product. Annealing the as-prepared α-Co(OH)2 leads to the formation of black powders. Figure 6 shows the XRD

lead to more defects that can act as favored sites for Co3O4 nucleation during annealing, resulting in a faster nucleation process. The obtained Co3O4 will thus have a smaller grain size. Figure 7a−c shows the morphology of 150-Co3O4, 180-Co3O4, and 210-Co3O4 characterized with SEM. The 150-Co3O4 is consisted of collapsed nanoparticles while the 210-Co3O4 shows a self-supporting mesoporous two-dimensional structure. The 180-Co3O4 morphology makes a compromise between them. The transformation of α-Co(OH)2 to Co3O4 is accompanied by intrinsic crystal contraction.36 To obtain Co3O4 nanoparticles and mesoporous nanosheets, the size of ionothermal synthesized α-Co(OH)2 precursor plays a key role. Low-magnification TEM images of 150-Co3O4, 180-Co3O4, and 210-Co3O4 are provided in Figure 8a,c,e. Judging from the contrast of Figure 8a,c, the nanoparticles of 150-Co3O4 and 180-Co3O4 collapse and stack together to form a mesoporous structure. The 210-Co3O4 still maintains two-dimensional structure. The insets of Figure 8a,c,e present SAED patterns. The rings in the SAED patterns can all be indexed to fcc Co3O4. HRTEM images of 150-Co3O4, 180Co3O4, and 210-Co3O4 are given in Figure 8b,d,f, respectively. A fast Fourier transform (FFT) filtered lattice image is given in the inset of Figure 8b as an example to demonstrate the good crystallinity of an individual nanoparticle. The grain size of 210-Co3O4 nanoparticles is about 5 nm (Figure 8f), and those of 150-Co3O4 and 180-Co3O4 are both more than 10 nm. The smaller size of 210-Co3O4 primary particle is consistent with its broadened XRD pattern (Figure 6c). To characterize the mesopores, a nitrogen adsorption− desorption technique was used. Figure 9 gives pore size distribution curve of 210-Co3O4 derived from the desorption branch and calculated using the BJH method. The inset is corresponding adsorption−desorption isotherm curves obtained at 77 K (pore size distribution and N2 adsorption−desorption curves of 150-Co3O4 and 180-Co3O4 are given in Figure S3). For BET surface area calculation, we select five points from the adsorption curve to guarantee the correlation coefficient of multipoint BET calculation is higher than 0.9999. Meanwhile, positive C constants were obtained to eliminate deviation which might be induced by a small amount of micropores in the samples. The BET surface areas were calculated to be 54.2, 43.6, and 113.5 m2 g−1 for 150-Co3O4, 180-Co3O4, and 210-Co3O4, respectively. We suggest the abnormal less surface area of 180Co3O4 is due to larger size compared with 150-Co3O4 and its more severe agglomeration compared with 210-Co3O4. It is worth noticing that the specific surface area of 210-Co3O4 annealed from α-Co(OH)2 is much higher than reported value for porous Co3O4 hexagonal nanosheets annealed from β-Co(OH)2 (69.7 m2 g−1).37 According to the pore size distribution curves, the pore size of 150-Co3O4 and that of 180-Co3O4 (Figures S3a and S3b) can be both attributed to the space among the stacked nanoparticles. The pore size distribution curve of 210-Co3O4 has extra distribution between 5 and 15 nm compared with that of both 150-Co3O4 and 180-Co3O4. It can be attributed to the mesopores in the nanosheets, which is consistent with the HRTEM image of 210-Co3O4 (Figure 8f). 3.4. Lithium Ion Storage Behavior of Co3O4. The electrochemical reduction of Co3O4 in a lithium cell goes through two rate/surface-dependent competitive paths as follows:

Figure 6. XRD patterns of 150-Co3O4 (a), 180-Co3O4 (b), and 210Co3O4 (c).

patterns of the annealed product. The XRD patterns of the three samples match well with Co3O4 (PDF#43-1003). Compared with the XRD patterns of 150-Co3O4 (Figure 6a) and 180-Co3O4 (Figure 6b), the peaks of the 210-Co3O4 (Figure 4c) are broadened, indicating a smaller grain size of the 210-Co3O4. Considering the above analysis of ionothermal synthesized α-Co(OH)2, the 210-α-Co(OH)2 contains more stacking faults. In addition, the unit cell elongates along the c-axis. These might 916

Co3O4 + x Li → LixCo3O4

(4)

Co3O4 + 2Li → 3CoO + Li 2O

(5)

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Figure 7. SEM images of 150-Co3O4 (a), 180-Co3O4 (b), and 210-Co3O4 (c).

broad peak in the CV curve (Figure 10a) appears associated with a continuous sloping voltage to 0.02 V in the first discharge curve (Figure 10b). Previous research concludes that reversible formation/dissolution of a gel-like film on the surface of oxide particles occur at low potential. Meanwhile, Maier and co-workers systematically described an interfacial charging mechanism, where lithium ions and electrons are stored at the interface between electrochemically generated nanometal and Li2O.40−42 Both mechanisms should contribute to the capacity in the low voltage region. In the first anodic scan, two peaks appear at 1.39 and 2.12 V. Since the SEI film should be stable below 1.8 V, the lower peak at 1.39 V can be attributed to partial oxidation of Co to CoO and LixCo3O4 intermediates.39 The peak at 2.12 V should be due to combined contribution of reversible dissolution of SEI film and oxidation of the cobalt intermediates back to Co3O4. The two anodic peaks at 1.39 and 2.12 V correspond to the two slopes in the charging curve (Figure 10b) from 1.1 to 2.0 V and from 2.0 to 3.0 V, respectively. In the second cycle, the main cathodic peak shifts to 1.14 V. This is due to reduced polarization induced by smaller size of electrogenerated nanograins. The third CV cycle is basically similar to the second CV cycle, indicating the electrode becomes stable after the first cycle. Figure 10c highlights a better cycling performance of 210Co3O4 compared to 150-Co3O4 and 180-Co3O4. All the three Co3O4 samples exhibit similar specific discharge capacity at about 950 and 750 in the first and second cycle, respectively, at current density of 0.2 C (1 C = 890 mAh g−1). However, under prolonged cycling, the capacity retention of 50th to second cycle for 210-Co3O4 is 72.3%, higher than 51.4% and 56.3% for 150-Co3O4 and 180-Co3O4, respectively. The enhanced cycling performance is also seen when the Co3O4 electrodes are cycled under different rates (Figure 10d). At high discharge rate, the capacity retention is not satisfactory because Co3O4 is a p-type semiconductor with low electron mobility. The 150-Co3O4 can barely deliver capacity at 2 C. Except for rate performance, the

A higher current density favors the formation of LixCo3O4. Both intermediates decompose into Co nanograins and Li2O on further reduction. The total Li-driven decomposition of Co3O4 can be described in eq 6:38 Co3O4 + 8Li+ + 8e− → 4Li 2O + 3Co

(6)

Figure 10a gives a representative CV curve of 210-Co3O4 assembled in a coin-type lithium cell (CV curves of 150-Co3O4 and 180 Co3O4 are given in Figures S4a and S4c). The CV curves of 150-Co3O4 and 180 Co3O4 (Figures S4a and S4c) are nearly identical to plate-like Co3O4 mesocrystal synthesized by Wang.13 For 150-Co3O4 and 180-Co3O4, shoulder peaks at 1.0 V (marked with black arrows in Figures S4a and S4c) can be seen. Wang et al. attributed this shoulder peak to the reduction from Co3O4 to CoO.13 However, the shoulder peak is not obviously seen for 210-Co3O4 (Figure 10a). Instead, we observed a very broad peak at 1.75 V. Considering the fact that 210-Co3O4 have much larger surface area (113.5 m2 g−1) which lowers the current density, it seems more possible that the appearance of the broad peak for 210-Co3O4 (Figure 10a) at 1.75 V should be mainly attributed to the formation of CoO while the shoulder peaks for 150-Co3O4 and 180-Co3O4 (Figures S2a and S2c) at around 1.0 V should mainly be attributed to the formation of LixCo3O4 because higher current density favors path 1.38 In this work the scan rate (0.1 mV s−1) of CV curves are slower than that of plate-like Co3O4 mesocrystal (0.5 mV s−1),13 which confirms our suggestion. The formation of the intermediates, no matter more LixCo3O4 or more CoO, leads to the steep slope ranging from 1.0 to 2.5 V in the first discharge curve of 210-Co3O4 (Figure 10b). The most prominent peak at 0.81 V for the 210-Co3O4 CV curve (Figure 10a) can be attributed to further reduction of the above-discussed intermediates to Co and Li2O as well as irreversible formation of SEI layer,39 corresponding to the long plateau at 0.9 V in the discharge curve. Furthermore, a 917

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Figure 8. TEM images of 150-Co3O4 (a), 180-Co3O4 (c), and 210-Co3O4 (e) with SAED pattern in the insets. HRTEM images of 150-Co3O4 (b), 180α-Co3O4 (d), and 210-α-Co3O4 (f). A fast Fourier transform (FFT) filtered lattice image is given in the inset of (b).

normalization used here, 0 refers to full delithiated state charged to 3.0 V and 1.0 refers to completely lithiated state. All the three Co3O4 exhibit lower hysteresis at lithiated state due to enhanced electronic conductivity with metallic Co formation. We note that 210-Co3O4 shows lower hysteresis at the lithiated state (normalized capacity > 0.6) and higher hysteresis at the delithiated state (normalized capacity < 0.3). At normalized capacity = 0.66, the phase should be CoO and Li2O if we do not consider LixCo3O4 and SEI film, and LixCo3O4 is more likely to be observed at the delithiated state. We infer that the lower hysteresis of 210-Co3O4 at normalized capacity higher than 0.6 is related to the formation of CoO intermediate phase. The higher hysteresis of 210-Co3O4 at normalized capacity lower than 0.3 V is related to less formation of LixCo3O4 phase. To further understand the electrochemical kinetics of the three Co3O4 electrodes, we performed electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration techenique (GITT), which combines transient and steady-state measurements.45 Nyquist plots (Figure 11a) are presented after we multiplied the Z′ and Z″ values obtained from these electrodes by the electrode’s mass ratio. The Nyquist plots of all the three Co3O4 all consist of a depressed semicircle where a high-frequency semicircle and a medium-frequency semicircle overlap each other. A long low-frequency line appears at the lowfrequency region. The EIS at high, medium, and low frequency

Figure 9. Pore size distribution of 210-α-Co3O4. The inset is the corresponding BET isotherm curve.

voltage discrepancy between charge/discharge curve, namely voltage hysteresis, is another barrier between laboratory and society, because it severely diminishes the round-trip efficiency of the electrode.43 We obtain the voltage hysteresis curve (Figure 10e) by subtracting the discharge curve at the second cycle from the charge curve at the first cycle after normalization.44 For the capacity 918

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Figure 10. (a) CV curve at a scan rate of 0.1 mV s−1 and (b) galvanostatic charge/discharge at 0.2 C at different cycles of 210-Co3O4 assembled in lithium ion cell. (c) Cycling performance at 0.2 C, (d) rate capability, and voltage polarization at 0.2 C of the Co3O4 samples.

conclude that the fundamental kinetic processes of 150-Co3O4, 180-Co3O4, and 210-Co3O4 do not show much difference in spite of their initial different morphologies, grain sizes, and surface areas. The GITT technique has been widely used to investigate thermodynamic and kinetic properties.49 Figure 11c displays the GITT curve of 210-Co3O4. Similar GITT curves of 150-Co3O4 and 180-Co3O4 are given in Figures S5a and S5c, respectively. The difference between the cutoff potential and the open-circuit potential measured after relaxation seen in the GITT curves can be used to roughly reflect polarization. This polarization slightly decreases with lithium insertion. But the polarization continuously increases during lithium extraction. This trend can be explained in terms of electronic conductivity. During lithium insertion, Co3O4 transforms into Co nanoparticles. In contrast, electronic transport continuously worsened when Co nanoparticles transform back into Co3O4 during lithium extraction. A similar trend was also observed for MnO2 electrode.50,51 To further investigate the staggering hysteresis in voltage observed between discharge and charge, we provide GITT potential−time profiles at third discharge pulse in Figure 11d. ΔE stands for voltage change in pseudoequilibrium before and after current pulse. The discharge pulse region contains two distinct parts (A and B). The potential drops rapidly in region A and linearly in region B. In region A, the potential almost exhibits a linear relationship with the square root of discharge titration time (inset of Figure 11d). It indicates a diffusion-controlled process. But it is still not clear which ion’s diffusion is the rate-determining step.

region reflects the impedance of lithium ion traveling through SEI layer, charge transfer resistance (Rct), and diffusion related Warburg impendence, respectively. Levi developed a equivalent circuit analogue based on a combination of a Voigt-type analogue and the generalized Frumkin and Melik-Gaykazyan impedance to simulate the impedance of lithium ion inserting into carbon electrode.46 The characteristic of a Voigt-type analogy is that it consists of several R||C in series to model the lithium ion migration through SEI film with multilayer structure evidenced by Aurbach and co-workers with XPS.47 A modified equivalent circuit is displayed in Figure 11b. Compared to the model developed by Levi, we used a constant phase element (CPE) to replace the pure capacitor in parallel with Rct because of the porosity of the prepared Co3O4.48 The CPE was marked as Cdl in Figure 11b to represent the double-layer capacitance of the porous electrode. In addition, we eliminate the Cint which was used to represent insertion capacitance of graphite electrode in Levi’s model because Co3O4 stores lithium ion based on conversion mechanism. The calculated impedance data (lines in Figure 11a) based on our modified equivalent circuit (Figure 11b) are in well accordance to the measured impedance data (dots in Figure 11a). It shows that the impedance of the Co3O4 comprises the lithium ion migration through multilayer SEI film, charge transfer resistance, and ion diffusion resistance. However, the electrochemical reaction involves formation and decomposition of complex intermediate phases. Oxygen and cobalt ions also move. It is still hard to determine accurate kinetic parameters with unambiguous physicochemical significance. We can qualitatively 919

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Figure 11. (a) Nyquist plots of 150-Co3O4, 180-Co3O4, and 210-Co3O4, where the presented Z′ and Z″ data have been multiplied with the mass of active materials. (b) The proposed equivalent circuit for simulation. (c) GITT test of 210-Co3O4 obtained by charging/discharging the cells for 2 h at C/ 20 and relaxed for 4 h. (d) Discharge and following rest potential during GITT at third current pulse. The inset is the plot of transient potential vs square root of titration time and the corresponding linear fitting curves.

expansion of 5% (Co3O4 + 2Li → LiCo3O4) and 21% (Co3O4 + 2Li → 3CoO + Li2O).38 Thus, the volumetric change during the formation of Co/Li2O from CoO intermediate should be less. We deduce that the formation of CoO intermediate provides a buffered reaction path for 210-Co3O4 electrode. Furthermore, the mesoporous nanosheets should be more robust for the buffering function than the collapsed nanoparticles under cycling. 3.5. Supercapacitive Performance of Co3O4. Co3O4 shows a pseudocapacitive behavior in alkali aqueous solution. It stores hydroxyl ion in a way similar to the storage of lithium ion as follows:52

The linear polarization in region B has been explained as strain accommodation energy of phase transformation in the research of LiFePO4 electrode. Obviously, during the third titration pulse at current of C/20, the polarization induced by mass transport resistance is about 0.3 V. It is 3 times larger than phase transformation-induced polarization estimated to be 0.1 V in Figure 11d. The similar shape of GITT potential−time profiles of 150-Co3O4 (Figure S5b) and 180-Co3O4 (Figure S5d) also indicates that the fundamental kinetic is not obviously changed by their initial different morphologies, grain sizes, and surface areas. From the above discussion, we found that the 210-Co3O4 has smaller grain size and larger surface area. CV test indicates that the discharge process of 210-Co3O4 involves formation of CoO intermediate. However, we could not find an obvious kinetic difference. To research how the 210-Co3O4 electrode showed better capacity retention under prolonged cycling, we disassembled the cells after 50 cycles and carried out SEM observation on the coated nickel foam. The SEM images of 150Co3O4, 180-Co3O4, and 210-Co3O4 are shown in Figures 12a−c, respectively, with magnified images in the insets. The surface of 210-Co3O4 electrode has much less crevices compared with 150-Co3O4 and 180-Co3O4 electrodes, indicating the better performance of 210-Co3O4 arises from alleviated volumetric expansion during cycling. The overall Co3O4 + 8Li → Co + 4Li2O reaction causes a molar volume expansion of around 100%. The two alternate biphasic steps come with a calculated

Co3O4 + OH− + H 2O → 3CoOOH + e−

(7)

CoOOH + OH− → CoO2 + H 2O + e−

(8)

The CV curves of 210-Co3O4 at scan rates ranging from 5 to 50 mV s−1 are presented in Figure 13a. At a scan rate of 5 mV s−1, two peaks appear at 0.31 and 0.46 V during cathodic scan, which should be attributed to the reduction from CoO2 to CoOOH and further to Co3O4, respectively. The two anodic peaks almost merge into one at 0.49 V. The CV curve indicates that Co3O4 show the pseudocapacitive behavior in aqueous alkali solution. When the scan rate reaches 50 mV s−1, the CV curve becomes distorted, which indicates the electron and mass transport at high rate is hindered. The 210-Co3O4 exhibits pseudocapacitances of 238.4, 213.6, 174.4, and 100 F g−1 at discharge current densities of 2, 4, 8, and 920

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Figure 12. SEM images of nickel foam current collector coated with 150-α-Co3O4 (a), 180-α-Co3O4 (b), and 210-α-Co3O4 (c) after cycling 50 runs in lithium cells. The insets are the corresponding magnified SEM images.

Figure 13. Pseudocapacitive performance of Co3O4 electrode tested in a three-electrode aqueous alkali system: (a) CV at different scan rates; (b) chronopotentiometry curves with different discharge rates of 210-Co3O4; (c) Ragone plots of Co3O4 samples; (d) cycling performance of Co3O4 samples.

20 A g−1. From the CV and chronopotentiometry curves, we found the pseudocapacitive performance of 210-Co3O4 is no better than that of 150-Co3O4 and 180-Co3O4 (see Figure S6a−d). A Ragone plot (power density vs energy density) is presented in Figure 13c to demonstrate the similar pseudocapacitive more clearly. The energy density and power density were calculated from chronopotentiometry curves using the equations

E=I

∫

ΔV dt /3.6M

P=I

∫

ΔV dt /M Δt −1

(9) (10) −1

where E (Wh kg ) is the energy density, P (kW kg ) is the power density, and 3.6 in eq 9 is the unit conversion factor (Wh kg−1 = 3.6 mA V s mg−1). 921

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The almost overlapped Ragone plot shows that the 150Co3O4, 180-Co3O4, and 210-Co3O4 have similar pseudocapacitive performance. However, the 210-Co3O4 showed better cycling performance just like its lithium storage behavior, as shown in Figure 13d. After 4000 cycles, the specific capacitance of 210-Co3O4 maintains at about 225 F g−1 while the 150-Co3O4 and 180-Co3O4 maintain at about 185 F g−1. We attribute the enhanced specific capacitance of 210-Co3O4 under cycling to the higher strain-mediated capability of the robust mesoporous nanosheets structure composed of nanosized grains.

ASSOCIATED CONTENT

S Supporting Information *

Digital pictures and EDAX of α-Co(OH)2, CV and chronopotentiometry curves of α-Co(OH) 2 , N 2 adsorption and desorption curves of 150-Co3O4 and 180-Co3O4, CV, charge/ discharge, and GITT curves of 150-Co3O4 and 180-Co3O4 in a Li-ion cell, CV and chronopotentiometry curves of 150-Co3O4 and 180-Co3O4. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSION In summary, we developed a facile and environmentally friendly ionothermal strategy to synthesize hydrotalcite-like α-Co(OH)2 as the precursor to generate Co3O4 with nanostructures. The interlayer chemistry and morphology of the α-Co(OH)2 can be tuned. With reaction temperature at 210 °C, more intercalated water molecule, CO32−, and some other complex species containing CO, OC−N, and C−N bonds can be introduced into α-Co(OH)2, resulting in a very large spacing (11.2 Å) of the (003) facet. The larger interlayer space is demonstrated to enhance the pseudocapacitive performance of the α-Co(OH)2. Meanwhile, these interlayer defects of α-Co(OH)2 can improve the Co3O4 nucleation during annealing, thus leading to the smaller grain size. In addition, the 210-α-Co(OH)2 nanosheets can be preserved in the annealed polycrystalline Co3O4 to form a selfsupporting mesoporous nanosheets structure. The mesoporous 210-Co3O4 nanosheet shows enhanced cycling performance as anode for lithium ion batteries. With transient and steady state electrochemical diagnosis, we believe that the enhanced cycling performance is due to the special morphology and the formation of CoO intermediate which triggers a new reaction path way with buffered volumetric change. The deduction was also verified by examining the pseudocapacitive performance of the mesoporous 210-Co3O4 nanosheet. This study provides enough evidence that the electrode with a mesoporous nanosheet structure constructed by fine grains could benefit the electrochemical performance due to the higher strain-mediated capability of the architecture.

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AUTHOR INFORMATION

Corresponding Author

*C.D.G.: tel, +86 571 87952573; e-mail, [email protected], [email protected]. Notes

The authors declare no cometing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51271169, 51001089), the Key Science and Technology Innovation Team of Zhejiang Province under Grant 2010R50013, and the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037). 922

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dx.doi.org/10.1021/jp411921p | J. Phys. Chem. C 2014, 118, 911−923