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Three-dimensional hierarchical wreath-like Co3O4@ TiO2 as an anode for lithium-ion batteries
Accepted Manuscript Three-dimensional hierarchical wreath-like Co3O4@ TiO2 as a anode for lithium-ion batteries Guozhen Liu, Xinguang Yuan, Yanmin Yang, Jianming Tao, Yubin Chi, Lixun Hong, Zhiya Lin, Yingbin Lin, Zhigao Huang PII:
S0925-8388(18)34374-3
DOI:
https://doi.org/10.1016/j.jallcom.2018.11.242
Reference:
JALCOM 48468
To appear in:
Journal of Alloys and Compounds
Received Date: 10 August 2018 Revised Date:
13 October 2018
Accepted Date: 18 November 2018
Please cite this article as: G. Liu, X. Yuan, Y. Yang, J. Tao, Y. Chi, L. Hong, Z. Lin, Y. Lin, Z. Huang, Three-dimensional hierarchical wreath-like Co3O4@ TiO2 as a anode for lithium-ion batteries, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.11.242. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Three-dimensional hierarchical wreath-like Co3O4@ TiO2 as a anode for lithium-ion batteries Guozhen Liu1,2,3, Xinguang Yuan1,2, Yanmin Yang1,2, Jianming Tao1,2, Yubin Chi1,2,
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Lixun Hong1,2, Zhiya Lin1,2, Yingbin Lin*1,2,3, Zhigao Huang1,2,3
1. College of Physics and Energy, Fujian Normal University, Fujian Provincial Key
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Laboratory of Quantum Manipulation and New Energy Materials, Fuzhou，350117, China.
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2. Fujian Provincial Engineering Technology Research Center of Solar Energy Conversion and Energy Storage, Fuzhou,350117, China. 3. Fujian
Provincial
Collaborative
Innovation
Center
for
Optoelectronic
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5.
* Corresponding author: Yingbin Lin Tel: +86-591-2286-8132
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4.
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Semiconductors and Efficient Devices, Xiamen, 361005, China
Fax: +86-591-2286-8132 E-mail: [email protected]
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Abstract Wreath-like Co3O4 particles consisting of microplates are synthesized by a facile solvothermal method and subsequently surface-modified with TiO2 ultrathin layer
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using a low-temperature hydrolysis process. Comparing with pure Co3O4, Co3O4@TiO2 exhibits superior electrochemical performances in terms of reversible
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capacity, rate capability and cycling stability. Co3O4@TiO2 exhibits high reversible capacity of 813.0 mAhg-1 at 500 mAg-1 after 180 cycles while the pristine Co3O4 only
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has a discharge capacity of 512.5 mAhg-1. The filling of TiO2 nanoparticles in porous Co3O4 sheets and the TiO2-coating on Co3O4 surface, could effectively suppress large volume expansion of Co3O4 and consequently enhance structural stability during the Li-ion
insertion/extraction
processes.
Analysis
from
the
electrochemical
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measurements reveals that the improved performances should be attributed to reduced the charge-transfer resistance and enhanced Li-ion diffusion kinetics because of
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TiO2-coating. In addition, the reduced work function induced by TiO2-coating is helpful to facilitate electron transfer in composites. Moreover, the built-in electric
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field resulting from the difference in work function between Co3O4 and TiO2, would facilitate electron-transfer and Li-ion migration across heterojunction interfaces.
Key words: lithium-ion batteries; CO3O4; TO2; diffusion coefficient; work function.
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ACCEPTED MANUSCRIPT 1. Introduction In recent years, rechargeable lithium-ion batteries with high energy density and long cycling life, have been paid increasing attention in energy storage systems such as consumer electronics, electric vehicles and smart grid-scale energy storage [1-3].
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Anode materials with high-capacity and long-term cycling are highly desirable for high-performance lithium-ion batteries. Comparing to the commercial graphite with a theoretical capacity of ~372 mAhg-1, transition metal oxides (such as Co3O4) are
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considered to be promising alternative anode materials for high-energy-density lithium ion batteries because of their high reversible capacity and chemical stability
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[4-6]. Among lots of transition metal oxides, cobalt oxide (Co3O4) has been regareded as a promising anode material due to its dramatically high specific capacity of 890 mAhg-1, based on the reversible electrochemical reaction (Co3O4+8Li++8e- ↔ 3Co+4Li2O) [7,8]. Unfortunately, Co3O4 also suffers from sluggish reaction kinetics,
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inferior electrical conductivity and large volumetric expansion during the charge/discharge processes, which induces electrode pulverization, severe capacity decay, poor rate capability and consequently hampers its fascinating application in
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high-energy lithium-ion batteries [9-11]. To address significant drawbacks above,
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considerable strategies have been carried out and preparation of hierarchical Co3O4 with surface-modification is proved to be an effective approach to improve electrochemical performances [12-15]. Hierarchical structure would not only alleviate the strain but also accommodate volume expansion during lithium insertion/extraction processes. For example, Li et al. [12] reported Co3O4 nanowires with a capacity of 836 mAhg-1 at the current of 200 mAg-1. Li et al. [13] reported that 2D-nanoporous Co3O4 nanosheets showed high reversible capacity of 1380 mAhg-1 at a current density of 500 mAg-1 and excellent rate capability (606 mAhg-1 at 10 Ag-1). Liu et al. -3-
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ACCEPTED MANUSCRIPT [14] found that flower-like clusters of Co3O4/C nanosheets delivered a high reversible capacity of 1082 mAhg-1 at 100 mAg-1 and excellent cycling stability. On the other hand, coating of passivation layer on Co3O4 surface is an important strategy to prevent repeated formation of SEI films and the structure degradation [16, 17]. As a promising
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anode, titanium oxide (TiO2) has been extensively investigated because of its low cost, nontoxicity and low volume expansion (3-4%) during the Li-ion insertion/extraction process [18, 19]. Therefore, high-capacity Co3O4 coated with stable-structure TiO2
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layer as hierarchical core/shell structure, should be highly desirable for high-performance lithium-ion batteries with high capacity and excellent cycle stability.
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It is expected that TiO2-coating layer acting as an interfacial barrier could effectively suppress the exothermic reaction between the active material and the electrolyte, resulting in significant improvement in cyclic performances.
Herein, wreath-like Co3O4 consisting of nanosheets has been surface-modified
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with TiO2 nanoparticles via the low-temperature hydrolysis. Such sandwich-like structure endows Co3O4@TiO2 with high specific capacity, excellent rate capability and good cyclic performance as well. The excellent electrochemical performances
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might be related with the enhanced charge transfer deriving from the beneficial
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features of hierarchical structures as well as the strong strain accommodation induced by the TiO2-layer coating, which relieves the large volume expansion of Co3O4. 2.
Experimental
2.1 Preparation of wreath-like Co3O4 and Co3O4@TiO2 Wreath-like Co3O4 consisting of microplates were synthesized by a facile solvothermal method. All chemicals of analytical grade were purchased from Aladdin Ltd. (Shanghai, China) and used as received without any further purification. Typically, 0.704 g of Co(NO)3·6H2O was thoroughly dissolved in 40 ml of absolute -4-
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ACCEPTED MANUSCRIPT methanol, followed by added dropwise of benzyl alcohol (5.9mL) under magnetic stirring. After stirring for 60 minutes, the obtained mixture was transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 180oC for 36 h. The precipitates were separated by centrifugation, washed with deionized water and
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alcohol several times, and subsequently dried at 100°C overnight. Finally, the resulted precursors were calcinated at 450 oC for 4 h in air to get wreath-like Co3O4 powders. Co3O4@TiO2 composites are synthesized by a hydrolysis process at low temperature
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using Co3O4 powders and tetrabutyl titanate (Ti(OC4H9)4) as precursors [20]. 0.1g of the as-prepared Co3O4 powders were dispersed in 40 ml of absolute ethanol and 1 ml
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of deionized water under vigorous stirring at 4 oC. Then, 21.31 µl of Ti(OC4H9)4 was dispensed in absolute ethanol (10 ml) and subsequently added dropwise into above solution. After continuously stirring for another 20 h at 4°C, the obtained precipitates were isolated by centrifugation, dried at 70 °C for 12 h and subsequently calcinated at
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450 °C for 4 h to get wreath-like Co3O4 surface-modified with TiO2 composites. The schematic illustration of the synthesis process for the Co3O4 and Co3O4@TiO2 is shown in Fig. 1.
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2.2 Materials characterizations
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The crystalline structures of the as-prepared samples were investigated by X-ray diffraction (XRD, Rigaku MinFlex II) with a Cu Kα radiation source (λ=0.15406 nm) and Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet, Nexus 870). Morphologies of the as-prepared samples were characterized by scanning electron microscopy (Hitachi SU8010) equipped with an energy-dispersive spectroscopy (EDS). The specific surface areas and pore size distribution of the as-prepared composites were investigated based on nitrogen sorption measurement using a Micromeritics Tristar 3020 analyzer. The TiO2 content in Co3O4@TiO2 composite was -5-
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ACCEPTED MANUSCRIPT determined by inductively coupled plasma OES spectrometer (ICP). The surface potentials of Co3O4 and Co3O4@TiO2 were characterized by Kelvin probe atomic force microscopy (KPAFM) (Bruker dimension ICON, Germany). 2.3 Electrochemical measurements
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The electrochemical performances of the as-prepared composites are evaluated with CR2025-type coin cell using metallic lithium foil as the combined reference and counter electrodes. The working electrodes were prepared by mixing 70 wt.% of the
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active materials (Co3O4 or Co3O4@TiO2), 20 wt.% super-P, and 10 wt.% polyvinylidene fluoride (PVDF) in N-methy1-2-pyrrolidone to form a slurry. The
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resulting slurry is cast onto a copper current collector and subsequently dried in vacuum at 110 oC for 12 h to remove the residual solvent. The electrolyte used in the cells was 1.0M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume). The separator was Cellgard 2300 microporous polyethylene
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membrane. Finally, all electrochemical cells were assembled in an Ar-filled glove box with concentrations of oxygen and moisture level below 1 ppm. Charge–discharge measurements were carried out galvanostatically in a LAND-CT2001A battery test
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system with a potential voltage window of 0.01-3.0 V. The cyclic voltammetry (CV)
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was performed using Arbin instruments BT-2000 battery testing station, and the electrochemical impedance spectra of the electrodes were determined by an electrochemical workstation (Zahner-Zennium) with an amplitude of 5 mV in the frequency range of 10 mHz to 100 KHz.
3 Results and discussion 3.1 material characterization
The typical surface morphologies of the as-prepared Co3O4 and Co2O4@TiO2 powders are characterized by field-emission scanning electron microscopy, shown in -6-
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ACCEPTED MANUSCRIPT Fig. 2. As shown in Fig.2(a, b), Co3O4 powders display wreath-like morphology consisting of self-assembled thin nanosheets with a thickness of ~50nm, in which porous Co3O4 nanoflowers grow on Co3O4 microplate.
Three-dimensional
hierarchical porous structure is expected to be benefit for Li-ion diffusion from the
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electrolyte into the active sites, electron transportation in composites and efficiently relieve large volume expansion during the Li-ion insertion/extraction processes [21]. After TiO2-coating, the composites maintain the nanosheet-bulit wreath-like
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morphologies as same to the pure one, shown in Fig.2(c). In comparison, Co3O4@TiO2 nanosheets display more smooth and integrated surface morphology. It
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is expected that the pores in porous Co3O4 sheet are filled with TiO2 nanoparticles and Co3O4 sheets are deposited by TiO2 layer. The cross-section SEM images of Co3O4@TiO2 nanosheet shown in Fig.2(f) reveals that the thickness of TiO2-coating layer is ca. 25 nm. Such sandwich-like structure would effectively suppress large
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volume expansion of Co3O4 during the Li-ion insertion/extraction processes and consequently enhance structural stability. Due to the existence of surface tension and residual air in the pores among Co3O4 sheets, the tetrabutyl titanate solution cannot
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penetrate into the pores thoroughly. So, It is reasonably to expected that the thickness
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of TiO2 ultrathin layer is becoming thinner from Co3O4 sheets outside to solid-core of Co3O4 nanoflower.
Fig. 3 shows the nitrogen adsorption-desorption isotherms of Co3O4 and
Co3O4@TiO2 powders. As shown in Fig. 3(a), the type IV hysteresis loops at relative pressure (P/P0) between 0.8 and 1.0 reveals the existence of the mesoporous structure for both composites [22, 23]. According to Brunauer-Emmett-Teller equation, the specific surface areas of Co3O4 and Co3O4@TiO2 are evaluated to be 30.23 and 25.07 m2g-1, respectively. Based on Barrett-Joyner-Halenda (BJH) model, the differential -7-
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ACCEPTED MANUSCRIPT pore volume in term of logarithm of pore diameter dV/(dlogD) of the Co3O4 and Co3O4@TiO2 powders was calculated and shown in Fig. 3(b). It is found that that Co3O4@TiO2 have larger average pore size (~30 nm) that that (~21 nm) of Co3O4. The reduction in surface area and increase in average pore size could be reasonably
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derived from the disappearance of micro-pores(2.3 nm) because of TiO2-coating. The analysis of nitrogen adsorption-desorption isotherms is consistent with the SEM images. Fig.4 presents the elemental distribution maps of Co3O4@TiO2 characterized
corresponding elements in Co3O4@TiO2 particles.
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by energy-dispersive spectroscopy, indicating the uniform distribution of the
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Fig.5 (a) presents the X-ray diffraction (XRD) patterns of Co3O4 and Co3O4@TiO2 composites. All of the diffraction peaks are indexed as a spinel structure (JCPDS card No. 42-1467) [24]. No visible differences in XRD patterns between two composites, which is attributed to the low content of TiO2 in composite. Furthermore,
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no secondary phase observed in XRD patterns indicates no chemical reaction occurs between Co3O4 and TiO2 during the calcination process. Fig. 5(b) shows FTIR spectra of Co3O4 and Co3O4@TiO2 powders. Two distinct absorptions observed around 550
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and 660 cm-1 in both samples are related to the stretching vibrations of the Co-O bonds [25, 26], further confirming the formation of Co3O4 phase. In addition, a small
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absorption around 760 cm-1 can be assigned to the vibrations of the anatase TiO2 lattice [27]. No peak-shifting between Co3O4 and Co3O4@TiO2 , shown in the inset of Fig. 5(b), further confirms no chemical reaction occurs between Co3O4 and TiO2 phases. The TiO2 content in Co3O4@TiO2 composite is determined to be ca. 2.83 wt.% by inductively coupled plasma OES spectrometer. Fig.6(a) shows the rate capabilities of Co3O4 and Co3O4@TiO2 electrodes at the various current densities of 200, 500, 1000 and 2000 mAg-1 in rising order for every -8-
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ACCEPTED MANUSCRIPT five successive cycles and subsequently followed by returning 1000, 500 and 200 mAg-1. It is found that the Co3O4@TiO2 electrodes exhibit better rate performances than that of the pristine Co3O4, especially at a higher rate. When the current densities increase from 200 mAg-1 to 2000 mAg-1, the Co3O4@TiO2 electrodes still deliver the
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average capacities of 1038.9, 1066.5, 1082, 1152.3 mAhg-1 respectively, which are larger than those of bare one (1032.1, 1060.1, 1052.9, 1001.3 mAhg-1) under the same conditions. However, when the current density goes back to 1000, 500 and 200 mA g-1,
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the Co3O4@TiO2 electrode also recovers 1130.3, 1207.2 and 1284.7 mAhg-1 while the discharge capacities of the Co3O4 electrode are only 1042.9, 1082.4 and 1140.8 mAg-1
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at the same respective current density. The improved rate performances of Co3O4@TiO2 would be attributed to the insulator-to-conductor transition of anatase TiO2 during the Li+ insertion possess [28, 29] and the stable structure as well. The increase in capacity observed at the initial 10 cycles, especially for Co3O4@TiO2, is
[30,31].
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strongly related to the enhanced Li-ion diffusion kinetics by the activation process
The electrical properties of coating layer should play a crucial role in the Li-ion
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kinetic behavior in surface-coated composites. Work function, defined with respected
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to the Fermi energy of the electrons, reflects the kinetic energy of the electrons to overcome the barrier and thus escape the composite. The effect of TiO2-coating on work functions of the as-prepared composites is investigated by surface potential characterized by Kelvin probe atomic force microscopy. Figs. 6(b, c) presents the surface potential maps of Co3O4 and Co3O4@TiO2 powders over a scan area of 200nm×200nm before cycling. According to our prior work [20,32], the work functions of Co3O4 and Co3O4@TiO2 powders are calculated and the corresponding results are displayed in Fig. 6(d). Here, the work functions of the SFM-tip ( φ tip ) is -9-
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ACCEPTED MANUSCRIPT calibrated by Au foil, whose work function ( φ Au ) is 5.31eV. It is found that Co3O4@TiO2 has a smaller work function (~5.42 eV) than that (~5.50 eV) of the Co3O4. The reduced work function of Co3O4@TiO2 could be explained phenomenologically based on the energy-band model [33]. The smaller work function
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suggests the less energy required for electrons to escape from the composites. As a result, the electrochemical performances of the composites are improved with surface-modification of TiO2.
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Fig.7(a) displays the cycling performances of Co3O4 and Co3O4@TiO2 electrodes
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at a current density of 500 mAg-1. In comparison with Co3O4, the capacity loss is significantly suppressed after coating with TiO2, especially after 70th cycle. The initial discharge capacity at 500 mAg-1 of Co3O4 is 983.9 mAh·g-1 and found to decrease to 512 mAh·g-1 after 180 cycles (i.e., only 52% of its initial discharge capacity). The discharge capacity of the Co3O4@TiO2 retains 85% of its initial
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discharge capacity after 180 cycles. It is clear that Co3O4@TiO2 has significant cycling stability than that of the pristine one after 70th cycle, which might result from the more stable structural stability induced by TiO2-coating. Fig. 7 (b, c) shows typical
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cyclic voltammetry profiles of Co3O4 and Co3O4@TiO2 electrode for the first five
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cycles at a scan rate of 0.2 mV s-1 over the voltage range of 0.01–3 V (vs. Li/Li+). In the first cycle, a strong cathodic peak at ca. 0.75V would be assigned to the initial reduction of Co3O4 to Co and the formation of solid electrolyte interphase (SEI) [34]. In the subsequent anodic scan, a pronounced oxidation peaks are observed at ca. 0.75V, which corresponds to the reversible oxidation of Co to cobalt oxide (Co + Li2O → CoOx +Li+ + 8e-)[35] . It is obvious that, from the second cycle onward, the CV curves overlap of Co3O4@TiO2 electrode is better than that of Co3O4 electrode,
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ACCEPTED MANUSCRIPT suggesting that the TiO2-coating helps to enhance lithiation/delithiation reversibility and cycle performance [36]. To investigate the influence of TiO2-coating on the charge-transfer kinetics in composites, electrochemical impedance measurements of Co3O4 and Co3O4@TiO2
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electrodes at the Li-ion insertion state are carried out, shown in Fig. 8(a). Both Nyquist plots are comprised of a depressed semicircle in high frequency region and a inclined line in the low frequency region and the obtained results could be fitted using
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the equivalent circuit (Fig. 8(a)), where Re, Rsf, Rct, and Rw are the solution resistance, the impedance of a solid-electrolyte interface (SEI) layer, the charge transfer
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resistance and Warburg impedance, respectively. Based on the equivalent circuit, the Rct values of Co3O4 and Co3O4@TiO2 electrodes are calculated as 134.7 Ω and 90.6 Ω respectively, reflecting the effective suppression on the formation of the resistive reaction layers. Furthermore, the much higher impedance-slope of Co3O4@TiO2 in
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low frequency range than that of the pristine one also reveal the improved Li-ion kinetics after TiO2 coating, shown in Fig. 8(b). Fig. 8(c, d) shows the surface morphologies of Co3O4 and Co3O4@TiO2 electrodes after 30 galvanostatic
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charge/discharge cycles. It is found that Co3O4 powders has more serious
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structure-deterioration (pulverization or aggregation) than that of Co3O4@TiO2 with increasing cycles due to the repeated volume change between metals and metal oxides. The aggregation of the active materials would reduce undoubtedly the contact areas between active materials and the electrolyte, and increase Li-ion diffusion length in composite. The sandwich-like TiO2-coating (Fig. 8(e)) help to remain hierarchical structure, suppress electrode pulverization and consequently offer more active sites for the lithium-ion insertion/extraction. The work functions of the as-prepared composites after cycling are evaluated -11-
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ACCEPTED MANUSCRIPT based on the cell model shown in Fig.9(a), in which the work electrode consists of Co3O4 and Co3O4@TiO2 powders. After cycling, the cells are disassembled in argon-filled glove box and the residual electrolyte on the electrode are washed with ethylene carbonate and ethanol several tmes, and subsequently dried in vacuum at
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100 °C for 12 h to remove the residual solvent. Figs. 9(b, c) shows the surface potential maps over a scan area of 200nm×200nm of Co3O4 and Co3O4@TiO2 powders after 30 cycles. Fig. 9(d) presents the surface potential image of Au foil
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acting as reference sample. Based on the measured surface potential profiles, the calculated work functions of Co3O4 and Co3O4@TiO2 powders are correspondingly
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shown in Fig. 9(e). Here, the work functions of the SFM-tip ( φ tip ) is calibrated by Au foil, whose work function ( φ Au ) is 5.31eV. The work function (~4.90 eV) of Co3O4@TiO2 is found to be smaller than that (~5.15 eV) of the Co3O4. The larger
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work function suggests the more energy required for electrons to escape from the composites. The reduced work function of Co3O4@TiO2 could be explained phenomenologically based on the energy-band model [32]. As shown in Fig.10(a), it
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is expected that electrons would transfer from the composite with smaller work function to the composite with larger work function until the Fermi levels are
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aligned[37,38]. As a result, a corresponding electric field(E) is built up between positively-charged TiO2 and negatively-charge Co3O4 due to electrostatic induction, shown in Fig.10(b). Such electric field could facilitate Li-ion diffusion from TiO2 to Co3O4, and electron transfer from Co3O4 to TiO2 across heterojunction interfaces. As a result, more electrons tends to transfer through TiO2 rather than Co3O4/Co3O4 interface in Co3O4 matrix during the lithium insertion process. Moreover, the sandwich-like structure of TiO2-coating Co3O4 could effectively suppress the pulverization of Co3O4 because of the large volumetric expansion in the charge/discharge process, and -12-
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ACCEPTED MANUSCRIPT consequently improve the electrochemical performances. To investigate the influence of TiO2-coating on the li-ion diffusion during the charge/discharge process, the impedance spectra under different discharge states for Co3O4 and Co3O4@TiO2 electrodes are continuously measured, shown in Fig.11(a-d).
according to the EIS profiles in the low frequency [39, 40].
RT 2
(
1
n F A 2 C Li DLi 2
Z re = R + σω −1 / 2
1/ 2
)
(1)
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σ=
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The diffusion coefficients (DLi) of the Li-ion kinetic of the cells could be calculated
(2)
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A, n and CLi are the surface area of the electrode, the number of electrons per molecule during oxidation and the molar volume of active material; R, T and F are the mass gas constant, absolute temperature and Faraday’s constant;σ, Zre andωare the Warburg factor, the real part of the impedance and the frequency.
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Based on the slope coefficient of Zre to ω−1/2, the corresponding lithium diffusion coefficients DLi of Co3O4 and Co3O4@TiO2 electrodes are calculated respectively. Fig.11(e)
shows
Li-ion
diffusion
kinetics
during
the
lithium-ion
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insertion/de-insertion process. Both electrodes demonstrate similar lithium-ion diffusion behavior. On the whole, Co3O4@TiO2 electrode has larger Li-ion
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diffusion coefficients than those of the bare one, indicating that TiO2-coating does readily facilitate the Li-ion diffusion in composites. 4. Conclusions
Wreath-like Co3O4 coated with TiO2 powders were prepared by solvothermal method and subsequent low-temperature hydrolysis process. In comparison, Co3O4@TiO2 exhibits higher reversible capacity, better rate capability and excellent cycling stability. Analysis from the electrochemical -13-
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ACCEPTED MANUSCRIPT measurements reveals that the improved performances should be attributed to enhanced Li-ion diffusion kinetics, reduced charge-transfer resistance and more structural stability with TiO2-coating. Moreover, analysis from Kelvin probe force microscopy measurements indicates that the built-in electric field
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resulting from the difference in work function would be help to facilitate electron-transfer and Li-ion migration across heterojunction interfaces Acknowledgements
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This work is supported by a grant from Key Project of Department of Science & Technology of Fujian Province (No.2014H0020), Program for New Century Excellent
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Talents in University of Fujian Province (No.JA14069), and Solar Energy Conversion & Energy Storage Engineering Technology Innovation Platform (No. 2018L3006). References
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for lithium-ion secondary batteries, J. Phys. Chem. Solids, 93 (2016) 52-58.
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Figure Captions Fig. 1
Schematic illustration of the preparation process for Co3O4 and Co3O4@TiO2 powders. SEM images of the as-prepared Co3O4 and Co3O4@TiO2 powders.
Fig. 3
(a) Nitrogen sorption isotherms and (b) pore diameter distribution of Co3O4 and Co3O4@TiO2 powders.
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Fig. 2
Element mapping images of Co3O4@TiO2 powders.
Fig. 5
(a) XRD patterns and (b) FTIR spectra of Co3O4 and Co3O4@TiO2 powders.
Fig. 6
(a) Rate capability of Co3O4 and Co3O4@TiO2 electrodes; (b,c) Surface
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Fig. 4
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potential maps over a scan area of 200nm×200nm of Co3O4 and Co3O4@TiO2 powders before cycling; (d) Work functions of Co3O4 and Co3O4@TiO2 electrodes. Fig. 7
(a) Cyclic performances and (b, c) cyclic voltammetry profiles of Co3O4 and
Fig. 8
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Co3O4@TiO2 electrodes;
(a) Typical EIS of Co3O4 and Co3O4@TiO2 electrode in the fully discharged state and the equivalent circuit for EIS fitting; (b) Real parts of the complex
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impedance Zre vs. ω−1/2 for Co3O4 and Co3O4@TiO2 electrodes; (c, d) SEM
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images of the Co3O4 and Co3O4@TiO2 powders after 30 cycles, respectively. Fig. 9
(a,b) Surface potential maps over a scan area of 200nm×200nm of Co3O4 and Co3O4@TiO2 powders after cycling; (c) surface potential image of Au foil acting as reference sample; (d) Work functions of Co3O4 and Co3O4@TiO2
electrodes. Fig. 10 (a) The energy-level model for explaining the improved electron transfer in Co3O4@TiO2 electrodes; (b) A built electric field(E) between Co3O4 and TiO2. -20-
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ACCEPTED MANUSCRIPT Fig. 11 (a-d) The impedance spectra of Co3O4 and Co3O4@TiO2 electrodes under different charge/discharge states; (e,f) Lithium ion diffusion coefficients at
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2.818V 1.878V 1.56V 1.275V 1.171V 1.099V 0.993V 0.893V 0.789V 0.686V 0.558V 0.421V 0.304V 0.192V 0.051V
ltag
e/
1000
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V
1000
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Vo
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0.074V 0.202V 0.318V 0.415V 0.554V 0.699V 0.807V 0.878V 0.984V 1.094V 1.171V 1.255V 1.586V 1.884V 2.901V
2.5
600 300 3.0
0.074V 0.215V 0.321V 0.406V 0.552V 0.699V 0.793V 0.884V 0.973V 1.09V 1.184V 1.25V 1.565V 1.884V 2.943V
0
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1500
1000
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Co3O4
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(511)
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Research highlights
Wreath-like Co3O4@TiO2 are prepared by solvothermal method low-temperature
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hydrolysis process. Co3O4@TiO2 exhibits high reversible capacity, better rate capability and cycling stability.
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TiO2-coating reduces work function of composite and facilitate charge transfer.
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and Li-ion migration.
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The built-in electric field at heterojunction interfaces facilitate electron-transfer
S0925-8388(18)34374-3
DOI:
https://doi.org/10.1016/j.jallcom.2018.11.242
Reference:
JALCOM 48468
To appear in:
Journal of Alloys and Compounds
Received Date: 10 August 2018 Revised Date:
13 October 2018
Accepted Date: 18 November 2018
Please cite this article as: G. Liu, X. Yuan, Y. Yang, J. Tao, Y. Chi, L. Hong, Z. Lin, Y. Lin, Z. Huang, Three-dimensional hierarchical wreath-like Co3O4@ TiO2 as a anode for lithium-ion batteries, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.11.242. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Three-dimensional hierarchical wreath-like Co3O4@ TiO2 as a anode for lithium-ion batteries Guozhen Liu1,2,3, Xinguang Yuan1,2, Yanmin Yang1,2, Jianming Tao1,2, Yubin Chi1,2,
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Lixun Hong1,2, Zhiya Lin1,2, Yingbin Lin*1,2,3, Zhigao Huang1,2,3
1. College of Physics and Energy, Fujian Normal University, Fujian Provincial Key
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Laboratory of Quantum Manipulation and New Energy Materials, Fuzhou，350117, China.
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2. Fujian Provincial Engineering Technology Research Center of Solar Energy Conversion and Energy Storage, Fuzhou,350117, China. 3. Fujian
Provincial
Collaborative
Innovation
Center
for
Optoelectronic
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* Corresponding author: Yingbin Lin Tel: +86-591-2286-8132
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Semiconductors and Efficient Devices, Xiamen, 361005, China
Fax: +86-591-2286-8132 E-mail: [email protected]
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Abstract Wreath-like Co3O4 particles consisting of microplates are synthesized by a facile solvothermal method and subsequently surface-modified with TiO2 ultrathin layer
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using a low-temperature hydrolysis process. Comparing with pure Co3O4, Co3O4@TiO2 exhibits superior electrochemical performances in terms of reversible
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capacity, rate capability and cycling stability. Co3O4@TiO2 exhibits high reversible capacity of 813.0 mAhg-1 at 500 mAg-1 after 180 cycles while the pristine Co3O4 only
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has a discharge capacity of 512.5 mAhg-1. The filling of TiO2 nanoparticles in porous Co3O4 sheets and the TiO2-coating on Co3O4 surface, could effectively suppress large volume expansion of Co3O4 and consequently enhance structural stability during the Li-ion
insertion/extraction
processes.
Analysis
from
the
electrochemical
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measurements reveals that the improved performances should be attributed to reduced the charge-transfer resistance and enhanced Li-ion diffusion kinetics because of
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TiO2-coating. In addition, the reduced work function induced by TiO2-coating is helpful to facilitate electron transfer in composites. Moreover, the built-in electric
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field resulting from the difference in work function between Co3O4 and TiO2, would facilitate electron-transfer and Li-ion migration across heterojunction interfaces.
Key words: lithium-ion batteries; CO3O4; TO2; diffusion coefficient; work function.
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ACCEPTED MANUSCRIPT 1. Introduction In recent years, rechargeable lithium-ion batteries with high energy density and long cycling life, have been paid increasing attention in energy storage systems such as consumer electronics, electric vehicles and smart grid-scale energy storage [1-3].
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Anode materials with high-capacity and long-term cycling are highly desirable for high-performance lithium-ion batteries. Comparing to the commercial graphite with a theoretical capacity of ~372 mAhg-1, transition metal oxides (such as Co3O4) are
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considered to be promising alternative anode materials for high-energy-density lithium ion batteries because of their high reversible capacity and chemical stability
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[4-6]. Among lots of transition metal oxides, cobalt oxide (Co3O4) has been regareded as a promising anode material due to its dramatically high specific capacity of 890 mAhg-1, based on the reversible electrochemical reaction (Co3O4+8Li++8e- ↔ 3Co+4Li2O) [7,8]. Unfortunately, Co3O4 also suffers from sluggish reaction kinetics,
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inferior electrical conductivity and large volumetric expansion during the charge/discharge processes, which induces electrode pulverization, severe capacity decay, poor rate capability and consequently hampers its fascinating application in
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high-energy lithium-ion batteries [9-11]. To address significant drawbacks above,
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considerable strategies have been carried out and preparation of hierarchical Co3O4 with surface-modification is proved to be an effective approach to improve electrochemical performances [12-15]. Hierarchical structure would not only alleviate the strain but also accommodate volume expansion during lithium insertion/extraction processes. For example, Li et al. [12] reported Co3O4 nanowires with a capacity of 836 mAhg-1 at the current of 200 mAg-1. Li et al. [13] reported that 2D-nanoporous Co3O4 nanosheets showed high reversible capacity of 1380 mAhg-1 at a current density of 500 mAg-1 and excellent rate capability (606 mAhg-1 at 10 Ag-1). Liu et al. -3-
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ACCEPTED MANUSCRIPT [14] found that flower-like clusters of Co3O4/C nanosheets delivered a high reversible capacity of 1082 mAhg-1 at 100 mAg-1 and excellent cycling stability. On the other hand, coating of passivation layer on Co3O4 surface is an important strategy to prevent repeated formation of SEI films and the structure degradation [16, 17]. As a promising
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anode, titanium oxide (TiO2) has been extensively investigated because of its low cost, nontoxicity and low volume expansion (3-4%) during the Li-ion insertion/extraction process [18, 19]. Therefore, high-capacity Co3O4 coated with stable-structure TiO2
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layer as hierarchical core/shell structure, should be highly desirable for high-performance lithium-ion batteries with high capacity and excellent cycle stability.
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It is expected that TiO2-coating layer acting as an interfacial barrier could effectively suppress the exothermic reaction between the active material and the electrolyte, resulting in significant improvement in cyclic performances.
Herein, wreath-like Co3O4 consisting of nanosheets has been surface-modified
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with TiO2 nanoparticles via the low-temperature hydrolysis. Such sandwich-like structure endows Co3O4@TiO2 with high specific capacity, excellent rate capability and good cyclic performance as well. The excellent electrochemical performances
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might be related with the enhanced charge transfer deriving from the beneficial
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features of hierarchical structures as well as the strong strain accommodation induced by the TiO2-layer coating, which relieves the large volume expansion of Co3O4. 2.
Experimental
2.1 Preparation of wreath-like Co3O4 and Co3O4@TiO2 Wreath-like Co3O4 consisting of microplates were synthesized by a facile solvothermal method. All chemicals of analytical grade were purchased from Aladdin Ltd. (Shanghai, China) and used as received without any further purification. Typically, 0.704 g of Co(NO)3·6H2O was thoroughly dissolved in 40 ml of absolute -4-
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ACCEPTED MANUSCRIPT methanol, followed by added dropwise of benzyl alcohol (5.9mL) under magnetic stirring. After stirring for 60 minutes, the obtained mixture was transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 180oC for 36 h. The precipitates were separated by centrifugation, washed with deionized water and
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alcohol several times, and subsequently dried at 100°C overnight. Finally, the resulted precursors were calcinated at 450 oC for 4 h in air to get wreath-like Co3O4 powders. Co3O4@TiO2 composites are synthesized by a hydrolysis process at low temperature
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using Co3O4 powders and tetrabutyl titanate (Ti(OC4H9)4) as precursors [20]. 0.1g of the as-prepared Co3O4 powders were dispersed in 40 ml of absolute ethanol and 1 ml
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of deionized water under vigorous stirring at 4 oC. Then, 21.31 µl of Ti(OC4H9)4 was dispensed in absolute ethanol (10 ml) and subsequently added dropwise into above solution. After continuously stirring for another 20 h at 4°C, the obtained precipitates were isolated by centrifugation, dried at 70 °C for 12 h and subsequently calcinated at
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450 °C for 4 h to get wreath-like Co3O4 surface-modified with TiO2 composites. The schematic illustration of the synthesis process for the Co3O4 and Co3O4@TiO2 is shown in Fig. 1.
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2.2 Materials characterizations
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The crystalline structures of the as-prepared samples were investigated by X-ray diffraction (XRD, Rigaku MinFlex II) with a Cu Kα radiation source (λ=0.15406 nm) and Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet, Nexus 870). Morphologies of the as-prepared samples were characterized by scanning electron microscopy (Hitachi SU8010) equipped with an energy-dispersive spectroscopy (EDS). The specific surface areas and pore size distribution of the as-prepared composites were investigated based on nitrogen sorption measurement using a Micromeritics Tristar 3020 analyzer. The TiO2 content in Co3O4@TiO2 composite was -5-
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ACCEPTED MANUSCRIPT determined by inductively coupled plasma OES spectrometer (ICP). The surface potentials of Co3O4 and Co3O4@TiO2 were characterized by Kelvin probe atomic force microscopy (KPAFM) (Bruker dimension ICON, Germany). 2.3 Electrochemical measurements
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The electrochemical performances of the as-prepared composites are evaluated with CR2025-type coin cell using metallic lithium foil as the combined reference and counter electrodes. The working electrodes were prepared by mixing 70 wt.% of the
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active materials (Co3O4 or Co3O4@TiO2), 20 wt.% super-P, and 10 wt.% polyvinylidene fluoride (PVDF) in N-methy1-2-pyrrolidone to form a slurry. The
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resulting slurry is cast onto a copper current collector and subsequently dried in vacuum at 110 oC for 12 h to remove the residual solvent. The electrolyte used in the cells was 1.0M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume). The separator was Cellgard 2300 microporous polyethylene
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membrane. Finally, all electrochemical cells were assembled in an Ar-filled glove box with concentrations of oxygen and moisture level below 1 ppm. Charge–discharge measurements were carried out galvanostatically in a LAND-CT2001A battery test
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system with a potential voltage window of 0.01-3.0 V. The cyclic voltammetry (CV)
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was performed using Arbin instruments BT-2000 battery testing station, and the electrochemical impedance spectra of the electrodes were determined by an electrochemical workstation (Zahner-Zennium) with an amplitude of 5 mV in the frequency range of 10 mHz to 100 KHz.
3 Results and discussion 3.1 material characterization
The typical surface morphologies of the as-prepared Co3O4 and Co2O4@TiO2 powders are characterized by field-emission scanning electron microscopy, shown in -6-
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ACCEPTED MANUSCRIPT Fig. 2. As shown in Fig.2(a, b), Co3O4 powders display wreath-like morphology consisting of self-assembled thin nanosheets with a thickness of ~50nm, in which porous Co3O4 nanoflowers grow on Co3O4 microplate.
Three-dimensional
hierarchical porous structure is expected to be benefit for Li-ion diffusion from the
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electrolyte into the active sites, electron transportation in composites and efficiently relieve large volume expansion during the Li-ion insertion/extraction processes [21]. After TiO2-coating, the composites maintain the nanosheet-bulit wreath-like
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morphologies as same to the pure one, shown in Fig.2(c). In comparison, Co3O4@TiO2 nanosheets display more smooth and integrated surface morphology. It
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is expected that the pores in porous Co3O4 sheet are filled with TiO2 nanoparticles and Co3O4 sheets are deposited by TiO2 layer. The cross-section SEM images of Co3O4@TiO2 nanosheet shown in Fig.2(f) reveals that the thickness of TiO2-coating layer is ca. 25 nm. Such sandwich-like structure would effectively suppress large
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volume expansion of Co3O4 during the Li-ion insertion/extraction processes and consequently enhance structural stability. Due to the existence of surface tension and residual air in the pores among Co3O4 sheets, the tetrabutyl titanate solution cannot
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penetrate into the pores thoroughly. So, It is reasonably to expected that the thickness
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of TiO2 ultrathin layer is becoming thinner from Co3O4 sheets outside to solid-core of Co3O4 nanoflower.
Fig. 3 shows the nitrogen adsorption-desorption isotherms of Co3O4 and
Co3O4@TiO2 powders. As shown in Fig. 3(a), the type IV hysteresis loops at relative pressure (P/P0) between 0.8 and 1.0 reveals the existence of the mesoporous structure for both composites [22, 23]. According to Brunauer-Emmett-Teller equation, the specific surface areas of Co3O4 and Co3O4@TiO2 are evaluated to be 30.23 and 25.07 m2g-1, respectively. Based on Barrett-Joyner-Halenda (BJH) model, the differential -7-
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ACCEPTED MANUSCRIPT pore volume in term of logarithm of pore diameter dV/(dlogD) of the Co3O4 and Co3O4@TiO2 powders was calculated and shown in Fig. 3(b). It is found that that Co3O4@TiO2 have larger average pore size (~30 nm) that that (~21 nm) of Co3O4. The reduction in surface area and increase in average pore size could be reasonably
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derived from the disappearance of micro-pores(2.3 nm) because of TiO2-coating. The analysis of nitrogen adsorption-desorption isotherms is consistent with the SEM images. Fig.4 presents the elemental distribution maps of Co3O4@TiO2 characterized
corresponding elements in Co3O4@TiO2 particles.
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by energy-dispersive spectroscopy, indicating the uniform distribution of the
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Fig.5 (a) presents the X-ray diffraction (XRD) patterns of Co3O4 and Co3O4@TiO2 composites. All of the diffraction peaks are indexed as a spinel structure (JCPDS card No. 42-1467) [24]. No visible differences in XRD patterns between two composites, which is attributed to the low content of TiO2 in composite. Furthermore,
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no secondary phase observed in XRD patterns indicates no chemical reaction occurs between Co3O4 and TiO2 during the calcination process. Fig. 5(b) shows FTIR spectra of Co3O4 and Co3O4@TiO2 powders. Two distinct absorptions observed around 550
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and 660 cm-1 in both samples are related to the stretching vibrations of the Co-O bonds [25, 26], further confirming the formation of Co3O4 phase. In addition, a small
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absorption around 760 cm-1 can be assigned to the vibrations of the anatase TiO2 lattice [27]. No peak-shifting between Co3O4 and Co3O4@TiO2 , shown in the inset of Fig. 5(b), further confirms no chemical reaction occurs between Co3O4 and TiO2 phases. The TiO2 content in Co3O4@TiO2 composite is determined to be ca. 2.83 wt.% by inductively coupled plasma OES spectrometer. Fig.6(a) shows the rate capabilities of Co3O4 and Co3O4@TiO2 electrodes at the various current densities of 200, 500, 1000 and 2000 mAg-1 in rising order for every -8-
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ACCEPTED MANUSCRIPT five successive cycles and subsequently followed by returning 1000, 500 and 200 mAg-1. It is found that the Co3O4@TiO2 electrodes exhibit better rate performances than that of the pristine Co3O4, especially at a higher rate. When the current densities increase from 200 mAg-1 to 2000 mAg-1, the Co3O4@TiO2 electrodes still deliver the
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average capacities of 1038.9, 1066.5, 1082, 1152.3 mAhg-1 respectively, which are larger than those of bare one (1032.1, 1060.1, 1052.9, 1001.3 mAhg-1) under the same conditions. However, when the current density goes back to 1000, 500 and 200 mA g-1,
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the Co3O4@TiO2 electrode also recovers 1130.3, 1207.2 and 1284.7 mAhg-1 while the discharge capacities of the Co3O4 electrode are only 1042.9, 1082.4 and 1140.8 mAg-1
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at the same respective current density. The improved rate performances of Co3O4@TiO2 would be attributed to the insulator-to-conductor transition of anatase TiO2 during the Li+ insertion possess [28, 29] and the stable structure as well. The increase in capacity observed at the initial 10 cycles, especially for Co3O4@TiO2, is
[30,31].
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strongly related to the enhanced Li-ion diffusion kinetics by the activation process
The electrical properties of coating layer should play a crucial role in the Li-ion
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kinetic behavior in surface-coated composites. Work function, defined with respected
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to the Fermi energy of the electrons, reflects the kinetic energy of the electrons to overcome the barrier and thus escape the composite. The effect of TiO2-coating on work functions of the as-prepared composites is investigated by surface potential characterized by Kelvin probe atomic force microscopy. Figs. 6(b, c) presents the surface potential maps of Co3O4 and Co3O4@TiO2 powders over a scan area of 200nm×200nm before cycling. According to our prior work [20,32], the work functions of Co3O4 and Co3O4@TiO2 powders are calculated and the corresponding results are displayed in Fig. 6(d). Here, the work functions of the SFM-tip ( φ tip ) is -9-
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ACCEPTED MANUSCRIPT calibrated by Au foil, whose work function ( φ Au ) is 5.31eV. It is found that Co3O4@TiO2 has a smaller work function (~5.42 eV) than that (~5.50 eV) of the Co3O4. The reduced work function of Co3O4@TiO2 could be explained phenomenologically based on the energy-band model [33]. The smaller work function
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suggests the less energy required for electrons to escape from the composites. As a result, the electrochemical performances of the composites are improved with surface-modification of TiO2.
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Fig.7(a) displays the cycling performances of Co3O4 and Co3O4@TiO2 electrodes
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at a current density of 500 mAg-1. In comparison with Co3O4, the capacity loss is significantly suppressed after coating with TiO2, especially after 70th cycle. The initial discharge capacity at 500 mAg-1 of Co3O4 is 983.9 mAh·g-1 and found to decrease to 512 mAh·g-1 after 180 cycles (i.e., only 52% of its initial discharge capacity). The discharge capacity of the Co3O4@TiO2 retains 85% of its initial
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discharge capacity after 180 cycles. It is clear that Co3O4@TiO2 has significant cycling stability than that of the pristine one after 70th cycle, which might result from the more stable structural stability induced by TiO2-coating. Fig. 7 (b, c) shows typical
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cyclic voltammetry profiles of Co3O4 and Co3O4@TiO2 electrode for the first five
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cycles at a scan rate of 0.2 mV s-1 over the voltage range of 0.01–3 V (vs. Li/Li+). In the first cycle, a strong cathodic peak at ca. 0.75V would be assigned to the initial reduction of Co3O4 to Co and the formation of solid electrolyte interphase (SEI) [34]. In the subsequent anodic scan, a pronounced oxidation peaks are observed at ca. 0.75V, which corresponds to the reversible oxidation of Co to cobalt oxide (Co + Li2O → CoOx +Li+ + 8e-)[35] . It is obvious that, from the second cycle onward, the CV curves overlap of Co3O4@TiO2 electrode is better than that of Co3O4 electrode,
-10-
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ACCEPTED MANUSCRIPT suggesting that the TiO2-coating helps to enhance lithiation/delithiation reversibility and cycle performance [36]. To investigate the influence of TiO2-coating on the charge-transfer kinetics in composites, electrochemical impedance measurements of Co3O4 and Co3O4@TiO2
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electrodes at the Li-ion insertion state are carried out, shown in Fig. 8(a). Both Nyquist plots are comprised of a depressed semicircle in high frequency region and a inclined line in the low frequency region and the obtained results could be fitted using
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the equivalent circuit (Fig. 8(a)), where Re, Rsf, Rct, and Rw are the solution resistance, the impedance of a solid-electrolyte interface (SEI) layer, the charge transfer
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resistance and Warburg impedance, respectively. Based on the equivalent circuit, the Rct values of Co3O4 and Co3O4@TiO2 electrodes are calculated as 134.7 Ω and 90.6 Ω respectively, reflecting the effective suppression on the formation of the resistive reaction layers. Furthermore, the much higher impedance-slope of Co3O4@TiO2 in
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low frequency range than that of the pristine one also reveal the improved Li-ion kinetics after TiO2 coating, shown in Fig. 8(b). Fig. 8(c, d) shows the surface morphologies of Co3O4 and Co3O4@TiO2 electrodes after 30 galvanostatic
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charge/discharge cycles. It is found that Co3O4 powders has more serious
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structure-deterioration (pulverization or aggregation) than that of Co3O4@TiO2 with increasing cycles due to the repeated volume change between metals and metal oxides. The aggregation of the active materials would reduce undoubtedly the contact areas between active materials and the electrolyte, and increase Li-ion diffusion length in composite. The sandwich-like TiO2-coating (Fig. 8(e)) help to remain hierarchical structure, suppress electrode pulverization and consequently offer more active sites for the lithium-ion insertion/extraction. The work functions of the as-prepared composites after cycling are evaluated -11-
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ACCEPTED MANUSCRIPT based on the cell model shown in Fig.9(a), in which the work electrode consists of Co3O4 and Co3O4@TiO2 powders. After cycling, the cells are disassembled in argon-filled glove box and the residual electrolyte on the electrode are washed with ethylene carbonate and ethanol several tmes, and subsequently dried in vacuum at
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100 °C for 12 h to remove the residual solvent. Figs. 9(b, c) shows the surface potential maps over a scan area of 200nm×200nm of Co3O4 and Co3O4@TiO2 powders after 30 cycles. Fig. 9(d) presents the surface potential image of Au foil
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acting as reference sample. Based on the measured surface potential profiles, the calculated work functions of Co3O4 and Co3O4@TiO2 powders are correspondingly
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shown in Fig. 9(e). Here, the work functions of the SFM-tip ( φ tip ) is calibrated by Au foil, whose work function ( φ Au ) is 5.31eV. The work function (~4.90 eV) of Co3O4@TiO2 is found to be smaller than that (~5.15 eV) of the Co3O4. The larger
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work function suggests the more energy required for electrons to escape from the composites. The reduced work function of Co3O4@TiO2 could be explained phenomenologically based on the energy-band model [32]. As shown in Fig.10(a), it
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is expected that electrons would transfer from the composite with smaller work function to the composite with larger work function until the Fermi levels are
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aligned[37,38]. As a result, a corresponding electric field(E) is built up between positively-charged TiO2 and negatively-charge Co3O4 due to electrostatic induction, shown in Fig.10(b). Such electric field could facilitate Li-ion diffusion from TiO2 to Co3O4, and electron transfer from Co3O4 to TiO2 across heterojunction interfaces. As a result, more electrons tends to transfer through TiO2 rather than Co3O4/Co3O4 interface in Co3O4 matrix during the lithium insertion process. Moreover, the sandwich-like structure of TiO2-coating Co3O4 could effectively suppress the pulverization of Co3O4 because of the large volumetric expansion in the charge/discharge process, and -12-
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ACCEPTED MANUSCRIPT consequently improve the electrochemical performances. To investigate the influence of TiO2-coating on the li-ion diffusion during the charge/discharge process, the impedance spectra under different discharge states for Co3O4 and Co3O4@TiO2 electrodes are continuously measured, shown in Fig.11(a-d).
according to the EIS profiles in the low frequency [39, 40].
RT 2
(
1
n F A 2 C Li DLi 2
Z re = R + σω −1 / 2
1/ 2
)
(1)
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σ=
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The diffusion coefficients (DLi) of the Li-ion kinetic of the cells could be calculated
(2)
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A, n and CLi are the surface area of the electrode, the number of electrons per molecule during oxidation and the molar volume of active material; R, T and F are the mass gas constant, absolute temperature and Faraday’s constant;σ, Zre andωare the Warburg factor, the real part of the impedance and the frequency.
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Based on the slope coefficient of Zre to ω−1/2, the corresponding lithium diffusion coefficients DLi of Co3O4 and Co3O4@TiO2 electrodes are calculated respectively. Fig.11(e)
shows
Li-ion
diffusion
kinetics
during
the
lithium-ion
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insertion/de-insertion process. Both electrodes demonstrate similar lithium-ion diffusion behavior. On the whole, Co3O4@TiO2 electrode has larger Li-ion
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diffusion coefficients than those of the bare one, indicating that TiO2-coating does readily facilitate the Li-ion diffusion in composites. 4. Conclusions
Wreath-like Co3O4 coated with TiO2 powders were prepared by solvothermal method and subsequent low-temperature hydrolysis process. In comparison, Co3O4@TiO2 exhibits higher reversible capacity, better rate capability and excellent cycling stability. Analysis from the electrochemical -13-
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ACCEPTED MANUSCRIPT measurements reveals that the improved performances should be attributed to enhanced Li-ion diffusion kinetics, reduced charge-transfer resistance and more structural stability with TiO2-coating. Moreover, analysis from Kelvin probe force microscopy measurements indicates that the built-in electric field
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resulting from the difference in work function would be help to facilitate electron-transfer and Li-ion migration across heterojunction interfaces Acknowledgements
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This work is supported by a grant from Key Project of Department of Science & Technology of Fujian Province (No.2014H0020), Program for New Century Excellent
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Talents in University of Fujian Province (No.JA14069), and Solar Energy Conversion & Energy Storage Engineering Technology Innovation Platform (No. 2018L3006). References
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Figure Captions Fig. 1
Schematic illustration of the preparation process for Co3O4 and Co3O4@TiO2 powders. SEM images of the as-prepared Co3O4 and Co3O4@TiO2 powders.
Fig. 3
(a) Nitrogen sorption isotherms and (b) pore diameter distribution of Co3O4 and Co3O4@TiO2 powders.
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Fig. 2
Element mapping images of Co3O4@TiO2 powders.
Fig. 5
(a) XRD patterns and (b) FTIR spectra of Co3O4 and Co3O4@TiO2 powders.
Fig. 6
(a) Rate capability of Co3O4 and Co3O4@TiO2 electrodes; (b,c) Surface
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Fig. 4
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potential maps over a scan area of 200nm×200nm of Co3O4 and Co3O4@TiO2 powders before cycling; (d) Work functions of Co3O4 and Co3O4@TiO2 electrodes. Fig. 7
(a) Cyclic performances and (b, c) cyclic voltammetry profiles of Co3O4 and
Fig. 8
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Co3O4@TiO2 electrodes;
(a) Typical EIS of Co3O4 and Co3O4@TiO2 electrode in the fully discharged state and the equivalent circuit for EIS fitting; (b) Real parts of the complex
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impedance Zre vs. ω−1/2 for Co3O4 and Co3O4@TiO2 electrodes; (c, d) SEM
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images of the Co3O4 and Co3O4@TiO2 powders after 30 cycles, respectively. Fig. 9
(a,b) Surface potential maps over a scan area of 200nm×200nm of Co3O4 and Co3O4@TiO2 powders after cycling; (c) surface potential image of Au foil acting as reference sample; (d) Work functions of Co3O4 and Co3O4@TiO2
electrodes. Fig. 10 (a) The energy-level model for explaining the improved electron transfer in Co3O4@TiO2 electrodes; (b) A built electric field(E) between Co3O4 and TiO2. -20-
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ACCEPTED MANUSCRIPT Fig. 11 (a-d) The impedance spectra of Co3O4 and Co3O4@TiO2 electrodes under different charge/discharge states; (e,f) Lithium ion diffusion coefficients at
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different charge/discharge states for Co3O4 and Co3O4@TiO2 electrodes.
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ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 1
-22-
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ACCEPTED MANUSCRIPT
M AN U
SC
RI PT
Figure 10
AC C
EP
TE D
(b)
-31-
Page 31 of 32
ACCEPTED MANUSCRIPT Figure 11
2.818V 1.878V 1.56V 1.275V 1.171V 1.099V 0.993V 0.893V 0.789V 0.686V 0.558V 0.421V 0.304V 0.192V 0.051V
ltag
e/
1000
2.0
2.5
V
1000
Ω
Vo
2000 1.5
3.0 0
0 0.5
3000
2000
1.0
(c)
900 600 300
600 300
2.5 3.0
V
0
TE D
1.5
(e)
Co3O4
Co3O4@TiO2
1.0
Charge
0.0
0.5
1.0
1.5
2.0
2.5
3.0
AC C
0.0
EP
0.5
Charge
600
300
1500 1200 900
1.0
Vo 1.5 2.0 lta ge /V
2 -1
2.0
-15
Diffusion coefficient/× 10 cm s
2 -1
Vo ltag e/
1.5
0
900
0 0.5
0.074V 0.202V 0.318V 0.415V 0.554V 0.699V 0.807V 0.878V 0.984V 1.094V 1.171V 1.255V 1.586V 1.884V 2.901V
2.5
600 300 3.0
0.074V 0.215V 0.321V 0.406V 0.552V 0.699V 0.793V 0.884V 0.973V 1.09V 1.184V 1.25V 1.565V 1.884V 2.943V
0
Ω
1.0
1500 1200 900
Zr e/
0 0.5
Co3O4@TiO2 1200
Z img / Ω
1200
1500
M AN U
Discharge
(d)
-15
Co3O4@TiO2
2.83V 1.947V 1.596V 1.266V 1.16V 1.101V 0.995V 0.896V 0.782V 0.677V 0.551V 0.48V 0.305V 0.199V 0.052V
Diffusion coefficient/× 10 cm s
1500
1000
SC
Vo 1.5 2.0 ltag 2.5 e/ V 3.0
Ω
3000 1.0
2000
Zr e/
0 0.5
Charge
Co3O4
/Ω
1000
3000
Zr e
Zimg / Ω
2000
(b)
RI PT
Discharge
Co3O4
Zr e/
3000
Z img / Ω
(a)
2.5
(f)
Co3O4 Co3O4@TiO2
2.0
Discharge
1.5 1.0 0.5 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Voltage/V
Voltage/V
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ACCEPTED MANUSCRIPT Figure 2
(a) Co3O4
M AN U
SC
RI PT
(b) Co3O4
(d) Co3O4@TiO2
(f) Co3O4@TiO2
AC C
EP
(e) Co3O4
TE D
(c) Co3O4@TiO2
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ACCEPTED MANUSCRIPT Figure 3
160 Co3O4
120
RI PT
Co3O4@TiO2
80
SC
40
0 0.0
0.2
M AN U
Volume Adsorbed / cm3g-1
(a)
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
0.30
Co3O4 Co3O4@TiO2
0.20 0.15 0.10
TE D
0.25
EP
3 -1
dV/dlogD / cm g ,STP
(b)
AC C
0.05 0.00
0
10
20
30
40
50
60
70
Pore diameter / nm
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ACCEPTED MANUSCRIPT Figure 4
(b) Co K
M AN U
SC
RI PT
(a) Co3O4@TiO2
(d) O K
AC C
EP
TE D
(d) Ti K
( a ) LiNi0.5Mn1.5O4
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ACCEPTED MANUSCRIPT
Co3O4
10
20
30
M AN U
PDF#73-1701
SC
Co3O4@TiO2
40
50
(533)
RI PT
(440)
(511)
(422)
(222)
Intensity / a.u.
(111)
(220)
(a)
(400)
(311)
Figure 5
60
70
80
2θ / Degree
100
(b)
Co3O4
40
760
TiO2
Co3O4@TiO2
TE D
60
120 100
Co3O4 Co3O4@TiO2
80 60
660 550
20
40
EP
Transmittance (%)
80
0
AC C
-20 4000
20
0 700
3500
650
3000
600
2500
550
2000
500
1500 -1
1000
500
Wavenumber / cm
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ACCEPTED MANUSCRIPT Figure 6
1400
( b ) Co3O4 Co3O4
1300
Co3O4@TiO2 -1
RI PT
Current density: mAg
1200 1100 1000 900
0
5
10
15
20
200
500
1000 2000 1000
500
200
25
30
SC
Discharge Capacity / mAhg-1
(a)
35
M AN U
Cycle number
( c ) Co3O4@TiO2
(d)
5.40
Co3O4
5.44 5.48 5.52 Work function / eV
5.56
AC C
EP
TE D
Counts / a.u.
Co3O4@TiO2
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ACCEPTED MANUSCRIPT Figure 7
(a)
1250 1000 750
RI PT
Discharge Capacity / mAhg
-1
1500
Co 3O 4
500
Co 3O 4@TiO 2
250
@ 500mAg
0
30
-1
60
90
120
150
0.2 0.1
1st
(b)
5th
M AN U
0.0
Current / mA
180
SC
Cycle Number
-0.1 -0.2 -0.3
Co3O 4
-0.4
@0.2mVs
-0.5
TE D
-0.6 0.0
0.5
1.0
1.5
2.0
-1
2.5
3.0
Voltage / V
1.0
1st
(c)
0.5
-0.5
AC C
Current / mA
EP
5th
0.0
-1.0 Co3O4@TiO2 -1
-1.5 -2.0 0.0
@0.2mVs
0.5
1.0
1.5
2.0
2.5
3.0
Voltage / V
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ACCEPTED MANUSCRIPT Figure 8
1800
2000 (b)
(a)
Co3O4
1600
Slope=332.86
Zre / Ω
1200 900
1200 800
RI PT
Co3O4@TiO2
Slope=227.67
600
Co3O4
400
300
Co3O4@TiO2
0
0
300
600
900
Zre / Ω
1200
1500
1800
0
1.0
1.5
2.0
2.5
ω
(c)
−1/2
3.5
4.0
1/2
/s
AC C
EP
TE D
M AN U
(d)
3.0
SC
Zimg / Ω
1500
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ACCEPTED MANUSCRIPT Figure 9
SC
RI PT
(b) Co3O4
(d) Au
TE D
M AN U
(c) Co3O4@TiO2
AC C
EP
Counts / a.u.
(e)
4.85
Co3O4
Co3O4@TiO2
4.90
4.95
5.00
5.05
5.10
5.15
5.20
Work function / eV
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ACCEPTED MANUSCRIPT
Research highlights
Wreath-like Co3O4@TiO2 are prepared by solvothermal method low-temperature
RI PT
hydrolysis process. Co3O4@TiO2 exhibits high reversible capacity, better rate capability and cycling stability.
SC
TiO2-coating reduces work function of composite and facilitate charge transfer.
AC C
EP
TE D
and Li-ion migration.
M AN U
The built-in electric field at heterojunction interfaces facilitate electron-transfer