Porous polyhedral and fusiform Co3O4 anode materials for high-performance lithium-ion batteries

Accepted Manuscript Title: Porous polyhedral and fusiform Co3 O4 anode materials for high-performance lithium-ion batteries Author: Guoyong Huang Shengming Xu Shasha Lu Linyan Li Hongyu Sun PII: DOI: Reference:

S0013-4686(14)01004-4 http://dx.doi.org/doi:10.1016/j.electacta.2014.05.023 EA 22715

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

23-2-2014 21-4-2014 5-5-2014

Please cite this article as: G. Huang, S. Xu, S. Lu, L. Li, H. Sun, Porous polyhedral and fusiform Co3 O4 anode materials for high-performance lithium-ion batteries, Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.05.023 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.

Porous polyhedral and fusiform Co3O4 anode

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materials for high-performance lithium-ion batteries

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China

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a

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Guoyong Huang,a Shengming Xu,a,b,* Shasha Lu,a Linyan Li,a Hongyu Sun c

Beijing Key Lab of Fine Ceramics, Tsinghua University, Beijing 100084, China

c

Beijing National Center for Electron Microscopy, School of Materials Science and Engineering,

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b

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Tsinghua University, Beijing 100084, China

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KEYWORDS: cobalt oxide, porous, polyhedral, fusiform, lithium-ion batteries

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ABSTRACT: Co3O4 is commonly used as a potential anode material for Li-ion batteries (LIBs). In this study, novel porous polyhedral and fusiform Co3O4 powders have been synthesized successfully through the hydrothermal method with different solvents followed by thermal treatment. It is shown that both of the polyhedrons (1.0-3.0 µm in side length) and the spindles (2.0-5.0 µm in length, 0.5-2.0 µm in width) are composed of similar irregular nanoparticles (20200 nm in diameter, 20-40 nm in thickness) bonded to each other. Evaluated by electrochemical measurements, both of them have high initial discharge capacities (1374.4 mAhg-1 and 1326.3

*

Corresponding

author:

Tel:

+86-10-62773585;

Fax:

+86-10-62773585;

E-mail:

[email protected] (S. Xu)

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mAhg-1) and enhanced cycling stabilities at the low rate (the capacity retention ratios at 0.1 C after 70 cycles are 91.6% and 92.2%, respectively). However, the rate capability of the spindles (93.8%, 90.1% and 98.9% of the second discharge capacities after 70 cycles at 0.5 C, 1 C and 2

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C, respectively) is better than the polyhedrons’ (only 76.2%, 42.1% and 59.3% under the same conditions). Remarkable, the unique morphologies and special structures may be extended to

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synthesize other similar transition metal oxides (NiO, Fe3O4, et al.) as high performance anodes

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for LIBs.

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1. Introduction

Co3O4, one of transition metal oxides, is accounted as a promising anode material for high

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performance Li-ion batteries (LIBs) due to its high theoretical specific capacity, high tap density and stable chemical property [1-4]. However, because of its large volume change upon insertion

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and extraction of lithium-ions and poor electrical conductivity, its capacity retention and rapid-

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rate capability should be improved further [5-8]. As you know, the morphology of electronic

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material (e. g., sphere [9], wire [10], platelet [11], rod [12], sheet [13,14], tube [15], et al.) usually affects the size of surface area, the number of reaction sites, the diffusion length of ions and electrons, and the volume expansion of Li-ions inserting and extracting, so it is one of crucial factors to impact its property [16,17]. And some special morphological Co3O4 powders with enhanced performance have been synthesized as follows. Uniform multi-shelled Co3O4 hollow microspheres have a high reversible capacity (1616 mAhg-1 at the 30th cycle under 50 mAg-1) [18]. Single-crystal Co3O4 nanocages with highly exposed {110} reactive facets exhibit good cycling stability (864 mAhg-1 after 50 cycles at 0.2 C) [19]. Mesoporous and single-crystal Co3O4 nanoplates (30 nm in thickness, 1 µm in width) with large surface area (118.6 m2g-1) and small average pore size (4.7 nm) exhibit superior cycling performance at low rate (1000 mAhg-1

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after 30 cycles at 0.2 C) [20]. Highly symmetric Co3O4 hexapods (~ 12 µm in length) assembled by numerous nanorods (150 nm in diameter) with huge surface area (134.84 m2g-1) exhibit passable rate capability (664 mAhg-1 after 50 cycles at 500 mAg-1) [21]. Hierarchical urchin-like

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Co3O4 spheres (5-8 µm in diameter) consisting of many nanowires and nanoparticles (10-50 nm) possess improved cycling stability (1190 mAhg-1 after 100 cycles at 0.1 C) and excellent rate

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capability (796 mAhg-1 at 5 C and 433 mAhg-1 at 10 C) [22]. A Co3O4 nanobelt array ( 20-50 nm

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in width with needle-like tips) was prepared with excellent rate capability (530 mAhg-1 and 320 mAhg-1 after 30 cycles at 15 C and 30 C, respectively) [23]. However, the reports about

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polyhedral and fusiform Co3O4 powders are very rare [24], and it is necessary to do research about the contrastive electrochemical performance of different morphological Co3O4 powders

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[25].

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Herein, through the hydrothermal method with different solvents followed by thermal

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treatment, unique porous polyhedral and fusiform Co3O4 powders have been synthesized successfully as anode materials for LIBs. The Co3O4 powders inherit the special morphologies of

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CoCO3 precursors and demonstrate the porous structure after heated. Evaluated by electrochemical measurements, the differences of their electrochemical performance such as rate capability and capacity retention are contrasted as expected. 2. Experimental Section

2.1. Preparation of samples

Porous polyhedral Co3O4 powders: Co(CH3COO)2•4H2O (2.50 g), CO(NH2)2 (3.00 g) and polyvinylpyrrolidone (PVP) (2.00 g) were dissolved in the mixed solvent (a volume ratio of deionized water/ethylene glycol = 1:9) step-by-step under vigorous stirring to obtain 100 mL transparent solution. Then, it was transferred into a Teflon-lined stainless steel autoclave (140 mL), and a thermal treatment was performed for the sealed autoclave in an electric oven at 160

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°C for 10 h. After the reaction was over and the autoclave was cooled naturally, the precursor in the autoclave was collected and washed by centrifugation for three cycles using deionized water and three cycles using pure ethanol. And it was dried in a vacuum oven at 60 °C for 24 h. At last,

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the precursor was transferred into a muffle electric furnace, another thermal treatment was performed at 600 °C in air for 10 h with a heating ramp of 10 °C·min-1. Then, the black product

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was collected and attired by hand.

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Porous fusiform Co3O4 powders: except using n-butyl alcohol instead of ethylene glycol, other operations were the exact same as the preparation of the porous polyhedral Co3O4 powders.

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2.2. Characterization of samples

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The crystal phases of samples were characterized by X-ray powder diffraction (XRD) (Rigaku, D/Max-2000) using CuKα radiation with graphite monochromator at a scanning rate of

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1°·min-1 with 2theta ranging of 10°-80°. The specific surface area and pore size distribution were

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calculated by Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method

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using the specific surface area and porosity analyzer (Micromeritics, Gemini VII 2390). The micro morphologies and others were observed by scanning electron microscope (SEM) (JEOL, JSM 6301 and JSM 5500) and transmission electron microscope (TEM) (JEOL, JSM 2100). 2.3. Electrochemical measurements

The electrochemical performance of Co3O4 powders as electrodes was evaluated through coin cells with lithium metal as the reference electrodes. The working electrode was composed of Co3O4, acetylene black (ATB) and polytetrafluoroethylene (PTFE) with a weight ratio of -1 Co3O4/ATB/PTFE = 7:2:1. The electrolyte solution is 1 mol·L LiPF6 dissolved in the mixture of

ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) with a volume ratio of EC/PC/DEC = 3:1:1. The electrode capacity was measured by the galvanostatic 4

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discharge-charge method at about 25 °C by the electrochemical test instruments (Land, CT2001A). And the electrochemical impedance spectroscopy (EIS) was obtained over a frequency range from 1 MHz to 0.01 Hz at AC voltage of 10 mV amplitude by electrochemical

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workstation (Princeton, Parstat2273).

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3. Results and discussion

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The possible preparation processes of porous Co3O4 powders with different shapes are illustrated by Scheme 1. And the processes include two steps. (1) Under the hydrothermal

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condition, CO(NH2)2 molecules could hydrolyze homogeneously, and CO32- anions are released. Then, CoCO3 particles are formed through the precipitation reaction of Co2+ and CO32-. With the

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influence of different solvents, the CoCO3 particles could grow up to form different morphologies. So the polyhedral CoCO3 precursor has been synthesized through ethylene glycol,

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while the fusiform CoCO3 precursor has been compounded using n-butyl alcohol. It indicates

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that the solvent may play an important role for the morphology of CoCO3, and it is a plausible

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explanation that the increase of viscosity in the reaction system restricts the growth of CoCO3 particles in some directions when the solvent is changed from ethylene glycol into n-butyl alcohol [26-27]. (2) After calcinated under 600°C in air, the CoCO3 precursors could easily turn to porous-structured Co3O4 powders owing to the decomposition reactions and the release of gas CO2 [28-30]. However, the special morphologies of samples are inherited perfectly. The X-ray powder diffraction (XRD) patterns of the precursor and the calcined product are shown in Fig. 1. The diffraction peaks are indexed as a pure hexagonal phase of CoCO3 (JCPDS no.78-0209), while the other diffraction peaks also match well with the standard pattern of cubic phase Co3O4 (JCPDS no.42-1467). No diffraction peaks of other impurities are obtained. Calculated from the data obtained by Brunauer-Emmett-Teller (BET) nitrogen adsorption 5

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isotherms, the specific surface areas of polyhedral CoCO3 precursor and Co3O4 product are about 3.06 m2g-1 and 5.29 m2g-1, respectively (Fig. 2a), at the same time, the specific surface areas of fusiform CoCO3 precursor and Co3O4 product are about 5.10 m2g-1 and 10.32 m2g-1, respectively

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(Fig. 2b). It indicates that the specific surface areas of products with new multi-porous structures are bigger than the precursors’ and the specific surface areas of the spindles are also bigger than

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the polyhedrons’. Meanwhile, measured from the data obtained by Barrett-Joyner-Halenda (BJH)

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method (insets in Fig. 3a and Fig. 3b), the average pore sizes of polyhedral and fusiform Co3O4 powders are about 1.91 nm and 2.45 nm, and both of them have narrow size distributions (more

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than 90% in the range of 1.00-5.00 nm).

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The scanning electron microscope (SEM) and transmission electron microscope (TEM) images of CoCO3 precursors with different shapes are shown in Fig. 4. It indicates that one

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sample is composed of many regular monodispersed polyhedral particles with the edge length

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range of 1.0-3.0 µm (Fig. 4a and Fig. 4c), and the other sample consists of massive fusiform particles with the length range of 2.0-5.0 µm and the width range of 0.5-2.0 µm (Fig. 4b and Fig.

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4d). In addition, it is further proved that the morphologies of the corresponding samples are polyhedral shape and fusiform shape clearly by the TEM images (Fig. 4e and Fig. 4f). It is shown that the Co3O4 samples are also regular polyhedral and fusiform shapes with multi-porous structures, greatly inheriting the morphologies and sizes of the corresponding precursors by the low-magnification SEM images (polyhedrons, Fig. 5a and Fig. 5c; spindles, Fig. 5b and Fig. 5d). Further, observed through the high-magnification SEM images (Fig. 5e and Fig. 5f), both of the polyhedron and the spindle are assembled by many irregular nanoparticles bonded to each other with the diameter range of 20-200 nm and the thickness of 20-40 nm, and lots of irregular pores can be clearly seen among the nanoparticles. Furthermore, it indicates that the porous Co3O4 polyhedron and spindle consist of nanoparticles again form the TEM images (Fig. 6a and Fig. 6

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6d). Shown by the high-resolution transmission electron microscope (HRTEM) images (Fig. 6b and Fig. 6e), the lattice fringe of polyhedron with the lattice spacing of 0.25 nm matches well with the (311) plane of cubic phase Co3O4, while the lattice fringe of spindle with the lattice

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spacing of 0.29 nm also matches well with the (220) plane. What is more, the corresponding fast Fourier transformation (FFT) patterns are clear regular diffraction spots shown by the insets in

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Fig. 6b and Fig. 6e, and similar clear regular diffraction spots are also shown by the selected area

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electron diffraction (SAED) patterns (Fig. 6c and Fig. 6f), which can be indexed to [0-11] and [11-4] zone axes. Therefore, the nanoparticles may be different single-crystal characteristics

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[31,32].

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The performance of Co3O4 electrodes with novel morphologies was evaluated by various electrochemical tests. The discharge and charge profiles of the polyhedral and fusiform Co3O4

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electrodes for the initial cycle in the voltage range from 0.01 V to 3.00 V (vs. Li+/Li) at different

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rates are shown in Fig. 7 (polyhedral shape, Fig. 7a; fusiform shape, Fig. 7b). All of the

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discharge and charge curves are similar change trends matched the previous reports: in the discharge curve, the potential value quickly falls to the plateau (~ 1.00 V) and then gradually declines to the cut-off voltage (0.01 V); in the charge curve, the potential value slowly goes up to the plateau (~ 2.00 V) and then gradually increases to the top voltage (3.00 V) [33-35]. In Fig. 7a, the initial discharge capacities at the rates of 0.1 C, 0.5 C, 1 C and 2 C are 1374.4 mAhg-1, 1112.1 -1 -1 -1 mAhg , 1118.6 mAhg and 916.6 mAhg , respectively; while the first charge capacities at the

same rates are 998.2 mAhg-1, 909.9 mAhg-1, 858.0 mAhg-1 and 672.7 mAhg-1, respectively. On -1 -1 the other hand, in Fig. 7b, the discharge capacities are 1326.3 mAhg , 1277.2 mAhg , 1216.9

mAhg-1 and 1055.7 mAhg-1 under the same conditions, respectively; and the charge capacities are -1

-1

-1

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990.5 mAhg , 945.9 mAhg , 923.8 mAhg and 819.7.7 mAhg , respectively. Compared with the polyhedral Co3O4, the initial discharge and charge capacities of fusiform Co3O4 are much higher 7

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at high rates such as 1 C and 2 C. As you see, both of polyhedral and fusiform Co3O4 powders exhibit extra capacities over the theoretical value of Co3O4 (890 mAhg-1), which is likely caused by the reversible formation/dissolution of the polymer/gel-like film contributing to an additional

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reversible capacity besides the electrochemical conversion reaction between cobalt oxide and Co

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[5,20,36].

The change trends of discharge capacity versus cycle number of the polyhedral and

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fusiform Co3O4 electrodes at different rates of 0.1 C, 0.5 C, 1 C and 2 C are shown by Fig. 8

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(polyhedral shape, Fig. 8a; fusiform shape, Fig. 8b). After 70 cycles, the capacity retention ratios of polyhedral Co3O4 electrodes (versus the second discharge capacities) are 91.6%, 76.2%,

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42.1% and 59.3%, respectively; while the capacity retention ratios of fusiform Co3O4 electrodes are 92.2%, 93.8%, 90.1% and 98.9%, respectively. The change trends of two shapes at the low

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rate (0.1 C) are very similar: both of them have high initial discharge capacities and enhanced

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cycling stabilities. However, at the high rates (0.5 C, 1 C and 2 C), the cycling stabilities of polyhedral shape are very poor, on the contrary, the discharge capacities of fusiform shape nearly

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keep the same as the second values even after 70 cycles. In other words, the rate capability of the spindle is much better than the polyhedron’s. To further investigate the rate capability, the polyhedral and fusiform Co3O4 electrodes were tested at various constant rates between 0.1 C and 8 C shown in Fig. 9. The average discharge capacities (except the first discharge capacity) of the polyhedral Co3O4 are 971.2 mAhg-1, 982.6 mAhg-1, 937.6 mAhg-1, 730.3 mAhg-1, 82.7 mAhg-1 and 40.2 mAhg-1 at 0.1 C, 0.5 C, 1 C, 2 C, 4 C and 8 C, respectively. On the other hand, under the -1 same conditions, the average discharge capacities of the fusiform Co3O4 are 944.1 mAhg , 964.0

mAhg-1, 927.3 mAhg-1, 844.6 mAhg-1, 665.7 mAhg-1 and 439.5 mAhg-1, respectively. Then, both of them can regain to deliver high discharge capacities (1046.5 mAhg-1 and 1046.1 mAhg-1) when the rate turns back to 0.1 C after 30 cycles. The excellent capacity retention of the fusiform shape

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at high rates (4 C and 8 C) is proved again. Basically, two superiorities of the spindles are here as follows. (1) The particle size of the fusiform shape is smaller than the polyhedral shape’s, and the specific surface area of the former is as two times as the latter’s. So the area of interface and

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the number of reaction sites of the former are more than the latter’s. (2) Compared with the intact polyhedrons, the spindles with loose stratified structures (Fig. 5f) can make diffusion lengths to

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be shorter and lithium-ions to diffuse much easier.

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To investigate the enhanced electronic conductivity, the electrochemical impedance

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spectroscopy (EIS) measurements of the polyhedral and fusiform Co3O4 electrodes were performed with the frequency range from 1 MHz to 0.01 Hz at the 1st and 70th cycle at 2 C

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shown in Fig. 10. All of the measured values have similar Nyquist plots, which are composed of semicircles at high-frequency region and approximate straight lines at low-frequency region. As

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you know, the semicircle represents the charge transfer resistance and the straight line indicates

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the lithium-ions diffusion process in electrodes [37-41]. And the semicircles’ diameters of fusiform Co3O4 electrodes are shorter than the polyhedral Co3O4 electrodes’ under the same

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conditions (especially the 70th cycle), which indicates that the charge transfer resistance values of the former are much smaller than the latter. And it is likely caused by the larger specific surface area and average pore size of the former than the latter’s to enlarge the contacted surface area with conductive additive (ATB) and promote the electrolyte diffusion. Therefore, it may be one of the reasons that the capacity retention ratios of the fusiform shape are better than the polyhedral shape’s at high rates. In addition, The SEM images of the Co3O4 electrodes (the mixtures of Co3O4/ATB/PVDF) after 70 cycles at 2 C are shown in Fig. 11 (polyhedral shape, Fig.11a; fusiform shape, Fig.11b), both of structures of the Co3O4 polyhedron and spindle after

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70 cycles at 2 C just have a little change, and they both display attractive morphological stability for the good cycling performance.

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4. Conclusions In summary, through the hydrothermal method with different solvents followed by thermal

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treatment, novel porous polyhedral and fusiform Co3O4 powders have been synthesized successfully. The specific surface areas of them are 5.29 m2g-1 and 10.32 m2g-1, and the average

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pore sizes are about 1.91 nm and 2.45 nm, respectively. In addition, the polyhedrons (1.0-3.0 µm in side length) and the spindles (2.0-5.0 µm in length, 0.5-2.0 µm in width) are both composed of

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similar irregular nanoparticles (20-200 nm in diameter, 20-40 nm in thickness) bonded to each

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other, and both of them greatly inherit the morphologies and sizes of the corresponding CoCO3 precursors. Evaluated by electrochemical measurements, both of them have high initial discharge

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capacities (1374.4 mAhg-1 and 1326.3 mAhg-1) and enhanced cycling stabilities at low rate (the

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capacity retention ratios at 0.1 C after 70 cycles are 91.6% and 92.2%, respectively). However, the rate capability of spindles (93.8%, 90.1% and 98.9% of the second discharge capacities after

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70 cycles at 0.5 C, 1 C and 2 C, respectively) is better than the polyhedrons’ (only 76.2%, 42.1% and 59.3% under the same conditions). Remarkable, the unique morphologies and special structures may be extended to synthesize other similar transition metal oxides (CoO, NiO, MnO, CuO, Fe3O4, et al.) as high performance anodes for LIBs. Acknowledgment

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51274130 and 51074096). References [1] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Tin-based amorphous oxide: a high-capacity

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129. [31] G.H. Zhang, Y.J. Chen, B.H. Qu, L.L. Hu, L. Mei, D.N. Lei, Q. Li, L.B. Chen, Q.H. Li, T.H. Wang, Synthesis of mesoporous NiO nanospheres as anode materials for lithium ion batteries, Electrochim. Acta 80 (2012) 140.

ip t

[32] D.Q. Liu, Z.B. Yang, P. Wang, F. Li, D.S. Wang, D.Y. He, Preparation of 3D nanoporous copper-supported cuprous oxide for high-performance lithium ion battery anodes, Nanoscale 5 (2013) 1917.

[33] X.G. Liu, S.W. Or, C.G. Jin, Y.H. Lv, W.H. Li, C. Feng, F. Xiao, Y.P. Sun, Co3O4/C nanocapsules with onion-

cr

like carbon shells as anode material for lithium ion batteries, Electrochim. Acta 100 (2013) 140.

[34] X.X. Zhang, Q.S. Xie, G.H. Yue, Y. Zhang, X.Q. Zhang, A.L. Lu, D.L. Peng, A novel hierarchical network-

us

like Co3O4 anode material for lithiumbatteries, Electrochim. Acta 111 (2013) 746.

[35] M.Y. Son, J.H. Kim, Y.C. Kang, Study of Co3O4 mesoporous nanosheets prepared by a simplespray-drying process and their electrochemical properties as anode material for lithium secondary batteries, Electrochim.

an

Acta 116 (2014) 44.

[36] X.W. Lou, D.D. Jim, Y. Lee, A.L. Archer, Thermal formation of mesoporous single-crystal Co3O4 nanoneedles and their Lithium storage properties, J. Mater. Chem. 18 (2008) 4397.

M

[37] C.N. He, S. Wu, N.Q. Zhao, C.S. Shi, E.Z. Liu, J.J. Li, Carbon-encapsulated Fe3O4 nanoparticles as a highrate lithium ion battery anode material, ACS Nano 5 (2013) 4459. [38] L. Hu, P. Zhang, H. Zhong, X.R. Zheng, N. Yan, Q.W. Chen, Foamlike porous spinel MnxCo3-xO4 material

te

Eur. J. 18 (2012) 15049.

d

derived from Mn3[Co(CN)6]2·nH2O nanocubes: a highly efficient anode material for lithium batteries, Chem. [39] L. Hu, H. Zhong, X.R. Zheng, Y.M. Huang, P. Zhang, Q.W. Chen, CoMn2O4 spinel hierarchical microspheres

Ac ce p

assembled with porous nanosheets as stable anodes for lithium-ion batteries, Sci. Rep. 2 (2012) 1. [40] P.J. Zuo, G.Y. Cheng, L.G. Wang, Y.L. Ma, C.Y. Du, X.Q. Cheng, Z.B. Wang, G.P. Yin, Ascorbic acid-assisted solvothermal synthesis of LiMn0.9Fe0.1PO4/C nanoplatelets with enhanced electrochemical performance for lithium ion batteries, J. Power Sources 243 (2013) 872. [41] J.X. Guo, L. Chen, X. Zhang, B. Jiang, L.Z. Ma, Sol-gel synthesis of mesoporous Co3O4 octahedra toward high-performance anodes for lithium-ion batteries, Electrochim. Acta 129 (2014) 410.

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ip t cr us an M (311)

Co3O4

10

20

(533) (622)

(440)

(511)

(422)

(104)

(222)

(400)

(220)

30

40

50

(122)

(116) (018)

(202)

(113)

(110)

(a)

CoCO3

(012)

(111)

(b)

60

(214) (208) (300)

Ac ce p

X-ray Intensity / Arb. Units

te

d

Scheme 1. The possible preparation processes of porous Co3O4 powders with polyhedral shape and fusiform shape.

70

80

2Theta (deg)

Fig. 1. XRD patterns of CoCO3 and Co3O4 powders.

15

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a

CoCO3

-1 3

Co3O4

2 -1

ip t

BET Surface Area: 5.29 m g

1

2 -1

BET Surface Area: 3.06 m g

cr

Volume Adsorbed (cm g )

2

0 0.10

0.15

0.20

0.25

us

0.05

Relative Pressure (P/P0) b

3

2 -1

an

3

-1

Volume Adsorbed (cm g )

BET Surface Area: 10.32 m g

2

2 -1

M

BET Surface Area: 5.10 m g

CoCO3

te

d

1

0.05

Ac ce p

0 0.00

0.10

Co3O4

0.15

0.20

0.25

Relative Pressure (P/P0)

Fig. 2. N2 adsorption isotherms of CoCO3 and Co3O4 powders. (a) polyhedral shape and (b) fusiform shape.

16

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a

14

Volume Adsorbed (cm g STP)

12 0.003

Average Pore Size: 1.91nm

3 -1

10

8

0.002

ip t

-1

dV/dr (cm g nm )

0.001

6 0.000 0

5

10

15

20

25

30

cr

Pore Size (nm)

4

2

Adsorption Desorption

0.0

0.2

0.4

0.6

us

3 -1

0.004

0.8

1.0

b

an

Relative Pressure (P/P0) 40

M

0.006

Average Pore Size: 2.45nm

3 -1

-1

dV/dr (cm g nm )

30

3 -1

0.004

d

0.002

20 0.000

te

Volume Adsorbed (cm g STP)

0.008

0

5

10

15

20

Pore Size (nm)

Ac ce p

10

0 0.0

0.2

Adsorption Desorption 0.4

0.6

0.8

1.0

1.2

Relative Pressure (P/P0)

Fig. 3. N2 adsorption/desorption isotherms of Co3O4 powders and the insets are the BJH pore-size distributions of the corresponding materials. (a) polyhedral shape and (b) fusiform shape.

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ip t cr us an M d te Ac ce p

Fig. 4. SEM and TEM images of CoCO3 powders. (a), (c) and (e) polyhedral shape. (b), (d) and (f) fusiform shape.

18

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ip t cr us an M d te Ac ce p

Fig. 5. SEM images of Co3O4 powders. (a), (c) and (e) polyhedral shape. (b), (d) and (f) fusiform shape.

19

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ip t cr us an M

Ac ce p

te

d

Fig. 6. TEM images, HRTEM images and SEAD patterns of Co3O4 powders. (a), (b) and (c) polyhedral shape. (d), (e) and (f) fusiform shape.

20

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4

a

1C

2C

0.5C 0.1C

+

Voltage (V) (vs. Li /Li)

3

ip t

Charge

2

Discharge

cr

1

0

300

600

2C

0.5C 1C 0.1C

900

us

0

1200

1500

-1

4

3

an

b

Voltage (V) (vs. Li /Li)

Specific Capacity (mAhg )

1C 0.5C 0.1C

M

+

2C

2

Charge

Discharge

0

te

d

1

300

Ac ce p

0

0.1C 1C 0.5C

2C 600

900

1200

1500

-1

Specific Capacity (mAhg )

Fig. 7. First discharge and charge profiles of Co3O4 in the voltage range of 0.01-3.00 V (vs. Li+/Li) at the rates of 0.1 C, 0.5 C, 1 C and 2 C. (a) polyhedral shape and (b) fusiform shape.

21

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2500

Specific Capacity (mAhg )

2000

0.1C 0.5C 1C 2C

ip t

1500

cr

1000

500

0 0

10

20

30

40

60

70

0.1C 0.5C 1C 2C

M

2500

2000

d

1500

te

1000

500

Ac ce p

-1

Specific Capacity (mAhg )

b

50

an

Cycle Number

us

-1

a

0

0

10

20

30

40

50

60

70

Cycle Number

Fig. 8. Change trends of discharge capacity versus cycle number of Co3O4 at different rates of 0.1 C, 0.5 C, 1 C and 2 C. (a) polyhedral shape and (b) fusiform shape.

22

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Polyhedral Shape Fusiform Shape

1200

0.1C

1000

0.5C

0.1C

1C 2C 2C

4C

ip t

800

600

8C

200

4C

0

5

10

15

20

25

30

35

an

Cycle Number

8C

cr

400

us

Specific Capacity/(mAhg-1)

1400

M

Fig. 9. Rate capabilities of polyhedral and fusiform Co3O4 powders at various rates.

1st (polyhedral Shape) 1st (Fusiform Shape) 70th (Polyhedral Shape) 70th (Fusiform Shape)

te

d

300

200

Ac ce p

Z"(ohm)

400

100

0

0

100

200

300

Z'(ohm)

Fig. 10. Nyquist plots of polyhedral and fusiform Co3O4 powders at the 1st and 70th cycle at 2 C measured with an amplitude of 10mV over the frequency range from 1 MHz to 0.01 Hz.

23

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ip t cr

Ac ce p

te

d

M

an

us

Fig. 11. SEM images of the Co3O4 electrodes (the mixtures of Co3O4/ATB/PVDF) after 70 cycles at 2 C. (a) polyhedral shape and (b) fusiform shape.

24

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