­­­ Synthesis and electrochemical performance of Co₃O₄/C composite anode for lithium ion batteries

GUO Hua-jun(郭华军), SUN Qian-ming(孙乾铭), LI Xin-hai(李新海),

WANG Zhi-xing(王志兴), PENG Wen-jie(彭文杰)

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

Received 24 March 2008; accepted 8 September 2008

Abstract: The Co₃O₄/acetylene black composite anodes were successfully prepared by combination of oxalate precipitation and pyrolysis of the precipitate. The composite and its precursor were characterized by thermo-gravimetric analysis(TGA), differential thermal analysis(DTA), X-ray diffractometry(XRD), scanning electronic microscopy(SEM) and electrochemical measurements. The effects of carbon content and calcination temperature on properties of the composite were investigated in detail. The cycling performance of the Co₃O₄ anode is improved remarkably by the addition of carbon. As the calcination temperature rises in the range of 300-450 ℃, the crystallinity of the composites increases, but their reversible capacity and cycling stability decrease. Being charged/discharged at a current density of 0.1C rate, the optimized Co₃O₄/C composite anode shows a large initial reversible capacity of 757 mA?h/g, and a capacity of 743 mA?h/g is observed after 10 cycles.

Key words: lithium ion batteries; anode; Co₃O₄; composite

1 Introduction

Lithium ion insertion materials have received considerable attention and have been widely used as active electrodes in lithium ion batteries, which have potential applications in portable electronic devices and electric vehicles. Negative electrodes in commercial lithium ion batteries employ different forms of carbon materials. However, with the growing demand for high capacity secondary batteries, the low capacity of carbon (theoretical capacity of 372 mA?h/g) has become a limiting factor. High capacity alternatives to carbonaceous materials have been sought for and much achievement has been made[1-4].

Recently, new anode materials, namely, transition metal oxides such as nickel oxides, cobalt oxides and tin oxides have been proposed for anodes of lithium ion batteries[1,5-8]. Among them, cobalt oxides have excellent electrochemical properties as lithium storage materials in lithium ion batteries because of their large reversible capacity[9-11]. A variety of methods, such as chemical spray pyrolysis, liquid-control-precipitation and oxidation of metal particles have been used in the preparation of cobalt oxides[12-15].

In this work, the Co₃O₄/acetylene black composite anodes were prepared by combination of oxalate precipitation and pyrolysis of the precipitate. The effects of carbon content and calcination temperature on properties of the composite were investigated in detail.

2 Experimental

The cobalt chloride hexahydrate was dissolved in distilled water, and polyglycol was used as dispersant with a concentration of 50 g/L. Then a certain amount of acetylene black was introduced into the solution. Finally, diammonium oxalate monohydrate was added and precipitation of cobalt oxalate occurred, and the molar ratio of cobalt chloride hexahydrate to diammonium oxalate monohydrate was 1.0?1.2. Agitation was employed during the precipitation and the mixture was kept at 40 ℃. The filtrated precipitate was dried in an oven at 120 ℃ for 4 h and the composite precursor of CoC₂O₄·2H₂O and acetylene black was obtained. The Co₃O₄/C composite anode powders were obtained by pyrolysis of the precursor at different temperatures for 2 h in air.

Thermogravimetry/differential thermal analysis (TG/DTA) was performed on METTLER TOLEDO TGA/SDTA851e thermal analyzer at a heating rate of 10 ℃/min. Powder X-ray diffraction(XRD) measurements were made with a Rigaku diffractometer. Scanning electron micrographs(SEM) were obtained with a JEOL JSM-5600LV spectrometer.

The Co₃O₄/C composite was mixed with poly(vinylidene difluoride) (PVDF) as binder in a mass ratio of 90?10. The Co₃O₄/C composite electrode was prepared by spreading the above mixture on copper foil. Charge-discharge measurements of the Co₃O₄/C composites were performed in the coin cells with the Co₃O₄/C composite as working electrode and lithium metal as counter electrode. A UP 3025 porous membrane of 25 ?m in thickness was used as separator, and the electrolyte was 1 mol/L LiPF₆ dissolved in a mixture of ethylene carbonate(EC), dimethyl carbonate(DMC) and methyl-ethyl carbonate(EMC) with a volume ratio of 1?1?1. The cells were discharged and charged between 3.0 and 0.01 V at a current density of 60 mA/g. AC impedance measurements were performed using a Shanghai Chenhua CHI660A electrochemistry station in the frequency range from 100 kHz to 10 mHz with the amplitude of 5 mV.

3 Results and discussion

The TG and DTA curves for the prepared precursor are shown in Fig.1. The broad peak located at 180.6 ℃ on the DTA curve is endothermic and corresponds to a sample mass loss of 18.1% in the range of 148-193 ℃ on the TG curve. This is due to the dehydration of chemically bonded water in the CoC₂O₄·2H₂O sample. A sharp exothermic peak observed at 282.5 ℃ on the DTA curve indicates the decomposition and oxidation reaction as follows:

3CoC₂O₄+2O₂→Co₃O₄+6CO₂                  (1)

Fig.1 TG and DTA curves of CoC₂O₄·2H₂O in air

Pure Co₃O₄ and Co₃O₄/C composite were prepared by calcining the precursors at 300 ℃ for 2 h, where the precursor of pure Co₃O₄ was CoC₂O₄·2H₂O and that of Co₃O₄/C composite was the mixture of CoC₂O₄·2H₂O and acetylene black with a molar ratio of 2?1. Fig.2 shows the SEM images of pure Co₃O₄ and the Co₃O₄/C composite. In both samples, the grains of Co₃O₄ present short strip appearance. Compared with the pure Co₃O₄ sample, a lot of tiny acetylene black particles are observed in the Co₃O₄/C composite sample and they are well distributed among the Co₃O₄ grains. The acetylene black on the surface of Co₃O₄ particles can not only increase the electric conductivity but also decrease the probability of agglomeration.

Fig.2 SEM images of pure Co₃O₄(a) and Co₃O₄/C composite(b)

The electrochemical performances of the pure Co₃O₄ and Co₃O₄/C composite were evaluated by galvanostatic charge-discharge in the voltage range of 0.01-3 V versus Li/Li⁺ at a constant current density of 60 mA/g. Fig.3 shows the initial charge-discharge curves of pure Co₃O₄ and Co₃O₄/C composite. During the first discharge, the profiles of both samples present a long voltage plateau around 1.2 V and then a sloping voltage profile from 1.2 V to the cut-off voltage of 0.01 V. The Co₃O₄/C composite shows an initial discharge capacity of 974 mA?h/g and a charge capacity (reversible capacity) of 757 mA?h/g, while the pure Co₃O₄ anode has an initial discharge capacity of 797 mA?h/g and a reversible capacity of 659 mA?h/g. The results indicate that the addition of acetylene black can obviously improve the charge-discharge performance of Co₃O₄ compound, and this can be attributed to the enhancement of conductivity of the Co₃O₄/C composite.

Fig.3 Charge-discharge curves of pure Co₃O₄(a) and Co₃O₄/C composite(b)

Fig.4 shows the cycling performance of pure Co₃O₄ anode and Co₃O₄/C composite anode. The reversible capacity of the pure Co₃O₄ declines drastically with the increase of cycle number. While the Co₃O₄/C composite anode shows good cycling stability with a capacity retention ratio of 98.15% after 10 cycles. This indicates that acetylene black in the composite plays an important role in the improvement of the cycling performance of Co₃O₄ anode.

Fig.4 Cycling performance of pure Co₃O₄(a) and Co₃O₄/C composite(b)

Fig.5 shows the impedance spectra for the pure Co₃O₄ and Co₃O₄/C composite at half charged state. The impedance spectra consist of a semicircle in high-frequency range and a line inclined at a constant angle to the real axis in low-frequency range. The semicircle in high-frequency range is due to the contact resistance at the anode and the charge transfer reaction at the interface of the anode/electrolyte. The inclined line in low-frequency range is attributed to Warburg impedance that is associated with lithium ion diffusion through the anode. As observed in Fig.5, the resistance of Co₃O₄/C composite is much smaller than that of the pure Co₃O₄. This indicates that the addition of carbon black decreases the contact resistance at the anode and the impedance for charge transfer reaction at the interface of the anode/electrolyte, then improves the charge-discharge performance of the Co₃O₄/C composite.

Fig.5 Impedance spectra of pure Co₃O₄(a) and Co₃O₄/C composite(b)

The effects of calcination temperature on the structure and properties of the Co₃O₄/C composites were investigated by calcining the precursor at different temperatures for 2 h, where the precursor was the mixture of CoC₂O₄·2H₂O and acetylene black with a molar ratio of 2?1. Fig.6 shows the XRD patterns of the composite samples calcined at different temperatures for 2 h. The diffraction peaks can be indexed to the Co₃O₄ with spinel structure. As the temperature rises from 300 ℃ to 450 ℃, the intensity of the diffraction peaks increases slightly, which suggests that the crystallinity of the Co₃O₄ compound rises.

Fig.7 shows SEM micrographs of the Co₃O₄/C samples calcined at different temperatures for 2 h. The Co₃O₄/C samples exhibit loose surface and the grains of Co₃O₄ are less than 1 μm. The grain size of Co₃O₄ increases with the increase of calcination temperature. This can be attributed to the better development of Co₃O₄ grains and more serious agglomeration of the small particles into large ones at higher temperature. In addition, large amount of disordered acetylene black is observed in the Co₃O₄/C sample prepared at 300 ℃. The acetylene black distributes on the surface of Co₃O₄ particles and surrounds the Co₃O₄ particles. With the increase of calcination temperature, the amount of acetylene black reduces greatly, and the acetylene black can hardly be seen in the SEM image of Co₃O₄ obtained at 450 ℃. This results from the oxidation and burning of the acetylene black at higher temperature.

Fig.6 XRD patterns of Co₃O₄ calcined at different temperatures for 2 h

Fig.7 SEM images of Co₃O₄/C composites calcined at different temperatures for 2 h: (a) 300 ℃; (b) 350 ℃; (c) 400 ℃; (d) 450 ℃

The initial charge-discharge curves of Co₃O₄/C composite calcined at different temperatures are shown in Fig.8. All Co₃O₄/C composites display large charge capacity (reversible capacity) above 700 mA?h/g, and large irreversible capacity occurs in the first charge- discharge cycle, which is the difference between the discharge capacity and charge capacity. This is related to the formation/decomposition of Li₂O as well as solid electrolyte interface(SEI) film formation[16]. Lithium reacts with Co₃O₄ during the first discharge and forms Li₂O and cobalt metal. Li₂O is known to be electrochemically inactive and the incomplete decomposition of Li₂O is responsible for the large irreversible capacity loss during cycling. The Co₃O₄/C composite prepared at 300 ℃ has the best initial charge- discharge performance with the highest reversible capacity of 757 mA?h/g, the lowest irreversible capacity of 169.2 mA?h/g and the largest initial coulombic efficiency of 81.75%.

Fig.8 Initial charge-discharge curves of Co₃O₄/C composites calcined at different temperatures: (a) 300 ℃; (b) 350 ℃;    (c) 400 ℃; (d) 450 ℃

Fig.9 shows the cycling performance of Co₃O₄/C composites prepared at different calcination temperatures. The cycling performance of Co₃O₄/C composites is improved with decreasing the calcination temperature. The Co₃O₄/C electrodes prepared at 450 ℃ display a fast capacity fading, while the Co₃O₄/C electrode prepared at 300 ℃ exhibits good cycling performance with a capacity of 743 mA?h/g and a capacity retention ratio of

Fig.9 Cycling performance of Co₃O₄/C composites calcined at different temperatures: (a) 300 ℃; (b) 350 ℃; (c) 400 ℃;    (d) 450 ℃

98.15% at the 10th cycle.

4 Conclusions

1) The Co₃O₄/C composite anode was synthesized by pyrolysis of the oxalate precursor. The Co₃O₄/C composite electrodes have high reversible capacity, which is above 700 mA?h/g and is twice as high as the commercial graphite anode material. The cycling performance of the Co₃O₄ anode is improved remarkably by the addition of carbon.

2) As the calcination temperature rises in the range of 300-450 ℃, the composites exhibit better crystalline structure, but their reversible capacity and cycling stability decrease. The optimized Co₃O₄/C composite anode shows a large initial reversible capacity of 757 mA?h/g, and a capacity retention ratio of 98.15% is observed after 10 cycles.

References

[1] Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon J M. Nanosized transition-metal oxides as negative- electrode materials for lithium-ion batteries [J]. Nature, 2000, 407: 496-499.

[2] ZHANG Han-ping, FU Li-jun, WU Yu-ping, WU Hao-qing. Research progress in anode materials for Li-ion batteries [J]. Battery, 2005, 35(4): 274-275. (in Chinese)

[3] Kim S S, Ikuta H, Wakihara M. Synthesis and characterization of MnV₂O₆ as a high capacity anode material for a lithium secondary battery [J]. Solid State Ionics, 2001, 139: 57-65.

[4] Yang J, Takeda Y, Imanishi N, Xie J Y, Yamamoto O. Morphology modification and irreverisibility compensation for SnO anodes [J]. Journal of Power Sources, 2001, 97/98: 216-218.

[5] Nuli Y N, Zhao S L, Qin Q Z. Nanocrystalline tin oxides and nickel oxide film anodes for Li-ion batteries [J]. Journal of Power Sources, 2003, 114: 113-120.

[6] Belliard F, Irvine T S. Electrochemical performance of ball-milled ZnO-SnO₂ systems as anodes in lithium-ion battery [J]. Journal of Power Sources, 2001, 97/98: 219-222.

[7] WANG G X, CHEN Y, KONSTANTINOV K, YAO J, AHN J　B, LIU H K, DOU S X. Nanosize cobalt oxides as anode materials for lithium-ion batteries [J]. J Alloys and Compounds, 2002, 340: L5-L10.

[8] LIU H C, YEN S K. Characterization of electrolytic Co₃O₄ thin films as anodes for lithium-ion batteries [J]. Journal of Power Sources 2007, 166: 478-484.

[9] Huang F, Zhan H, Zhou Y H. Studies of nanosized Co₃O₄ as anode materials for lithium-ion batteries [J]. Chinese Journal of Chemistry, 2003, 21(10): 1275-1279.

[10] DO J S, WENG C H. Electrochemical and charge/discharge properties of the synthesized cobalt oxide as anode material in Li-ion batteries [J]. Journal of Power Sources 2006, 159: 323-327.

[11] YUAN Z Y, HUANG F, FENG C Q, SUN J T, ZHOU Y H. Synthesis and electrochemical performance of nanosized Co₃O₄ [J]. Mater Chem Phys, 2003, 79: 1-4.

[12] BAHLAWANE N, RIVERA E F, KOHSE-HOINGHAUS K, BRECHLING A, KLEINEBERG U. Characterization and tests of planar Co₃O₄ model catalysts prepared by chemical vapor deposition [J]. Appl Catal B: Environmental, 2004, 53(4): 245-255.

[13] OMATA K, NUKUI N, HOTTAI T, YAMADA M. Cobalt-magnesia catalyst by oxalate co-precipitation method for dry refoeming of methane under pressure [J]. Catalysis Communications, 2004, 5: 771-775.

[14] YANG Y P, LIU R S, HUANG K L, WANG L P, LIU S Q, ZENG W W. Preparation and electrochemical performance of nanosized Co₃O₄ via hydrothermal method [J]. Tans Nonferrous Met Soc, 2007, 17(6): 1334-1338.

[15] LI W Y, XU L N, CHEN J. Co₃O₄ nanomaterials in lithium-ion batteries and gas sensors [J]. Adv Funct Mater, 2005, 15(5): 851-857.

[16] FU L J, LIU H, LI C, WU Y P, RAHM E, HOLZE R, WU H Q. Surface modifications of electrode materials for lithium ion batteries [J]. Solid State Sciences, 2006, 8(2): 113-128.

(Edited by YANG Bing)