Improving the electrochemical properties of natural graphite spheres by coating with a pyrolytic carbon shell

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1 NEW CARBON MATERIALS Volume 23, Issue 1, March 2008 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2008, 23(1): RESEARCH PAPER Improving the electrochemical properties of natural graphite spheres by coating with a pyrolytic carbon shell LIU Shu-he, YING Zhe, WANG Zuo-ming, LI Feng, BAI Shuo, WEN Lei, CHENG Hui-ming* Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China Abstract: Two kinds of modified natural graphite (MNG) spheres with a pyrolytic carbon shell on smooth or granular surface were obtained using fluidized bed chemical vapor deposition of acetylene by adjusting the reaction parameters. The core of the MNG has a highly ordered graphite structure and the shell has a disordered structure. Compared with natural graphite (NG) spheres, MNG with a core-shell structure shows improved first cycling efficiency and cyclability. Especially, the MNG spheres with a granular surface possess 84% retention of the first discharge capacity after 41cycles, owing to effective decrease of the contact resistance and increase of contact area between the MNG spheres. Key Words: Fluidized bed chemical vapor deposition; Core-shell structure; Pyrolytic carbon; Modified natural graphite; Lithium ion secondary battery 1 Introduction As the most popular portable device power, high power density and energy density lithium ion battery is highly needed. Therefore, electrode materials, especially anode materials such as high energy density materials (nitrides, tin oxides, tin-based alloys, silicon, and their composites) and improved carbon materials are attracting great attention. The commercial carbon materials used as anode include petroleum coke, mesocarbon microbeads (MCMBs), natural graphite (NG), and so on. Because MCMBs are expensive compared to NG [1], much attention is directed to NG. However, the large irreversible capacity and poor cycling life have been persistent problems for the wide application of NG anodes [2, 3]. Several research results [2 12] have confirmed that the modification of NG is an effective approach to improve its electrochemical performance. Coating NG with non-graphitic carbon is an effective route [3 12], which can form a core-shell structure: the core is NG and the shell is coating carbon. One method in this route is the chemical vapor deposition (CVD) of hydrocarbons on the surface of graphite [3, 8 12], especially fluidized bed CVD for its large production [13]. In a previous study, core-shell structured modified natural graphite (MNG) was obtained by a fluidized bed reactor with a shell thickness of ~250 nm. It was found that the first coulombic efficiency (CE) and cyclability were significantly improved [10]. However, little attention has been paid to the effect of deposition parameters, such as fluidized bed parameters and reaction time on the structure and properties of the coated materials [3, 13]. In this study, the influence of CVD deposition parameters on the structure and electrochemical performance of the MNG was explored. Two forms of coated morphologies (smooth surface and granular surface) of MNG were obtained by changing the fluidizing condition. It was found that, compared with the MNG having smooth surface, the MNG with granular surface in an appropriate amount of coated carbon has an improved electrochemical performance (both the first CE and cyclability). 2 Experimental In this experiment, a fluidized bed CVD process was used, in which the original NG was in the fluidization condition to achieve homogeneous coating (Fig.1). To control surface morphology, two fluidization types were used in the experiments: agitated fluidized bed (AFB), in which the bare NG spheres were put into the deposition zone, and the propeller-blade churned up the NG spheres during the deposition process; vibrated fluidized bed (VFB), in which the bare NG spheres were put into the cold zone before deposition, and an electric vibrator was used to knock on the gas inlet pipe to fluidize the NG spheres in the deposition zone. Acetylene was used as the carbon source, Ar as the diluting gas, and spherical NG with d 50 ~18 µm as raw material. The total flow rate of gas was m 3 /min with 5 25% acetylene, and the reaction temperature was 1000 o C. The morphologies of the original NG and MNG were observed by SEM (JSM-6301F), and analyzed by XRD (D/max 2500 PC) with Cu-Kα radiation and laser Raman spectroscopy (Labram HR 800) with nm He-Ne laser. Specific surface areas (SSA) of the samples were determined by the Brunauer Emmelt Taller (BET) method from N 2 adsorption with a surface area analyzer (ASAP2010M). The amount of depos- Received date: ; Revised date: *Corresponding author. Tel: ; Fax: ; cheng@imr.ac.cn Copyright 2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

2 ited carbon was determined by TG-DTA (STA 449C Jupiter) method [3, 14]. After MNG were ground and dispersed in alcohol, a drop of the mixture was put onto the Cu grid for TEM observation at 200 kv (JEOL 2010). (DMC) in 1:1 by volume. Lithium foils were used as counter electrode and a porous separator (Celgard 2400) was sandwiched between the working electrode and counter electrode. The cells were galvanostatically discharged (intercalation)/charged (de-intercalation) between and 2.5 V versus Li+/Li. All cycling tests of the samples were performed at 0.1C (35 mah/g) except those of rate capability. 3 Results and discussion 3.1 Structure of MNG Fig.1 Schematic of the apparatus for the fluidized bed CVD Half-cells were assembled in an Ar-filled glove box (Unilab, H 2 O and O 2 < ). The working electrode was made by coating slurry of the samples, carbon black (60 nm in diameter) and polyvinylidene (PVDF) in a weight ratio of 90:5:5 on copper foils. The electrolyte was 1mol/L LiPF 6 in the mixed solvent of ethylene carbonate (EC) and dimethyl carbonate Fig.2 shows the morphologies of the original NG and MNG. Many cracks and step-like undulations can be clearly seen on the surface of the original NG spheres (Fig.2a). The surface of MNG (Fig.2b) prepared by AFB was smoother than that of the original NG. The pyrolytic carbon covered the surface and filled the cracks of the original NG, which acted as a shell to protect the inner core from the penetration of electrolyte [6, 15]. The morphologies of these MNG were similar to the core (original NG) and shell (pyrolytic carbon) structure in the previous report [10]. The MNG sample was observed by HRTEM (Fig.3), which showed the typical core-shell structure directly: the well-developed graphitic layers in the core and amorphous structure in the shell (Fig.3a). The granule grown in the VFB process also had an amorphous structure (Fig.3b). Fig.2 Morphologies of the original NG (a), MNG spheres by AFB mode (b) and MNG spheres by VFB mode (c, d)

3 Fig.3 HRTEM images of the cross-section of the MNG sphere prepared by AFB (a) and the granule prepared by VFB (b). However, the fluidized CVD can influence the morphologies of MNG greatly as shown in Fig.2b and c. In the VFB mode, although the MNG was covered by pyrolytic carbon as well, the granule-like structure was randomly formed on the surface. The diameter of the granules was from 300 to 650 nm (Fig.2c, d). From SEM observations, it was found that contacts of MNG spheres were formed among many granules, which can reduce the contact resistance of the MNG spheres, as evidenced by Han and Lee [16]. The difference between the AFB and VFB mode is the amount of NG spheres in the reaction zone, which causes a difference of A/V ratio, where A is the deposition surface area and V is the reactor volume. In the AFB mode, all the original NG spheres were in the deposition zone and the A/V ratio were bigger than that in VFB mode because only part of the NG spheres were in the deposition zone in the VFB mode. Generally speaking, there are two kinds of pyrolytic carbon deposition mechanisms in CVD: surface growth model (heterogeneous surface reaction) and nucleation model (homogeneous gas phase reaction) [17]. When the A/V ratio is large, heterogeneous surface reaction of pyrolytic carbon deposition is dominated; however, homogeneous gas phase reaction is dominated when A/V ratio is small [17 19]. In the AFB, the A/V ratio is large, so heterogeneous surface growth is prominent and the smooth surface is formed. The small species formed in the pyrolysis of acetylene not only saturates the dangling bond of the graphite, but also penetrates into the cracks of the NG spheres and deposits in them, which is just like the densification of carbon felt [20]. While in the VFB, the NG spheres are passed through the deposition zone in a pulse mode, and only part of them are in the deposition zone instantaneously. Therefore the A/V ratio of VFB is smaller than that of AFB and homogeneous nucleation of the carbon species predominates. This mode of deposition mainly forms granules on the graphite surface. In fact, heterogeneous surface growth of pyrolytic carbon on NG spheres also synchronously occurs with the predominating homogeneous nucleation in VFB [21]. In this way, both the coarse (granular) coating and smooth coating are formed on the NG spheres. In the VFB, all the NG spheres pass through the hot zone in a replacement way with prolonged reaction time, and so all of them are covered by granular pyrolytic carbon. As known, the amount of pyrolytic carbon increases with increasing reaction time, which determines the thickness and completeness of the coating on the surface. Depending on the pyrolytic rate in AFB and VFB modes, different reaction time was chosen. The reaction time in AFB was 0.5, 1, and 2 h, and the samples were denoted as NGa 0.5, NGa 1, and NGa 2, respectively; in VFB, the reaction time was 0.5, 2, and 4 h, and the samples were denoted as NGv 0.5, NGv 2, and NGv 4, respectively. TG-DTA analysis results indicated that the mass fraction of pyrolytic carbon in MNG were 7.3, 13.4 and 26.2% in NGa 0.5, NGa 1, and NGa 2, and 2.0, 9.1, and 19.0% in NGv 0.5, NGv 2, and NGv 4, respectively. Fig.4 (a) shows the XRD spectra of the original NG and MNG. With the reaction time increasing, the (002) peaks of MNG became broader, which showed increasing amount of amorphous structure in MNG. This feature can also be identified from the Raman spectra of the MNG samples (Fig.4b), where the value of R (the ratio of D band and G band), which represents the ratio of disordered structure to graphitic structure, also increased monotonously from 0.4 of the original NG to 1.4 of NGa 2 and 0.9 of NGv 4. Fig.4 XRD (a) and Raman (b) spectra of the original NG and MNG

4 The MNG samples had a decreasing SSA and increasing coating thickness (δ, half of the difference of d 50 between MNG and NG) with increasing reaction time (Fig.5). The SSA and δ of the AFB samples decreased from 5.7 (original NG) to 0.4 m 2 /g and increased from 0.13 to 0.56 µm, respectively. The SSA and δ of the VFB samples decreased from 5.7 to 2.3 m 2 /g and increased from 0.05 to 1.22 µm, respectively. It clearly showed that AFB was more effective in reducing BET surface area than VFB because of the heterogeneous surface growth on the surface and in the crack. Due to the granules formed on the surface of the VFB MNG, the coating thickness of VFB samples increased more than that of AFB samples. to the first discharge capacity of sample for a certain cycling number) of the MNG were greatly improved compared with that of the original NG. Fig.6 The relationship of the first discharge capacity (DC) and the first CE of MNG with the coating time Fig.5 Relationship of SSA and δ with reaction time 3.2 Electrochemical properties The changes of the first discharge capacity and the first CE of MNG with the reaction time are shown in Fig.6. The first discharge capacity of MNG was lower than that of NG, and decreased monotonously with the increase of coating time. For AFB samples, the first discharge capacity decreased from 410 mah/g of NG to 310 mah/g of NGa 2. For VFB samples, the first discharge capacity decreased from 410 mah/g of NG to 389 mah/g of NGv 4. The decrease of the first discharge capacity of the MNG with increasing reaction time mainly results from three reasons: first, the decrease of SSA results in the decrease of the first irreversible discharge capacity [22,23]; second, the pyrolytic carbon depresses the co-intercalation of solvated lithium ions [15]; third, the first discharge capacity of pyrolytic carbon ( mah/g [24,25]) is lower than that of NG.. It can be found that the first discharge capacity of NGv series was decreased more mildly with the increasing reaction time than that of NGa series, possibly due to the smaller proportion of the pyrolytic carbon in the NGv series. As discussed before [10], the low SSA and amorphous carbon structure can improve the first CE, so the first CE of all the MNG is higher than that of the original NG (Fig.6). However, the first CE of both series showed a relative maximum at an appropriate reaction time. Cycling measurements for 41 cycles (Fig.7) showed that the capacity retentions (i.e. the ratio of the last discharge capacity Fig.7 Cyclic performance of the NG and MNG. Discharge capacities have been normalized to those in the first cycle After 41 cycles, the capacity retention of the NG was 24.7%. However, the capacity retention of NGa 0.5, NGa 1 and NGa 2 was 33.9%, 34.9%, and 43.1% and the capacity retention of NGv 0.5, NGv 2, and NGv 4 was 71.6%, 84.0%, and 48.4%. Comparing two series, it was found that the cyclability of the NGv series was much higher than that of NGa series. For all the samples, NGv 2 had the largest capacity retention (84.0%). According to the capacity retention results of two MNG series, there was an appropriate amount of pyrolytic carbon coating to effectively improve the cyclability of the MNG. Since short reaction time may form a very thin or incomplete coating film, solvated lithium ions also can intercalate in the MNG [6, 15] and cause the exfoliation of the graphite during cycling; so it only has a limited improvement in cyclability than NG. On the other hand, if the NG is coated by a very thick shell, the cyclability of the anode is less improved, because of breaking of the shell and forming of new SEI film. So the best cyclability of MNG will be obtained with appropriate coating carbon amount.

5 Compared with the NGa series, the NGv series had better cyclability. It was proposed that the improvement is related to the coating morphology. The granular structure of NGv can reduce the contact resistance through multi-point connection between the MNG spheres. The contact area of the neighboring MNG spheres in NGv series can be greatly increased compared with the smooth surface of the NGa series. Effective contact not only reduces the resistance but also keeps the conductive network during the charge/discharge. Therefore a good conducting network can always be retained in the cycling of the NGv series. This phenomenon was also observed in the modification of the synthetic graphite and silicon powders by CVD carbon coating [16, 26]. Fig. 8 shows the rate capability at different C-rate ranging from 0.1 to 1.1C. The NGv 2 sample showed much higher capacity than NG and NGa 1, especially in the C discharge rate. The reason for this performance improvement by coating carbon on the surface of NG is mainly related to reducing the bulk resistance of electrode and the impedance of charge transfer by forming a conductive network. Moreover, the granules in sub-micron scale in the NGv series samples is effective in increasing the adhesion amongst the MNG spheres by organic binder, termed as anchoring effect, which improves the cyclability by increasing the resistance to the failure of electro-conduction amongst the MNG spheres during the charge/discharge cycling [24]. Furthermore, the granules deposited on the cracks or on the edge layers of the NG spheres can also act as anchors, and refrains the exfoliation of the graphite layers by lithium ion intercalation in cycling. To maximally increase the contact area amongst MNG spheres and the cyclability of MNG, random and uniform distribution of the granules on the NG surface should be emphasized. Fig.8 The discharge capacity of NG and MNG anodes (NGa1 and NGv2) at different C-rates Because of the poor conductance of cathode materials, their electrochemical performances are usually poor [27]. The electrochemical performance of cathode materials can be highly improved by carbon coating [28 30]. Therefore, the fluidized CVD method also can be used in improving the electrochemical performance of cathode materials, such as LiCoO 2, LiMnO 2, and LiFePO 4, by enhancing their electrical conductivity through a smooth and granular carbon coating. 4 Conclusions NG spheres coated by pyrolytic carbon with smooth and granular surface can be prepared by changing the parameters of fluidized bed CVD. The SSA of the MNG spheres decreased and coating thickness increased with the increasing reaction time. The carbon coating can improve the first discharge capacity, first CE, and cyclability of NG using lithium ion battery anode materials. An appropriate granular coating was better for improving the electrochemical properties of NG. Especially, the MNG spheres with granular surface were with 84.0% retention of the first discharge capacity after 41 cycles, owing to the effective decrease of contact resistance between the MNG spheres. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No and ). References [1] Shim J, Striebel K A. Cycling performance of low-cost lithium ion batteries with natural graphite and LiFePO 4. J Power Sources, 2003, : [2] Wu Y P, Jiang C, Wan C, et al. Anode materials for lithium ion batteries by oxidative treatment of common natural graphite. Solid State Ionics, 2003, 156: [3] Yoshio M, Wang H, Fukuda K, et al. Effect of carbon coating on electrochemical performance of treated natural graphite as lithium-ion battery anode material. J Electrochem Soc, 2000,147: [4] Guo K K, Pan Q M, Fan S B. Poly(acrylonitrile) encapsulated graphite as anode materials for lithium ion batteries. J Power Sources, 2002, 111: [5] Pan Q M, Guo K K, Wang L Z, et al. Ionic conductive copolymer encapsulated graphite as an anode material for lithium ion batteries. Solid State Ionics, 2002,149: [6] Zhou Y F, Xie S, Chen C H. Pyrolytic polyurea encapsulated natural graphite as anode material for lithium ion batteries. Electrochimica Acta, 2005, 50: [7] Wang G P, Zhang B, Yue M, et al. A modified graphite anode with high initial efficiency and excellent cycle life expectation. Solid State Ionics, 2005, 176: [8] Yoshio M, Wang H Y, Fukuda K. Spherical carbon-coated natural graphite as a lithium-ion battery anode material. Angew Chem Int Ed, 2003, 42: [9] Yoshio M, Wang H, Fukuda K, et al. Improvement of natural graphite as a lithium-ion battery anode material, from raw flake to carbon-coated sphere. J Mater Chem, 2004, 14: [10] Zhang H L, Liu S H, Li F, et al Electrochemical performance of pyrolytic carbon-coated natural graphite spheres. Carbon,

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