Electrochimica Acta 56 (2011) 6464–6468
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Nano-CaCO3 templated mesoporous carbon as anode material for Li-ion batteries Bin Xu a,∗ , Lu Shi b , Xianwei Guo c , Lu Peng b , Zhaoxiang Wang c,∗ , Shi Chen b , Gaoping Cao a , Feng Wu b , Yusheng Yang a a b c
Research Institute of Chemical Defense, Beijing 100191, China Beijing Key Laboratory of Environmental Science and Engineering, Beijing Institute of Technology, Beijing 100081, China Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 20 December 2010 Received in revised form 10 February 2011 Accepted 26 April 2011 Available online 12 May 2011 Keywords: Lithium ion battery Mesoporous carbon Capacity Cycle performance
a b s t r a c t Mesoporous hard carbon is obtained by pyrolyzing a mixture of sucrose and nanoscaled calcium carbonate (CaCO3 ) particles. The microstructure of the carbon is characterized by N2 adsorption/desorption, Hg porosimetry, ﬁeld-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Raman spectroscopy. The electrochemical performances of the carbon as an anode material for lithium ion batteries are evaluated by galvanostatic charge/discharge and cyclic voltammetry tests. It is shown that this mesoporous carbon possesses high capacity, good cycling performance and rate capability, indicating the promising application of nano-CaCO3 particle as template in massive fabrication of mesoporous carbon anode materials for lithium ion batteries. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Due to the high energy density and long cycle life, lithium ion batteries have been widely used in consumer electronics such as cellular phones and laptop computers . In order to meet the ongoing market demand, considerable attention has been paid to develop advanced electrode materials with higher capacity, longer cycling durability and better rate performance. Graphite is the most common anode material in nowadays commercial lithium ion batteries due to its low cost and low lithium intercalation potential. However, the lithium-storage capacity of graphite is limited to 372 mAh g−1 , corresponding to the formation of LiC6 . Other alternative anode materials with higher speciﬁc capacities, such as Sn, Si and the recently found transition metal oxides that store lithium via electrochemical conversion reaction (CoOx and FeOx , for example) suffer from quick capacity decay (for the metal anode) and high delithiation potential (for the oxide anodes) during cycling [2–7]. Therefore, disordered carbons are still regarded potential highcapacity anode materials for lithium ion batteries though there are a lot of challenges to meet before they can be applied [8–19]. To improve the rate capability of anode materials, various methods have been developed such as decreasing the particle size,
∗ Corresponding authors. Fax: +86 10 66705840. E-mail addresses: [email protected]
(B. Xu), [email protected]
(Z. Wang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.04.130
creating mesoporous structure or coating with carbon materials. As well-developed mesoporous structures facilitate the penetration of the non-aqueous electrolyte and the diffusion of the lithium ions, some mesoporous metal oxides have been prepared and indeed present good rate performances [20–22]. Recent studies [10–12] indicate that mesoporous carbons prepared by silica template method show both good rate capability and very high capacities. Hu et al.  synthesized hierarchical porous carbon monoliths with an initial reversible capacity of 900 mAh g−1 at C/5 and good rate performance, by nanocasting with silica monolith as a hard template. The carbon with ordered mesopores of ca. 6.7 nm in diameter by soft template method  even delivers a reversible capacity as high as 1048 mAh g−1 . The capacity retains at 500 mAh g−1 after 50 cycles. However, the complicated process of material synthesis and the removal of the silica templates with corrosive HF discourage the practical application of this method. Recently, we reported a novel method of mesoporous carbon preparation by dispersing CaCO3 nanoparticles in a carbon precursor . The CaCO3 nanoparticles, as a hard template, are very cheap and can be easily removed with HCl aqueous solution instead of corrosive HF, making our method very simple, cheap and easy for mass production of mesoporous carbon. In this paper, we report the lithium storage behavior of such mesoporous hard carbon with nano-CaCO3 as the template. It will be seen that such prepared mesoporous carbon has high speciﬁc capacity, good cycling stability and rate performance.
B. Xu et al. / Electrochimica Acta 56 (2011) 6464–6468
2. Experimental 2.1. Preparation and physical characterization Commercial hydrophilic CaCO3 nanoparticles (ca. 40 nm) and sucrose were used as template and precursor, respectively. The CaCO3 nanoparticles were dispersed in a sucrose aqueous solution (CaCO3 /sucrose = 4:6 in weight) by mechanical stirring. The mixture was then carbonized at 800 ◦ C in nitrogen (99.999%) for 2 h. After removal of the hard template with diluted HCl, mesoporous carbon was obtained. The speciﬁc surface area and pore structure of the carbon were determined with the nitrogen adsorption/desorption isotherms at 77 K (Autosorb 1) and Hg porosimetry (Poremaster GP60). Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-5600LV scanning electron microscope operated at an acceleration voltage of 10 kV. X-ray diffraction (XRD) was measured by a Rikagu B/max-2400X with Cu K␣ radiation. 2.2. Electrochemical evaluation Electrode sheets were prepared by casting a slurry of the mesoporous carbon, carbon black and poly(vinylidene ﬂuoride) (PVDF) at a weight ratio of 85:10:5 onto a Cu foil and dried in vacuum. Test cells were assembled using lithium foil as the counter electrode and 1 mol L−1 LiPF6 /EC + DEC solution as the electrolyte (EC for ethylene carbonate and DEC for diethyl carbonate). The cyclic voltammetry (CV) was recorded on a Solartron 1280B electrochemical workstation. The galvanostatic charge/discharge test was carried out on a Land battery tester at a current density of 50 mA g−1 between 0.05 and 3.00 V vs. Li/Li+ . 3. Results and discussion Fig. 1 shows the nitrogen (77 K) adsorption/desorption isotherms and the Hg porosimetry of the prepared carbon. It presents a type IV isotherm according to IUPAC classiﬁcation with a sharp capillary condensation step at very high relative pressures (p/p0 > 0.9) and an H1-type hysteresis loop, indicating the existence of large pores in the material . The BET surface area and pore volume reach 606 m2 g−1 and 0.974 cm3 g−1 , respectively. Hg porosimetry analysis demonstrates that the carbon has a narrow pore size distribution with a mean value of 50 nm, close to the particle size of the CaCO3 nanoparticles. The mesoporous feature of the carbon is supported with the morphology observation. SEM imaging (Fig. 2) shows that the carbon has large interconnected mesopores of 40–50 nm in diameter, roughly the size of the CaCO3 nanoparticle. This proves that the CaCO3 nanoparticles in the precursor indeed act as hard templates for the mesoporous carbon. Clearly our CaCO3 hard template method has obvious advantages over the previous silica template method. It is much more difﬁcult to impregnate/inﬁltrate the carbon precursor into the nanochannels of the porous silica template entirely than to disperse CaCO3 nanoparticles in the carbon precursor. Actually, CaCO3 nanoparticles have been widely used as a ﬁller for rubber production. In addition, it is much easier to remove the CaO (HCl as solvent) than SiO2 (HF as solvent). Therefore, the CaCO3 hard template method is simple, cheap and easy for mass production of mesoporous carbons . The local structure of the carbon was characterized by XRD and Raman spectra. Two broad diffraction peaks are observed at 2 ≈ 23◦ for the (0 0 2) diffraction and at 2 ≈ 43◦ for the (1 0 0) diffraction, respectively (Fig. 3a), indicating a non-graphitized structure. Consistent with the XRD results, a strong band is
Fig. 1. (a) Nitrogen (77 K) adsorption/desorption isotherms of the carbon; (b) pore size distribution by Hg porosimetry of the carbon.
observed at 1350 cm−1 , the D-band characteristic of disordered carbons, as well as the G-band at 1600 cm−1 of graphitic carbons (Fig. 3b). The large intensity ratio ID /IG (=0.88) indicates that the mesoporous carbon is highly amorphous, La = 5.0 nm, according to the well-known empirical equation La = 4.4 IG /ID (nm) (La is for the size of the graphitic carbon). The cyclic voltammetry of the mesoporous carbon between 0.05 and 3.00 V at 0.1 mV s−1 scan rate is shown in Fig. 4. In the reduction segments of the ﬁrst cycle, two strong reduction peaks appear at ca. 0.7 V and 0.4 V, respectively. The 0.7 V peak is assigned to the decomposition of the electrolyte and the formation of solid electrolyte interphase (SEI) layer. The peak at 0.4 V, even stronger than the one at 0.7 V, might be originated from the reaction of the hanging groups of the porous carbon and the lithium ions. No corresponding oxidation peaks are observed in the recharge segment of the CV proﬁles. Therefore, these two reactions are responsible for the obvious capacity loss in the ﬁrst cycle. Delithiation mainly occurs between 0.05 and 1.50 V. After the ﬁrst cycle, these two cathodic peaks disappear and the areas of the cathodic and anodic peaks tend to be the equal to each other, implying the stability of the SEI layer and the structure of the porous carbon. Fig. 5 shows the galvanostatic voltage proﬁles of the carbon in the ﬁrst ﬁve cycles between 0.05 and 3.0 V at 50 mA g−1 . Consistent with the CV proﬁles, a vague plateau is observed in the ﬁrst discharge but disappears in the subsequent cycles. The discharge capacity is as high as 1860 mA g−1 , but the charge capacity is only 825 mA g−1 . The initial coulombic efﬁciency of the material is only 44.3%. Low coulombic efﬁciency in the ﬁrst cycle is a common
B. Xu et al. / Electrochimica Acta 56 (2011) 6464–6468
Fig. 2. SEM images of the CaCO3 nanoparticles (a) and the mesoporous hard carbon (b).
problem and hinders the practical application of hard carbon anode materials [10,25]. It is supposed to be originated from the electrolyte reduction on the porous carbon with high speciﬁc surface areas (606 m2 g−1 ), i.e. solid electrolyte interface (SEI) formation, and residual electrochemically active surface groups such as C–H in the low-temperature pyrolyzed carbon materials [10–13,16–19]. In the subsequent cycles, the cycling efﬁcient increases sharply. The coulombic efﬁciency goes up rapidly to 90% in the third cycle and maintains a high value for 50 cycles. Fig. 6 shows the cycling performance of the mesoporous carbon. The charge capacity (reversible capacity) fading is very slow, indicating the excellent cycling performance of the material. The reversible capacity decreases from 825 mA g−1 in the 1st cycle to 672 mA g−1 in the 10th cycle and then remains at ca. 630 mA g−1 for 50 cycles, much higher than the hierarchically porous carbon monoliths  and ordered mesoporous carbon . Different models [8,26,27] have been proposed to explain the excess capacity of disordered carbon anode over the theoretical value of 372 mAh g−1 of graphite anodes, such as formation of lithium multi-layers on graphene sheets, Li2 covalent molecules, Li–C–H bonds, and metallic lithium clusters in microcavities. Actually the exact mechanism of high speciﬁc lithium storage capacity of disordered carbon is in argument ever since the ﬁnding that hard carbon can be a promising high-capacity anode material of lithium ion batteries. As for the present CaCO3 nanoparticle templated mesoporous carbon, the capacitive lithium storage at the walls of the nanopores (like that in electrochemical double-layer
Fig. 3. XRD (a) and Raman (b) patterns of the mesoporous carbon.
capacitors) might be another important contributor to the total capacity because the speciﬁc surface area of the porous carbon is high (>600 m2 g−1 ) and the pores are large enough (40–50 nm) for the electrolyte to enter. The large mesopores facilitate the penetration of the nonaqueous electrolyte into the pores and the diffusion of the lithium ion, beneﬁcial to improving the rate capability of the material.
Fig. 4. The voltammetry of the mesoporous carbon in the ﬁrst ﬁve cycles between 0.05 and 3.00 V at 0.1 mV s−1 .
B. Xu et al. / Electrochimica Acta 56 (2011) 6464–6468
Fig. 5. The galvanostatic discharge/charge curves of the mesoporous carbon between 0.05 and 3.00 V at 50 mA g−1 .
These indicate that the mesoporous carbon is an attractive candidate of anode materials for lithium ion batteries. The CaCO3 nanoparticle templated mesoporous carbon presents high capacity, good cycling stability and rate capability as an anode material for lithium ion batteries. Further work should be focused on increasing the initial coulombic efﬁciency of the hard carbon. As the irreversible capacity is closely related to the amount of dangling groups at the surface of the material, increasing the pyrolysis temperature will decrease the dangling groups and decrease the loss of lithium due to reaction with these groups. However, because the lithium storage mechanism and the reversible capacity are related to the content of hydrogen in the material, increasing the pyrolysis temperature will decrease the reversible capacity of the material. In our previous study , we found that ﬁlling the pores of hard carbon spheres with nanoscaled metallic tin particles can increase the initial coulombic efﬁciency to 96%. This was attributed to the decomposition of lithium alkline carbonate (ROCO2 Li) at ca. 3.0 V vs. Li+ /Li, a component of the SEI ﬁlm and also the reduction product of the electrolyte. Some other probably methods, such as porosity optimism and electrolyte additives will also be tried. 4. Conclusion
Fig. 6. The cycling performance of the mesoporous carbon at 50 mA g−1 .
CaCO3 nanoparticles were employed as hard templates for the preparation of mesoporous carbon with sucrose as the carbon precursor. The mesoporous carbon presents high capacity, good cycling stability and rate capability as an anode material for lithium ion batteries. The reversible capacity is as high as 825 mAh g−1 in the ﬁrst cycle and remains at 633 mAh g−1 after 50 cycles. As the current density increases, the speciﬁc capacity decreases slowly and a reversible capacity of 338 mAh g−1 is obtained at 1 A g−1 , and 260 mAh g−1 at 2 A g−1 . Considering the low cost of the CaCO3 nanoparticles and the facility of removing the hard templates after carbonization, the CaCO3 nanoparticle is a much better hard template than the conventional silica template. These make the CaCO3 nanoparticle template method a much more practical strategy on mass producing high-performance mesoporous carbon materials than the hierarchical and the silica template methods for lithium ion batteries. Acknowledgements
Fig. 7 shows the discharge/charge capacity of the carbon cycled at enhanced rate. The speciﬁc capacity decreases slowly with the increase of the current density. A speciﬁc charge capacity of 338 mAh g−1 is obtained at 1 A g−1 and 260 mAh g−1 at 2 A g−1 .
This work was ﬁnancially supported by the National 973 Program (2009CB220100) and the National Science Foundation of China (NSFC, 50802112, 21073233). References                
Fig. 7. The rate performance of the mesoporous carbon between 0.05 and 3.00 V.
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