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J. Phys. Chem. C 2008, 112, 14216–14219
Carbon-Coated Macroporous Sn2P2O7 as Anode Materials for Li-Ion Battery Yueming Li and Jinghong Li* Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Tsinghua UniVersity, Beijing 100084, China ReceiVed: May 20, 2008; ReVised Manuscript ReceiVed: July 6, 2008
Carbon-coated macroporous SnP2O7 characterized by SEM, XRD, and FTIR was prepared by facile synthesis, aiming to take advantages of macroporous structure and carbon coating. The performances as anode materials for Li-ion battery were investigated by galvanostatic discharge/charge and electrochemical impedance spectroscopy (EIS). The charge/discharge experiments showed that the cycle performances of carbon-coated macroporous SnP2O7 electrode were largely enhanced as compared to uncoated macroporous SnP2O7 electrode, which delivered 392.3 and 122.8 mAh g-1 at the 20th cycle, respectively. The EIS test exhibited that the charge-transfer resistance of carbon-coated macroporous SnP2O7 was smaller than that of uncoated macroporous SnP2O7. The results showed that carbon-coated macroporous SnP2O7 might have potential application as anode materials for Li-ion battery. 1. Introduction Today, Li-ion battery has become one of the most widely used batteries in modern life.1 However, the rapid development of electronic devices requires Li-ion batteries with high specific energy.2,3 Although commercially used graphite anodes have good electrochemical properties, their theoretical capacity (372 mAh g-1) is insufficient to satisfy the market requirements. Thus, there is urgent need to exploit novel anode materials for Li-ion batteries. A lot of new anode materials including metal oxide and sulfide have been studied to improve the performance of Li-ion battery.4-9 Among these materials, tin-based anode materials such as metallic tin,10,11 and tin oxides,12-14 have attracted much attention because of their higher theoretical capacity, cheapness, and no toxicity. However, a major drawback affects practical applications of these materials, that is, the large volume expansion-contraction that accompanies the lithium alloying-dealloying process. These volume variations resulted in severe mechanical strains, which greatly limited the cycling life of electrodes.15 A promising approach is to introduce an inactive matrix, which can buffer the volume expansion during the alloying processes. The typical examples are tin-based alloy16-18 and tin phosphates (including SnP2O7, Sn2P2O7, Sn3(PO4)2, and amorphous Sn2BPO6, etc.) with improved cycle performance,19-21 although the mechanism was not exactly the same. On the other hand, the capacity faded usually very fast when the upper cutoff voltage was higher than 1.3 V. The studies indicated that mesoporous tin phosphate exhibited enhanced cycle stability at the upper cutoff voltage of 2.5 V.22,23 Furthermore, the large extrinsic irreversible capacity from the side reactions in the mesoporous tin-phosphate anode was drastically decreased by introducing amorphous carbon coating in mesopores of tin phosphate.24 It was also reported that macroporous materials could benefit charge-transfer reaction rates because of the relatively large surface areas resulted from the architecture, favoring the reaction of electrochemistry.25,26 Thus, it is quite interesting to synthesize macroporous tin phosphate and macroporous tin phosphate/carbon composites and study the performance as anode materials for Li-ion battery. * Corresponding author. Phone: 86-10-6279-5290. E-mail: jhli@ mail.tsinghua.edu.cn.
Here, we report the synthesis of carbon-coated SnP2O7 by facile method, and the performance as anode materials for Li battery was also investigated. The results showed that the carbon-coated macroporous SnP2O7 exhibited higher capacity and better cycle performance as compared to uncoated macroporous SnP2O7 at the upper cutoff voltage of 3.0 V. 2. Experimental Section 2.1. Synthesis of Uncoated/Carbon-Coated Macroporous SnP2O7. To prepare macroporous SnP2O7, precursors were first prepared by hydrothermal synthesis. In a typical synthesis of precursors, 2 mL of H3PO4 (85%), sucrose (3.4 g), and SnCl4 · 5H2O (3.5 g) were dissolved in distilled water (20 mL) under stirring. The mixture was transferred into a Teflon-lined stainless steel autoclave and heated at the temperature of 190 °C for 24 h. After the mixture was cooled to room temperature, viscous and black precipitates were formed in the autoclave. The precipitates were washed with distilled water and absolute alcohol two times, respectively. The resulting black slurry was centrifugated at 5000 rpm for 30 min and then heated at 60 °C overnight. Finally, the resulting products were transferred into the muffle furnace and heated at 550 °C for 10 h, yielding the white SnP2O7 powders. To synthesize carbon-coated macroporous SnP2O7, 0.1 g of asprepared macroporous SnP2O7 and 0.1 g of sucrose were dissolved in 1 mL of distilled water under stirring. The mixture was heated at 60 °C for 12 h in air and then heated in the tube furnace at the rate of 10 °C/min to 400 °C and maintained at this temperature for 2 h under Ar atmosphere, yielding black composite powders. 2.2. Preparation of Electrodes and Electrochemical Measurements. To evaluate electrochemical performance, composite electrodes were constructed by mixing the active materials, conductive carbon black, and polyvinylidene fluoride (PVDF) in the weight ratio 80:15:5. The mixture was prepared as slurry in N-methyl pyrrolidinone and spread onto copper foil using the doctor blade technique. The electrode was dried under a vacuum at 120 °C for 8 h. The loading material on anode electrode is about 3 mg cm-2 with the electrode thickness of about 70 µm. The cells were assembled inside the argon-filled glovebox using a lithium metal foil as the counter electrode and the reference electrode, and microporous polypropylene as
10.1021/jp804438v CCC: $40.75 2008 American Chemical Society Published on Web 08/15/2008
Sn2P2O7 as Anode Materials for Li-Ion Battery
J. Phys. Chem. C, Vol. 112, No. 36, 2008 14217
Figure 1. Preparation scheme of carbon-coated macroporous SnP2O7.
the separator. The electrolyte used was 1 M LiPF6 in a 1:1 weight ratio ethylene carbonate (EC):dimethyl carbonate (DMC) solvent. Assembled cells were allowed to soak overnight and then began electrochemical testing on a Land battery testing unit (Wuhan, China). Galvanostatic charge and discharge of the assembled cells were performed at the current density of 100 mA/g between voltage limits of 0.01 and 3 V (vs Li) at room temperature. Electrochemical impedance spectroscopy (EIS) measurements (PARSTAT 2273 Advanced electrochemical system, Princeton Applied Research) were carried out at the open circuit potential with an amplitude of 10 mV AC potential in the frequency range from 100 mHz to 100 KHz. 2.3. Characterization. The morphologies of the samples were observed by a JSM-7401 field emission scanning electron microscope with energy dispersive X-ray spectroscopy (EDX). Powder X-ray diffraction (XRD) measurements of the samples were performed on a Bruker D8-Advance X-ray powder diffractometer using a graphite monochromator with Cu KR radiation (λ ) 1.5406 Å). The data were collected between scattering angles (2θ) of 10-80° at a scanning rate of 8°/min. Fourier-transform IR (FT-IR) spectra were carried out through a Perkin-Elmer spectrophotometer operating in the infrared domain 400-4000 cm-1 by a KBr matrix.
Figure 2. XRD patterns of precursor of macroporous SnP2O7 (a), macroporous SnP2O7 (b), carbon-coated macroporous SnP2O7 (c), and standard SnP2O7 phase in JCPDS (d).
3. Results and Discussion The formation of carbon-coated macroporous SnP2O7 was prepared through a process as shown in Figure 1. First, precursors that were composed of carbon spheres coated with Sn(HPO4)2 · (H2O) were obtained by one-pot hydrothermal reaction of H3PO4, SnCl4, and sucrose solution in certain amount. Next, macroporous SnP2O7 was prepared via calcinating thecentrifugatedprecursors.Finally,thecarbon-coatedmacroporous SnP2O7 was obtained by carbonizing the mixtures of as-prepared SnP2O7 and sucrose solution. The general methods to synthesize macroporous materials were often complicated, multistep experiments, that is, to prepare polymer or silica microsphere template first, then to coat desired materials to the template via layer-by-layer assembly or chemical deposition, and finally to remove template.27,28 For our method, the precursors (carbon spheres coated with Sn(HPO4)2 · (H2O)) of macroporous SnP2O7 can be prepared in a single step of hydrothermal synthesis, which greatly reduced the experimental steps. XRD measurements were performed to study the phases of precursors prepared by hydrothermal synthesis, macroporous SnP2O7 and carbon-coated macroporous SnP2O7. The XRD pattern of the precursor shown in Figure 2a exhibits diffraction peaks corresponding to monoclinic phase Sn(HPO4)2 · (H2O) (ICDD-JCPDS no. 83-110). The diffraction peaks of XRD
Figure 3. SEM images of Sn(HPO4)2 · (H2O)/carbon composite (a), macroprous SnP2O7 (b, low; c, high) magnification, carbon-coated macroprous SnP2O7 (d), EDX patterns of Sn(HPO4)2 · (H2O)/carbon composite (e), macroporous SnP2O7 (f), and carbon-coated macroporous SnP2O7 (g).
pattern for macroporous SnP2O7 shown in Figure 2b could be readily indexed to cubic phase SnP2O7 (ICDD-JCPDS no. 291352), and the peak positions and relative intensities of observed patterns were basically consistent with standard phase in JCPDS (Figure 2d). During the process of calcination, the Sn(HPO4)2 · (H2O) and carbon in the precursor converted into SnP2O7 and CO2, and thus macropores were obtained. As compared to the XRD pattern of macroporous SnP2O7, the peaks of carbon-coated macroporous SnP2O7 shown in Figure 2c had no obvious change except the intensity of peaks decreased a little, which might be caused because most carbon forming in the preparation condition belonged to the amorphous form. The morphologies and microstructures of samples were observed by SEM. As seen from Figure 3a, the precursor was composed of carbon spheres coated with coarse Sn(HPO4)2 · (H2O) layer, and still there existed a few single
14218 J. Phys. Chem. C, Vol. 112, No. 36, 2008
Figure 4. FT-IR spectra of macroporous SnP2O7 (a) and carbon-coated macroporous SnP2O7 (b).
Sn(HPO4)2 · (H2O). It is well-known that carbon spheres would be formed if sucrose was under hydrothermal environment. 29-31 During the formation of the carbon spheres, tin phosphate grew on the surface of the carbon spheres. The spheres were not monodispersed, and the diameter ranged from 2 to 5.6 µm. As seen from Figure 3b, as-prepared SnP2O7 exhibited macroscopic network structures such as honeycomb. A closer observation in Figure 3c shows diameters of most macropores were below 2 µm, which become much smaller as compared to the diameters of precursors. The serious shrinkage during the calcination process might be probably caused by further dehydration of the loosely cross-linked structure of the carbon spheres.29 As shown in Figure 3d, the carbon layer, which was blacker than SnP2O7, can be clearly seen in the macropores of macroporous SnP2O7. EDX was carried out for an elemental chemical analysis for these three samples, respectively, and the result proved the existence of Sn, P, O, and C elements seen from Figure 3e-g. For the precursor of macroporous SnP2O7, the high carbon contents originated from carbon spheres. Also, a small quantity of carbon in macroporous SnP2O7 was caused by the conductive adhesive, while the higher carbon contents originated from the carbonization of sucrose. The further EDX data analysis showed that mole ratios of Sn:P for three samples were close to 1:2, which were consistent with the ratios of molecular formulas and further proved the involved preparation processes. From FT-IR spectra of macroporous SnP2O7 shown in Figure 4a, main bands dominate the spectrum in the range from 1300 to 700 cm-1. A strong band was located in the high frequency region, for example, 1145 cm-1, followed by an intense and narrow band at 1015 cm-1. In the lower frequency part of this region, a weak and narrow band was located at 760 cm-1. According to previous references,32-34 the band at 1145 cm-1 was attributed to asymmetric vibration mode νas (PO3), close to the reported data of 1150 cm-1,34 while the band at 1015 cm-1 was attributed to symmetric vibration mode νs (PO3), in agreement with the published data of 1015 cm-1. The band at 760 cm-1 was related to the vibration mode νas of the bridging band (P-O-P), which was close to the published data of 744 cm-1.34 The peak at ∼540 cm-1 corresponded to the bending vibration (δPO2) of PO2.35 As compared to the IR spectrum of macroporous SnP2O7, the spectrum of composite shown as Figure 4b was similar in the range from 1300 to 400 cm-1 relating to the characteristic peaks of SnP2O7. However, the characteristic peaks (2960, 2930, and 2875 cm-1) of aliphatic groups appeared in the IR spectrum of composite. Furthermore, the bands at around 1380 and 1600 cm-1 indicated the formation of carbon.36,37 To test the potential applicability of the as-synthesized macroporous SnP2O7 and SnP2O7/carbon composites in lithium
Li and Li
Figure 5. The first and second cycle voltage profiles of macroporous SnP2O7 (a), carbon-coated macroporous SnP2O7 electrode (b), at a current density of 100 mA/g.
Figure 6. Cycle performance curves at the current density of 100 mA/g (∆, 2 charge and discharge capacities of SnP2O7 electrode; 0, 9 charge and discharge capacities of carbon-coated SnP2O7 electrode).
batteries, we investigated the storage properties with respect to Li insertion/extraction using the galvanostatic dischargecharge method. The newly assembled cells had open potentials about 3 V. Figure 5 shows the voltage profiles of uncoated/ carbon-coated macroporous SnP2O7 for the first and second cycles at a rate of 100 mA/g between 0.01 and 3 V (vs Li/Li+) at room temperature. The first discharge capacity of macroporous SnP2O7 and SnP2O7/carbon composite electrodes reached 1085 and 1263 mAh g-1, respectively. The second discharge capacity of SnP2O7 electrode decreased rapidly to 380.4 mAh g-1, while the one of the composite electrode remained at 620.5 mAh g-1. Although irreversible capacity would inevitably exist, the irreversible capacity of composites electrode was greatly reduced as compared to SnP2O7 electrode. These irreversible capacities resulted from the irreversible reduction of Sn(IV) to Sn(0) shown by the plateau at about 1.3-1.4 V, breakdown of the phosphate phases, and some side reactions with the electrolytes during the first reduction process.19,20,38 The macroporous framework was favored to introduce carbon into the interior of SnP2O7. The above results show that the carbon coating was also efficient to reduce the irreversible capacity for macroporous tin pyrophosphate. The cycle performance was also tested for both electrodes, as shown in Figure 6. The results show that the Coulombic efficiencies for both electrodes are nearly 100%. The discharge capacity of macroporous SnP2O7 faded fast, which was related to the upper cutoff voltage, and the discharge capacity only remained at 122.8 mAh g-1 at the 20th cycle, which is close to previous results cycled 0 to 1.4 V at 150 mA g-1 by Lee et al.21 On the other hand, carbon-coated macroporous SnP2O7 electrode showed remarkably good capacity retention as compared to the SnP2O7 electrode, with the cell remaining at 392.3 mAh g-1 at the 20th
Sn2P2O7 as Anode Materials for Li-Ion Battery
J. Phys. Chem. C, Vol. 112, No. 36, 2008 14219 permit carbon to enter the interior of SnP2O7. The coated carbon not only helped to reduced irreversible capacity but also could reversely store lithium. These results showed that carbon-coated macroporous materials might have potential applications in energy devices such as Li-ion electrode materials. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20675044) and 863 Project (2006AA05Z123). References and Notes
Figure 7. Nyquist plots of macroporous SnP2O7 (∆) and carbon-coated macroporous SnP2O7 electrode (*) at the open circuit potential (inset: equivalent circuit mode plot; Rs, resistance of the electrolytes; Rct, charge-transfer resistance; Zw, Warburg resistance; Q, constant phase element).
cycle, although the upper cutoff voltage was 3 V. The experiment showed cycle performance could be greatly enhanced by carbon coating even at a higher cutoff voltage, because the carbon itself has lithium interaction/deinteraction behaviors and could reduce the irreversible capacity of composite electrode.39 The improved electrochemical performance might be caused by the synergetic effect of carbon and macroporous SnP2O7. The ac impedance spectra of carbon-coated and uncoated macroporous SnP2O7 are shown in Figure 7. As seen from the curves, the typical characteristics of both Nyquist plots were one semicircle in the high frequency range and a sloping straight line in the low frequency. An intercept at the Zre axis in high frequency corresponded to the resistance of the electrolytes (Rs). The semicircle in the middle frequency range indicated the chargetransfer resistance (Rct), relating to the charge transfer through the electrode/electrolyte interface. Also, the inclined line in the low frequency represented the Warburg impedance (Zw), which was related to solid-state diffusion of Li ion in the electrode materials. A simplified equivalent circuit mode (inset of Figure 7) was built to analyze the impedance spectra. A constant phase element (CPE) was related to the capacitance caused by the double layer. From the Nyquist plots, it can be found that the radius of the semicircle of carbon-coated SnP2O7 electrode was smaller than that of uncoated SnP2O7 electrode, which indicated the charge-transfer resistance (Rct) of carbon-coated SnP2O7 electrode was smaller than that of uncoated SnP2O7 electrode. 4. Conclusions In summary, carbon-coated macroporous SnP2O7 was prepared by a facile method, which reduced the trivial experimental step to prepare macroporous materials as compared to the conventional method. Also, the studies of carbon-coated SnP2O7 composites as anode materials for Li-ion battery showed that the irreversible specific capacity was reduced and the cycle performance was greatly enhanced as compared to uncoated macroporous SnP2O7. The carbon-coated macroporous SnP2O7 electrode could deliver 392.3 mAh g-1 of discharge capacity, while it was only 122.8 mAh g-1 for uncoated macroporous SnP2O7 electrode at the 20th cycle with the current density of 100 mA/g. The EIS tests also showed that the charge-transfer resistance was reduced for carbon-coated macroporous SnP2O7. This improved electrochemical performance might be related to the macroporous structure, which could provide a large surface area favoring the good contact with electrolytes and
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