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In Situ Synthesis of LiFePO4/Carbon Fiber Composite by Chemical Vapor Deposition with Improved Electrochemical Performance C. Y. Wu, G. S. Cao,* H. M. Yu, J. Xie, and X. B. Zhao Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China ABSTRACT: A novel in situ route was employed to synthesize LiFePO4/carbon fiber (CF) composites. The route combined high-temperature solid-phase reaction and chemical vapor deposition (CVD) using LiOH 3 H2O and FePO4 3 2H2O as the precursors for LiFePO4 growth and acetylene (C2H2) as carbon source for CF growth. The formation mechanism of the LiFePO4/CF composites was investigated by X-ray diffraction and scanning electron microscopy. The electrochemical performance of the composites was studied by galvanostatic cycling and cyclic voltammetry. The results showed that the in situ CF growth could be realized by the catalytic effect of the Fe2O3 intermediate. The LiFePO4/CF composite with 450 °C CVD reaction showed the best electrochemical performance. This sample exhibited a stable discharge capacity of 122 mA h g1 at 10C and a slow capacity fade of only 5.5% after 1170 cycles at 1C, indicating an attractive application in high-power Li ion batteries.
1. INTRODUCTION Since first reported by Padhi et al.,1,2 olivine-type LiFePO4 has received ever increasing attention as a promising cathode material for large-size Li ion batteries from both economic and environmental points of view. Li+ can be extracted from LiFePO4 and inserted into FePO4 with a flat plateau of around 3.5 V vs Li/ Li+, yielding a theoretical capacity of 170 mA h g1, higher than those of LiCoO2 and LiMn2O4. The main problem that restricts the practical application of LiFePO4 is its poor electronic conductivity and low Li ion diffusion rate. Carbon coating has been found to be the most effective way to increase the electronic conductivity, as first reported by Ravet et al.3 By carbon coating, the electrochemical performance of LiFePO4 could be improved remarkably.47 As proved by Doeff et al.8,9 and Julien et al.10 using Raman spectra, the carbon structure plays a critical role in determining the electrochemical performance of the LiFePO4/C composites. Various forms of carbon have been used to coat LiFePO4, such as carbon black,6,7,11,12 graphite,1113 pyrolytic carbon,35,14 and porous carbon.15 One-dimensional carbon materials, such as carbon fibers (CFs)16,17 and carbon nanotubes (CNTs),1826 were also considered as the potential conductive additives for LiFePO4 due to their high electronic conductivity. Bhuvaneswari et al. found that the CF-added LiFePO4 showed improved electrochemical performance compared with the acetylene-black-added one.16 Whittingham et al. reported that the CNT-added LiFePO4 also showed promising electrochemical performance.17 In these works, the route that introduced CFs or CNTs into LiFePO4 can only be considered as an ex situ process, because the CFs or CNTs were presynthesized. In this work, a novel in situ route has been proposed to synthesize LiFePO4/CF composites, during which, LiFePO4 and CFs could form simultaneously. The in situ formation r 2011 American Chemical Society
mechanism of the composites and their electrochemical performance will be investigated.
2. EXPERIMENTAL SECTION 2.1. Preparation of Samples. LiFePO4/CF composites were prepared by solid-state reaction combined with chemical vapor deposition (CVD) with in situ growth of CFs. The precursors were mixed by ball-milling LiOH 3 H2O (analytical reagent) and FePO4 3 2H2O (analytical reagent) with a 1.02:1 molar ratio for 10 h at 300 rpm in deionized water. The mixture of the precursors was then spray-dried at 240 °C. The zfor the solid state reaction with CVD is as follows: first, the dried mixture was heated from room temperature to 400500 °C at a heating rate of 10 °C min1 and maintained at this temperature for 30 min in flowing N2/C2H2 (10% C2H2) with a flow rate of 100 cm3 min1; second, the temperature was increased to 650 at 10 °C min1 and kept at 650 °C for 4 h in flowing N2, followed by cooling to room temperature naturally. The samples with CVD reaction at 400, 450, and 500 °C are named LFPC-1, LFPC-2, and LFPC-3, respectively. 2.2. Characterization. The crystalline structure of the products was characterized by X-ray diffraction (XRD) on a Rigaku D/Max-2550pc powder diffractometer equipped with Cu Kα radiation (λ = 1.541 Å). The morphology of the products was observed by field emission scanning electron microscopy (FE-SEM) on a FEI-sirion microscope and transmission electron microscopy (TEM) on a JEM-2110 microscope. Raman spectra Received: June 1, 2011 Revised: August 14, 2011 Published: October 03, 2011 23090
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Figure 1. XRD patterns (a) and Raman spectra (b) of the CVD products.
were collected on a Jobin-Yvon Labor Raman HR-800 Raman system by exciting a 514.5 nm Ar ion laser. The carbon content was measured on a Flash EA 1112 tester. The powder samples were pressed into pellet samples with a relative density of about 90% by hot pressing under vacuum. The electronic conductivity of pellet samples was measured by four-probe method at room temperature. 2.3. Electrochemical Performance. LiFePO4/CF, acetylene black, and polyvinylidene fluoride (PVDF) binder in a weight ratio of 75:15:10 were mixed in N-methylpyrrolidone (NMP) and stirred for 2 h to make a slurry. The working electrodes were prepared by spreading the slurry on Al foils followed by drying at 100 °C for 5 h under vacuum. The CR2025-type coin cells were assembled in an Ar-filled glovebox using metallic lithium as anode, 1 M LiPF6 solution in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume) as electrolyte, and Celgard 2300 porous polypropylene film as separator. Galvanostatic cycling was performed between 2.5 and 4.3 V vs Li/Li+ at various C rates on a PCBT-138-32D battery tester, where 1C corresponds to 170 mA g1. Cyclic voltammetry (CV) was conducted between 2 and 4.2 V at 0.1 mV s1 using a CHI660C electrochemical workstation. All of the electrochemical measurements were performed at room temperature.
3. RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of the products prepared by solid-phase reaction with CVD reaction. For comparison, the standard patterns of LiFePO4 are also given. Note that all the patterns of the products can be indexed to olivine-type LiFePO4 with space group Pmnb with minor Li3PO4 impurity. No diffraction peaks related to carbon can be found, indicating that it is not well crystallized or at a low content. It is generally accepted that the formation of LiFePO4 is based on the reaction 2LiOH þ 2FePO4 þ C f 2LiFePO4 þ CO þ H2 O
ð1Þ
when using LiOH 3 H2O and FePO4 3 2H2O as the precursors in the presence of carbon. In this work, the C comes from the pyrolysis of C2H2: C2 H2 f 2C þ H2
ð2Þ
The carbon contents of LFPC-1, LFPC-2, and LFPC-3 are 2.1, 3.7, and 5.7 wt %, respectively. Note that reasonable carbon contents were obtained during the 30 min CVD reaction and that increasing the CVD temperature will deposit more carbon. Figure 1b gives the Raman spectra of the three samples. Two
Table 1. Electronic Conductivity of the LiFePO4/CF Composites sample electronic conductivity (S cm1)
LFPC-1 0.013
LFPC-2 0.49
LFPC-3 0.61
bands at around 1350 and 1580 cm1 appear for all the products, corresponding to the disordered (D) band and graphitic (G) band of carbon materials. The Raman spectra further confirms the presence of the deposited carbon. The electronic conductivities of the LiFePO 4/CF composites were measured by the four-probe method and are presented in Table 1. As expected, the electronic conductivities of the composites are much higher than that of pure LiFePO4 (about 1091010 S cm1).2729 Figure 2 shows some typical SEM images of LFPC-2. Only the SEM images of LFPC-2 are given, since the three samples exhibit similar morphology. It is clear that the product is composed of nanosized particles and fiberlike materials. The nanoparticles are entangled by the fiberlike materials to form aggregates. The diameter of the fiberlike materials in the aggregations is around 1030 nm. It seems that the small aggregates are bridged by the thicker fiberlike materials (50100 nm in diameter) to form larger aggregates. According to the XRD and Raman analyses, the nanoparticles are considered to be LiFePO4, while the fiberlike materials are CFs. As a result, a three-dimensional (3D) conductive network for the LiFePO4 particles was formed by the interentangled CFs. It is expected that the CFs can maintain an intimate contact with the LiFePO4 particles owing to this in situ synthetic route. The composite was further observed by TEM, as seen in Figure 3. It is clear from the figure that the LiFePO4 nanoparticles are entangled by the CFs (Figure 3a). Figure 3b shows the TEM images of the two adjacent LiFePO4 nanoparticles connected by a carbon layer of 3 nm thickness. Thus, the CFs that bridge the detached (also adjacent particles in some cases) particles and carbon layers that connect the neighboring particles constitute the 3D electronically conductive channels for the LiFePO4 particles. A good electronic conductivity, therefore, can be anticipated for the LiFePO4/CF composites, which is critical for high-power application. The in situ growth mechanism of CFs is investigated by XRD and TEM. Figures 4 and 5 show the XRD patterns and SEM images, respectively, of the CVD products using different precursors. Clearly, LiFePO4 (Figure 4a) and fiberlike structures (Figure 5a) 23091
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Figure 2. SEM images of the LFPC-2 sample. The scale bars in parts a, b, c, and d correspond to 2 μm, 2 μm, 1 μm and 500 nm, respectively.
Figure 3. TEM images of the LFPC-2 sample. The scale bars in parts a and b correspond to 100 and 20 nm, respectively.
can form when using FePO4 3 2H2O and LiOH 3 H2O as the precursors. In contrast, No fiberlike materials appear (Figure 5b) when using presynthesized LiFePO4 as the precursor under the same CVD conditions. It means that LiFePO4 itself cannot catalytically grow CFs. FePO4 3 2H2O cannot act as the catalyst either to grow CFs, as confirmed by the SEM observation (Figure 5c). After the CVD reaction, it was converted to Fe2P2O7 (Figure 4c). The possibility of LiOH 3 H2O as the catalyst is also excluded. Generally, transition metals or transition metal oxides, for instance Fe2O3, can be used as catalyst to grow CFs or CNTs. In a separate experiment, fiberlike materials form when using nanosized Fe2O3 (2040 nm, Alfa Aesar) as the precursor during the CVD reaction (Figure 5d). The Fe2O3 was transformed into Fe3C, as shown in Figure 4d. On the basis of the above analyses, a possible mechanism is suggested that the growth of CFs was catalyzed by the Fe2O3 intermediate during the formation of
Figure 4. XRD patterns of the CVD products using different precursors: (a) FePO4 3 2H2O and LiOH 3 H2O, (b) presynthesized LiFePO4, (c) FePO4 3 2H2O, and (d) Fe2O3.
LiFePO4 via the following reactions: 3LiOH þ FePO4 f FeðOHÞ3 þ Li3 PO4
ð3Þ
2FeðOHÞ3 f Fe2 O3 þ 3H2 O
ð4Þ
The formation of Li3PO4 impurity confirms our assumption (Figure 1a). In a control experiment, Li3PO4 did not form when glucose was used as carbon source instead of C2H2. In our previous work, CFs also formed in the CVD reaction when using Fe2O3, Li3PO4, and FePO4 as the precursors.30 Furthermore, the formation of Fe(OH)3 was confirmed by checking the ball-milling products of FePO4 3 2H2O and LiOH 3 H2O. The low carbon 23092
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Figure 5. SEM images of the CVD products using different precursors: (a) FePO4 3 2H2O and LiOH 3 H2O, (b) presynthesized LiFePO4, (c) FePO4 3 2H2O, and (d) Fe2O3. The scale bars in parts a, b, c, and d correspond to 1 μm, 1 μm, 500 nm, and 1 μm, respectively.
Figure 6. CV plots of the CVD products.
content (below 6 wt %) and the trace amount of Li3PO4 suggest that the growth rate of LiFePO4 is much rapider than that of CFs under the present CVD conditions. This is favorable for the preparation of LiFePO4 with a high purity and a reasonable carbon content. Figure 6 shows the CV plots of the LiFePO4/CF composites. All the samples exhibit very sharp and highly symmetrical current peaks, indicating rapid electrochemical kinetics due to a good crystallization of the LiFePO4 and a high electronic conductivity of the composites. Among the three samples, LFPC-2 demonstrates the smallest potential difference between the cathodic and anodic current peaks, indicating the best electrode kinetics. It is suggested that the different kinetics originates from the subtle difference in the microstructure of LiFePO4, CFs, and carbon layer due to the different CVD temperatures. The exact mechanism, however, is unclear yet.
The charge (Li extraction) and discharge (Li reinsertion) curves at various rates are presented in Figure 7. In these electrochemical tests, 1C corresponds to 170 mA g1 and the gravimetric capacity is based on the weight of the LiFePO4/C composites (including C layers and CFs). Figure 7a shows the charge and discharge performance of the three samples at 0.1C. All the samples exhibit flat charge (3.46 V) and discharge (3.41 V) plateaus, indicative of good crystallization of LiFePO4. The LFPC-2 sample yields the highest discharge capacity of 168 mA h g1, close to the theoretical value of LiFePO4. The high capacity means that only a trace amount of Fe source participates in the catalytic growth of CFs. In contrast, the relatively low capacity for LFPC-3 suggests that a higher proportion of Fe source was consumed to catalytically grow CFs, in agreement with its higher carbon content. At higher current rates (1C, 5C, 10C), the LFPC-2 sample also gives the highest capacity (parts b, c, d of 7). The discharge capacities of LFPC-2 at 1C and 5C reach 150 and 134 mA h g1, respectively. Even at a 10C rate, a discharge capacity of 122 mA h g1 can be obtained for LFPC-2, while for samples LFPC-1 and LFPC-3, the values rapidly decrease to 105 and 82 mA h g1 (Figure 7d), respectively, when the current density is increased to 10C. In addition, the LFPC-2 sample also exhibits the smallest polarization among the three samples. The different electrochemical performance of the samples can be attributed to their different microstructures arising from different CVD temperatures. Figure 8a compares the rate capability among the three samples. It is obvious that the LFPC-2 sample displays the best rate capability. Although all the samples exhibit a good cycling stability at each current rate, the LFPC-2 sample shows the 23093
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Figure 7. Chargedischarge curves of the CVD products at various rates: (a) 0.1C for charge and 0.1C for discharge, (b) 1C for charge and 1C for discharge, (c) 5C for charge and 5C for discharge, and (d) 1C for charge and 10C for discharge.
Figure 8. Rate capability among the three samples (a) and cycling stability of the LFPC-2 sample at 1C rate (b).
highest capacity. The good rate capability can be ascribed to the efficient 3D conductive network constructed from the interentangled CFs and carbon layers that bridge and connect the LiFePO4 particles. The best rate capability for LFPC-2 indicates that it has the most efficient conductive channels among the three samples. Figure 8b shows the long-term cycling stability of LFPC-2 at 1C. A capacity loss of only 5.5% can be achieved after 1170 cycles at 1C, indicating an excellent cycling stability for this sample. The attractive rate capability and cycling stability of
LFPC-2 suggest that this material will show a promising application as cathode for high-power Li ion batteries.
4. CONCLUSIONS In this work, LiFePO4/CF composites were successfully synthesized by an in situ route that combines solid-phase reaction with CVD treatment using LiOH 3 H2O, FePO4 3 2H2O, and C2H2 as the precursors. The results showed that the formation 23094
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The Journal of Physical Chemistry C of LiFePO4 and CFs takes place simultaneously, with the former exhibiting dominantly more rapid growth rate than the latter under the applied CVD conditions, leading to a good crystallization of LiFePO4 and a reasonable carbon content. The CFs are interentangled forming an efficient 3D conductive network. Besides CFs, a carbon layer is also deposited on the surface of LiFePO4 particles during the CVD process, further enhancing the electronic conductivity of the composites. Among the three CVD products, the sample treated at 450 °C exhibits the highest capacity and the best rate capability. This sample also shows an excellent cycling stability with only 5.5% capacity loss during 1170 cycles at 1C. The excellent electrochemical performance is attributed to the efficient 3D conductive network composed of the interentangled CFs and carbon layers that bridge and connect the LiFePO4 particles. The attractive electrochemical performance of the LiFePO4/CF composite suggests that this material shows a promising application as cathode for high-power Li ion batteries.
’ AUTHOR INFORMATION Corresponding Author
*Tel./fax: +86-571-87951451. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by Zijin Program of Zhejiang University, China, the Fundamental Research Funds for the Central Universities (No. 2010QNA4003), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20100101120024), the Foundation of Education Office of Zhejiang Province (No. Y201016484), and the Qianjiang Talents Project of Science Technology Department of Zhejiang Province (2011R10021).
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