Ortho-Rich Polyacene

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J. Phys. Chem. C 2010, 114, 3297–3303

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The Optimum Nanomicro Structure of LiFePO4/Ortho-Rich Polyacene Composites Li-Qun Sun,†,‡ Ming-Juan Li,†,‡ Rui-Hui Cui,†,‡ Hai-Ming Xie,*,†,‡ and Rong-Shun Wang*,†,‡ Institute of Functional Materials, Department of Chemistry, Northeast Normal UniVersity, Changchun, Jilin 130024, People’s Republic of China, LIB Engineering Laboratory, Materials Science and Technology Center, Changchun, Jilin 130024, People’s Republic of China ReceiVed: October 31, 2009; ReVised Manuscript ReceiVed: December 28, 2009

An optimum nanomicro structure of LiFePO4/o-polyacene (LiFePO4/O-PAS) with a high proportion of ortho linkages has been constructed using special processing technologies. O-PAS produces a thin, uniform conductive network because of its unique structure with low-branching and long-range order. The nanomicro structure of the composite cooperates with O-PAS to improve the electronic conductivity and lithium ion mobility, decrease the specific surface area, and improve the quality of the prepared electrode, which are the key factors for the large scale manufacture of lithium iron phosphate materials. 1. Introduction Electrochemical energy storage devices with both high energy and power densities have become the most important focus in current research, owing to their potential use in powering electric vehicles (EVs), hybrid electric vehicles, and dispersed energy storage systems. Since the pioneering work by Goodenough et al.,1 considerable attention has been paid to the olivine-structured LiFePO4 for application as a cathode material in Li-ion batteries. This is because of its better features over conventional cathodes.2-18 The high safety, long operation lifetime, and environmental benign of LiFePO4 are all requirements of cathode materials used in power batteries for EVs. However, weak intrinsic electronic conductivity (ca. 10-9-10-10 S cm-1 at room temperature),2 the low lithium ion mobility in LiFePO4, and the difficult slurry pasting of LiFePO4 in electrode preparation were identified as major problems preventing its commercial application. Numerous approaches were applied to improve the performance of this material. Conductive carbon is the technology most commonly used to improve electronic conductivity because of its low cost and small effect on charge-discharge processing. However, most carbon materials failed to completely and effectively coat the electrode. This leads to a partly broken and hence insufficient electronically conducting network.19,20 To obtain improved coatings, high carbon contents have to be required. The reason for this lies in the different surface properties of carbon, which vary from sp2-type21 to sp3-type22,23 in the final samples. Many conducting polymers, such as polyaniline, polypyrrole, and polyacene (PAS), have been studied as conductive carbon sources or electrode materials directly for rechargeable batteries because they are electrochemically active and permit penetration of the electrolyte into the polymer mass.24,25 Among the conducting polymers, PAS is one of the most important for use as a highly conductive carbon material. Previously, we reported that PAS shows intrinsic high conductivity because it contains many special sp2-type π bonds.21 However, this PAS material was found to have an amorphous and incomplete structure, * To whom correspondence should be addressed. E-mail: wangrs@ nenu.edu.cn and [email protected]. Tel.: +86-431-85099511. Fax: +86-431-85099511. † Northeast Normal University. ‡ Materials Science and Technology Center.

which decreases the proportion of sp2-type carbon it contains. The discharge and rate capacities of LiFePO4 cathodes increased with an increased amount of sp2-type carbon and a decreased level of disorder.26 With the aim of increasing the content of sp2-type carbon and further improving the electrochemical performance, we report here a new strategy to a LiFePO4 material containing ortho-rich polyacene (O-PAS). Researchers have attempted to reduce the diffusion length of Li+ by controlling the particle size and morphology.27-30 Ultrafine LiFePO4 nanoparticles with homogeneous particle size indeed show an obvious effect on the lithium extraction/insertion process in the electrochemical reaction. However, they also possess a large specific surface area that hinders the slurry pasting used for electrode preparation. We have designed a simple and low-cost route to construct the optimum nanomicro structure of a LiFePO4/O-PAS material using special processing conditions. Nanosized primary particles work to shorten the lithium ion migration path in LiFePO4. Microsized secondary particles reduce the specific surface area of the particles in the sample, improving the processability of the cathode slurry. 2. Experimental Section 2.1. Synthesis. High o-phenol formaldehyde resin (O-PF) was synthesized by using an optimal amount of Co(CH3COO)2 as catalyst, a pH of 5-6, and a reaction time of 8 h. The molar ratio of phenol to formaldehyde was controlled at 1:0.8 to ensure a higher proportion of ortho-ortho links in the phenolic resin. After heat treatment at 700 °C, the transformation from O-PF to O-PAS was complete. The synthesis of PF has been described in detail in the literature.21 LiFePO4/O-PAS and LiFePO4/PAS were synthesized with use of ferric oxide (Fe2O3), phosphoric acid (H3PO4), and lithium hydroxide (LiOH · H2O) with a molar ratio of 1:2:2. The precursors were mixed with 5 wt % of O-PF and 10 wt % of PF, which were separately dissolved in small quantities of ethanol. The mixtures were first coarsely ground and then fine ground for 5 h separately, which helped to form the nanosized primary particles. The raw slurry was spray-dried and granulated at 180 °C, using air as the carrier gas. The nozzle diameter of the spray-drier was adjusted to control the size of the secondary particles between 10 and 20 µm. The same raw materials were

10.1021/jp910422g  2010 American Chemical Society Published on Web 02/01/2010

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Figure 1. SEM images of the prepared LiFePO4 with different carbon coatings: (a) sp3-sucrose, (b) CNTs, (c) mainly sp2-PAS, and (d) sp2-O-PAS.

used to synthesize LiFePO4/CNT and LiFePO4/sucrose. The precursors were mixed with 3 wt % of CNT and 10 wt % of sucrose, which were separately dissolved in small quantities of ethanol. The mixtures were ball-milled for 10 h and then dried at 80 °C. Finally, the uniform precursors were calcined under N2 gas for 4 h at 700 °C using a heating rate of 3 °C min-1. 2.2. Structural Characterization and Electrochemical Measurements. FTIR absorption spectra were recorded with a Fourier transform interferometer (D/MAX-IIIC) over the wavenumber range 500-4000 cm-1. The ratio of o,o′-linkages and p,p′-linkages in the high ortho resin was obtained with a NMR spectrometer (Bruker, 600 MHz). An integrated Raman microscope (HR-800) was used to analyze the structure and composition of the LiFePO4 and carbon materials. The powder samples were pressed into disks (with a typical diameter of 15 mm and a thickness of 2 mm) at 12 MPa and then the electronic conductivity of the samples was measured with use of a fourpoint probe meter (SDY-5). Brunauer-Emmet-Teller (BET) surface area measurements were carried out on a Micromeritics ASAP 2010 surface area analyzer at 77K. Prior to measurement, the samples were degassed for 12 h at 523 K under 10-6 Torr to remove absorbed moisture. The morphology of the powders was observed on a scanning electron microscope (SEM; Hitachi S-3500 V) and a high-resolution transmission electron microscope (HR-TEM; Jeol TEM-2000FXII). Coin cells (type 2025) were assembled in an argon-filled glovebox. The cathodes were prepared by mixing 90 wt % of LiFePO4-based powders with 5 wt % of carbon black and 5 wt % of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). The mixed slurry was pasted onto aluminum foil and dried overnight at 393 K in an oven. The dried coated foil was roller pressed and circular discs were punched out. Lithium metal was used as the anode and a 1 M solution of LiPF6 in

ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 v/v) was used as the electrolyte. Galvanostatic cycling tests of the assembled cells were carried out on a Land battery test system in the voltage range of 2.5-4.2 V (vs. Li+/Li). 3. Results and Discussion 3.1. Surface Characteristics and Electrochemical Studies. To investigate the coating abilities of different carbon materials, the surface physical properties of the prepared samples were compared (Figure 1). Here, three kinds of carbon are classified by their corresponding structure and are labeled as follows: sp3type amorphous carbon, carbon nanotubes (CNTs), and sp2type organic macromolecular polymers. The different structural features of the three carbon samples result in different coating of the electrode. The sp3-type (Figure 1a) carbon sample can be defined as partially carbon-coated and partially carbon-doped, and the CNT (Figure 1b) sample can be defined as mainly carbon-mixed. These particles have line or point connections, while connections in the macromolecule sp2-type carbon are defined as planar. This is caused by the special layered structure of sp2-type carbon with a π-bond (Figure 1c,d), and results in more effective coverage. Further comparison of the coating features of PAS (Figure 1c) and O-PAS (Figure 1d) allows better understanding of their differences. Because of the low-branched, highly ordered planar structure of O-PAS, the optimum amount of carbon is just 1.6% in the calcined LiFePO4 sample. This amount is sufficient to produce an ideal coating that is thinner and shows porous carbon networks as well as improving electronic conductivity. The content of carbon is up to 5% in the best sample of LiFePO4/ PAS, in which PAS forms a thicker but incomplete layer on the surface of LiFePO4.

LiFePO4/Ortho-Rich Polyacene Composites

Figure 2. Comparison of the first charge-discharge profiles of the prepared LiFePO4 composites recorded at a rate of C/10 between 2.5 and 4.2 V.

J. Phys. Chem. C, Vol. 114, No. 7, 2010 3299 Figure 2 shows the first charge-discharge profiles and cyclability of four samples containing different carbon-based coatings. The as-synthesized LiFePO4/CNT and LiFePO4/ sucrose samples exhibit a discharge capacity of 102.5 and 135.5 mAh g-1, respectively, with a large voltage difference between the charge and discharge curves. The lower discharge capacity of these samples could be partly caused by the poorer surface coverage of CNT and sucrose compared with that obtained with PAS. The excellent capacity of the samples containing PAS could be related to significant improvements in the ordered structure of the carbon coating and the electronic conductivity as reported elsewhere.21,31 The differences between the O-PAS and PAS samples will be discussed in the following sections. 3.2. Molecular Structures and Characteristics of PAS and Its Precursors. Organic macromolecular polymers have the tendency to pyrolyze to form more highly graphitized carbons

Figure 3. Molecular structures of (a) the phenolic formaldehyde resin (PF), (b) polyacenic semiconductor (PAS), (c) the high o-phenolic formaldehyde resin (O-PF), and (d) o-polyacenic semiconductor (O-PAS).

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Figure 4. Determination of ortho substituents in the composites: (a) IR spectra of LiFePO4/PF, (b) IR spectra of LiFePO4/O-PF, (c) spectrum of O-PF, and (d) Raman spectra of LiFePO4/O-PAS and LiFePO4/PAS.

in aromatic ring systems. We have reported that PF can generate a two-dimensional planar PAS semiconductor with sp2 character by high temperature pyrolysis.21 Although the ideal structure of PAS was given, in reality it still contained some branches and amorphous parts. In this work we focus on realizing lowbranched PAS to improve its proportion of sp2 carbon. To reduce the number of branches in traditional PF we synthesized O-PF, which was used as a carbon source to synthesize in situ O-PAS-coated LiFePO4 cathode materials. In alkaline conditions, phenol reacts with formaldehyde to generate highly branched PF (Figure 3a). O-PF, which has a linear, lowbranched structure (Figure 3c), is obtained in acidic conditions when the percentage of phenol exceeds 20%. After being processed at high temperatures, O-PF generates highly ordered O-PAS (Figure 3d), which has a ratio of sp2 carbon that is much higher than that of conventional PAS. O-PAS consists of (2n + 3) benzene rings and n five-membered rings (n ) 1, 2, 3, ...), so it can form a complete carbon network on the bare surface of LiFePO4 because of its low-branched structure with long-range order. It uniformly disperses over the activated LiFePO4, inhibiting the growth of LiFePO4 particles and forming an ideal coating. 3.3. Spectral Characterization of the LiFePO4/Carbon Composites. The chemical structures of conventional PF and O-PF were confirmed by using FTIR. Figure 4 shows that the spectra of the two resins are approximately the same between 4000 and 1000 cm-1. However, the functional group region between 850 and 500 cm-1 shows obvious differences. The absorption bands at 753 and 812 cm-1 are characteristic of orthosubstituted and para-substituted benzene rings of the phenolic formaldehyde resin, respectively. These two absorption peaks

13

C NMR

have similar intensities in PF (Figure 4a). In O-PF, the absorption intensity of the band at 753 cm-1 is much higher than that of the 812 cm-1 band (Figure 4b), indicating that there is only a small amount of para-substitution in O-PF. This is also confirmed by the 13C NMR spectrum of O-PF shown in Figure 4c. The band at 159 ppm was attributed to residual phenol and the band at 40 ppm was caused by the solvent. The region from 30 to 45 ppm shows peaks caused by the methylene bridges between aromatic rings in O-PF. Particularly, the bands at around 42, 35, and 32 ppm were attributed to the methane protons in p,p′-, o,p′-, and o,o′-diphenylmethane-type methylene bridges, respectively. From the relative intensities of these peaks, it can be seen that o,o′-linkages formed predominantly, and the ratio of p,p′-linkages was very low. Figure 4d shows the Raman spectra of the LiFePO4/PAS and nanomicro structured LiFePO4/O-PAS composites. The peaks at around 600 and 950 cm-1 can be attributed to iron oxide and the characteristic symmetric PO43- stretching vibration of LiFePO4, respectively.32 The bands in the range of 1170-1460 and 1470-1730 cm-1 are attributed to the D-band (sp2 disorderinduced phonon mode) and G-band (sp2 graphite band) of carbon, respectively.32 The D and G fractions of the composites were estimated by fitting the Raman spectra. Each component is a convolution of a Gaussian distribution, and the contribution of the background was subtracted by using the Shirley method. The integral intensity ratios of the D band to the G band for LiFePO4/O-PAS and LiFePO4/PAS were estimated to be about 0.78 and 0.93, respectively. The lower D/G ratio of carbon for LiFePO4/O-PAS means it should have better electrical conductivity than LiFePO4/PAS.

LiFePO4/Ortho-Rich Polyacene Composites

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Figure 5. (a-c) SEM images of LiFePO4/O-PAS at different magnifications indicating the particle size and morphology; (d) a typical TEM image of LiFePO4/O-PAS showing the even coating; (e, f) high magnification images of the nanomicro structure and the carbon networks on the surface of LiFePO4 and between the nanosized primary particles.

TABLE 1: BET Analysis of LiFePO4/O-PAS (5 wt %) Samples with Random and Spherical Morphology Obtained by Different Preparation Methods sample LFP LFP LFP LFP LFP LFP

+ + + + + +

3.0 wt % PF 5.0 wt % PF 10.0 wt % PF 3.0 wt % O-PF 5.0 wt % O-PF 10.0 wt % O-PF

residual carbon (wt %) 1.26 2.51 4.15 1.35 2.55 4.48

3.4. Morphology and BET Surface Area of the Composites. The size and shape of the particles in the products were examined by SEM and TEM. Panels a and b of Figure 5 show typical low-magnification FE-SEM images of the LiFePO4/OPAS sample. It can be clearly seen that the sample is mainly composed of uniform sphere-like microstructures with diameters ranging from 10 to 20 µm. Figure 5c-f displays the morphology of an individual microsized secondary spherical particle, which has an outer surface coated by a nanonetwork of O-PAS and is internally connected by interlaced O-PAS. The typical TEM image of LiFePO4/O-PAS (Figure 5d) shows clearly that the LiFePO4 particles (black) are completely and uniformly coated with a layer of O-PAS (gray) with a thickness of ∼20 nm. Panels e and f of Figure 5 show the nanomicro composite consists of nanosized primary LiFePO4 particles about 200-300 nm in diameter embedded in a nanosized O-PAS network. The nanosized primary particles together with the thinner reticulated

conductivity (S cm-1) 5.2 × 1.6 × 8.5 × 8.7 × 2.31 3.78

10-2 10-1 10-1 10-1

first discharge capacity (mAh g-1) 141 150 156 151 161 154

conductive carbon connect to form porous micrometer-sized particles with a regular spherical shape. This not only makes the spherical particles more permeable to the electrolyte solution, which further improves the lithium ion mobility, but also facilitates the slurry attaching more firmly to the electrode due to the decreasing of the specific surface area from 30.5 m2 g-1 with random morphology to 14.2 m2 g-1 (Table 1). Decreasing the specific surface area of nanomicro spherical LiFePO4 has great significance to its application. The capacity of the composites has no obvious correlation with the specific surface area.34 However, an appropriate specific surface area facilitates firm attachment of the slurry onto the aluminum foil. The lower specific surface area of the nanomicro sphere-like sample helps to overcome the surface tension in the slurry and to allow an electrode that is smooth and of even thickness to be obtained. Simultaneously, the tap density increases to 1.48 g cm-3 for

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Figure 6. Typical charge/discharge profiles of LiFePO4/O-PAS and LiFePO4/PAS at a current rate of 0.5 C; the inset shows the flat region magnified. Charge/discharge curves show little polarization because of the high electronic conductivity of the samples.

TABLE 2: Conductivities of the LiFePO4/O-PAS and LiFePO4/PAS Cathode Materials with Various Proportions of Carbon morphology random spherical

BET surface area (m2 g-1)

tap density (g cm-3)

30.5 14.2

1.22 1.48

the spherical sample from 1.22 g cm-3 for the random LiFePO4/ O-PAS composite. 3.5. Electrochemical Characterization. To test the potential application of the olivine-type LiFePO4/O-PAS and LiFePO4/ PAS cathodes in high-power lithium ion batteries, we investigate the electrochemical performance with respect to Li+ insertion/ extraction. Figure 6 compares the first charge-discharge profiles and cyclability of LiFePO4/O-PAS and LiFePO4/PAS. They exhibit a very flat discharge plateau at 3.4-3.5 V and display a very small charge/discharge polarization at a current density of 34 mA g-1. The LiFePO4/O-PAS nanomicro composite exhibits a high discharge capacity of 161 mAh g-1, close to the theoretical value of 170 mAh g-1, with little difference between the charge and discharge curves. The polarization between the charge and discharge plateau is reduced to 38 mV from 79 mV for the LiFePO4/PAS sample, indicating that the kinetics of LiFePO4 are indeed improved by using O-PAS. The prepared LiFePO4/ PAS exhibits a reversible capacity of 156 mAh g-1 with a significant voltage difference. The polarization loss could be caused by the low electronic conductivity of this sample. Table 2 shows the conductivities of the LiFePO4/O-PAS and LiFePO4/ PAS cathode materials. Increasing the carbon content increased the conductivity of the sample. The conductivities of the O-PAScoated LiFePO4 samples are nearly eleven orders of magnitude greater than that of pure LiFePO4 (10-9 S cm-1), while that of PAS-coated LiFePO4 is 8 orders of magnitude higher. Figure 7 compares the rate capabilities (at rates of 0.2 to 20 C) of LiFePO4/O-PAS and LiFePO4/PAS. The LiFePO4/O-PAS nanomicro structured composite exhibits better rate capability than the LiFePO4/PAS sample because of the improved kinetics provided by the structurally ordered, electronically conducting O-PAS coating on LiFePO4. Figure 8a shows the rate performance of the LiFePO4/O-PAS and LiFePO4/PAS electrodes at rates of up to 20 C. Although the specific capacity decreases with increasing current density, the capacity retention is high for all the different rates. It can also be seen that O-PAS and PAS have significantly different

Figure 7. Comparison of the rate capabilities of (a) LiFePO4/O-PAS and (b) LiFePO4/PAS.

Figure 8. (a) Rate performance of LiFePO4/O-PAS and LiFePO4/PAS at different current densities from 34 mA g-1 to 3400 mA g-1; (b) cycling performance of LiFePO4/O-PAS, cycled at rates of 0.5 and 8 C between 2.5 and 4.2 V (vs. Li+/Li). The inset shows the corresponding Coulombic efficiency profiles and the discharge voltage plateau of LiFePO4/O-PAS at rates of 0.5 and 8 C, respectively.

effects on the electrochemical behavior of LiFePO4. At low rates, both samples exhibit comparable performance. However, at higher rates the superior performance of the O-PAS-coated composites is revealed. Specific reversible capacities of 131 and

LiFePO4/Ortho-Rich Polyacene Composites 112.4 mAh g-1 were obtained at rates of 5 and 10 C for the O-PAS-coated sample, respectively, whereas these values decrease from 121 (5 C) to 95 mAh g-1 (10 C) for the PAScoated sample. Even at 20 C, a reversible capacity of 80 mAh g-1 is achieved for the O-PAS sample, whereas for the PAScoating sample the specific reversible capacity is lower than 40 mAh g-1. LiFePO4/O-PAS also exhibits excellent cyclability with no noticeable decrease on performance over 800 cycles (Figure 8b). The discharge capacity loss is less than 1% at 0.5 C and 3% at 8 C. The Coulombic efficiency (the ratio of discharge capacity to charge capacity) remains close to 100%, and the discharge voltage plateaus were maintained separately at 3.4 (0.5 C) and 3.0 V (8 C) as shown in the inset of Figure 8b. The O-PAS coating exhibits remarkable properties. These results highlight the fact that the higher discharge capacities and better rate capabilities of LiFePO4/O-PAS versus LiFePO4/ O-PAS cathodes are directly correlated with an increased amount of sp2-type carbon domains and a decreased level of disorder in the O-PAS planes. 4. Conclusion A simple and low-cost synthesis in the presence of an orthorich polyacenic semiconductor (O-PAS) allowed the preparation of nanomicro structured LiFePO4 powders with outstanding features as cathode materials for Li-ion batteries. LiFePO4/OPAS nanomicro composites exhibited higher initial discharge capacity, better cyclability, and higher rate capabilities compared with as-prepared LiFePO4-PAS. This is because of a high degree of ordering and a high ratio of sp2-type carbon atoms in O-PAS. The study demonstrates that both a high electronic conductivity and a short lithium ion diffusion length are critical to achieve high-performance LiFePO4 cathodes. The presence of O-PAS leads to the preparation of powders with finely dispersed LiFePO4, which further improved the electron conductivity. Nanosized primary particles in the nanomicro structured LiFePO4 composite facilitated lithium ion diffusion. Moreover, the nanomicro particles greatly improve the slurry pasting and tap density of LiFePO4. The whole procedure is simple, yet very effective at producing LiFePO4 powders with desirable properties. The success of this model design could also provide a new way to synthesize other electrode materials. Acknowledgment. This work was supported by a project issued by the National Key Technologies R&D Program (grant No. 2009BAG19B00) and the Science and Technology Development Program of the Jilin Province (grant Nos. 20076020 and 20080304). References and Notes (1) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188. (2) Chung, S.-Y.; Blokingand, J. T.; Chiang, Y.-M. Nat. Mater. 2002, 1, 123.

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