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J. Phys. Chem. B 2006, 110, 10236-10240
ARTICLES Polymer-Functionalized Multiwalled Carbon Nanotubes as Lithium Intercalation Hosts Xiaohong Wang,† Hewen Liu,*,†,‡ Yi Jin,§ and Chunhua Chen§ Department of Polymer Science and Engineering, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China, State Key Laboratory of Polymer Physics and Chemistry, Changchun 130022, People’s Republic of China, and Department of Materials Science and Engineering, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: January 2, 2006; In Final Form: March 24, 2006
Multiwalled carbon nanotubes (MWNTs) functionalized with a hyperbranched aliphatic polyester and two different poly(ethylene glycol)s were synthesized by the reactions of carbonyl chloride groups on the surface of MWNTs and hydroxyl groups of polymers. Electrochemical intercalation of lithium in the three materials was investigated with galvanostatic charge-discharge experiments. The hyperbranched polymer-functionalized MWNT as an electrode material for lithium batteries showed a significant improvement over linear polymerfunctionalized MWNTs in lithium insertion/deinsertion capacity and cycle stability. The MWNT functionalized with linear poly(ethylene glycol) showed a high initial capacity of lithium insertion/deinsertion but had the highest capacity fade rate among the materials. Because the polymers were chemically localized in the electrode-electrolyte interface, the comparison between hyperbranched and linear polymer-modified MWNTs manifested the important influence of the electrode-electrolyte interface on the electrochemical properties of lithium batteries.
1. Introduction The development of materials for rechargeable lithium batteries is of an enormous scientific importance. Much interest in this field has been concentrated on polymer electrolytes and nanosized electrode materials for lithium insertion.1 Carbon nanotubes (CNTs) as electrode materials show great promise for the optimization of the performances of lithium batteries, due to the theoretically large numbers of nanoscale sites for intercalant atoms on the CNTs.2-4 Polymer electrolytes typically based on poly(ethylene oxide) (PEO) containing a lithium salt have received considerable attention.5 Besides other demands, an ideal battery has both a high-energy capacity and a long cycle time, which are significantly affected by the properties of the solid electrolyte-electrode interface.6 Efforts have been devoted to understanding the influence of the electrolyte-electrode interface on the performances of lithium batteries.7 In this work we modified multiwalled carbon nanotubes (MWNTs) via a “graft-to” method by chemically attaching different polymers including two poly(ethylene glycol)s with different molecular weights and a hyperbranched polyester based on 2,2-bis(methylol)propionic acid to the surfaces of MWNTs. These polymers are chemically localized in the electrolyteelectrode interfaces when these chemically modified MWNTs were used as electrodes for lithium batteries. The electrochemi* Author to whom correspondence should be addressed. Phone: +86551-3607780. E-mail:
[email protected]. † Department of Polymer Science and Engineering, University of Science and Technology of China. ‡ State Key Laboratory of Polymer Physics and Chemistry. § Department of Materials Science and Engineering, University of Science and Technology of China.
cal properties of these polymer-functionalized MWNTs in terms of the charge-discharge performances of electrochemical cells using polymer-functionalized MWNTs as electrode materials were investigated. Little work has been reported on chemically functionalized CNTs used as electrode materials for lithium batteries, though chemical functionalization of carbon nanotubes, especially binding polymers to the carbon nanotubes, is a very attractive area. Different linear polymers8-10 and some highly branched polymers were grafted to the CNTs.11-13 One author of this work has paid much attention to the synthesis and characterization of hyperbranched polymers, which are a class of highly branched polymers with relatively open structures and numbers of end groups.14 Functionalization by hyperbranched polymers results in a layered, treelike polymer film that is thickly packed.15 Moreover, these hyperbranched films contain a fairly open structure, allowing the metal ions to permeate the film more easily than in conventional polymer films. The hyperbranched polymer used in this work was commercially available as Boltorn H20, whose structures,16,17 surface properties,18 phase behavior,19 and rheological properties have been investigated.20 2. Experimental Section 2.1. Materials. MWNTs were provided by Sun Nanotech with a purity greater than 95%. The received MWNTs have a diameter of 30-60 nm and a length of 50-60 µm. The MWNTs were synthesized by chemical vapor deposition (CVD) using dimethylbenzene as the carbon source and ferrocene as the catalyst precursor materials. The reaction was conducted at 700800 °C. Monomethyl poly(ethylene glycol) with a molecular weight of 350 (PEG350) and poly(ethylene glycol) with a
10.1021/jp0600155 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/09/2006
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SCHEME 1
molecular weight of 2000 (PEG2000) were purchased from Fluka. The aliphatic hyperbranched polyester based on 2,2-bis(methylol)propionic acid (Boltorn H20) was provided by Perstorp AB in Sweden, with a molecular weight of 2100 and theoretically 16 primary hydroxyl groups. Thionyl chloride was purchased from Sinopharm Group Chemical Reagent Co. Ltd. 2.2. Characterization. Fourier transform infrared spectra (FTIR) were recorded on an Eouinox55 model Fourier transform infrared spectrometer. 1H NMR was performed on an AVANCE 300 spectrometer. Tetramethylsilane (TMS) was used as a reference. Thermogravimetric analysis (TGA) was performed with a Diamond thermogravimetric analyzer (Perkin-Elmer) in an N2 atmosphere at a heating rate of 10 °C/min. 2.3. Synthesis. A typical procedure (Scheme 1) for fabricating polymer-functionalized MWNTs is described as follows. 2.3.1. Preparation of MWNTs with Carboxyl Groups from MWNTs. MWNTs with carboxy groups (MWNT-COOHs) were prepared by oxidization of MWNTs in a mixture of concentrated sulfuric acid and nitric acid under sonication, which has been used widely elsewhere.11,12,21 In a typical experiment, 1.5 g of crude MWNTs was added to the mixture of 15 mL of 60% HNO3 and 45 mL of 98% H2SO4. The mixture was treated an with ultrasonic bath (400 kHz) for 3 h, then was refluxed for 24 h. After that, the mixture was diluted with distilled water, then was vacuum-filtered through a 0.45 µm Millipore polyvinylidene fluoride membrane, and was repeatedly washed with distilled water. The product was dried under vacuum for 24 h at 100 °C. 2.3.2. Preparation of MWNTs with Carbonyl Chloride Groups from MWNT-COOHs. Approximately 0.500 g of MWNTCOOHs was suspended in 15 mL of SOCl2. This suspension was stirred at 65 °C for 24 h. The solid was separated by filtration and washed with anhydrous toluene to yield the MWNTs with carbonyl chloride groups (MWNT-COCls). 2.3.3. Synthesis of MWNT-PEG350 and MWNT-PEG2000 from MWNT-COCl. Approximately 1 g of MWNT-COCls was added to a solution of carefully dried PEG350 (8 mL) in anhydrous toluene (20 mL). The mixture was stirred at 80 °C for 24 h, followed by repeatedly washing with hot toluene and then tetrahydrofuran (THF). The solid was dried under vacuum for 24 h at 100 °C. MWNT-PEG2000s were synthesized by reaction of MWNT-COCls and PEG2000 according to the same procedure. 2.3.4. Synthesis of MWNT-H20 from MWNT-COCl. A solution of hyperbranched polymer Boltorn H20 (3 g) in hot anhydrous THF (100 mL) was added to 0.0996 g of MWNTCOCls. The mixture was stirred at 50 °C for 24 h under nitrogen protection. The reaction mixture was repeatedly washed with amounts of hot THF and was Millipore-filtered rapidly to obtain MWNT-H20s. The dark solid was dried under vacuum for 24 h at 80 °C (Chart 1). 2.3.5. Preparation of the Electrochemical Analysis Cells. The MWNT powders (90 wt %) were mixed with 10 wt % poly(tetrafluoroethylene) (PVDF) binder in N-methyl 2 pyrrolidinone (NMP) to obtain slurries. Slurry was cast onto a copper foil to form an electrode laminate that was vacuum-dried at 120 °C. CR2032 coin-cells with the configuration of Li/electrolyte/ MWNTs were assembled in an argon-filled glovebox (MBraun
CHART 1: MWNT Functionalized with Hyperbranched Aliphatic Polyester Based on 2,2-Bis(methylol)propionic Acid
SCHEME 2
TABLE 1: Average Discharging (T1) and Charging (T2) Time in the Cycling Galvanostatic Charge-Discharge Analysis material
weight (mg)
average T1 (h)
average T2 (h)
MWNT-PEG350 MWNT-PEG2000 MWNT-H20
6.30 7.29 5.22
4.2 3.3 4.8
4.3 3.4 4.9
Lab Master130). The mass of MWNTs in each cell was about 6 mg. Electrolytes consisted of 1 M LiPF6 dissolved in a 50/50 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). These cells were cycled on a multichannel battery cycler (Neware BTS-610) at 25 °C at a current density of 0.2 mA/ cm2 in the voltage range between 0 and 3 V. The programmed charge-discharge cycle is illustrated in Scheme 2. The discharge (T1) and charge time (T2) depend on the specific capacity and weight of the electrode materials. Average T1 and T2 values for the different electrode materials used in this work are listed in Table 1. 3. Results and Discussion 3.1. Characterization of Polymer-Functionalized Multiwalled Carbon Nanotubes. The formation of MWNT-COOHs can be confirmed by the appearance of the CdO vibration at around 1720 cm-1 (Figure 1c).22 Stretching vibrations of O-H (3400 cm-1), C-H (2870, 2920 cm-1), C-O (1000-1200 cm-1), and CdO (1722 cm-1) can be found in MWNT-H20 (Figure 1a). In MWNT-PEG2000, the stretching vibrations of O-H (3470 cm-1), C-H (2870, 2920 cm-1), and C-O (1080 cm-1) can be assigned (Figure 1b). The NMR spectra of MWNT-PEG350 and MWNT-H20 were illustrated in Figure 2. The resonance signals of -CH2- at 3.78
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Figure 1. FTIR spectra of (a) MWNT-H20, (b) MWNT-PEG2000, and (c) MWNT-COOH.
Figure 3. High-resolution TEM images of MWNT-H20. The scale bar represents 110 nm in part A and 6 nm in part B.
Figure 2. 1H NMR at 300 MHz of MWNT-PEG350 (top) and MWNTH20 in CDCl3 (bottom).
ppm and -CH3 at 3.50 ppm for MWNT-PEG350 were assigned. Because of the complicated structures, Boltorn H20 usually showed a complicated proton NMR spectrum, so did MWNTH20. In the 1H NMR spectra of H20 in THF solution, the protons of the methyl group in the terminal, linear, and dendritic repeat units resonate at 1.02, 1.07, and 1.17 ppm, respectively.16 However, the three signals for the methyl groups in MWNTH20 in CDCl3 were found at 0.92, 1.28, and 1.60 ppm, respectively. The signals for the protons of the hydroxyl groups of H20 in CDCl3 appeared at about 1.5 ppm. The methylene groups attached to esters and ethers resonated between 2 and 4.0 ppm.17 Shown in Figure 3 are transmission electrom microscopy (TEM) images of a MWNT-H20 on a holey-carbon-coated copper grid at a higher resolution. Polymer layers covering carbon nanotubes can be well distinguished in the pictures. The quantity of the polymers attached to the surface of the MWNTs could be determined from thermogravimetric analysis (TGA). According to TGA results (Figure 4), the attached quantity increased with the molecular mass of the polymers. The average grafting degree calculated from Figure 4 is 0.19 ( 0.04 mmol/g. Boltorn-H20 formed a thick film on the surface of MWNTs. Due to the coverage of H20, the characteristic G and D bands in the Raman spectrum of MWNT-H20 became weak, which was also observed in the literature (Figure 5).12 After the H20 film had been removed by thermal decomposition,
Figure 4. TGA curves of (a) MWNT, (b) MWNT-COOH, (c) MWNTPEG350, (d) MWNT-PEG2000, and (e) MWNT-H20.
the TGA residue of MWNT-H20 showed again the typical G and D bands of MWNTs, indicating the main structure of the MWNTs was not changed by chemical modification and thermal decomposition during the TGA. 3.2. Galvanostatic Charge-Discharge Analysis. The electrochemical intercalation/deintercalation of lithium was investigated by galvanostatic charge-discharge experiments performed with button cells with reference electrodes of lithium metal foil, working electrodes of MWNTs bound with 10% PVDF, and electrolytes of 1 M LiPF6 (Chart 2). During discharge, Lithium ions spontaneously shuttle from the negative insertion electrode (lithium metal foil) into the electrolyte and from the electrolyte into the positive insertion electrode
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Figure 5. Raman spectra of MWNT, MWNT-H20, and TGA residue of MWNT-H20.
CHART 2: Schematic Illustration of the Electrochemical Cell
Figure 6. Specific capacity for Li insertion in MWNT-PEG2000, MWNT-PEG350, MWNT-COOH, MWNT, and MWNT- H20.
TABLE 2: Li Insertion (Cins) and Deinsertion (Cde) Capacities (with units of (mA h)/g) of the MWNTs 1 cycle electrodes MWNT MWNT-PEG350 MWNT-PEG2000 MWNT-COOH MWNT-H20
5 cycles
90 cycles
Cins Cde Cde/Cins Cins Cde Cde/Cins Cins Cde Cde/Cins 219 794 588 310 401
55 374 106 123 155
0.25 0.47 0.18 0.40 0.39
216 327 112 352 307
195 302 100 342 306
0.90 0.92 0.88 0.97 1.0
250 132 163 251 293
244 131 159 254 290
0.98 0.99 0.97 1.0 1.0
(MWNTs). Electrons are injected into the positive and taken from the negative electrode when Li ions reach the MWNT electrodes, and vice versa during charge. The electrochemical cycle investigation of capacities of intercalation/deintercalation can reveal the reversible insertion properties. According to the literature, graphite and carbonaceous materials as anodes in lithium batteries had a theoretical energy capacity of 372 (mA h)/g, partially limited by the thermodynamical equilibrium saturation composition of LiC6.23 The electrochemical intercalation of Li in CNTs was strongly dependent on the synthesis conditions and microstructures of CNTs.24 Table 2 listed the capacities at the different chargedischarge cycles for the functionalized MWNTs. The MWNTs used in this work possessed an insertion capacity (discharging) of 219 (mA h)/g at the first charge-discharge cycle, corresponding to a saturation composition of Li0.6C6. MWNT-COOH showed a significant improvement (42%) over the original MWNT at the first cycle. An assumption has been made that more structural defects were obtained in MWNT-COOH as electrochemical active areas for lithium insertion.25 PEG can serve as the electrolyte solvent in lithium cells. At the first cycle, MWNT-PEG350 showed a saturation composition of Li2.1C6 when discharging and LiC6 when charging. However, longer polymer chains hence stronger molecular interaction resulted in more condensed coverage over MWNTs, which could hinder the movement of Li+. MWNT-PEG2000 showed a saturation composition of Li1.6C6 and a very low deinsertion/insertion efficiency at the first cycle. MWNT-H20 showed an initial insertion capacity of 410 (mA h)/g (Li1.1C6), and the deinsertion/insertion efficiency was among the highest
in Table 2, which was perhaps because of many channels accessible for Li+ inside hyperbranched films. Figure 6 illustrates the insertion capacity versus cycle number for the different MWNTs. As illustrated in Figure 6, MWNTH20 showed a very stable capacity of an average of 290 (mA h)/g (Li0.8C6), which was bout 30% larger than that of pure MWNTs and about 17% larger than that of MWNT-COOHs. Both MWNT-PEG350 and MWNT-PEG2000 showed poor cyclability. MWNT-PEG350 showed capacity fading at a fade rate of about 2-3 (mA h)/g per cycle. MWNT-PEG350 and MWNT-PEG2000 had the same charge-discharge behavior after about 70 cycles, at which same natured films might be formed over the electrode surface. MWNT-COOH possessed large initial irreversible capacity however showed nearly the same behavior as the original MWNTs after about 80 cycles. Polymer functionalization might affect the formation and conformation of the solid electrolyte interface (SEI).8 Functionalization by hyperbranched polymers resulted in a stable monomolecular layer with open structures, which accounted for the good reversible capacity and excellent cyclability. 4. Conclusions In conclusion, as a working electrode against lithium metal, the hyperbranched polymer-H20-modified MWNT electrode material showed a good reversible capacity and excellent capacity retention. The stable and relatively open hyperbranched layer over MWNTs was attributed to the improvement of capacity and cycling stability of MWNT-H20. High initial capacity but large capacity fade rate was found in MWNT-PEGs, which indicated that attention should be paid to the MWNTPEG interface. The observation is interesting because PEG (or PEO)-based polymer electrolytes are currently under considerable consideration. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Project No. 50573072).
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