Letter pubs.acs.org/NanoLett
Polymer−Graphene Nanocomposites as Ultrafast-Charge and -Discharge Cathodes for Rechargeable Lithium Batteries Zhiping Song,†,⊥ Terrence Xu,† Mikhail L. Gordin,† Ying-Bing Jiang,‡ In-Tae Bae,§ Qiangfeng Xiao,∥ Hui Zhan,⊥ Jun Liu,¶ and Donghai Wang*,† †
Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Center for Micro-Engineered Materials, University of New Mexico, Albuquerque, New Mexico 87131, United States § Small Scale Systems Integration and Packaging Center, State University of New York at Binghamton, Binghamton, New York 13902, United States ∥ General Motor Technical Center, Warren, Michigan 48092, United States ⊥ Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China ¶ Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: Electroactive polymers are a new generation of “green” cathode materials for rechargeable lithium batteries. We have developed nanocomposites combining graphene with two promising polymer cathode materials, poly(anthraquinonyl sulfide) and polyimide, to improve their high-rate performance. The polymer−graphene nanocomposites were synthesized through a simple in situ polymerization in the presence of graphene sheets. The highly dispersed graphene sheets in the nanocomposite drastically enhanced the electronic conductivity and allowed the electrochemical activity of the polymer cathode to be efficiently utilized. This allows for ultrafast charging and discharging; the composite can deliver more than 100 mAh/g within just a few seconds.
KEYWORDS: Lithium battery, cathode, polymer, graphene, nanocomposite also been emphasized recently as a new generation of “green” lithium battery electrodes due to their sustainability and environmental benignancy. The electrochemical redox mechanism of an organic cathode is based on the reversible redox reaction of the organic functional group, such as quinone,4,8,9,11 anhydride,5,7,10 and nitroxide radical,12,13 accompanied by the association and disassociation of Li+ ions or electrolyte anions. Scheme 1 shows the electrochemical redox reactions of Li ions with poly(anthraquinonyl sulfide) (PAQS)9 and polyimide (PI)10 based on the quinone and anhydride functional groups. A polymer with a stable skeleton and highly electroactive functional group can potentially be a high-power cathode candidate because its redox reaction intrinsically has faster kinetics than inorganic intercalation cathodes. For example, anthraquinone can show electrochemical redox activity at a fast scan rate of 1000 mV/s in cyclic voltammetry tests in acetonitrile,14 and nitroxide radical polymer can still retain 97% of its theoretical capacity even at a charge rate of 1200 C
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echargeable lithium batteries have been identified as one of the most promising energy storage techniques, especially as power sources for emerging applications such as plug-in hybrid and electrical vehicles. Besides high energy density, high power density is another indispensable requirement in these applications. In conventional Li-ion batteries, cathode materials are typically lithium transition-metal oxides or phosphates (e.g., LiCoO2 or LiFePO4) that can reversibly de/reintercalate Li+ ions.1 Although many inorganic intercalation cathode materials have been demonstrated to show high capacity retention at moderate C-rates such as 1 or 5 C, they generally fail under more extreme charge−discharge conditions comparable to those of supercapacitors, for example, releasing all their energy within several seconds. This can be ascribed to the relatively slow lithium ion diffusion kinetics in the bulk particles of inorganic materials. For these materials, the most efficient way to improve the ultrafast-charge and -discharge performance is synthesizing nanosized-particle cathodes to shorten the Li-ion transport pathway,2,3 which may cause other problems such as low crystallinity and complicated synthesis. Besides the conventional inorganic materials, organic cathode materials, including small molecules4−7 and polymers,8−13 have © 2012 American Chemical Society
Received: November 10, 2011 Revised: March 12, 2012 Published: March 26, 2012 2205
dx.doi.org/10.1021/nl2039666 | Nano Lett. 2012, 12, 2205−2211
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Letter
that graphene-based composites can greatly improve the specific capacity, cycling stability, and rate capability of several metal oxide anodes such as TiO2,18 SnO2,19,20 Co3O4,21,22 and Mn3O4,23 and of LiFePO424 cathodes. For polymer cathode materials, there are many reports on polymer−CNT composites synthesized by in situ polymerization.25−29 Researchers have also made many attempts to create polyaniline−graphene composites as high-performance electrodes for supercapacitors.30−33 Recently, Guo et al.34 attempted to use graphene to enhance the electrochemical performance of a nitroxide radical polymer cathode through a dispersion− deposition process. However, the active material loading in the electrode composite is only 10%, while the graphene loading is as high as 60%. It is still necessary to develop polymer−graphene nanocomposite cathodes that can fully utilize graphene and obtain significantly enhanced electrochemical performance with low graphene loading. In this paper, we use PAQS and PI (Scheme 1) as examples to demonstrate a general one-pot synthesis of polymer−graphene nanocomposites with highly dispersed graphene, which show excellent ultrafast-charge and -discharge performance as high-power cathodes for rechargeable lithium batteries. We also illustrate the effect of graphene content on the electronic conductivity, surface area, and electrochemical performance of the nanocomposites. Typical approaches to synthesize polymer−graphene composites are solvent blending, melt blending, and in situ polymerization.16 Solvent blending is mostly suitable for soluble polymers and melt blending always leads to poor dispersion of graphene in the composite. In order to develop polymer−graphene nanocomposites with well-dispersed graphene to fully utilize its high surface area and high electronic conductivity, in situ polymerization was adopted, as illustrated in Scheme 2. Functionalized graphene sheets (FGSs) prepared by thermal expansion of graphite oxide35−37 were used in this study because they show higher electronic conductivity than chemically reduced graphene oxide. A mixture of single-layer and multilayer graphene sheets was used without purification in synthesis of the nanocomposites, and the existence of multilayer graphene sheets does not affect the knowledge obtained from this study. The polar aprotic solvent 1-methyl-2pyrrolidinone (NMP) was chosen as the solvent because it is not only one of the best solvents for dispersing pristine graphene,38,39 but also a good solvent in the synthesis of PAQS and PI.9,10 This eliminated the need to add surfactants or
Scheme 1. Electrochemical Redox Reactions of Li Ions with PAQS and PI Based on Quinone and Anhydride Functional Groups, Respectively
and discharge rate of 60 C in aqueous electrolyte.13 Moreover, the robust backbone of the polymer can prevent the unwanted dissolution in nonaqueous electrolyte that is always suffered by small molecules, and thereby achieve good cycling stablility.9 In our previous study, some polymers have already shown high capacity, high Coulombic efficiency, and good cycling stability,9,10 but the intrinsic electronic insulation and low mass density of the polymers resulted in unsatisfactory highrate performance and necessitated addition of large amounts of conductive carbon (e.g., the mass ratio of PAQS/acetylene black is 1:1 in ref 9) during electrode fabrication for high utilization of the active materials. Conductive coating (e.g., carbon, metal oxides, and conductive polymers) and incorporating conductive additive to form composites have been demonstrated as efficient approaches to improving the electron transport in electrodes, and thus improving the high-rate performance. So far, graphene and graphene sheets, one or a few layers of atomically thick, two-dimensional sheets composed of sp2 carbon atoms arranged in a honeycomb structure,15 have been identified as excellent conductive additives in nanocomposites due to their extraordinary electronic conductivity.16 Other advantages of graphene and graphene sheets in formation of nanocomposites include its high surface area (theoretical value of 2630 m2/g)17 for improved interfacial contact and potential of low manufacturing costs compared to mesoporous carbon and carbon nanotubes (CNTs). Many successful examples show
Scheme 2. In Situ Polymerization Process of PAQS-FGS or PI-FGS Nanocomposite
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dx.doi.org/10.1021/nl2039666 | Nano Lett. 2012, 12, 2205−2211
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polymer matrix, with no obvious aggregation even at the high FGS loading of PAQS-FGS-b. Figure 1d is a high-magnification TEM image of PAQS-FGS-b showing a graphene sheet coated with a polymer layer. Elemental mapping using electron energy loss spectroscopy (EELS) (Figure 1e,f) was used to establish the distribution of polymer on the graphene surface by detecting carbon and sulfur signals from PAQS. The rather uniform distribution of sulfur along with carbon over the whole area of PAQS-FGS-b confirms the existence and homogeneous coating of PAQS on the FGS surface. TEM investigation also showed the polymer coating of PI samples to be uniform, and the morphology differences are mainly from the polymer itself. The flowerlike particles (Figure 1g) with particle size of 0.5−1 μm are observed in pure PI samples. For the PI-FGS-b nanocomposite, both SEM and TEM images (Figure 1h,i) consistently show that FGSs are homogeneously coated with flowerlike PI polymer particles (∼100 nm in size) smaller than those found in pure PI. The uniform polymer coating on the FGS surface and the corresponding excellent dispersion of graphene in the nanocomposite can mainly be attributed to the NMP solvent, which facilitates both graphene dispersion and the polymerization process, and to the noncovalent π−π interaction between the graphene surface and the backbones of PAQS and PI owing to the conjugated aromatic rings.40 The electrochemical performance of the polymer−graphene nanocomposites was characterized by galvanostatic charge− discharge tests of CR2016-type coin cells (see Supporting Information). The cathodes contained 60 wt % active material (polymer or polymer−graphene composite), 30 wt % conductive carbon, and 10 wt % polytetrafluoroethylene (PTFE) binder. PAQS and its nanocomposites were tested sequentially at rates of 0.1 C, 0.5 C, 2 C, 10 C, and 0.1 C, charging and discharging between 1.5 and 3.5 V (vs Li+/Li) for 20 cycles at each rate. As shown in Figure 2a, the discharge capacity of PAQS, after the initial several activation cycles, stabilizes at 177 mAh/g and a utilization ratio of 79% can be achieved relative to the theoretical capacity of 225 mAh/g. The utilization ratio is defined as a ratio of discharge capacity contributed by polymer at 0.1 C based on polymer mass relative to the theoretical capacity of the polymer (see Table 1). In contrast, the reversible capacity of the PAQS-FGS-a and PAQS-FGS-b nanocomposites is 187 and 165 mAh/g at 0.1 C based on the whole mass of the composite, respectively. Under the same test conditions, FGS itself gives a specific discharge capacity of only 32 mAh/g at 0.1 C between 1.5 and 3.5 V (Figure S4 in the Supporting Information), which is consistent with previous reports.18,41 Thus the discharge capacity contributed by the graphene sheets is only 2 and 8 mAh/g for PAQS-FGS-a (6 wt % FGS) and PAQS-FGS-b (26 wt % FGS), respectively. Deducting this FGS capacity from the total, the addition of graphene can be seen to significantly improve the utilization ratio of PAQS to 88 and 95% for PAQS-FGS-a and PAQS-FGS-b, respectively (Table 1). Moreover, it is found that the rate performance becomes much better as the FGS content increases. In particular, PAQS-FGS-b discharged at 10 C still retains 90% of its discharge capacity at 0.1 C. Since this performance exceeded our expectations, we further tested these samples at more extreme current rates of up to 200 C. In Figure 2b we compare the discharge specific capacity versus C-rate of PAQS and all its nanocomposites. Comparing PAQS-FGS-a and PAQS-CNT-c, we observe that the nanocomposite containing graphene shows better performance than that containing a similar amount of carbon nanotubes, which is
perform graphene surface modification to improve its dispersion, which is a big impediment for many graphenebased nanocomposites synthesized in aqueous solution.18,20,22,23 In our specific synthetic process (Scheme 2), FGSs were first dispersed well in NMP by sonication. 1,5-Dichloroanthraquinone (DCAQ) or 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) was then added as precursor of PAQS or PI, respectively, and completely dissolved in NMP. After the addition of a condensing agent (anhydrous sodium sulfide for PAQS or ethanediamine for PI), the polymer chains formed through a polycondensation reaction at high temperature. In the final product, FGSs are coated with the in situ formed polymer and homogeneously embedded in the polymer matrix. In the obtained polymer−graphene nanocomposite, the welldispersed graphene functions as an electron transport pathway and provides intimate contact with the polymer to enable fast charge transfer and thus ultrafast charging and discharging. This one-pot synthesis of polymer−graphene nanocomposite is similar to that of bare polymer except for the presence of the graphene and an extra sonication step, so it can easily be extended to other polymers synthesized in polar aprotic solvent. In order to study the effect of FGS content on the electrochemical performance of the nanocomposites, we have synthesized two composite samples for each polymer with different FGS weight percentage, named PAQS-FGS-a (6 wt % FGS), PAQS-FGS-b (26 wt % FGS), PI-FGS-a (6 wt % FGS), and PI-FGS-b (11 wt % FGS). PAQS-CNT-c (5 wt % CNT) was also synthesized through the same approach for comparison between graphene and carbon nanotubes (Table 1). Table 1. Physical Properties of Polymers and Polymer− Graphene Nanocomposites sample PAQS PAQSFGS-a PAQSFGS-b PAQSCNT-c PI PI-FGS-a PI-FGS-b
additive content (wt %)
electronic conductivity (S/cm)
surface area (cm2/g)
utilization ratioa (%)
0 6
