A team of researchers from the US and China have developed novel polymer−graphene nanocomposites as high-rate cathode materials for rechargeable lithium batteries. Compared to the pure polymer, the polymer-graphene nanocomposites possess much higher active material utilization ratios and superior ultrafast-charge and -discharge ability; one of the materials maintained a discharge capacity of 100 mAh/g even at 100 C, when the whole discharge process takes only 16 seconds.
The team, including researchers from Penn State University, the University of New Mexico, SUNY Binghamton, General Motors Technical Center, Wuhan University (China), and Pacific Northwest National Laboratory, combined graphene with two promising polymer cathode materials, poly(anthraquinonyl sulfide) (PAQS) and polyimide (PI), to improve their high-rate performance. A paper on their work appears in the ACS journal Nano Letters.
Besides the conventional inorganic materials [for Li-ion battery cathodes], organic cathode materials, including small molecules and polymers, have 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, anhydride, and nitroxide radical, accompanied by the association and disassociation of Li+ ions or electrolyte anions.
...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, and nitroxide radical polymer can still retain 97% of its theoretical capacity even at a charge rate of 1200 C and discharge rate of 60 C in aqueous electrolyte. 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 stability.
—Song et al.
The researchers had earlier reported some polymers showing high capacity, high Coulombic efficiency, and good cycling stability, but the high-rate performance was unsatisfactory. While other work has shown the ability of graphene to function as a conductive additive, active material loading in the electrode composite was only 10%, while the graphene loading was as high as 60%.
In this new study, Song et al. demonstrated a general one-pot synthesis of polymer−graphene nanocomposites with highly dispersed graphene. They synthesized the polymer–graphene nanocomposites 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, allowing for ultrafast charging and discharging.
Their cathodes contained 60 wt % active material (polymer or polymer−graphene composite), 30 wt % conductive carbon, and 10 wt % polytetrafluoroethylene (PTFE) binder. They characterized their polymer−graphene nanocomposites by galvanostatic charge−discharge tests of CR2016-type coin cells.
They found that 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. One of the PAQS materials maintained a discharge capacity of 100 mAh/g at 100 C. The capacity of PI-graphene materials was 172 and 205 mAh/g, corresponding to a polymer utilization ratio of 49 and 62%, respectively.
On the basis of the above electrochemical performance, we conclude that formation of graphene-based nanocomposites can significantly improve not only the specific capacity, but also the rate capability of polymer cathode materials, especially at ultrahigh current rates.
...The outstanding high-rate performance is explained by the fast redox kinetics, the fast electron transfer, and the benefits to Li− ion conduction due to increased surface area in the polymer− graphene nanocomposites. Since the battery performance of these kinds of polymer−graphene nanocomposites is so attractive and the synthesis is very simple, it gives important insights into improving battery performance of other polymer cathodes.
—Song et al.
This work was primarily supported by Penn State startup fund and the US Department of Energy (DOE) under the Batteries for Advanced Transportation Technologies (BATT) Program.