|Comparison of capacity retention and Coulombic efficiency of GNRs and the GNRs/SnO2 composite at a rate of 100 mA/g. Credit: ACS, Lin et al. Click to enlarge.|
Researchers in the James M Tour Group at Rice University have synthesized a composite made from graphene nanoribbons (GNRs) and tin oxide (SnO2) nanoparticles (NPs) for use as improved anode materials for lithium-ion batteries (LIBs). A paper on their work is being published in the Journal of the American Chemical Society.
The conductive GNRs boost the lithium storage performance of SnO2 NPs. The composite exhibits reversible capacities of more than 1,520 mAh/g and 1,130 mAh/g for the first discharge and charge, respectively—more than the theoretical capacity of SnO2. The reversible capacity retains ~ 825 mAh/g at a current density of 100 mA/g with a Coulombic efficiency of 98% after 50 cycles. Further, the composite shows good power performance with a reversible capacity of ~580 mAh/g at the current density of 2 A/g— much higher than the theoretic capacity of graphite (372 mAh/g).
The researchers attributed the high capacity, good power performance and retention to the uniformly distributed SnO2 NPs along the high-aspect-ratio GNRs. The GNRs act as conductive additives that buffer the volume changes of SnO2 during cycling.
Graphite, the standard commercialized anode material for LIBs, has a theoretical specific capacity of only 372 mAh/g, which limits its applications in LIBs. Therefore, new anode materials with high specific capacity such as Si (4200 mAh/g), Sn (994 mAh/g) and SnO2 (782 mAh/g) have been intensively investigated. However, the enormous volume expansion and structural changes during repeated alloying/dealloying causes significant capacity fading during cycling. To address this problem, researchers have employed several approaches.
One tactic is the development of electrode materials based upon nanostructures that minimize the strain during volume expansion. Another approach is to integrate the electrode material with a carbonaceous matrix such as amorphous carbon, mesoporous carbon, graphene, or carbon nanotubes (CNTs).
Graphene nanoribbons (GNRs), a quasi-one-dimensional form of graphene, exhibit tunable electrical properties through dimension confinement as well as edge morphology or functionalization.18 Interestingly, GNRs have been theoretically and experimentally shown to enhance lithium storage through edge effects. Moreover, GNRs, having large aspect ratios and high surface area, might provide an excellent conductive matrix with good mechanical flexibility for the metal oxide to accommodate the volume changes during change/discharge cycles. Here, we used our recently developed conductive GNRs to prepare anode composite materials with SnO2 nanoparticles for LIBs.—Lin et al.
|Scheme for the synthesis of the GNRs/SnO2 composite. Credit: ACS, Lin et al. Click to enlarge.|
Rice professor James Tour and his colleagues developed a method for unzipping nanotubes into GNRs, revealed in a 2009 cover story in the journal Nature. Since then, the researchers have figured out how to make graphene nanoribbons in bulk and are moving toward commercial applications—including for use in battery electrode materials.
In the new experiments, the Rice lab mixed graphene nanoribbons and tin oxide particles about 10 nanometers wide in a slurry with a cellulose gum binder and a bit of water, spread it on a current collector and encased it in a button-style battery.
Graphene nanoribbons make a terrific framework that keeps the tin oxide nanoparticles dispersed and keeps them from fragmenting during cycling. Since the tin oxide particles are only a few nanometers in size and permitted to remain that way by being dispersed on GNR surfaces, the volume changes in the nanoparticles are not dramatic. GNRs also provide a lightweight, conductive framework, with their high aspect ratios and extreme thinness.—Prof. Tour
The researchers said the work is a starting point for exploring composites made from GNRs and other metal oxides—such as MnO2 and Fe2O3—for lithium storage applications. The lab plans to build batteries with other metallic nanoparticles to test their cycling and storage capacities.
Boeing, the Air Force Office of Scientific Research, Sandia National Laboratory and the Office of Naval Research supported the research.
Jian Lin, Zhiwei Peng, Changsheng Xiang, Gedeng Ruan, Zheng Yan, Douglas Natelson, and James M. Tour (2013) Graphene Nanoribbon and Nanostructured SnO2 Composite Anodes for Lithium Ion Batteries. ACS Nano doi: 10.1021/nn4016899