Adding Graphene to Metal Oxides Significantly Improves Li-ion Electrode Specific Capacity at High Charge/Discharge Rates
|Specific capacities of two TiO2-FGS (functionalized graphene sheets) hybrids at different charge / discharge rates compared to control TiO2. Credit: ACS, Wang 2009. Click to enlarge.|
Researchers at the US Department of Energy’s (DOE) Pacific Northwest National Laboratory have found that adding graphene—sheets made up of single carbon atoms—to titanium dioxide (TiO2) results in lithium-ion electrode materials that significantly outperform standard titanium dioxide materials. They presented the results of their work on these hybrid TiO2-graphene systems earlier this week at the Micro Nano Breakthrough Conference in Portland, Oregon.
As attractive as Li-ion batteries are for application in electric vehicles and renewable energy applications, many potential electrode materials are limited by slow Li-ion diffusion, poor electron transport in electrodes, and increased resistance at the interface of electrode/electrolyte at high charge. One avenue researchers are exploring to improve that performance is to introduce hybrid nanostructured electrodes that interconnect nanostructured electrode materials with conductive additive material.
While the hybrids or nanocomposites offer significant advantages, some of the candidate materials to improve the conductivity, such as RuO2 and CNTs [carbon nanotubes], are inherently expensive. In addition, conventional carbon additives at high loading content (20 wt% or more) are still needed to ensure good electron transport in fabricated electrodes. To improve high-rate performance and reduce cost of the electrochemically active materials, it is important to identify high surface area, inexpensive, and highly conductive nanostructured materials that can be integrated with electrochemical active materials at nanoscale.—Wang et al.
TiO2 is an attractive electrode material. It is abundant, low cost, and environmentally benign. It is also structurally stable during the insertion and extraction of lithium ions, and is intrinsically safe by avoiding lithium electrochemical deposition. Graphene has excellent electronic conductivity and mechanical properties, and may be the ideal conductive additive for hybrid nanostructured electrodes, the researchers suggested.
We demonstrate the use of graphene as a conductive additive in self-assembled hybrid nanostructures to enhance high rate performance of electrochemical active materials. We choose metal oxide TiO2 as a model electrochemical active oxide material, but this method can be applied to other materials as well.—Wang et al.
The PNNL team developed a one-step synthesis approach for the self-assembly of the metal oxide-graphene hybrid nanostructures. They found that the hybrid materials showed enhanced Li-ion insertion/extraction kinetics, with specific capacity more than doubled at high charge rates, as compared with the pure TiO2 material. The high rate performance is important for applications where fast charge and discharge is needed, such as in load leveling utility applications.
The attributed the improved capacity at high charge-discharge rate to increased electrode conductivity in the presence of the percolated graphene network embedded into the metal oxide electrodes.
The high rate properties obtained from the graphene nanocomposite materials are comparable to some of the best results reported in the literature using RuO2 or carbon nanotubes with a higher carbon (Super P) additive content. The simple self-assembly approach, and the potential low manufacturing cost of graphene may provide a new pathway for large scale applications of novel hybrid nanocomposite materials for energy storage.
...we expect that the self-assembly approach discussed here can be applied to other metal oxide-graphene hybrid nanostructures to study synergetic properties and improve the performance of oxide electrodes in electrochemical energy storage and conversion.—Wang et al.
Donghai Wang, Daiwon Choi, Juan Li, Zhenguo Yang, Zimin Nie, Rong Kou, Dehong Hu, Chongmin Wang, Laxmikant V. Saraf, Jiguang Zhang, Ilhan A. Aksay and Jun Liu (2009) Self-Assembled TiO2–Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion. ACS Nano, 3 (4), pp 907–914 doi: 10.1021/nn900150y