May 18, 2012
Graphite foam makes good battery electrode
Researchers in the US have shown that ultrathin graphite foam can be used as a new type of working cathode in lithium-ion batteries. The lightweight and highly conducting foam appears to be electrochemically stable, is easy to prepare and cheap, and might even be up to rivalling conventional cathode materials, such as aluminium and nickel foils.
“The ultrathin graphite (UGF) is extremely stable in electrolytes such as LiPF6-organic carbonate at potentials of up to 5 V,” team member Hengxing Ji told nanotechweb.org. “The ultralight foam also has higher power and energy densities than many conventional electrode materials, such as aluminium and nickel foils.”
The researchers, led by Rodney Ruoff of the University of Texas at Austin, loaded the UGF with lithium iron phosphate (LFP) by drop casting a slurry of LFP, carbon black and polyvinylidene fluoride dispersed in a solvent. The mix was then dried and the cathode assembled with a separator, anode, electrolyte and cell shell to make a fully working battery. This process is very much like the ones used to make lithium-ion batteries.
However, the similarity ends there. The cathode of a state-of-the-art lithium-ion battery single cell is usually made of a material such as LFP casted on a current collector (typically aluminium foil – around 20–30µm thick) for conducting electrons placed between the cathode and the outside circuit. Aluminium foil, despite its high electrical conductivity, cannot efficiently collect electrons because of its planar structure, which ultimately limits the power density of batteries that employ this material. The problem is further exacerbated for thicker cathodes, often used in an attempt to increase energy density.
What is more, aluminium corrodes in many electrolyte solutions, something that leads to slow self-discharge of the cathode and overall degradation of the battery.
The UGF could provide an alternative to aluminium foils because it does not suffer from any of these drawbacks, says Ji. The graphitic material provides an interconnected network of highly conducting struts (around 1.3 × 105 Sm–1 at room temperature) that greatly promote electron conduction inside the cathode, which improves power density. And, because the surface-to-volume ratio of UGF is high compared with its mass (it has a density of just 9.5 mg cm–3), much less material is required compared with the equivalent weight needed for a cathode made from aluminium foil. The energy density is thus also improved.
“Accounting for the total mass of the electrode, we found that the maximum specific capacity of the UGF/LFP cathode was 23% higher than that of Al/LFP cathodes and 170% higher than that of Ni-foam/LFP cathode,” said Ji.
And that is not all: the fact that graphite is extremely stable in a variety of electrolytes means that batteries employing this material do not corrode and thus do not self-discharge. This is particularly good news for devices operating at high voltages.
“Electric vehicles and electric hybrid vehicles demand batteries with both high power and energy densities and our UGF might be employed in such applications,” added Ji.
The team now plans to further optimize the pore size and wall thickness in UGF to increase power and energy densities even more. “Such improvements mean that the foam might also be used to make other electrochemical energy storage devices, such as fuel cells and supercapacitors.”
The current results were published in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org