|Gravimetric Ragone plot comparing energy and power characteristics of CNF electrodes based on the pristine and discharged electrode weight with that of LiCoO2. Source: Mitchell et al. Click to enlarge.|
A team at MIT, led by Carl V. Thompson and Yang Shao-Horn, has synthesized carbon nanofiber (CNF), binder-free electrodes for lithium-air batteries that yield high gravimetric energies up to ~2500 W h kgdischarged-1 at powers up to ~100 W kgdischarged-1—among the highest values reported for Li–O2 batteries to date (including carbon-only and catalyst containing electrodes).
This translates to an energy enhancement ~4 times greater than the state-of-the-art lithium intercalation compounds such as LiCoO2 (~600 W h kgelectrode-1, the researchers said. They report on their study in a paper published in the RSC journal Energy & Environmental Science.
The carbon nanofiber electrodes are substantially more porous than other carbon electrodes, and can therefore more efficiently store the solid oxidized lithium (Li2O2) that fills the pores as the battery discharges.
In addition, the nanofiber structure allowed for the clear visualization of the morphological evolution of Li2O2 particles as a function of rate and depth-of-discharge and also of the removal of Li2O2 particles during charging.
The visualization of Li2O2 morphologies upon discharge and disappearance upon charge represents a critical step toward understanding key processes that limit the rate capability and low round-trip efficiencies of Li–O2 batteries, which are not currently understood within the field.—Mitchell et al.
Li-air (or Li-O2) batteries are receiving a great deal of attention and funding as a high-density energy storage solution, especially for electric vehicle applications. Despite their promise, however, Li–O2 batteries face substantial practical challenges, the authors note, including a large voltage hysteresis (70% round trip efficiency); poor rate capability performance at high power (>1 kW kgelectrode-1; and poor cycle life (typically <100 cycles).
The work on the CNF electrode complements extensive recent research focused on the development of catalysts and tunable electrode morphologies to increase round-trip efficiency and discharge capacity respectively, demonstrating that the design of novel all-carbon electrode structures can be an equally promising route for improving the discharge performance of Li–O2 batteries.
In earlier lithium-air battery research that Shao-Horn and her students reported last year, they demonstrated that carbon particles could be used to make efficient electrodes for lithium-air batteries. In that work, the carbon structures were more complex but only had about 70% void space.
In the paper published last year, the team had estimated the kinds of improvement in gravimetric efficiency that might be achieved with lithium-air batteries; this new work realizes this gravimetric gain, Shao-Horn says. Further work is still needed to translate these basic laboratory advances into a practical commercial product, she cautions.
Ji-Guang Zhang, a laboratory fellow in battery technology at the Pacific Northwest National Laboratory, called the CNF work “original and high-quality work.” He added that this research “demonstrates a very unique approach to preparing high-capacity electrodes for lithium-air batteries.”
Robert R. Mitchell, Betar M. Gallant, Carl V. Thompson and Yang Shao-Horn (2011) All-carbon-nanofiber electrodes for high-energy rechargeable Li–O2 batteries. Energy Environ. Sci., Advance Article doi: 10.1039/C1EE01496J