The Thorny Problem of Lithium Batteries
Rechargeable batteries utilizing lithium metal foil could mean higher-mileage electric vehicles, yet this requires curbing lithium dendrite formation. These dendrites grow like thorns on the lithium metal, diminishing battery performance. Scientists at the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center, have employed a novel approach using tiny porous carbon tubes coated with a protective aluminum oxide (Al2O3) layer as a stable lithium metal host to effectively inhibit dendritic growth. This is a nanostructured solution as these carbon tubes are less than 100 nanometers in diameter.
In a reversible lithium battery, lithium ions are repeatedly transported between two electrodes, known as the anode and cathode. Lithium ions stored in the anode have a higher chemical potential energy than the cathode. When lithium ions are transported to the anode during the charging process, this is similar to storing energy in a compressed spring. This stored energy is released when lithium ions are transported back to the cathode during the discharge process. The separator and current collectors ensure that electrical current flows through an external circuit between the electrodes to power a device.
Stability of the electrodes and reversibility of lithium ion transport are the main drivers for battery performance. Lithium metal is a desirable anode material for high energy density applications such as electric vehicles. When paired with oxide cathodes, lithium metal offers high voltages and high capacities for powering devices. Unfortunately, battery failure and safety concerns caused by lithium dendritic growth have limited the feasibility of using lithium metal foil.
For the common planar copper anode current collector, non-uniform lithium deposition forms thorn-like metal extending outwards from the surface. These dendrites expose more lithium to the liquid electrolyte, resulting in unwanted side reactions and irreversible capacity loss. They can even puncture the separator and extend to the cathode. Dendrites forming an inner bridge between the two electrodes permit electrons to flow freely. When taken altogether, these problems result in unsafe battery operation, making it unreliable for many applications.
Using carbon nanotubes-aluminum oxide core-shell structures as the current collector (see figure), the team demonstrated that lithium deposits more uniformly compared to the flat surface. No dendrite formation was observed even after many repeated cycles of plating and stripping lithium from the electrode. The high surface area carbon nanotubes form a 3D sponge that readily holds lithium in the porous structure — like a sponge holding water — and suppresses dendrite growth.
To improve performance even further, the scientists grew a conformal aluminum oxide layer, using a thin film deposition technique, as a protective outer shell. This outer shell shields lithium from harmful reactions with the liquid electrolyte. Coulombic efficiency, a measure of the reversibility of the lithium plating process, confirmed that this nanostructure solution improves performance for extending the battery’s lifetime.
These NEES scientists provide insight into how nanostructures can improve battery operation. “Compared with planar current collectors (such as copper foil), nanostructure materials like this carbon nanotube sponge achieve higher safety with inhibited lithium dendritic growth,” said Ying Zhang, the first author on the paper. While overcoming the safety hazards of lithium metal is a long process, this team’s work shows a clear path to improving the reliability and safety of lithium electrodes in batteries.
Zhang Y, B Liu, E Hitz, W Luo, Y Yao, Y Li, J Dai, C Chen, Y Wang, C Yang, H Li and H Liangbing. 2017. “A Carbon-Based 3D Current Collector with Surface Protection for Li Metal Anode.” Nano Research 10(4): 1356-1365. DOI: 10.1007/s12274-017-1461-2
This work was supported as part of the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Sciences.