The energy density of traditional lithium-ion batteries is approaching a saturation point that cannot meet the demands of the future—in electric vehicles, for example. Lithium metal batteries can provide double the energy per unit weight when compared to lithium-ion batteries. The biggest challenge hindering their use is the formation of lithium dendrites over the lithium metal anode. These dendrites often continue to grow until they pierce the separator membrane, causing the battery to short-circuit and ultimately destroying it.

Scientists at Friedrich Schiller University in Jena, together with colleagues from Boston University (BU) and Wayne State University (WSU), have now succeeded in preventing dendrite formation and thus at least doubling the lifetime of a lithium metal battery. The researchers report on their method in an open-access paper in the journal Advanced Energy Materials.

Here, the use of an ultrathin (≈1.2 nm) carbon nanomembrane (CNM) which contains sub-nanometer sized pores as an interlayer to regulate the mass transport of Li-ions is demonstrated. Symmetrical cell analysis reveals that the cell with CNM interlayer cycles over 2x longer than the control experiment without the formation of Li dendrites. Further investigation on the Li plating morphology on Cu foil reveals highly dense deposits of Li metal using a standard carbonate electrolyte. A smoothed-particle hydrodynamics simulation of the mass transport at the anode–electrolyte interface elucidates the effect of the CNM in promoting the formation of highly dense Li deposits and inhibiting the formation of dendrites. A lithium metal battery fabricated using the LiFePO₄ cathode exhibits a stable, flat voltage profile with low polarization for over 300 cycles indicating the effect of regulated mass transport.

—Rajendran et al.

During the charge transfer process, lithium ions move back and forth between the anode and the cathode. Whenever they pick up an electron, they deposit a lithium atom and these atoms accumulate on the anode. A crystalline surface is formed, which grows three-dimensionally where the atoms accumulate, creating the dendrites. The pores of the separator membrane influences the nucleation of dendrites. If ion transport is more homogeneous, dendrite nucleation can be avoided, the researchers said.

That’s why we applied an extremely thin, two-dimensional membrane made of carbon to the separator, with the pores having a diameter of less than one nanometer. These tiny openings are smaller than the critical nucleus size and thus prevent the nucleation that leads to the formation of dendrites. Instead of forming dendritic structures, the lithium is deposited on the anode as a smooth film.

—Professor Andrey Turchanin, University of Jena, co-corresponding author

There is no risk of the separator membrane being damaged by this and the functionality of the battery is not affected.

a) Regular battery separators with microscale porosity cause non-uniform lithium transport during the battery charge-discharge cycles resulting in needle-like growth of metallic lithium. This leads to short circuits and premature failure of lithium metal batteries. b) By introducing a carbon nanomembrane on the regular battery separator, the growth of lithium needles can be suppressed. The sub-nanometer-sized pores in carbon nanomembranes regulate the transport of lithium ions during the battery charge-discharge cycles, resulting in the deposition of a smooth film and the battery life can be increased significantly. Image: Turchanin et al./Wiley

The researchers recharged test batteries fitted with the hybrid separator membrane; even after hundreds of charging and discharging cycles, there was no dendritic growth, said Dr. Antony George from the University of Jena.

The key innovation here is stabilizing electrode/electrolyte interface with an ultra-thin membrane that does not alter current battery manufacturing process. Interface stability holds key in enhancing the performance and safety of an electrochemical system.

—Associate Professor Leela Mohana Reddy Arava, WSU, co-corresponding author

The research team has applied for a patent for their method. The next step is to see how the application of the two-dimensional membrane can be integrated into the manufacturing process. The researchers also want to apply the idea to other types of batteries.

Resources

• Rajendran, Z. Tang, A. George, A. Cannon, C. Neumann, A. Sawas, E. Ryan, A. Turchanin & L. M. R. Arava (2021) “Inhibition of Lithium Dendrite Formation in Lithium Metal Batteries via Regulated Cation Transport through Ultrathin Sub-Nanometer Porous Carbon Nanomembranes,” Advanced Energy Materials doi: 10.1002/aenm.202100666