Illustration of caged lithium ions in a new polymer membrane for lithium batteries. Image: Artem Baskin/Berkeley Lab.
Illustration of caged lithium ions in a new polymer membrane for lithium batteries. Image: Artem Baskin/Berkeley Lab.

Membranes that allow certain molecules to pass quickly through while blocking others are key enablers for energy technologies ranging from batteries and fuel cells to resource refinement and water purification. For example, membranes separating the two terminals in a battery help to prevent short circuits while still allowing the transport of the charged particles, or ions, needed to maintain the flow of electricity.

The most selective membranes – those with very specific criteria for what may pass through – suffer with low permeability for the working ion in a battery, which limits the battery's power and energy efficiency. To overcome the trade-offs between membrane selectivity and permeability, researchers are developing ways to increase the solubility and mobility of ions within the membrane, therefore allowing a higher number of them to transit through the membrane more rapidly. Doing so could improve the performance of batteries and other energy technologies.

Now, as reported in a paper in Nature, a team of researchers has designed a polymer membrane with molecular cages built into its pores to hold positively charged ions from a lithium salt. These cages, called 'solvation cages', are made up of molecules that together act as a solvent surrounding each lithium ion – much like how water molecules surround each positively charged sodium ion in the familiar process of table salt dissolving in liquid water.

The team, led by researchers at the US Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab), found that solvation cages increased the flow of lithium ions through the membrane by an order of magnitude compared to standard membranes. The new membrane could allow high-voltage battery cells to operate at higher power and more efficiently, important factors for both electric vehicles and aircraft.

"While it's been possible to configure a membrane's pores at very small length scales, it's not been possible until now to design sites to bind specific ions or molecules from complex mixtures and enable their diffusion in the membrane both selectively and at a high rate," said Brett Helms, a principal investigator in the Joint Center for Energy Storage Research (JCESR) and a staff scientist in Berkeley Lab's Molecular Foundry, who led the work.

The research is supported by JCESR, a DOE Energy Innovation Hub whose mission is to deliver transformational new concepts and materials for electrodes, electrolytes and interfaces that will usher in high-performance next-generation batteries for transportation and the grid. In particular, JCESR provided the motivation to understand how ions are solvated in the porous polymer membranes used in energy storage devices, Helms said.

To pinpoint the design for a cage in a membrane that would solvate lithium ions, Helms and his team looked to a widely practiced drug-discovery process. In drug discovery, it's common to build and screen large libraries of small molecules with diverse structures to pinpoint one that binds to a biological molecule of interest. Reversing that approach, the team hypothesized that by building and screening large libraries of membranes with diverse pore structures, it would be possible to identify a cage to temporarily hold lithium ions. Conceptually, the solvation cages in the membranes are analogous to the biological binding site targeted by small-molecule drugs.

Helms' team devised simple but effective strategies for introducing functional and structural diversity across multiple length scales in the polymer membranes. These strategies included designs for cages with different solvation strengths for lithium ions, as well as arrangements of cages in an interconnected network of pores. "Before our work, a diversity-oriented approach to the design of porous membranes had not been undertaken," said Helms.

Using these strategies, Miranda Baran, a graduate student researcher in Helms' research group and lead author on the paper, systematically prepared a large library of possible membranes at the Molecular Foundry. She and her co-authors experimentally screened each one to discover a leading candidate whose specific shape and architecture made its pores best suited for selectively capturing and transporting lithium ions. Then, working with Kee Sung Han and Karl Mueller at the Environmental Molecular Sciences Laboratory, a DOE user facility at Pacific Northwest National Laboratory, Baran and Helms revealed, using advanced nuclear magnetic resonance techniques, how lithium ions flow within the polymer membrane, compared with other ions in the battery.

"What we found was surprising. Not only do the solvation cages increase the concentration of lithium ions in the membrane, but the lithium ions in the membrane diffuse faster than their counter anions," said Baran, referring to the negatively charged particles that are associated with the lithium salt when it enters the membrane. The solvation of lithium ions in the cages helped to form a layer that blocked the flow of those anions.

To further understand the molecular reasons for the new membrane's behavior, the researchers collaborated with Artem Baskin, a postdoctoral researcher working with David Prendergast, another investigator in JCESR. They performed calculations, using computing resources at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC), to determine the precise nature of the solvation effect that occurs as lithium ions associate with the cages in the membrane's pores. This solvation effect causes lithium ions to concentrate more in the new membrane than they do in standard membranes without solvation cages.

Finally, the researchers investigated how the membrane performed in an actual battery, and determined the ease with which lithium ions are accommodated or released at a lithium-metal electrode during the battery's charge and discharge. Using X-ray tools at Berkeley Lab's Advanced Light Source, they observed lithium flow through a modified battery cell whose electrodes were separated by the new membrane. The X-ray images showed that, in contrast to batteries with standard membranes, lithium was deposited smoothly and uniformly at the electrode, indicating that the battery charged and discharged quickly and efficiently thanks to the solvation cages in the membrane.

Future work by the Berkeley Lab team will expand the library of membranes and screen it for enhanced transport properties for other ions and molecules of interest in clean energy technologies. "We also see exciting opportunities to combine diversity-oriented synthesis with digital workflows for accelerated discovery of advanced membranes through autonomous experimentation," said Helms.

This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.