Using polymers to build a better battery

Rechargeable lithium-ion batteries have become ubiquitous in recent years, appearing in widely used products such as mobile phones. The Adolphe Merkle Institute’s Soft Matter Physics group is investigating solid polymer electrolytes (SPEs) for use in these batteries to make them safer and more powerful. 

Typical lithium-ion (Li-ion) batteries use a volatile liquid electrolyte. This entails risks, occasionally leading to short circuits or even fire, and reduces a battery’s cycle life. Liquid electrolytes are also incompatible with higher energy metal anodes such as lithium metal, the use of which would increase the battery’s energy density— the amount of energy stored in a given system per unit volume—compared to currently employed graphite anodes. For batteries to grow into larger transportation and grid storage roles, where they are forecast to be a disruptive force in favor of the green economy, fundamental safety and energy density issues therefore need to be resolved. SPEs effectively address these, according to AMI alumnus Dr. Preston Sutton. “Nanostructured SPEs have the potential to provide both good ionic conductivity and good mechanical properties,” he explains. Unlike other electrolyte systems, such as liquids, gelled polymers, and inorganic solids, the benefits of nanostructured SPEs include relatively low volatility (increased safety), compatibility with metallic lithium (increased energy density), and relatively low‐cost materials and manufacturing.

SPEs have not been widely adopted thus far because their ambient temperature conductivity is found to be lacking for more general-purpose usage. Researchers are therefore usually targeting conductivity improvements, while also maintaining the energy and safety advantages of polymer electrolytes. The Soft Matter Physics researchers investigated the suitability of a block copolymer, specifically a triblock terpolymer, as an electrolyte for Li‐ion batteries. Block copolymers (BCPs), consisting of two or more different polymers connected at each end, can self-assemble into various nanostructures.

“We chose the terpolymer because of its 3D continuous conductivity network, which offers conceptual advantages compared to conventional structures. But we were surprised by the outcome,” says group leader Dr. Ilja Gunkel.

The Li‐ion conductivity across very thick samples fell short of the anticipated performance, and appeared to be limited by the nature of the surfaces between which the electrolytes were sandwiched. Inspired by this observation, the AMI researchers showed that modifying the surface area of the electrode that comes into contact with the SPE surface with a specific polymer brush results in higher and more reproducible conductivity values. “The importance of this discovery is also the increased resolution in accurately identifying conductivity so that better designs can be established,” says Sutton. “While it is only one piece of the SPE puzzle, it is extremely relevant for reliable characterization.”

These results are of practical relevance for next-generation solid‐state energy storage devices. They demonstrate that the properties of the electrode surface, previously ignored, are important for the design of SPE‐containing devices with high energy density.

Reference: Sutton, P.; Bennington, P.; Patel, S. N.; Stefik, M.; Wiesner, U. B.; Nealey, P. F.; Steiner, U.; Gunkel, I. Surface Reconstruction Limited Conductivity in Block-Copolymer Li Battery Electrolytes. Advanced Functional Materials 2019, 29 (48), 1905977.