It is becoming evident that moving beyond lithium ion batteries is going to require the combined efforts of research labs and battery manufacturers around the world. The extent of such collaboration was made clear in a recent press release from the University of Pennsylvania, detailing research into a solid electrolyte for use in lithium metal batteries.
Presently, commercial lithium ion batteries use a carbon graphite anode electrode and a metal oxide cathode electrode. They are separated by a liquid organic solvent that can pass lithium ions between the electrodes while preventing electrons from making the journey. The organic solvent of the electrolyte is flammable—resulting in a potential for a fire in the event that a lithium ion battery is punctured.
The anode side of a lithium ion battery is made from layers of graphite. Lithium ions are inserted between the material’s layers during charging and are released during discharge. Battery researchers realize that replacing the graphite anode with metallic lithium would allow many more lithium ions to flow during discharge, producing a battery with at least twice the capacity. But during the charging stage of a lithium metal battery, spiky crystalline structures, called dendrites, form on the metal surface. These dendrites can grow through the liquid electrolyte, reaching the cathode and shorting out the battery.
A Solid Approach
A worldwide search is on for a solid or semi-solid electrolyte that can prevent dendrite growth while allowing the easy passage of lithium ions without conducting electrons.
One possibility that has received great attention is the use of an electrolyte made from a polymer material. Such a material is called a solid polymer electrolyte (SPE). This is the approach taken by the research group at the University of Pennsylvania. The starting point for the team was Nafion, a sulfonated tetrafluoroethylene-based copolymer that allows the passage of positive ions (cations) without allowing the transmission of electrons. Nafion is commonly used as a proton exchange membrane (PEM) in hydrogen fuel cells.
“Nafion is something of a fluke,” explained Karen Winey, chair of the Department of Materials Science and Engineering at the University of Pennsylvania, in the press release produced by the university. “Its structure has been the subject of debate for decades, and will likely never be fully understood or controlled,” she added.