Over the past decade, lithium-ion batteries have seen stunning improvements in their size, weight, cost, and overall performance. (See Physics Today, December 2019, page 20.) But they haven’t yet reached their full potential. One of the biggest remaining hurdles has to do with the electrolyte, the material that conducts Li+ ions from anode to cathode inside the battery to drive the equal and opposite flow of charge in the external circuit.
Most commercial lithium-ion batteries use organic liquid electrolytes. The liquids are excellent conductors of Li+ ions, but they’re volatile and flammable, and they offer no defense against the whisker-like Li-metal dendrites that can build up between the electrodes and eventually short-circuit the battery. Because safety comes first, battery designers must sacrifice some performance in favor of not having their batteries catch fire.
A solid-state electrolyte could solve those problems. But what kind of solid conducts ions? An ordered crystal won’t do—when every site is filled in a crystalline lattice, Li+ ions have nowhere to move to. A solid electrolyte therefore needs to have a disordered, defect-riddled structure. It must also provide a polar environment to welcome the Li+ ions, but with no negative charges so strong that the Li+ ions stick to them and don’t let go.
For several years, Jenny Pringle, Maria Forsyth, and colleagues at Deakin University in Australia have been exploring a class of materials, called organic ionic plastic crystals (OIPCs), that could fit the bill. As a mix of positive and negative ions, an OIPC offers the necessary polar environment for conducting Li+. And because the constituent ions are organic, the researchers have lots of chemical leeway to design their shapes so they can’t easily fit together into a regular lattice but are forced to adopt a disordered, Li+-permeable structure.
One drawback of OIPCs, however, is that their component ions themselves can move around under the battery’s electric field. When that happens, the ion mobility can decrease the Li+ transport and diminish the battery performance.
Now Pringle, Forsyth, research fellow Faezeh Makhlooghiazad, and their colleagues are exploring a new approach that could overcome that difficulty: yoking an OIPC’s positive and negative components together in a single molecule. Because the molecule as a whole has no net charge, it stays put under the battery’s electric field, leaving the Li+ ion as the only mobile species.
Molecules with positively and negatively charged parts are called zwitterions, from the German word Zwitter, meaning “hermaphrodite.” They’ve been explored before in the context of battery electrolytes, but so far only as additives to other electrolyte materials. Most known zwitterions form ordered crystals in their pure state, so they’re not suitable for conducting Li+.
The Deakin researchers used the design principles from their work on OIPCs to create new zwitterions, including the one shown in the figure, that are prone to forming disordered structures. As the micrograph (taken at room temperature) shows, the pure zwitterionic material is full of grain boundaries and defects. When warmed to 57 °C, it undergoes a phase transition to an even more disordered solid form. And when mixed with just a small amount of Li salt, it exhibits a promising Li+ conductivity.
The researchers stress that they’re just getting started. The design terrain of zwitterionic materials is still largely underexplored, in part because of the challenges in synthesizing the twice-charged molecules. But with the help of colleagues in industry, the Deakin researchers have identified new strategies for synthesizing zwitterionic materials that are even more disordered. (F. Makhlooghiazad et al., Nat. Mater., 2021, doi:10.1038/s41563-021-01130-z.)
