From Toothpaste to EV Batteries
You probably know that fluoride – or more specifically, sodium fluoride, which is a compound of fluorine – as a common ingredient in toothpaste. When brushing our teeth, fluoride helps protect against tooth decay. But it has lots of other practical uses, and scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have discovered a use for fluoride that could help protect next-generation EV batteries against performance degradation.
While today’s EV batteries generally use lithium-ion chemistry, an all-new generation of EV batteries is just over the horizon. We’ve documented some of these chemistries, such as lithium-metal, here on GreenCars. Lithium-metal batteries use an anode made of lithium metal instead of the graphite which is normally found in lithium-ion batteries. Combined with a cathode containing nickel, manganese, and cobalt, lithium-metal batteries have up to double the energy density of a lithium-ion battery.
Lithium-Metal Batteries: Advantages
Lithium-metal batteries can in theory provide much longer range, and thanks to their significantly higher density, they’re lighter, increasing efficiency and performance. Thanks to their smaller size and lower weight, lithium-metal batteries are also flexible enough to be installed in long-haul trucks and even aircraft, paving the way for zero-emissions uses beyond just automobiles.
The problem with lithium-metal batteries is that their high energy density can decline rapidly with many charge and discharge cycles. Today’s lithium-ion cells lose a small percentage of their performance and range over time, which is one reason they are so popular; buyers expect that a car will perform just as well a few years down the road as when new.
So how can an ingredient from toothpaste help? The secret involves the electrolyte – the liquid through which the lithium ions move between the cathode and the anode when charging and discharging. In a lithium-metal battery, the electrolyte usually is a liquid that contains a lithium-containing salt that has been dissolved in a solvent. The electrolyte doesn’t form a protective layer on the anode surface during charging cycles.
Extending Useful Life of EV Batteries
Just like it helps form a protective layer on your teeth, fluoride is the key to forming a protective layer on the lithium-metal anode’s surface during charging cycles. This layer, called a solid electrolyte interphase (SEI), protects the anode, and allows lithium ions to pass freely in and out while charging and discharging – without any degradation.
Led by John Zhang, group leader in the Chemical Sciences and Engineering division at Argonne National Laboratory, the team discovered that a fluoride solvent would maintain a robust protective layer on the anode for hundreds of cycles. The solvent couples a positively-charged fluorinated component with a negatively-charged one into what is called an ionic fluid. The combination of the two solvents is what maintained high performance in the lithium-metal battery for hundreds of cycles of testing.
The Department of Energy Office of Science provided access to its high-performance computing resources to facilitate testing of the ionic fluid at the atomic level. Simulations revealed that the fluorine-infused compounds stick to and accumulate on the anode and cathode surfaces of the battery before any charging and discharging cycles. Then, during the early stages of a charge or discharge cycle, a resilient SEI layer forms on the anode, protecting it and extending its life.
Safer and More Environmentally Friendly
With the help of the sophisticated simulations, the team tuned the proportion of fluoride and lithium salt to create a layer that was not too thick and not too thin, allowing ions to flow in and out of the electrodes efficiently while affording sufficient protection. The new electrolyte is low-cost, as well; it can be made with great purity and yield in one simple step, compared to multiple steps for current electrolytes. The fluoride-based electrolyte also is more environmentally friendly, as it uses less solvent, which is volatile and can release contaminants into the environment. Safety is another advantage, as the new electrolyte isn’t flammable.
The discovery of this new fluorinated electrolyte at the Argonne National Laboratory could help bring lithium-metal batteries to market sooner – which could considerably boost the adoption of electric cars, as battery sizes and prices would come down. Plus, the usefulness of the new electrolyte could extend to other types of advanced battery systems in the future.