PNNL scientists have created a chemical cocktail that could help electric cars power their way through the extreme temperatures that degrade the efficiency of current lithium-ion batteries.
Wu Xu and colleagues have demonstrated combinations of compounds in electrolytes that, in the laboratory, allow lithium-ion batteries to function very well both in sub-zero temperatures (Fahrenheit), which are reached often during the winter in the northern United States and other parts of the world, and in very hot temperatures like those in the summer in Phoenix, Ariz. Those are temperatures where today’s batteries, used to power electric cars, cell phones, laptops, and other devices, often retain little charge and expend it quickly.
Scientists expect the finding to allow batteries for electric cars to operate more efficiently in extreme temperatures. The electrolytes developed by Xu’s team allow lithium-ion batteries to work well below zero, even down to about -40 degrees Celsius (-40 degrees Fahrenheit), and at temperatures of 60 degrees Celsius (140 degrees Fahrenheit) – making them attractive for commercial lithium-ion batteries.
The compounds the team developed are found in a battery’s electrolyte, the liquid material that bathes the electrodes and plays a critical role shuttling around the lithium ions that make the battery work. The fluid has several important functions and is typically a mix of well-known compounds, such as lithium hexafluorophosphate as the conducting salt, organic carbonates as the solvent, and closely held additives that boost performance.
But several constraints limit the ability of lithium-ion batteries to power vehicles and other devices in very cold or hot environments. For batteries of limited size and in varied and demanding environments – such as those that power cars – the demands on the electrolyte are steep.
In a recent article in ACS Applied Materials & Interfaces, Xu’s team demonstrated electrolytes that were viable at much lower temperatures than current materials.
The protective power of an electrolyte
Their work focused on creating a mix that kept other important parts of the battery, the anode and cathode, functioning as efficiently as possible while allowing the electrolyte to continue doing its job. That job includes shuttling ions to their destinations efficiently and keeping wayward ions from going where they shouldn’t.
The electrolyte serves an important function protecting the electrodes. As the battery operates, molecules from the electrolyte build up what is known as the SEI (solid electrolyte interface) on the anode or CEI (cathode electrolyte interface) on the cathode. These are the thin layers of materials that help protect those devices from chemical damage – from valuable lithium ions going into unwanted chemical reactions, for example, or from processes that eat away at the integrity of the electrodes.
“It’s a little bit like how lamination protects a photograph,” said Xu. “You want a very thin but very strong layer that prevents damage to what lies beneath.”
But scientists first face a balancing act when creating the combination of solvents and additives that will be most effective in low temperatures. The most common solvent, ethylene carbonate, does a great job at room temperature but is not effective at low temperatures. A common substitute, propylene carbonate, works much better at low temperatures but is more likely to allow damage to the graphite anode. Researchers face another balancing act with vinylene carbonate, a common additive that works well at room and high temperatures but is not effective at low temperatures either.
Benefits of additives add up
So Xu and team tested several combinations of additives and found a few that combine the best features: protecting a battery’s key elements, keeping the protective layers around the electrodes thin but strong, operating over a wide range of temperatures, and doing so over many cycles.
Among the additives the team found that were effective when used in combination with others:
- cesium hexafluorophosphate or CsPF6, which helps build the ultrathin but strong protective structure on the anode
- fluoroethylene carbonate or FEC, which does the same and improves battery performance at low temperatures
- tris(trimethylsilyl) phosphite or TTMSPi, which helps form the protective structure on the cathode and helps prevent damage generated by some chemical activity
- 1,3-propane sultone or PS, which helps reduce the harmful effects of unwanted chemical reactions on the cathode.
The most effective combination forms protective layers on both the anode and cathode and expands the electrolyte’s effectiveness down to about -40 degrees Fahrenheit and up to 140 degrees Fahrenheit.
“The key here is the synergistic effect we demonstrated when additives are used in particular combinations,” said Xu.
He added: “We need to widen the temperature stability window so that batteries like the ones in electric cars are effective everywhere – from Phoenix on the warmest day of the year to Minneapolis on the coldest."
The work is sponsored by DOE’s Office of Energy Efficiency and Renewable Energy through the Technology Commercialization Fund Program. The microscopy and spectroscopy measurements were done at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility located at PNNL.