Rechargeable lithium-ion batteries power phones, laptops, other personal electronics and electric cars, and are even used to store energy produced by solar panels. But if the temperature of these batteries rises too high, they stop working and can catch fire.
That’s partly because the electrolyte inside them, which transports lithium ions between the two electrodes as the battery charges and discharges, is flammable.
“One of the biggest challenges in the battery industry is this safety issue, so a lot of effort goes into trying to make a battery electrolyte that is safe,” said Rachel Z Huang, a graduate student at Stanford University and first author of report published November 30 in Mater.
Huang developed a nonflammable electrolyte for lithium-ion batteries with 19 other researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University. Their work showed that batteries containing this electrolyte continued to function at high temperatures without starting a fire.
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Salty SAFEty
Conventional lithium-ion battery electrolytes are made from lithium salt dissolved in a liquid organic solvent, such as ether or carbonate. Although this solvent improves battery performance by helping to move lithium ions around, it is also a potential fire starter.
Batteries generate heat as they operate. And if there are holes or defects in a battery, it will heat up quickly. At temperatures above 140 degrees F, the small molecules of solvent in the electrolyte begin to evaporate, transforming from liquid to gas and inflating a battery like a balloon – until the gas catches fire and the whole thing goes to up in flames.
Over the last 30 years researchers have developed non-flammable electrolytes, such as polymer electrolytes, which use a polymer matrix instead of the classic salt-solvent solution to move ions around. However, these safer alternatives do not move ions as efficiently as liquid solvents, so their performance has not measured up to that of conventional electrolytes.
Rachel Huang works in the lab. (Jian-Cheng Lai/Stanford University)
The team wanted to produce a polymer-based electrolyte that could offer safety and performance. And Huang had an idea.
He decided to add as much as he could of a lithium salt called LiFSI to a polymer-based electrolyte designed and synthesized by Jian-Cheng Lai, a postdoctoral scholar at Stanford University and co-first author of the paper.
“I wanted to see how much I could add and test the limit,” Huang said. Typically less than 50% by weight of a polymer-based electrolyte is salt. Huang bumped that number to 63%, creating one of the saltiest polymer electrolytes ever.
Unlike other polymer-based electrolytes, this one also contained flammable solvent molecules. However, the general electrolyte, known as Solvent Anchored Non-Flammable Electrolyte (SAFE), was non-flammable at high temperatures during tests in a lithium-ion battery.
SAFE works because the solvents and the salt work together. The solvent molecules help to conduct ions, resulting in performance comparable to that of batteries containing conventional electrolytes. But, instead of failing at high temperatures like most lithium-ion batteries, batteries containing SAFE continue to function at temperatures between 77-212 degrees F.
Meanwhile, the abundance of added salts act as anchors for the solvent molecules, preventing them from evaporating and catching fire.
Standard battery materials (left) catch fire when exposed to flame, but a new material designed by SLAC and Stanford researchers (right) does not. (Jian-Cheng Lai/Stanford University)
“This new finding highlights a new way of thinking for polymer-based electrolyte design,” said Zhenan Bao, a professor at Stanford University and a researcher with the Stanford Institute for Materials and Energy Sciences (SIMES) who advises Huang. “This electrolyte is important for the future development of batteries that are high energy density and safe.”
Staying gooey
Polymer-based electrolytes can be solid or liquid. Importantly, the solvents and salt in SAFE plasticise its polymer matrix into a goo-like liquid, just like conventional electrolytes.
One advantage: Gooey electrolyte can fit into existing commercially available lithium-ion battery parts, unlike other nonflammable electrolytes that have emerged. Solid state ceramic electrolytes, for example, must use specially designed electrodes, making them costly to produce.
“With SAFE there is no need to change any of the manufacturing settings,” Huang said. “Of course, if it is ever used for production it needs optimization for the electrolyte to fit into the production line, but the work is much less than any of the other systems.”
Yi Cui, a professor at SLAC and Stanford and a SIMES researcher who is also advising Huang, said, “This very exciting new battery electrolyte is compatible with existing lithium ion battery cell technology and would have a major impact on consumer electronics and electrical transport. “
One application of SAFE may be in electric cars.
If the multiple lithium-ion batteries in an electric car sit too close together, they can heat each other up, which could eventually lead to overheating and fire. But, if an electric car contains batteries filled with an electrolyte such as SAFE that is stable at high temperatures, its batteries can be packed close together without worrying about overheating.
As well as mitigating fire risk, this means less space for cooling systems and more space for batteries. More batteries increase the overall energy density, meaning the car could go longer between charges.
“So it’s not just a security benefit,” Huang said. “This electrolyte could also allow you to pack a lot more batteries.”
Time will tell which other battery powered products might get a little SAFE.
This research was funded by the DOE’s Office of Energy Efficiency and Renewable Energy under the Battery Materials Research Program and the Battery 500 Consortium.
Citation: Huang et al., Issue, November 30, 2022 (10.1016/j.matt.2022.11.003)
For questions or comments, please contact the SLAC Office of Communications at communications@slac.stanford.edu.
By Chris Patrick, Courtesy of the SLAC National Accelerator Laboratory, operated by Stanford University for the US Department of Energy’s Office of Science.
Related story: EV Batteries Keep Getting Better & Better
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What is lithium salt in batteries?
Lithium salts are one of the main components of electrolytes within a battery. They are based on an inorganic anion and a lithium cation, which is responsible for the lithium conductivity in the system.
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Can we use NaCl in battery?
NaCl can be activated to an electrode by electrochemically stimulating it in a crystalline structure [26]. NaCl has been used in several components of sodium-ion batteries, including electrolyte [28, 29], as a raw material for electrodes and electrode doping [30].
Why would a battery not need a salt bridge?
Therefore, there is no need for salt bridges and porous separators because lead acid batteries use the same electrolyte for each electrode.
What lithium salt is used in batteries?
Lithium hexafluorophosphate (LiPF6) is currently the dominant Li salt used in rechargeable lithium-ion batteries (LIBs) based on graphite anode and 34 V cathode material.
What are lithium battery electrolytes made of? Potassium hydroxide is the electrolyte in common household alkaline batteries. The most common electrolyte in lithium batteries is a lithium salt solution such as lithium hexafluorophosphate (LiPF6).
What are the 3 chemicals used to make lithium-ion batteries? Essential raw materials used in the manufacture of Li-ion batteries (LIBs) include lithium, graphite, cobalt, and manganese. As the use of electric vehicles increases, the production of LIB cells for vehicles is becoming an increasingly important source of demand.
What lithium is used for batteries? Lithium cobalt oxide is the most common lithium battery type as found in our electronic devices.
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