Scientists at Stanford University and the Department of Energy's SLAC National Accelerator Laboratory have employed a novel approach to make lithium-ion batteries lighter, safer, and more efficient. The redesigned battery components, known as current collectors made of copper or aluminum foil, have reduced weight by 80% and can also extinguish large fires.
Researchers say that if this technology is adopted, it could achieve two main goals of battery research: extending the driving range of electric vehicles; and reducing the risk of fires in laptops, mobile phones, and other devices. When batteries are charged at ultra-fast speeds, it can cause damage to more types of batteries, leading to fires.
The research team described their work in the October 15 issue of Nature Energy.
“Current collectors have always been considered to be self-sustaining, and until now they have not been successfully used to improve battery performance,” said YiCui, a professor at Stanford University and a researcher at the Stanford Institute for Materials and Energy Sciences (SIMES).
"But in our research, reducing the collector weight by 80% can increase the energy density of lithium-ion batteries by 16-26%." This is a significant leap compared to the average increase of 3% in recent years.
Whether cylindrical or pouch-shaped, lithium-ion batteries have two current collectors, one for each electrode. These collectors distribute the current flowing into or out of the electrodes, and in some high-power or ultra-thin batteries, they account for 15% to 50% of the weight. Reducing battery weight is desirable in itself, as it allows for lighter devices and reduces the weight that electric vehicles must carry; each unit of weight stores more energy, enabling electronic devices and electric vehicles to run longer between charges.
Reducing battery weight and flammability can also significantly impact recycling by lowering the cost of transporting recycled batteries, Cui said.
Battery industry researchers have been trying to reduce the weight of current collectors by making them thinner and more porous, but these attempts have had unintended side effects, such as making batteries more fragile or chemically unstable or requiring more electrolyte, increasing costs, says Dr. Choi, a postdoctoral researcher in his lab who conducted experiments with visiting scholar Lien-Yang Zhou.
Regarding safety concerns, he said, "People also try to add flame retardants to the battery electrolyte, which is the flammable part, but you can only add so much, otherwise it will become viscous and no longer conduct ions well."
Design a polymer foil sandwich
After brainstorming on this problem, Cui, Ye, and graduate student Liu Yayuan designed an experiment to manufacture and test a current collector based on a lightweight polymer called polyimide. Polyimide is fire-retardant and can withstand the high temperatures generated by fast charging. The flame retardant triphenyl phosphate (TPP) was embedded in the polymer, and then an ultra-thin layer of copper was coated on both sides of the polymer. The copper not only performs the normal current distribution function but also protects the polymer and its flame retardant.
Ye said that compared to the current version, these changes have reduced the weight of the current collector by 80%, which means that the energy density of various types of batteries has increased by 16-26%, and its current conduction performance is as good as that of ordinary collectors without degradation.
Ye explained that when a pouch battery made with a lighter is exposed to an open flame, it will ignite and burn intensely until all the electrolyte is burned away. However, in the battery equipped with a new type of flame retardant collector, the fire never actually ignites; the flame produced is very weak and extinguishes within seconds, and it does not reignite even when scientists attempt to rekindle it.
Cui said a major advantage of this method is that the new current collector is easy to manufacture and inexpensive because it replaces some of the copper with a cheap polymer. He said that scaling it up to commercial production "should be very feasible." The researchers have already filed a patent application through Stanford University, and Cui said they will contact battery manufacturers to explore the possibility.
