In the past 10 years, lithium-ion battery cell energy density has almost tripled. Get more energy (kilowatt-hours) out of the same mass (kilograms) of batteries and you can build an electric vehicle (EV) that goes farther with better performance. Researchers have spent the last decade finding ways to improve the chemistry of the batteries for more power and reducing the weight of the components—resulting in a win-win.
Lithium-ion cells are actually quite complex. Inside a compact package, they contain a positive electrode (cathode), a negative electrode (anode), an electrolyte solution, and a separator that allows ions to pass through it while blocking the flow of electrons.
In commercial lithium-ion batteries, the anode is a form of carbon graphite that is spread onto a conducting sheet of metal, usually aluminum. Similarly, the cathode is often made from a paste of nickel metal oxide, along with the oxides of transition metals such as cobalt, manganese, and aluminum. This paste is spread onto a thin sheet of copper, which is used to collect excess electrons during discharge and provide them for current flow. The thin sheets of aluminum and copper that collect the battery current can account for 15% to as much as 50% of the battery cell weight.
Now, scientists at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have re-engineered one of the heaviest battery components—those sheets of copper or aluminum foil current collectors. A Stanford University press release describes an entirely new approach to make the collectors up to 80% lighter, resulting in lithium-ion batteries that are lighter and more efficient.
“Researchers in the battery industry have been trying to reduce the weight of current collectors by making them thinner or more porous, but these attempts have had unwanted side effects, such as making batteries more fragile or chemically unstable or requiring more electrolyte, which raises the cost,” said Yusheng Ye, a postdoctoral researcher in the Stanford lab.
“The current collector has always been considered dead weight, and until now it hasn’t been successfully exploited to increase battery performance,” said Yi Cui, a professor at SLAC and Stanford and investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) who led the research. “But in our study, making the collector 80% lighter increased the energy density of lithium-ion batteries – how much energy they can store in a given weight – by 16-26%. That’s a big jump compared to the average 3% increase achieved in recent years,” he said.
The researchers designed experiments for making and testing current collectors based on a lightweight polymer called polyimide, which stands up to the high temperatures created by fast battery charging. Polyamide also resists fire which provided an additional benefit to the new current collector design. In addition, a fire retardant called triphenyl phosphate, or TPP was embedded in the polymer, which was then coated on both surfaces with an ultrathin layer of copper. “People have also tried adding fire retardant to the battery electrolyte, which is the flammable part, but you can only add so much before it becomes viscous and no longer conducts ions well,” said Cui. The copper not only does its usual job of collecting current, but also protects the polymer and its fire retardant.
“When exposed to an open flame from a lighter, pouch batteries made with today’s commercial current collectors caught fire and burned vigorously until all the electrolyte burned away,” Ye said. “But in batteries with the new flame-retardant collectors, the fire never really got going, producing very weak flames that went out within a few seconds, and did not flare up again even when the scientists tried to relight it,” according to the Stanford release.
The approach also has some extra plusses. “One of the big advantages of this approach,” Cui said, “is that the new collector should be easy to manufacture and also cheaper, because it replaces some of the copper with an inexpensive polymer.” Lighter batteries are also easier to transport, both on their way to installation in an EV, and on their way to recyclers at the end of their life. Scaling it up for commercial production should not provide any problems, Cui indicated.