Craig Arnold puts the squeeze on batteries
Your laptop easily ran for six hours when you bought it three years ago. Now, you would be lucky if it stayed powered for two hours. That is, if you haven’t bought a new battery already. Most everyone has dealt with the frustration of fading battery capacity. The short life of lithium-ion batteries, which now power our cell phones and laptops, also has limited their use in electric cars.
In an effort to make batteries last as long as the gadgets and cars they power, Craig Arnold, a Princeton professor of mechanical and aerospace engineering, has found a surprising link between battery life and the day-to-day physical forces acting on an overlooked battery component.
“People think of batteries as chemical devices,” said Arnold, who is affiliated with the Andlinger Center for Energy and the Environment and the Princeton Environmental Institute. “What we’re looking at is, How do mechanics or physical forces affect the electrochemical performance of the system? If we can understand why a laptop battery fails we can start hoping to create one that never needs to be replaced.”
Past efforts by others to study the mechanics of a lithium-ion battery have focused on the battery’s electrodes, the "plus" and "minus" connectors. Many studies suggest that the chemical changes and physical wear-and-tear of the electrodes are responsible for shortening battery life.
However, Arnold and his graduate student Christina Peabody (who received her Ph.D. in August) were the first to connect battery performance with the mechanics of a part called the separator. They found that typical forces acting on a battery compress the separator, which limits the flow of lithium ions and dramatically diminishes charge capacity.
“The mechanics of the separator can very much affect the lifetime of batteries,” Arnold said. “That’s an interesting and surprising result. The role of the separator in this sense hadn’t really been appreciated before.”
Taking their work one step further, Arnold and Peabody recommend ways to mitigate this effect: modify the surface of the electrodes or change the separator’s properties.
The detailed findings of the engineers’ work appeared in the October 1 issue of the Journal of Power Sources. Arnold presented the findings this month at the Electrochemical Society Meeting in Boston.
Commercial lithium-ion batteries contain layers of different materials that are rolled up and tightly packed into a metal container or foil pouch. There are three main layers: a positive electrode, a negative electrode, and a separator membrane that keeps the electrodes from touching one another.
During charge and discharge, lithium ions move from one electrode to the other. Meanwhile, the separator, a thin polymer sheet with tiny nanometer-sized pores, allows the ions to flow through and is considered a non-active component because it is not involved in charge storage.
In fact, the separator was not what the Princeton researchers had intended to study. They started out wanting to see what happens to the entire battery structure when it is under mechanical stress.
Many different forces act on a battery over its lifetime. For example, the weight of other batteries can press on a battery when they are stacked inside large packs such as those used in electric cars. More important though, Arnold said, is “an interesting and more subtle place where batteries start feeling force in their normal charging and discharging.” That is because when electrodes take in lithium ions during charge and discharge, they swell and wind up pushing against the separator.
The researchers wanted to simulate, in a short time, what happens to a battery over years of enduring such pressures. To do that, they placed batteries in a big press and squeezed them tightly for several hours to see how the components deformed.
They found that, under a certain pressure, the separator pores start closing more and more over time. This is because atoms and molecules in the polymer material gradually rearrange themselves, a phenomenon called "creep."
The researchers observed that the pores close faster under higher pressures and higher temperatures. Creep sets in at relatively low forces that could be generated if for instance someone were to simply hold the separator between his fingers and pinch it. The swelling of electrodes can exert a higher pressure than the pinching action, Arnold said.
Next, the researchers put the separator between electrodes to make a basic battery cell. They charged and discharged it and found that at constant currents, the gradual closing of pores blocked the flow of lithium ions and decreased the cell’s measured storage capacity.
“Our main conclusion is that the stresses on a battery primarily affect the separator,” Arnold said. “Their effect is to start closing separator pores, and this type of pore closure leads to a decrease in battery capacity for a given discharge rate. Under very high stresses or long times, it can even shut off the battery entirely.”
There are ways to prevent the capacity loss associated with separator compression, according to Arnold. One solution is to find better materials for the separator. Most of the research and development on separators has involved creating materials that are robust and can withstand the tension of being rolled. But, Arnold said, it might be prudent to make separators that are resistant to creep. One possibility would be creating composites of polymers mixed with non-polymeric materials that are creep-resistant. “This becomes a job for people who make materials,” he said. “We’ve identified some important properties to investigate further.”
Another “neat little trick,” would be to play with the overall structure of the battery itself to minimize the effects of stress. Arnold said that small changes in the electrode or packaging design can have a significant effect on the mechanics of the system. “We’re just starting to understand the range of possibilities,” he said. The researchers are continuing to test these ideas with the hopes of battling the effects of creep on the separator.
Arnold is optimistic that this work will have important consequences for future battery research and development. “Lithium ion batteries have been around for a long time,” he said. “Looking at them from a different angle can reveal unexpected ways to overcome long-standing challenges.”
Funding for Arnold’s research is from the National Science Foundation, the Carbon Mitigation Initiative, and the Grand Challenges Initiative.
Funding for the video above, by Volker Steger, is from the High Meadows Foundation Sustainability Fund.