Developing batteries for electric cars is tricky, and current batteries keep electric car ranges low. The lithium-ion batteries currently being used in cars are very expensive and their low energy density can’t make the cars go very far.
But new research on a different kind of battery, lithium-sulfur, may at least bring the price down. Researchers at Stanford University, led by material science and engineering professor Yi Cui, are currently developing a new Li-S battery with much better capacity retention than typical sulfur batteries.
The new batteries retain more of their charge after more cycles. While most current Li-S batteries lose much of their capacity after a few dozen cycles, Cui’s new batteries keep over 80 percent of their 1180 mAh/g after more than 300 cycles. The team also said that they could get similar batteries to keep that much power after thousands of cycles.
The researchers found a new mechanism that was causing traditional Li-S batteries to lose capacity so quickly. When a Li-S battery charges, the lithium sulfide in the battery’s cathode must bind to the surface of the cathode. In the batteries Cui was testing, this was on the inner surface of the hollow carbon nanofiber of the cathode, and created a good flow of charge through the electrical contact. The team found that the battery’s lithium sulfide detaches from the nanotube while discharging power, leading to a loss of electrical contact, which led to a smaller capacity every time the battery was recharged.
One obstacle in the way of fixing this is the sulfur cathode’s fragility. The sulfur compound is very sensitive to air and moisture, and will sublime in a vacuum. This makes it hard to study at the nanoscale (under a microscope). But a development by the team for a previous study, the hollow carbon nanofiber structure of the anode, allowed the researchers to study the cathode, using a transmission electron microscope (TEM), while keeping the sulfur intact.
The team sought to solve the problem by added amphiphilic polymers to the carbon nanofiber surface. Amphiphilic objects are hydrophilic and lipophilic, binding to both water and fats, like soap. This helps the lithium sulfides bind to the carbon surface, by creating anchoring points for the sulfides.
This solution had great results. The new batteries had less than three percent decay for the first 100 cycles, and this performance remained strong with less than 20 percent decay after 300 cycles. Of course, this isn’t close to the lithium-ion batteries, which boast lifetimes closer to 10,000 cycles.
This much higher life is required for applications like electric cars, which use expensive batteries that can’t realistically be changed every few years. But much cheaper lithium-sulfide batteries with competitive lifespans may be available in the future.
“The Li-S batteries become pretty promising for electric vehicles,” Cui told Phys Org. “The life cycle needs to improve further. The lithium metal anodes’ safety problem needs to be solved, [but] it is possible to get around lithium metal anodes with sulfur anodes.”
