Researchers have found the reasons behind the capacity loss in lithium-ion batteries, which are widely used in electronic devices. This insight could lead to the creation of electric vehicles that can operate for longer periods without needing to be charged.
Batteries gradually lose their power, which is why older mobile phones deplete faster. Despite being common, the reasons for this phenomenon are not fully understood.
Recently, a global team of scientists, led by an engineer at the University of Colorado Boulder, has uncovered the core mechanism that causes battery deterioration. This breakthrough could assist researchers in creating superior batteries that would enable electric vehicles to travel further and last longer, while also improving energy storage solutions that facilitate the shift to clean energy.
The research findings were shared in the journal Science on September 12.
“We are contributing to the advancement of lithium-ion batteries by understanding the molecular processes that lead to their decline,” stated Michael Toney, the lead author of the paper and a professor in the Department of Chemical and Biological Engineering. “Enhancing battery performance is crucial for transitioning our energy systems from fossil fuels to more sustainable energy sources.”
For years, engineers have focused on creating lithium-ion batteries—the most prevalent type of rechargeable battery—without cobalt. Cobalt is a costly rare mineral whose extraction has raised serious environmental and human rights concerns. In the Democratic Republic of Congo, which produces over half of the world’s cobalt, many miners are children.
To date, scientists have attempted to substitute cobalt with other elements, like nickel and magnesium, in lithium-ion batteries. However, these alternatives often demonstrate even greater self-discharge rates, a phenomenon where the battery’s internal chemical reactions diminish stored energy and, over time, degrade its capacity. Because of self-discharge, the lifespan of most EV batteries is typically seven to ten years before they require replacement.
Toney, who is also a member of the Renewable and Sustainable Energy Institute, and his team examined the reasons behind self-discharge. In a standard lithium-ion battery, lithium ions, which carry electrical charges, move from the anode (one side of the battery) to the cathode (the opposite side) via an electrolyte medium. This ion movement generates an electric current that powers electronic devices. When the battery is charged, the flow of these charged ions reverses, returning them to the anode.
Previously, it was believed that batteries experienced self-discharge because not all lithium ions made it back to the anode during charging, thereby reducing the available charged ions needed to sustain the electric current and provide power.
Using the Advanced Photon Source, a high-powered X-ray facility at the U.S. Department of Energy’s Argonne National Laboratory in Illinois, the research team discovered that hydrogen molecules from the battery’s electrolyte migrate to the cathode and occupy the positions that lithium ions typically attach to. This prevents lithium ions from binding effectively to the cathode, thereby weakening the electric current and reducing the battery’s capacity.
Transportation is the largest contributor to greenhouse gas emissions in the U.S., responsible for 28% of the country’s emissions in 2021. In response, many car manufacturers are shifting focus from gasoline vehicles to producing more electric vehicles (EVs). However, EV makers are confronted with various challenges, including limited driving range, higher production costs, and shorter battery lifespans compared to traditional vehicles. In the U.S. market, a standard all-electric car can travel approximately 250 miles on a single charge, which is about 60% of the range of a gasoline vehicle. The discoveries from this new study could help tackle these issues, according to Toney.
“Consumers desire vehicles with extended driving ranges. Some of the lower cobalt batteries could offer enhanced ranges, but we must also ensure they maintain performance over time,” he noted, emphasizing that decreasing cobalt usage could also lower costs and address concerns related to human rights and energy justice.
With a clearer understanding of the self-discharge process, engineers can investigate various strategies to mitigate it, such as applying a protective coating on the cathode to inhibit hydrogen molecules from entering or experimenting with different electrolytes.
“Now that we have identified the factors causing battery degradation, we can guide the battery chemistry community on necessary improvements for battery design,” Toney concluded.
