Revealing the Underlying Mechanism of Battery Self-Discharge

In a study published in Science, an international team of researchers led by an engineer from CU Boulder discovered the fundamental process responsible for battery degeneration.

Batteries lose capacity over time, which is why older cell phones tend to run out of charge more quickly. Despite its prevalence, this phenomenon is not fully understood.

The study could help scientists develop improved batteries, enabling electric cars to travel farther and last longer, while also advancing energy storage technologies that are crucial for accelerating the transition to renewable energy.

We are helping to advance lithium-ion batteries by figuring out the molecular-level processes involved in their degradation. Having a better battery is very important in shifting our energy infrastructure away from fossil fuels to more renewable energy sources.

Michael Toney, Study Co-Corresponding Author and Professor, Department of Chemical and Biological Engineering, University of Colorado

Engineers have spent years developing cobalt-free lithium-ion batteries, the most common type of rechargeable battery. Cobalt is a rare and expensive material, and its mining is linked to severe environmental and human rights issues, particularly in the Democratic Republic of the Congo, where over half of the world's cobalt is sourced, and many miners are children.

Scientists have attempted to replace cobalt in lithium-ion batteries with metals like nickel and magnesium. However, these alternatives tend to have higher rates of self-discharge—where the battery's internal chemical reactions deplete stored energy, causing capacity to decline over time. Due to self-discharge, most electric vehicle (EV) batteries need replacement within seven to ten years.

Toney, a fellow of the Renewable and Sustainable Energy Institute, and his colleagues set out to investigate the cause of self-discharge. In a typical lithium-ion battery, lithium ions transport charges between the anode and cathode through an electrolyte, generating electric current to power devices. Recharging reverses the flow, returning the ions to the anode.

Previously, scientists believed self-discharge occurred because not all lithium ions returned to the anode after charging, reducing the ions available to generate current.

Using the Advanced Photon Source, a powerful X-ray facility at the U.S. Department of Energy’s Argonne National Laboratory, the research team discovered that hydrogen molecules from the electrolyte migrate to the cathode, occupying the spaces where lithium ions usually bond. This reduces the available bonding sites for lithium ions, weakening the electric current and diminishing battery capacity over time.

We discovered that the more lithium you pull out of the cathode during charging, the more hydrogen atoms accumulate on the surface. This process induces self-discharge and causes mechanical stress that can cause cracks in the cathode and accelerate degradation.

Gang Wan, Study First Author, Stanford University

Transportation is the largest source of greenhouse gases in the United States, accounting for 28 % of total emissions in 2021. To reduce pollution, many manufacturers have committed to transitioning from gasoline vehicles to EVs.

However, EV manufacturers face several challenges, including limited driving range, higher production costs, and shorter battery life compared to conventional cars. In the U.S., a typical all-EV has a range of around 250 miles per charge, which is nearly 60 % longer than a fuel-powered vehicle. Toney believes the new study could help address these issues.

Highlighting that reducing cobalt usage can lower costs and address energy justice and human rights concerns, Toney said, “All consumers want cars with a large driving range. Some of these low cobalt-containing batteries can potentially provide a higher driving range, but we also need to make sure they do not fall apart in a short period of time.”

With a clearer understanding of the self-discharge mechanism, engineers can explore solutions such as using alternative electrolytes or coating the cathode with a material that blocks hydrogen molecules.

Toney concluded, “Now that we understand what is causing batteries to degrade, we can inform the battery chemistry community on what needs to be improved when designing in batteries.”

Co-authors of the study include Oleg Borodin, Travis Pollard, and Marshall Schroeder from DEVCOM Army Research Laboratory; Chia-Chin Chen from National Taiwan University; Zihua Zhu and Yingge Du from Pacific Northwest National Laboratory; and Ye Zhang from the University of Houston.

Journal Reference:

Wan, G., et al. (2024) Solvent-mediated oxide hydrogenation in layered cathodes. Science. doi.org/10.1126/science.adg4687.