In a breakthrough that’s expected to lead to better batteries and driving ranges for electric vehicles (EVs), researchers used X-rays to discover why rechargeable batteries degrade over time and suggest ways to improve their performance.

“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 batteries,” Michael Toney, a professor in the Department of Chemical and Biological Engineering at the University of Colorado Boulder, told The Independent.

Toney’s research team, which included partnerships with Argonne National Laboratory and other universities to conduct the study, published its findings in an article titled “Solvent-mediated oxide hydrogenation in layered cathodes” in the academic journal Science.

Lithium-ion batteries are currently a crucial component of the electric vehicles that climate advocates say are key to reducing global emissions. Though technological advancements like solid state batteries could replace them one day, lithium-ion batteries remain the dominant power source for leading EV companies like Tesla. But like other batteries we use in our day-to-day lives, lithium ion batteries wear out over time and eventually hold less charge than they did when new.

The root of the issue is batteries’ tendency to “self-discharge,” by which a battery loses charge even when not in use. Self-discharge eventually “leads to continuous degradation of battery performance,” Zonghai Chen, an Argonne senior chemist, explained in a release. Chen added that self-discharge is “experienced by all rechargeable electrochemical devices.”

But why batteries self-discharge, and what complex chemical reactions cause it to happen, has been poorly understood. So the research team used state-of-art X-ray facilities to fill knowledge gaps, and found that hydrogen molecules from a battery’s electrolyte—a liquid or gel that transfers ions through the battery to store or release a charge—were contaminating the cathode, one of the terminals to and from which electrons flow. This contamination would then interfere with the flow of lithium ions carrying charge through the battery.

Additional findings of the study clarify that the hydrogen atoms that did the contaminating were primarily sourced from solvents in the electrolyte that contain methylene, a chemical compound of carbon and hydrogen. The researchers also showed that batteries charged to a higher voltage would create more of a chemical layer on the cathode surface to further interfere with the battery’s charging and discharging, compared to when a lower voltage was considered a full charge.

The researchers say that revealing the cause behind self-discharge in batteries could lead to solutions for preventing or reducing self-discharge and cathode degradation, which would in turn improve battery longevity and performance in low-carbon technologies like EVs. Possible solutions include limiting the voltage at which a battery is considered fully charged, using solvents that do not contain methylene, and applying a protective coating to cathodes.

“By mitigating self-discharge, we can design a smaller, lighter, and cheaper battery without sacrificing end-of-life battery performance,” Chen said.