While the lithium-ion battery could help save the planet, it is in some ways like any other battery: it degrades with time and operation, taking a toll on its lifespan.
Along with enabling much of our digital and mobile lifestyle, lithium-ion batteries power most electric vehicles (EVs). For that reason, extending the battery’s lifetime is critical to widespread adoption of EVs in the transition away from fossil fuel-burning cars. Scientists are working to find the causes of battery degradation with the goal of extending battery lifespan.
Specifically, scientists at the U.S. Department of Energy's (DOE) Argonne National Laboratory are collaborating with other U.S. laboratories and academic institutions to study a phenomenon called self-discharge. This is a series of chemical reactions in the battery that causes performance loss over time, shortening the battery's lifespan.
During self-discharge, the charged lithium-ion battery loses stored energy even when not in use. For example, an EV that sits for a month or more may not run due to low battery voltage and charge.
"Self-discharge is a phenomenon experienced by all rechargeable electrochemical devices," said Zonghai Chen, an Argonne senior chemist. "The process slowly consumes precious functional battery materials and deposits undesired side products on the surface of the battery components. This leads to continuous degradation of battery performance."
To find the cause of self-discharge, scientists need to identify the complex chemical mechanisms that trigger the degradation process in the battery. Lithium-ion batteries are rechargeable and use lithium ions to store energy. The cathode and the electrolyte are two key components in lithium-ion batteries. The battery's longevity can be influenced by the degradation of cathodes.
While scientists are making significant progress in understanding lithium-ion batteries, there is an ongoing debate on what causes the self-discharge phenomenon.
The prevailing wisdom on cathode degradation centers on two areas: a loss of lithium or oxygen release from cathodes. Meanwhile, theoretical studies have predicted that electrolytes tend to decompose on cathode surfaces. This has created a critical knowledge gap between the decomposition of the electrolyte and the degradation of the cathode within lithium-ion batteries.
Recently, a research team across several academic universities and national laboratories including Argonne, DOE's SLAC National Accelerator Laboratory and the DEVCOM U.S. Army Research Laboratory (ARL) published a new paper in Science bridging this knowledge gap. This research validates a cathode hydrogenation mechanism as a pathway to the self-discharge that leads to battery degradation. The research was funded by DOE's Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office.
Scientists say they could not have validated their findings without access to the Advanced Photon Source (APS) at Argonne, one of the world's premier storage-ring-based high-energy X-ray light source facilities. The APS is a DOE Office of Science user facility. The light sources use electrons circling in a storage ring at near the speed of light to produce X-ray beams that allow scientists to unveil the battery's inner workings at an atomic level.
"We are deeply grateful to the state-of-art X-ray facilities and support available at the Advanced Photon Source. It is the ideal pairing of the X-ray studies and electrochemistry that enables our discoveries on how cathode hydrogenation occurs in lithium-ion batteries and impacts self-discharge," said study lead Gang Wan, a physical science research scientist at Stanford University.
A New Pathway to Self-Discharge Leading to Battery Degradation
While the inner workings are more complicated, batteries basically convert electrochemical energy directly to electrical energy. Batteries consist of an anode, electrolyte, separator and cathode.
The electrolyte transfers ions, or charge-carrying particles, between the cathode and anode that store the lithium. Self-discharge occurs in both the cathode and anode. The cathode material is critical, since it determines how much energy the battery can store. In their new research, the team used layered lithium transition metal oxides, a prototype cathode material.
"Finding the right chemistry for these cathode materials is necessary to improve the battery's chemical stability and reduce the rate of self-discharge," said co-author Michael F. Toney, professor of chemical engineering and materials science and a fellow in the Renewable and Sustainable Energy Institute at the University of Colorado Boulder. "Degradation of the cathode reduces the battery's lifetime."
In their research, this team discovered experimental and computational proof of a mechanism that triggers self-discharge: cathode hydrogenation, or the process of dynamically transferring the protons and electrons from the electrolyte solvent into highly charged layered oxides in the cathode. The mechanism explains the chemical nature of the contamination products on the cathode that lead to battery degradation.
Along with Chen's early seminal paper investigating the decomposition mechanism of cathode materials using high-energy X-ray diffraction, this new study sheds light on the cathode hydrogenation-based degradation mechanism.
Based on their results, scientists can further develop bottom-up approaches to reduce self-discharge and cathode degradation, with the goal of lengthening battery life.
"By mitigating self-discharge, we can design a smaller, lighter and cheaper battery without sacrificing end-of-life battery performance," Chen said.
Advanced Photon Source Helps Validate Research Findings
Argonne beamline scientists Cheng-Jun Sun, Shelly Kelly and Zhan Zhang used the APS to work with Wan to design the X-ray spectroscopy and scattering experiments that validated the landmark findings.
"X-ray spectroscopy measurements allow an atomic view of the nickel, manganese and cobalt metal atoms within the cathode," Kelly said. "Using the APS, we could see the effect of the accumulation of protons at the surface of the cathode, which ultimately results in self-discharge."
The APS, which welcomes more than 5,500 scientists from around the world in a typical year, is currently undergoing a massive upgrade that will replace the current electron storage ring with a new, more powerful model. When completed later in 2024, the upgrade will increase the brightness of the APS X-ray beams by up to 500 times.
"The research team, which includes a number of longtime APS users, is excited to embrace the new and exciting opportunities brought by the APS upgrade to target the grand challenges in energy sciences, including building better batteries," Wan said.
Other senior co-authors include Oleg Borodin, a scientist at ARL, and Kang Xu, a fellow of the Materials Research Society and the Electrochemical Society and an ARL fellow emeritus who was a former team leader at ARL and is now chief scientist at SES AI.
The research team dedicated their paper to the late George Crabtree and the late Peter Faguy. Crabtree, an Argonne Senior Scientist and Distinguished Fellow, served as director of the DOE's Joint Center for Energy Storage Research from 2012 to 2023. Faguy, an electrochemist at the DOE, served as the DOE project manager on this research.