Stanford researchers identify low-cost method to extend the life of lithium-metal batteries and increase range of EVs by up to 600 miles.

Electric vehicles may soon run on lithium-metal (Li-metal) batteries with ranges that could double the capacity of batteries currently used by EV manufacturers, according to a recent study completed by Stanford University researchers.

The scientists published an article about their discovery, "Recovery of isolated lithium through discharged state calendar aging," Feb. 7 in the journal "Nature." They found that a new generation of lithium-metal batteries could enable electric cars and trucks to travel up to 600 miles on a single charge.

"Rechargeable Li-metal batteries have the potential to more than double the specific energy of the state-of-the-art rechargeable (lithium-ion) batteries, making Li-metal batteries a prime candidate for next-generation high-energy battery technology," the researchers wrote.

And unlike lithium-ion batteries, which rapidly lose their ability to store energy after relatively few cycles of charging and discharging, Li-metal battery technology could enable EVs to operate for years.

After testing a variety of new materials and techniques to improve EV batteries' cycle life, the researchers said they have identified a potential low-cost solution – drain a Li-metal battery and let it rest for several hours.

The approach, described in the Stanford study, restored EV battery capacity and boosted overall performance.

"We were looking for the easiest, cheapest, and fastest way to improve Li-metal cycling life," Wenbo Zhang, the study's co-lead author, told a reporter recently.

Zhang, a Stanford Ph. D. student in materials science and engineering, said that by resting the battery in the discharged state, its lost capacity could be recovered and cycle life increased.

"These improvements can be realized just by reprogramming the battery management software, with no additional cost or changes needed for equipment, materials, or production flow," Zhang said.

Guide for future research

Yi Cui, the Fortinet Founders Professor of Materials Science and Engineering at Stanford's School of Engineering and the study's senior author, said the results could provide EV manufacturers with practical insights on adapting Li-metal technology to real-world driving conditions.

Cui is also professor of energy science and engineering at the Stanford Doerr School of Sustainability.

"Li-metal batteries have been the subject of a lot of research," said Cui. "Our findings can help guide future studies that will aid in the advancement of Li-metal batteries towards widespread commercial adaptation."

A Li-metal battery differs from a conventional lithium-ion battery, which consists of a graphite anode and a metal cathode – separated by a liquid or solid electrolyte that shuttles lithium ions back and forth.

In a Li-metal battery, the graphite anode is replaced with electroplated Li-metal, which enables the battery to store twice the energy of a lithium-ion battery in the same amount of space. The Li-metal anode also weighs less than the graphite anode, which is important for EVs. Li-metal batteries can hold at least a third more energy per pound than lithium-ion units.

"A car equipped with a Li-metal battery would have twice the range of a lithium-ion battery powered vehicle of equal size – 600 miles per charge versus 300 miles, for example," observed co-lead author Philaphon Sayavong, a Ph. D. student in chemistry.

"In EVs, the goal is to keep the battery as lightweight as possible while extending the vehicle range," he explained.

Doubling the traveling distance on a single charge could eliminate range anxiety for drivers who are reluctant to purchase EVs. Unfortunately, continuous charging and discharging causes Li-metal batteries to degrade quickly, rendering them useless for routine driving, the scientists said.

When the battery is discharged, micron-sized bits of Li-metal become isolated and get trapped in the solid electrolyte interphase (SEI), a spongy matrix that forms where the anode and electrolyte meet.

"The SEI matrix is essentially decomposed electrolyte," Zhang explained. "It surrounds isolated pieces of Li-metal stripped from the anode and prevents them from participating in any electrochemical reactions. For that reason, we consider isolated lithium dead."

Repeated charging and discharging results in the build-up of additional dead lithium, causing the battery to rapidly lose capacity.

"An EV with a state-of-the-art Li-metal battery would lose range at a much faster rate than an EV powered by a lithium-ion battery," Zhang said.

Discharge and rest

In previous work, Sayavong and his colleagues discovered that the SEI matrix begins to dissolve when the battery is idle. Based on that finding, the Stanford team focused on any chemical reaction that might occur if the battery was allowed to rest while discharged.

The first step was to completely discharge the battery so there was zero current running through it.

"Discharging strips all the metallic lithium from the anode, so all you're left with are inactive pieces of isolated lithium surrounded by the SEI matrix," Zhang said.

The next step was to let the battery sit idle.

"We found that if the battery rests in the discharged state for just one hour, some of the SEI matrix surrounding the dead lithium dissolves away," Sayavong said. "So, when you recharge the battery, the dead lithium will reconnect with the anode, because there's less solid mass getting in the way."

Reconnecting with the anode brings dead lithium back to life, enabling the battery to generate more energy and extend its cycle life.

"Previously, we thought that this energy loss was irreversible," Cui said. "But our study showed that we can recover lost capacity simply by resting the discharged battery."

Using time-lapse video microscopy, the researchers visually confirmed the disintegration of residual SEI and subsequent recovery of dead lithium during the resting phase.

Practical applications

The average American driver spends about one hour behind the wheel every day, so the idea of resting a car's battery for several hours is feasible.

A typical EV may have 4,000 batteries arranged in modules controlled by a battery management system, an electronic brain that monitors and controls battery performance. In a Li-metal battery, the existing management system can be programmed to discharge an individual module completely so that it has zero capacity left.

This approach would not require expensive, new manufacturing techniques or materials.

Wenbo Zhang

"You can implement our protocol as fast as it takes you to write the battery management system code," Zhang said. "We believe that in certain types of Li-metal batteries, discharged-state resting alone can increase EV cycle life significantly."

Yi Cui is also a professor of photon science at SLAC National Accelerator Laboratory, director of the Sustainability Accelerator in the Stanford Doerr School of Sustainability, and co-director of the StorageX Initiative in the Stanford Precourt Institute for Energy.

Other Stanford co-authors of the study are Professor Stacey F. Bent and graduate students Xin Xiao, Solomon T. Oyakhire, Sanzeeda Baig Shuchi, Rafael A. Vilá, David T. Boyle, Sang Cheol Kim, Mun Sek Kim, Sarah E. Holmes, Yusheng Ye, and Donglin Li.

Funding was provided by the U.S. Department of Energy Battery Materials Research Program, Battery500 Consortium, the National Academy of Sciences Ford Foundation Fellowships, the National Science Foundation Graduate Research Fellowship Program, and the Enhancing Diversity in Graduate Education and Knight-Hennessy Scholars programs at Stanford.