Stanford’s breakthrough in lithium-metal battery technology promises to extend electric vehicle range and battery life through a simple rest protocol, increasing commercial viability.
Next-generation electric vehicles could run on lithium-metal batteries that can travel 800 to 1,100 km on a single charge, which is twice the range of conventional lithium-ion batteries in today’s electric vehicles.
But lithium metal technology has serious drawbacks: the battery quickly loses its ability to store energy after relatively few cycles of charge and discharge – highly impractical for drivers who expect rechargeable electric cars to last for years.
Scientists have tested a variety of new materials and techniques to improve battery life. Now researchers at Stanford University have discovered a cheap solution: simply drain the battery and let it rest for several hours. This simple approach is described in a study published today (Feb. 7) in the journal Naturerestored battery capacity and improved overall performance.
“We were looking for the easiest, cheapest, and fastest way to improve the lifespan of lithium metal,” said co-lead author Wenbo Zhang, a PhD student at Stanford in materials science and engineering. “We found that allowing the battery to rest in a discharged state can restore lost capacity and extend its lifespan. These improvements can be achieved by simply reprogramming the battery management software, without additional costs or changes to equipment, materials or production flow.”
The study’s results could provide EV manufacturers with practical insights into adapting lithium metal technology to real-world driving conditions, says senior author Yi Cui, the founder of Fortinet, professor of Materials Science and Engineering in the School of Engineering, and professor of energy and technology. at the Stanford Doerr School of Sustainability.
“Lithium metal batteries have been the subject of a lot of research,” says Cui. “Our findings can help guide future studies that will aid the advancement of lithium-metal batteries toward widespread commercial adoption.”
Lithium metal versus lithium-ion technology
A conventional lithium-ion battery consists of two electrodes – a graphite anode and a lithium metal oxide cathode – separated by a liquid or solid electrolyte that transports lithium ions back and forth.
In a lithium-metal battery, the graphite anode is replaced with electroplated lithium metal, allowing it to store twice as much energy as a lithium-ion battery in the same amount of space. The lithium metal anode also weighs less than the graphite anode, which is important for EVs. Lithium metal batteries can hold at least one-third more energy per pound than lithium-ion batteries.
“A car equipped with a lithium-metal battery would have twice the range of an equivalent-sized lithium-ion vehicle – 600 miles per charge versus 300 miles, for example,” says co-lead author Philaphon Sayavong, a PhD candidate in chemistry. “With electric vehicles, the goal is to keep the battery as light as possible while increasing the range of the vehicle.”
Doubling the range could alleviate range concerns among drivers who are reluctant to purchase electric vehicles. Unfortunately, constant charging and discharging causes lithium metal batteries to deteriorate quickly, rendering them useless for routine driving. When the battery is discharged, micron-sized pieces of lithium metal become isolated and trapped in the solid electrolyte interphase (SEI), a spongy matrix that forms where the anode and electrolyte meet.
“The SEI matrix is essentially made up of decomposed electrolyte,” explains Zhang. “It surrounds isolated pieces of lithium metal that have been stripped from the anode and prevents them from participating in electrochemical reactions. For that reason, we consider isolated lithium to be dead.”
Repeated charging and discharging results in the build-up of extra dead lithium, causing the battery to quickly lose capacity. “An EV with a state-of-the-art lithium-metal battery would lose range much faster than an EV powered by a lithium-ion battery,” said Zhang.
Discharge and rest
In previous work, Sayavong and his colleagues found that the SEI matrix begins to dissolve when the battery is idle. Based on that finding, the Stanford team decided to see what would happen if the battery was allowed to rest while it was discharged.
“The first step was to completely discharge the battery so that no more current flows through it,” said Zhang. “Discharging removes all the metallic lithium from the anode, leaving you with just inactive bits of isolated lithium surrounded by the SEI matrix.”
The next step was to leave the battery stationary.
“We found that if the battery is left in a discharged state for just one hour, some of the SEI matrix around the dead lithium dissolves,” says Sayavong. “So when you charge the battery, the dead lithium will reconnect to the anode because there is less solid mass in the way.”
Reconnecting to the anode brings dead lithium back to life, allowing the battery to generate more energy and extend its life.
“We previously thought this energy loss was irreversible,” Cui said. “But our research shows that we can restore lost capacity by simply letting the discharged battery rest.”
Using time-lapse video microscopy, the researchers visually confirmed the disintegration of residual SEI and the subsequent recovery of dead lithium during the rest phase.
The average American driver spends about an hour behind the wheel every day, so the idea of letting your car’s battery rest for a few hours is feasible.
A typical EV may have 4,000 batteries, arranged into modules controlled by a battery management system, an electronic brain that monitors and controls battery performance. With a lithium metal battery, the existing management system can be programmed to completely discharge an individual module so that it no longer has any capacity.
This approach does not require expensive new production techniques or materials, Zhang added.
“You can implement our protocol as quickly as it takes to write the code for the battery management system,” he said. “We believe that for certain types of lithium metal batteries, simply resting in a discharged state can significantly extend the life of the EV cycle.”
Reference: “Recovery of isolated lithium from aging by discharged state calendar” by Wenbo Zhang, Philaphon Sayavong, 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, Donglin Li, Stacey F. Bent and Yi Cui, February 7, 2024, Nature.
Yi Cui is also a professor