Even though lithium metal batteries could double the range of electric vehicles (EVs), their quick degradation during operation poses a massive challenge. However, researchers at Stanford have discovered that you can improve the battery’s life cycle by simply letting it rest in the discharged state.
The implications of being able to use lithium metal batteries instead of lithium-ion batteries in EVs are profound. Future EVs could travel twice as far on a single charge, completely eliminating range anxiety.
“A car equipped with a lithium metal battery would have twice the range of a lithium-ion vehicle of equal size – 600 miles (965km) per charge versus 300 miles (482km),” said PhD student in chemistry Philaphon Sayavong.
It sounds great, right? Why aren’t they in all EVs, then? Well, to understand the problem and why what researchers discovered is significant, you’ll need to understand the fundamental difference between the two battery types.
The conventional lithium-ion batteries that we’ve all begun to rely on (not just in EVs but in our everyday lives) consist of two electrodes – a graphite anode and a lithium metal oxide cathode. They are separated by a solid or liquid electrolyte that shuttles lithium ions back and forth.
In a lithium metal battery, electroplated lithium metal replaces the graphite anode. This gives it the ability to store twice as much energy as a lithium-ion battery in the same amount of space at a reduced weight.
The major problem with lithium metal batteries is that they suffer from rapid capacity loss after only a few charge-discharge cycles. Obviously, when we buy an EV, we expect that the vehicle will last quite a number of years, so this effectively renders using lithium metal batteries useless in EVs until a solution is found.
The capacity loss is caused by micron-sized bits of lithium metal that become isolated when the battery is discharged. They get trapped in the solid electrolyte interphase (SEI), a spongy matrix that forms where the anode and electrolyte meet inside the battery.
“The SEI matrix is essentially decomposed electrolyte,” explains Stanford PhD student in materials science and engineering, Wenbo Zhang. “It surrounds isolated pieces of lithium metal stripped from the anode and prevents them from participating in any electrochemical reactions. For that reason, we consider isolated lithium dead.”
This means that every time you charge and run down the battery, more dead lithium builds up, causing the battery to lose capacity quite rapidly. “An EV with a state-of-the-art lithium metal battery would lose range at a much faster rate than an EV powered by a lithium-ion battery,” Zhang said.
Previous work by the Stanford University team revealed that the SEI matrix dissolves when the battery is idle, prompting them to explore the effects of what would happen if the battery was allowed to rest while discharged.
This leads us to this promising breakthrough…
Researchers discovered that allowing lithium metal batteries to rest for several hours in a discharged state did indeed enhance their life cycle, potentially revolutionising the range of next-generation EVs.
“The first step was to completely discharge the battery so there is zero current running through it,” Zhang said. “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.”
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,” said Yi Cui, the Fortinet Founders Professor of Materials Science and Engineering in the School of Engineering and Professor of Energy Science and Engineering in the Stanford Doerr School of Sustainability. “But our study showed that we can recover lost capacity simply by resting the discharged battery.”
According to Zhang, these improvements could be realised just by reprogramming the battery management software, with no additional cost or changes needed for equipment, materials, or production flow.
“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 lithium metal batteries, discharged-state resting alone can increase EV cycle life significantly.”
“Lithium metal batteries have been the subject of a lot of research. Our findings can help guide future studies that will aid in the advancement of lithium metal batteries towards widespread commercial adaptation,” Cui said.
I certainly hope so. Whether you’re for EVs or against them, there’s no denying that the technology is fascinating and advances in battery range will go a long way to eliminating range anxiety and ensuring that EVs are a more practical and appealing option.
I expect that it can be a useful solution??? My ? is if a vehicle has one battery how would that work ?, as generally we are conditioned to having our vehicles instantly available for use I expect that it could be overcome by splitting batteries or two 2 batteries I feel that we are a long way from a useful solution ,as going by mobile phones yes battery life has increased but where is the phone where the battery lasts for a week or longer I have not yet seen that , but i expect in time and in the meantime I will stick with my old green diesel which works well as a practical solution here in Australia.