Ellen Phiddian
Lithium-metal batteries are a promising alternative to lithium-ion batteries. They could be even more energy-dense than lithium-ion, but for now they degrade very quickly, leading to short lifecycles.
A team of US researchers has found a simple way to make lithium-metal batteries last longer: discharge them completely, then give them a short rest.
“We were looking for the easiest, cheapest, and fastest way to improve lithium-metal cycling life,” says Wenbo Zhang, a PhD student in materials science and engineering at Stanford University, US, and co-lead author on a new paper in Nature.
“We discovered that by resting the battery in the discharged state, lost capacity can be recovered and cycle life increased.
“These improvements can be realised just by reprogramming the battery management software, with no additional cost or changes needed for equipment, materials, or production flow.”
Lithium-metal batteries use metallic lithium in their anodes (the part of the battery that supplies electrons) – making them different to lithium-ion batteries, which use lithium-based compounds in battery cathodes (the part of the battery that accepts electrons). By weight, they can hold much more power than lithium-ion batteries.
“A car equipped with a lithium metal battery would have twice the range of a lithium-ion vehicle of equal size – 600 miles per charge versus 300 miles, for example,” says co-lead author Philaphon Sayavong, a PhD student in chemistry at Stanford.
Current commercial lithium-metal batteries are mostly single-use only. This is because the lithium metal and other compounds inside the battery react with one another and degrade very quickly when the battery discharges.
Specifically, tiny bits of lithium get stuck in the electrolyte – the substance which charged particles move through to make electricity flow. The researchers call this a solid electrolyte interphase, or SEI, matrix.
“The SEI matrix is essentially decomposed electrolyte,” says 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.”
Researchers have proposed complicated chemical recipes and detailed electrical techniques to stop this degradation.
But previous research from Savayong and colleagues had found that the SEI matrix starts dissolving on its own when the battery is inactive. The researchers decided to investigate this further.
“The first step was to completely discharge the battery so there is zero current running through it,” says Zhang.
“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.”
Then, they let the discharged battery sit.
“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,” says Sayavong.
“So when you recharge the battery, the dead lithium will reconnect with the anode, because there’s less solid mass getting in the way.”
This increased the battery’s depleted capacity. The researchers used video microscopy to confirm what was going on.
“Previously, we thought that this energy loss was irreversible,” says senior author Yi Cui, a professor of materials science at Stanford.
“But our study showed that we can recover lost capacity simply by resting the discharged battery.”
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