Engineering news
It’s long been thought that the main reasons why the performance of lithium-ion batteries degrades over time is because of the growth of a layer of material called the solid electrolyte interphase between the lithium anode and the electrolyte.
But a research team led by the University of California San Diego believes they’ve found an alternative answer, and that failures are caused by lithium metal deposits breaking off the anode during discharging and becoming trapped in such a way that the battery can no longer access them.
The researchers developed a technique to measure the amounts of inactive lithium on the anode – a first in the field of battery research – and study their micro- and nano-structures. "By figuring out the major underlying cause of lithium metal battery failure, we can rationally come up with new strategies to solve the problem," says first author Chengcheng Fang, a materials science and engineering PhD student at UC San Diego. "Our ultimate goal is to enable a commercially viable lithium metal battery."
Lithium metal batteries have long been the holy grail of battery research, promising twice the capacity of today’s lithium-ion batteries. But because lithium metal is so reactive, attempts to create a battery based on it have faltered because they can only undergo a limited number of charging cycles before they stop working.
This has usually been blamed on the SEI layer, but although many ways to control and stabilise this layer have been developed, they have not fully resolved the problem. "The cells still fail because a lot of inactive lithium is forming in these batteries. So there is another important aspect that is being overlooked," says Y. Shirley Meng, a senior author of the research, which was published in
Nature, and a nanoengineering professor at UC San Diego.
Instead, the culprits are lithium metal deposits that break off of the anode when the battery is discharging and then get trapped in the SEI layer. There, they lose their electrical connection to the anode, becoming inactive lithium that can no longer be cycled through the battery. This trapped lithium is largely responsible for lowering the Coulombic efficiency of the cell.
To measure how much unreacted lithium was being trapped, researchers added water to a sealed flask containing a sample of inactive lithium formed on a cycled battery. Any unreacted lithium reacted with the water in a fashion familiar to GCSE science students, producing hydrogen gas, which could be measured to calculate the amount of trapped lithium.
The researchers hope their method could become the new standard for evaluating efficiency in lithium metal batteries. "One of the problems battery researchers face is that testing conditions are very different across labs, so it's hard to compare data. It's like comparing apples to oranges. Our method can enable researchers to determine how much inactive lithium forms after electrochemical testing, regardless of what type of electrolyte or cell format they use," Meng says.
By investigating the structure of trapped lithium at the nanoscale, the researchers hope to unlock greater performance from lithium metal batteries. "Control of the micro- and nanostructure is key," Meng says. "We hope our insights will stimulate new research directions to bring rechargeable lithium metal batteries to the next level."
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