Researchers have increased the battery life of a promising electric vehicle to a record level, an important step toward the goal of lighter, less expensive, long-lasting batteries for electric vehicles in the future. The work was reported on June 28 in the magazine nature energy.
Such batteries – a goal of research groups around the world – are seen as an important part of the solution to limiting the effects of climate change, and scientists are exploring a dizzying array of options.
One solution on the horizon is a lithium-metal battery for electric vehicles. These batteries contain nearly twice the energy of their widely used lithium-ion counterparts, and are lighter in weight. This combination offers the attractive prospect of an electric vehicle that is lighter and goes further on a single charge. But lithium-metal batteries in the lab were plagued by premature death, lasting only a fraction of the time of current lithium-ion batteries.
Now, a team of scientists at the US Department of Energy’s Pacific Northwest National Laboratory has created a lithium-metal battery that lasts 600 cycles, much longer than other reported results. This means that it can be fully charged and discharged 600 times before it dies.
It’s a big step forward for a promising technology, but lithium metal technology isn’t yet ready for prime time. While the lithium-ion batteries used in electric cars today contain less energy, they also last much longer, usually at least 1,000 cycles. But cars won’t go as far on a single charge as they do with an efficient lithium-metal battery.
The new research was conducted by the Department of Energy’s Battery500 Consortium Innovation Center, a multi-institutional effort led by PNNL to develop electric vehicle batteries that are lighter, more energy-intensive, and less expensive than those currently in use. PNNL leads the consortium and is responsible for incorporating the latest developments from partner organizations into devices known as high-power bag cells and for demonstrating improved performance under real-world conditions.
Lithium Metal: Thin slices of lithium translate to longer life
The PNNL team found a way to increase battery life with a surprising approach. Instead of using anodes that contain more lithium, the team used incredibly thin strips of lithium, only 20 microns wide, which are much thinner than the width of a human hair.
“Many people think that thicker lithium will enable the battery to run longer,” said Ji Xiao, who along with Jun Liu, director of Battery500 Consortium, co-author of the paper. “But that’s not always true. There is an optimized thickness of each lithium metal battery depending on the cell’s power and design.”
The lithium-metal battery created by the Battery500 team has an energy density of 350 watt-hours per kilogram (watt/kg) – very high but not unprecedented. The value of the new results is related to battery life. After 600 cycles, the battery has retained 76 percent of its initial capacity.
Just four years ago, an experimental lithium-metal battery could operate for 50 cycles. It has increased rapidly; Two years ago, the PNNL team achieved 200 cycles — and now 600. Furthermore, the PNNL battery is a pod cell, which more closely reflects real-world conditions than a coin cell, and is a less realistic type of device used in many battery research projects. .
Lithium metal: Why is thickness so important?
The team’s decision to experiment with thin film lithium chips was based on their detailed understanding of the molecular dynamics of the anode as described in nature energy paper.
Scientists have found that thicker chips directly contribute to battery failure. This is due to the complex interactions around a film on the anode known as the solid electrolyte interphase, or SEI. The SEI is a byproduct of side reactions between lithium and electrolytes. It acts as an important gatekeeper allowing certain molecules to move from the anode to the electrolyte and back again while keeping other molecules in place.
It’s an important job. SEI action effectively allows the passage of certain lithium ions but limits unwanted chemical reactions that reduce battery performance and accelerate cell failure. The researchers’ primary goal was to reduce unwanted side reactions between the electrolyte and lithium metal – to encourage biochemical reactions while suppressing unwanted ones.
The team found that thin lithium chips are adept at creating what one might call good SEI, while thicker chips have a greater chance of contributing to what one might call harmful SEI. In their paper, the researchers used the terms “wet SEI” and “dry SEI.” The wet version retains contact between the liquid electrolyte and the anode, making important electrochemical reactions possible.
But in the dry version, the liquid electrolyte does not fully reach the lithium. Simply put, because the lithium strips are thicker, the electrolyte needs to flow into the deep pockets of the lithium, and when that happens, it leaves other parts of the lithium dry. This stops important reactions from occurring, effectively suffocating the necessary electrochemical reactions, and directly contributes to the premature death of the battery.
It is an important problem, especially in real-world batteries such as bag cells, where the amount of electrolyte available is 20 to 30 times less than that used in pilot coin cells.
Consider how a layer of grease will gradually build up in the pan if it is not cleaned thoroughly after each use. Over time, the layer builds up and acts as a barrier, reducing energy flow and making the surface less effective. In the same way, the unwanted dry layer of SEI prevents the efficient transfer of necessary energy within the battery.
Long-lasting, stable, solid-state lithium battery
Chaojiang Niu et al, Balancing interfacial reactions to achieve a long cycle life in high-energy lithium-metal batteries, nature energy (2021). DOI: 10.1038 / s41560-021-00852-3
Submitted by Pacific Northwest National Laboratory
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