Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL) have created a lithium-metal battery that lasts for 600 cycles.
Lithium-metal batteries hold almost twice the energy, are safer and lighter than lithium-ion, but still have some way to go to match the 1,000 cycle life.
The PNNL team replaced anodes with thin strips of lithium— 20 microns wide— to produce a lithium-metal pouch battery with an energy density of 350Wh/kg.
After 600 cycles, the battery retained 76% of its initial capacity.
The work was published in the journal Nature Energy.
Jie Xiao, who along with Jun Liu, the director of the Battery500 Consortium, is a corresponding author of the paper, said: “Many people have thought that thicker lithium would enable the battery to cycle longer, but that is not always true.
“There is an optimised thickness for each lithium-metal battery depending on its cell energy and design.”
Four years ago, an experimental lithium-metal battery could operate for 50 cycles, that was increased to 200 two years ago by the PNNL team.
Making the battery last longer
The scientists found that thicker strips contribute directly to battery failure due to reactions around the solid electrolyte interphase (SEI).
The SEI allows certain lithium ions to pass through, but limits unwanted chemical reactions that reduce battery performance and accelerate cell failure.
A primary goal for researchers was to reduce unwanted side reactions between the electrolyte and the lithium metal.
The team found that thinner lithium strips are adept at creating “good SEI”, while thicker strips have a higher chance of contributing to “harmful SEI”.
In their paper, the researchers use the terms “wet SEI” and “dry SEI”— the former retaining contact between the liquid electrolyte and the anode, making important electrochemical reactions possible.
But in the dry version, the liquid electrolyte doesn’t reach all of the lithium because the thick lithium strips means the electrolyte needs to flow into deeper pockets of the lithium, and as it does so, it leaves other portions of the lithium dry.
Today’s electric vehicle batteries are aroud 200-250Wh/kg; Battery500 is aiming for a cell level of 500Wh/kg.
A report from PNNL noted the combination offers the enticing prospect of an electric vehicle that would be lighter and go much farther on a single charge.
The report noted: “It’s a big step forward for a promising technology, but lithium-metal technology is not yet ready for prime time.
“While the lithium-ion batteries used in electric vehicles today hold less energy, they last longer, typically at least 1,000 cycles. But vehicles won’t go as far on one charge as they would with an effective lithium-metal battery.”
The research was done through DOE’s Innovation Center for Battery500 Consortium, a multi-institution effort led by PNNL to develop electric vehicle batteries.
PNNL leads the consortium and is responsible for integrating the latest advances from partner institutions into devices known as high-energy pouch cells and demonstrating improved performance under realistic conditions.
BEST first reported on the Battery500 consortium in 2016 when a consortium of US companies received $50million in federal funding to develop next generation lithium-metal batteries.
The firms received up to $10 million a year over five years from the Department of Energy's Office of Energy Efficiency and Renewable Energy.
Last month, BEST reported how a team from Harvard University in the US had designed a lithium-metal solid-state battery that could be cycled at least 10,000 times.
The researchers paired a multilayer battery that sandwiches materials of varying stabilities between the anode and cathode with a commercial, high energy density cathode material.
Responding to Harvard's research, a PNNL spokesman told BEST: "The report that you cite analyses the energy of just one component of a battery— the cathode only. The report from PNNL discusses the energy of the entire battery. This is a more relevant measure of battery function.
"Another way to put it is that the cell level energy measured by the PNNL team is based on the entire pouch cell, while the measure in the other report is based on the cathode only. An interesting question would be to know the cell level energy if the Harvard team had converted its energy based on the weight of the whole cell."