Nanoengineers at the University of California, San Diego have developed a four-point plan to speed the commercialisation of solid-state batteries.
The researchers focused on inorganic solid electrolytes such as ceramic oxides or sulfide glasses.
The path to commercialisation of solid-state batteries will involve: creating stable solid electrolyte chemical interfaces; new tools for in operando diagnosis and characterisation; scalable and cost-effective manufacturability; and batteries designed for recyclability.
“It’s critical that we step back and think about how to address these challenges simultaneously because they are all interrelated,” said Shirley Meng, a nanoengineering professor at the UC San Diego Jacobs School of Engineering.
“If we are going to make good on the promise of all-solid-state batteries, we must find solutions that address all these challenges at the same time.”
The findings were published this month in Nature Nanotechnology. The article summarised the team’s work over the past three years.
Previous solid-state electrolytes exhibited conductivity values too low for practical applications, however today’s technologies are showing ionic conductivities exceeding those of conventional liquid electrolytes, ie greater than 10mS cm-1.
However, most highly conductive solid electrolytes are electrochemically unstable and present concerns when applied against electrode materials used in batteries.
Meng said: “At this point, we should shift our focus away from chasing higher ionic conductivity. Instead, we should focus on stability between solid-state electrolytes and electrodes.”
The team used cryogenic methods to keep battery materials cool to mitigate their decomposition under the electron microscope probe— solid electrolytes and lithium metal can be sensitive to electron beam damage.
Another tool used to characterise solid electrolyte interfaces was X-ray tomography that can allow observation of lithium dendrites buried within the solid electrolyte without opening or disrupting the battery.
Previously, battery characterisation typically relied on using probes such as X-rays, or electron or optical microscopy, useful in commercial lithium-ion batteries where the electrolytes are transparent.
“We have a much easier time observing today’s lithium-ion batteries. But in all-solid-state batteries, everything is solid or buried. If you try the same techniques for all-solid-state batteries, it’s like trying to see through a brick wall,” said Darren H. S. Tan, a nanoengineering Ph.D. candidate at the UC San Diego Jacobs School of Engineering.
The researchers developed flexible and stable solid electrolytes that could withstand scalable manufacturing processes by merging multiple fields of expertise to combine ceramics used in traditional material sciences with polymers used in organic chemistry.
Previous promising materials have either been expensive or difficult to scale up, for example, many become highly brittle when made thin enough for roll-to-roll manufacturing, which demands thicknesses of under 30 micrometers.
Batteries also need to be designed with their full life-cycle in mind and second-life applications when they drop below 80% capacity.
“Cost-effective reusability and recyclability must be baked into the future advances that are needed to develop all-solid-state batteries that provide high energy densities of 500 watt-hours per kg or better,” said UC San Diego nanoengineering professor Zheng Chen. “It’s critical that we don’t make the same recyclability mistakes that were made with lithium-ion batteries.”