Lithium ions move through a common battery material in unusual ways that defy accepted wisdom, according to a new study (Nat. Mater. 2018, DOI: 10.1038/s41563-018-0168-4). Using powerful X-rays to observe the motion of lithium ions at nanometer scales, a research team discovered that the ions flow in multiple directions in lithium iron phosphate, leading to a mechanism that causes the battery material to degrade over time. The findings could lead to longer-lasting, faster-charging batteries.
Lithium iron phosphate is the cathode material of choice for rechargeable batteries used in power tools, remote-control model cars, and solar-powered lights. Batteries made of the material hold less energy per kilogram than those with cobalt-based cathodes, which are used in electric vehicles and portable electronics. But lithium iron phosphate is cheaper and safer than cobalt materials, and also lasts twice as long, for around 1,000 charge cycles.
This lifetime could be even longer, potentially tens of thousands of charging cycles, says William Chueh, a materials scientist and engineer at Stanford University. But during battery discharge, when lithium ions move from the electrolyte into a lithium iron phosphate cathode, the ions tend to collect in certain hotspots in the material, creating lithium-rich and lithium-poor regions. This causes the lithium iron phosphate crystal to break apart over time. The battery also charges slowly because the ions do not move through the electrode uniformly and freely.
Not much is known about exactly how ions move in lithium iron phosphate. So Chueh and colleagues, including chemical engineer Martin Z. Bazant of Massachusetts Institute of Technology and chemist M. Saiful Islam of the University of Bath, conducted X-ray experiments to find out where the ions go. The team bombarded plate-like particles of lithium iron phosphate with high-energy X-rays from a synchrotron light source at SLAC National Accelerator Laboratory. The resulting diffraction patterns revealed the material’s atomic and electronic structure, and scanning transmission X-ray microscopy showed how ions move through the structure at the nanoscale.
Until now, battery researchers thought that the ions quickly diffuse into the lithium iron phosphate material in a direction that is perpendicular to the interface between the electrode and the electrolyte. That hypothesis is based on the fact that tunnels in the material’s crystal structure run in that direction, Chueh says.
Turns out that is not the case. When the researchers analyzed their X-ray data, they found that organic solvent and water molecules in the electrolyte toss lithium ions around on the electrode surface. As a result, when the ions move from electrolyte to electrode, they first move along the surface of the electrode and then tunnel into the material when they reach the lithium hotspot regions.
“Now that we’ve discovered this pathway, we’re trying to find ways to shut it down,” Chueh says. This could involve coating the electrode surface or adding certain chemical species to the electrolyte that prevent the ions from moving along the electrode surface, forcing them to flow perpendicularly into the material as researchers had previously thought.
“Lithium iron phosphate already has a good lifetime, but our finding could further extend it,” Chueh says. To demonstrate the value of reaching tens of thousands of recharge cycles, he points to what batteries would have to do to store energy collected by photovoltaic cells. “One would need to run it for about 300 days for 20 to 30 years, which translates to 10,000 recharge cycles.”
Alexej Jerschow, a chemist who does battery diagnostics research at New York University says that the X-ray techniques allowed the researchers to probe the material under conditions found in working batteries. “I think this work shines light on the behavior of not just lithium iron phosphate, but also other related materials, and would thus have a lot of impact beyond batteries as well,” he says.