Thinner Lithium Electrolytes from Lower Temperature Process might Improve Solid-State Batteries

July 12, 2019 by Scott McMahan

MIT Researchers have devised a new pulsed laser deposition technique to produce thinner lithium electrolytes using less heat.

According to the researchers, the use of thinner lithium electrolytes could allow faster charging and might lead to higher-voltage solid-state lithium-ion batteries.

Key to the new processing technique for the solid-state battery electrolyte is alternating layers of the active electrolyte lithium garnet component (Li6.25Al0.25La3Zr2O12, known as LLZO) with layers of lithium nitride (Li3N).

First, a pulsed laser deposition process builds up the layers at about 300°C (572°F). Then, in an annealing process, they are heated to 660C and slowly cooled.

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During the annealing process, almost all of the nitrogen atoms burn off into the atmosphere. The lithium atoms from the original nitride layers combine to make lithium garnet, forming a single lithium-rich, ceramic thin film.

The garnet film's extra lithium content lets the material maintain the cubic structure necessary for the quick movement of positively charged lithium ions (cations) through the electrolyte.

The findings were reported recently in a Nature Energy paper published online by MIT Associate Professor Jennifer L. M. Rupp and her students Reto Pfenninger, Michal M. Struzik, Inigo Garbayo, and collaborator Evelyn Stilp.

"The really cool new thing is that we found a way to bring the lithium into the film at deposition by using lithium nitride as an internal lithiation source," Rupp, the work's senior author, said. Rupp holds joint MIT appointments in the departments of Materials Science and Engineering and Electrical Engineering and Computer Science.

"The second trick to the story is that we use lithium nitride, which is close in bandgap to the laser that we use in the deposition, whereby we have a very fast transfer of the material, which is another key factor to not lose lithium to evaporation during a pulsed laser deposition," Rupp explained.

Safer technology

Lithium batteries with commonly used electrolytes that combine a polymer and a liquid pose a fire risk when the liquid is exposed to air.

Solid-state batteries can replace the often-used liquid polymer electrolytes of consumer lithium-ion batteries with a safer solid material.

"So we can kick that out, bring something safer in the battery, and decrease the electrolyte component in size by a factor of 100 by going from the polymer to the ceramic system," Rupp explained.

Although other methods using heating in a sintering process produce lithium-rich ceramic materials on larger pellets or tapes can yield a dense microstructure that preserves a high lithium concentration, they require more heat and result in bulkier material.

The new technique that Rupp and her students developed produces a thin film that is approximately 330nm thick (less than 1.5 hundred-thousandths of an inch).

"Having a thin film structure instead of a thick ceramic is attractive for battery electrolyte in general because it allows you to have more volume in the electrodes, where you want to have the active storage capacity. So the holy grail is thin and being fast," she said.

The lithium (garnet) oxide thin films processed using Rupp's techniques show nanometer-scale grain structures. This smaller grain size lets Rupp fabricate thinner electrolytes for batteries.

Faster ionic conduction

The faster the conductivity of an electrolyte the better a solid-state battery performs. The unit of measurement for lithium-ion conductivity is given in Siemens.

The new multilayer deposition method makes a lithium garnet (LLZO) material that the researchers say shows the fastest ionic conductivity yet for a lithium-based electrolyte compound of about 2.9 x 10-5 Siemens (0.000029 Siemens) per centimeter. This ionic conductivity is competitive with solid-state lithium battery thin-film electrolytes based on LIPON (lithium phosphorus oxynitride).

"Having the lithium electrolyte as a solid-state very fast conductor allows you to dream out loud of anything else you can do with fast lithium motion," Rupp said.

Aluminum Added

A small fraction of aluminum was added to the lithium garnet formulation because aluminum is known to stabilize the highly conductive cubic phase of this high-temperature ceramic.

In addition to using Raman spectroscopy analysis, they used another technique, known as negative-ion time-of-flight secondary ion mass spectrometry (TOF-SIMS). This technique revealed that the aluminum maintains its position at what were originally the interfaces between the lithium nitride and lithium garnet layers before the heating expelled the fused material and the nitrogen.

"When you look at large-scale processing of pellets by sintering, then everywhere where you have a grain boundary, you will find close to it a higher concentration of aluminum. So we see a replica of that in our new processing, but on a smaller scale at the original interfaces," Rupp says. "These little things are what adds up, also, not only to my excitement in engineering but my excitement as a scientist to understand phase formations, where that goes and what that does," Rupp said.

Oxford Professor Bruce noted the novelty of the approach commenting, "I'm not aware of similar nanostructured approaches to reduce diffusion lengths in solid-state synthesis."

"Although the paper describes specific application of the approach to the formation of lithium-rich and therefore highly conducting garnet solid electrolytes, the methodology has more general applicability, and therefore significant potential beyond the specific examples provided in the paper," Bruce added.

After demonstrating the unique processing and high conductivity of the lithium garnet electrode, the next step will be to assess the material in an actual battery to investigate how the material reacts with a battery cathode and how stable it is. "There is still a lot to come," Rupp predicts.

While the work will likely first apply to batteries, Rupp predicts another decade of advances based upon applications of her processing techniques to devices for artificial intelligence, neuromorphic computing, and fast gas sensors. "The moment the lithium is in a small solid-state film, you can use the fast motion to trigger other electrochemistry," she said.

Several companies have already shown an interest in using the new electrolyte approach.

Reference Materials

Pfenninger, R., Struzik, M., Garbayo, I., Stilp, E., Rupp, J. L.M. A low ride on processing temperature for fast lithium conduction in garnet solid-state battery films. Nature Energy, 4, pages 475-483 (2019).