Researchers at MIT have devised a new pulsed laser deposition technique to make thinner lithium electrolytes using less heat, promising faster charging and potentially higher-voltage solid-state lithium ion batteries. The method enables future solid-state battery architectures with more room for cathode volumes by design, and reduces the processing temperature. The findings are reported in a paper in Nature Energy.
Key to the new technique for processing the solid-state battery electrolyte is alternating layers of the active electrolyte lithium garnet component (chemical formula, Li6.25Al0.25La3Zr2O12, or LLZO) with layers of lithium nitride (chemical formula Li3N). First, these layers are built up using a pulsed laser deposition (PLD) process at about 300 degrees Celsius (572 degrees Fahrenheit). Then they are heated to 660 ˚C and slowly cooled, a process known as annealing.
Schematic of the experimental approach for the employment of a multilayer structure to deposit cubic Li6.25Al0.25La3Zr2O12 via PLD. Thin layers of Li3N were incorporated to compensate for Li loss at elevated temperatures. Pfenninger et al.
During the annealing process, nearly all of the nitrogen atoms burn off into the atmosphere and the lithium atoms from the original nitride layers fuse into the lithium garnet, forming a single lithium-rich, ceramic thin film.
The extra lithium content in the garnet film allows the material to retain the cubic structure needed for positively charged lithium ions (cations) to move quickly through the electrolyte.
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. 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.—MIT Associate Professor Jennifer Rupp, senior author
Although other methods to produce lithium-rich ceramic materials on larger pellets or tapes, heated using a process called sintering, can yield a dense microstructure that retains a high lithium concentration, they require higher heat and result in bulkier material. The new technique pioneered by Rupp and her students produces a thin film that is about 330 nanometers thick.
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 be thin and be fast. There is no need in a solid-state battery to have a large electrolyte.—Jennifer Rupp
What is needed is an electrolyte with faster conductivity. The unit of measurement for lithium ion conductivity is expressed in Siemens (S). The new multilayer deposition technique produces a lithium garnet (LLZO) material that shows the fastest ionic conductivity yet for a lithium-based electrolyte compound, about 2.9 x 10-5 S cm-1.
This ionic conductivity is competitive with solid-state lithium battery thin film electrolytes based on LIPON (lithium phosphorus oxynitride electrolytes) and adds a new film electrolyte material to the landscape.
Co-authors Michal Struzik and Reto Pfenninger carried out processing and Raman spectroscopy measurements on the lithium garnet material. These measurements were key to showing the material’s fast conduction at room temperature, as well as understanding the evolution of its different structural phases.
One of the main challenges in understanding the development of the crystal structure in LLZO was to develop appropriate methodology. We have proposed a series of experiments to observe development of the crystal structure in the [LLZO] thin film from disordered or amorphous phase to fully crystalline, highly conductive phase utilizing Raman spectroscopy upon thermal annealing under controlled atmospheric conditions. That allowed us to observe and understand how the crystal phases are developed and, as a consequence, the ionic conductivity improved.—Michal Struzik
Their work shows that during the annealing process, lithium garnet evolves from the amorphous phase in the initial multilayer processed at 300 ˚C through progressively higher temperatures to a low conducting tetragonal phase in a temperature range from about 585 ˚C to 630 ˚C, and to the desired highly conducting cubic phase after annealing at 660 ˚C. Notably, this temperature of 660 ˚C to achieve the highly conducting phase in the multilayer approach is nearly 400 ˚C lower than the 1,050 ˚C needed to achieve it with prior sintering methods using pellets or tapes.
One of the greatest challenges facing the realization of solid-state batteries lies in the ability to fabricate such devices. It is tough to bring the manufacturing costs down to meet commercial targets that are competitive with today's liquid-electrolyte-based lithium-ion batteries, and one of the main reasons is the need to use high temperatures to process the ceramic solid electrolytes.
This important paper reports a novel and imaginative approach to addressing this problem by reducing the processing temperature of garnet-based solid-state batteries by more than half—that is, by hundreds of degrees. Normally, high temperatures are required to achieve sufficient solid-state diffusion to intermix the constituent atoms of ceramic electrolyte. By interleaving lithium layers in an elegant nanostructure the authors have overcome this barrier.
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.—Professor Peter Bruce, the Wolfson Chair of the Department of Materials at Oxford University, who was not involved in this research
After demonstrating the novel processing and high conductivity of the lithium garnet electrode, the next step will be to test the material in an actual battery to explore how the material reacts with a battery cathode and how stable it is.
MIT and ETH have jointly filed for two patents on the multi-layer lithium garnet/lithium nitride processing. This new processing method, which allows precise control of lithium concentration in the material, can also be applied to other lithium oxide films such as lithium titanate and lithium cobaltate that are used in battery electrodes.
This work was funded by the MIT Lincoln Laboratory, the Thomas Lord Foundation, Competence Center Energy and Mobility, and Swiss Electrics.
Reto Pfenninger, Michal Struzik, Iñigo Garbayo, Evelyn Stilp & Jennifer L. M. Rupp (2019) “A low ride on processing temperature for fast lithium conduction in garnet solid-state battery films” Nature Energy 4, pages 475–483 doi: 10.1038/s41560-019-0384-4