Most battery researchers agree that the next step in advanced lithium batteries will include solid-state electrolytes. Current commercial lithium ion batteries use electrolytes composed of liquid organic solvents that are flammable. This can compromise the safety of large scale lithium ion cells, such as those used in electric vehicles (EVs) or for grid storage.
Solid electrolytes also have the potential to increase battery capacity and performance by a factor of 2 to 3 through the use of lithium metal as the anode. Commercial cells presently use carbon graphite anodes that intercalate lithium ions during charging and release them during discharge. This approach avoids the formation of spiky dendritic crystals that would form on the surface of lithium metal during charging. The carbon anode cannot release as many lithium ions as lithium metal would, and thus has a lower performance level. The dendrite crystals, which can grow freely from the surface of lithium metal into the liquid electrolyte, would be limited and restricted by a dense solid electrolyte.
On the left, the crystal structure of cubic garnet-type Li7-xLa3Zr2-xTaxO12 that is finding use as a solid-state electrolyte for advanced lithium batteries. On the right, the temperature dependence of ionic conduction is shown. (Image source: Toyohashi University of Technology)
If solid-state batteries are the next big thing, what is holding them back? A team at the Toyohashi University of Technology in Japan notes, in a university news release, that a solid electrolyte must do two things. It must have high ionic conductivity (above 1 mS/cm at room temperature) and it must have good electrochemical stability (especially after many charge and discharge cycles).
There are two types of ceramic materials that are under consideration for solid electrolytes. Sulfide-based solid electrolytes have high lithium ion conductivity, but can be chemically unstable—particularly when exposed to air. Oxide solid electrolytes are much more stable chemically, but often exhibit poor ionic conductivity. Ryoji Inada and his colleagues at the Department of Electrical and Electronic Information Engineering at Toyohashi University have worked to develop a garnet-type, fast ionic conducting oxide as the solid electrolyte for an all-solid-state battery.
The garnet-type fast lithium-ion conducting oxide developed by Inada, Li7-xLa3Zr2-xTaxO12 (x = 0.4-0.5, called LLZTO), is a good candidate for a solid electrolyte because of its good ionic conducting properties and high electrochemical stability. Yet it has some other issues that needed to be addressed. The high-temperature sintering at 1000-1200°C that is needed for densification creates side reactions at the interface with the possible cathode materials. This prevents the option of co-sintering the solid electrolyte with the cathode.
To overcome this limitation, Inada and his team fabricated a lithium trivanadate (LiV3O8, called LVO) thick-film cathode on garnet-type LLZTO by using an aerosol deposition method. This room-temperature technique produces a film of material by impacting ceramic particles onto a substrate. Control of the particle size and shape allows a dense and thick ceramic film to be built up without any thermal treatment. By using aerosol deposition, Inada was able to add a thick-film cathode, with a thickness of 5-6 μm, to the previously sintered LLZTO oxide-based solid electrolyte. Lithium metal foil was attached to the opposite side of the solid electrolyte, forming a test battery cell.
The properties of the test cells were measured at 50°C and 100°C. At the lower temperature, a battery capacity of 100 milliamp-hour/gram (mAh/gm) was measured. At 100°C, however, the capacity rose to a promising 300 mAh/gm—about what an LVO cathode would deliver with a conventional liquid electrolyte.
The results from the Toyohashi University experiments have some significant implications. They showed that an oxide-based solid electrolyte can produce high lithium ion conduction while having good chemical stability. They also found a way to produce a thick film LVO cathode with high adhesion and good electrochemical properties when aerosol is deposited onto the solid electrolyte substrate.
Although solid-state electrolytes are not yet ready for prime time, materials researchers are finding solutions to the problems that they pose. This work, and the continuing efforts by research teams around the world, is bringing the promise of solid-state electrolytes closer to reality.
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.
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