Lithium-air batteries, with high energy density, low weight and useful stability, are a major candidate for future electric car batteries.
However, commercialization may not happen for another fifteen years or more given current limitations, so improvements in the meantime must be found elsewhere.
Toyota is researching solid-state lithium-ion batteries as a mid-term alternative, while lithium-air's bugs are ironed out.
According to Green Car Congress (via Charged EVs), Toyota's prototype cells have energy density of 400 Wh/L--some way short of the 1,000 Wh/L energy density of lithium-air, but ahead of traditional lithium-ion batteries.
And, unlike lithium-air, such batteries could be commercially viable by 2020, with "substantial improvement" by 2025.
Solid-state batteries, as their name suggests, use a solid, rather than liquid electrolyte between the anode and cathode.
There are several advantages to this, not least elimination of the risk of electrolyte leakage and the high thermal stability of the solid electrolyte.
Other advantages--noted by authors Dr. Chihiro Yada and Claudia Brasse in the research paper ATZelektronik worldwide--include a long life-cycle, and improved packaging efficiency.
This packaging efficiency stems from a cell design that allows in-series stacking and bi-polar structures. By reducing the dead space between single cells, engineers can also improve the battery's energy density for a given size.
Researchers are beginning to overcome one of solid state batteries' previous limitations, too--that of transfer resistance between the cathode and the solid-state electrolyte. Sulfide-based electrolytes are currently leading the way.
Toyota's initial prototype is more ready for the sidewalk than it is the road right now, in the form of an electric kick-board scooter--but it moves, it's powered by the battery and serves as a working example of the technology.
Work will continue in the meantime, but Yada and Brasse still consider solid-state an innovative solution for new-generation batteries. It could be the ideal interim solution until lithium-air batteries are ready for the market.