Although lithium ion batteries have seen some significant improvements over the last few years, they still have a number of weaknesses, including fragility, sensitivity to operating temperatures, bulky support structures, and flammable electrolytes. As a result, researchers (and a few start-ups) have been attempting to develop updated versions based on different chemistries. One option is a lithium-air battery, where one of the electrodes moves charges by allowing the lithium to react with oxygen, saving the space involved with a standard electrode. The primary alternative is a solid-state battery, where the liquid electrolyte is replaced by a solid version.

A number of solid-state lithium electrolytes have been identified, but these have their own issues: low ionic conductivity, temperature sensitivity, and chemical instability. As such, they've typically performed significantly worse than existing lithium-ion technology. A Japanese group that includes some researchers at Toyota has now found a solid electrolyte that's also a superionic conductor, and show that it may have what it takes to function in batteries.

"What," I'd imagine you're asking, "is a superionic conductor?" These solids have internal pores within their crystal structure that allow ions to pass through, even at temperatures below the melting point of the solid. When the ions aren't being driven through the structure, they remain in place, allowing them to serve as a charge storage area. The new material uses lithium as its charge carrier, but embeds it in a crystal lattice made of germanium, phosphorus, and sulfur (the chemical formula is Li 10 GeP 2 S 12 . The authors have determined its crystal structure, which contains an ordered array of components like GeS 4 , PS 4 , and LiS 6 . This array creates a series of channels that contain individual lithium and sulfur ions, which are apparently mobile.

When it comes to the ionic conductivity of lithium, the material performs just as well as standard lithium-ion batteries, which is about double the performance of previously published solid-state materials. And its performance doesn't appear to be nearly as sensitive to temperature as existing battery technology. The material worked nicely at 100°C, and showed a relatively gradual decline as the temperatures dropped to -100°C; standard lithium batteries tend to drop off dramtically, performance wise, a bit below freezing.

A battery built with the material showed stable performance, but the authors only took it through eight charge/discharge cycles, so it's difficult to say whether this really has what it takes to power anything electronic. Although the details are sparse, it appears to be easy to make: just put the right ratio of chemicals in a vacuum and heat it to 550°C.

The authors send a bit of a mixed message when wrapping up, as well. They start off by saying that the new material should enable us to get a better understanding of how ions move within solid, which will aid the development of a new generation of battery materials. That sounds a bit like they don't expect their own electrolyte to make it to market. But in the very next sentence they claim their new material is "promising for applications requiring batteries with high powers and energy densities, and for pure electric and hybrid electric vehicles and other electrochemical devices that require high safety, stability and reliability." So, maybe we'll see this in future Toyotas yet.

Nature Materials, 2011. DOI: 10.1038/NMAT3066 (About DOIs).