The batteries in phones and electric cars have a liquid inside. This liquid can catch fire. In a solid-state battery, the liquid is changed to a solid. People hope these batteries will be safer and charge faster.

But there is a big problem. The solid parts do not stay close together easily. So large batteries for cars are still difficult to make.

News about electric cars often mentions all-solid-state batteries, which promise longer driving ranges and shorter charging times. The idea is simple: replace the liquid inside a lithium-ion battery with a solid. In today's batteries, ions swim through a liquid called an electrolyte, but this liquid contains flammable chemicals. In a solid-state battery, ions pass through gaps in a solid crystal instead.

Scientists once believed ions moved too slowly in solids. However, in 2011 a Japanese research team, including Toyota, discovered a solid material in which ions moved about as fast as in a liquid. Even so, it is hard to keep solids in close contact, especially because electrodes swell and shrink during charging. Large batteries for cars are still being developed.

All-solid-state batteries have become a fixture of electric vehicle headlines, but the underlying concept is straightforward: the liquid electrolyte in a conventional lithium-ion cell is replaced by a solid one. Because the organic solvents in liquid electrolytes are flammable, a solid alternative could reduce fire risk, tolerate higher temperatures, simplify cooling systems and, depending on materials, enable faster charging and higher energy density. For many years, however, ions were thought to move too sluggishly through solids to be practical. That view changed in 2011, when a team led by Ryoji Kanno, working with Toyota, reported in Nature Materials a sulfide material whose ionic conductivity rivaled that of liquid electrolytes; in 2016 the group demonstrated high-power solid-state cells.

The main remaining obstacle lies at the interfaces between solid components. Unlike a liquid, a solid cannot flow into every gap, and because electrode materials expand and contract during each cycle, contact can be lost and performance degraded. Some sulfide materials also react with moisture, and many laboratory cells are tested under high pressure that differs from real conditions. Small solid-state batteries are already used in some electronic components, but mass production for electric cars remains a work in progress.