Japanese site Nikkei reports that Sony has developed a battery that's based on a sulphur compound rather than lithium. Not only does this mean that more power can be stored, but batteries can also be smaller in size. This in turn could lead to thinner smartphones.
Unfortunately, we won't see this new technology any time soon. Sony is currently safety testing its batteries, which could take some time. It may be 2020 before this new battery is used in a smartphone that we can buy.