Question

I found out today that a mix of sodium and potassium can be heated until molten and used as a thermal battery. Are there other substances with similar properties, ie something better than batteries for storing energy?

Answer 1

There is a neat picture of a liquid metal battery on the MIT OCW 3.091SC Introduction to Solid State Chemistry home page:
http://ocw.mit.edu/courses/materials-science-and-engineering/3-091sc-introduction-to-solid-state-chemistry-fall-2010/index.htm

Batteries are a big part of electrochemistry and they can be made from a lot of things. Some of batteries might be surprising if your picture of a "battery" is a AA or lithium ion found in electronics.

There are many ways of storing energy. Biochemical energy is stored in fat, protein and sugar in the human body. Hydrocarbons such as gasoline store energy. Heat from nuclear reactions can be used to produce steam which drives turbines that generate electrical power.

Naturally, notions of "better" or "worse" depend on what you are going for. Power supplies for mobile devices need to be compact but price is more important for (massive) stationary industrial power supplies.