Could Mercury be used, as it is liquid at room temperature? The question then is, which metal has the lowest melting point in the set-up?
The efficiency might be similar to pumped storage systems, but to have the same energy capacity, surely the liquid metal batteries would cost a huge amount more than hillsides and water.

Large quantities of liquid magnesium and liquid antimony eh?
What could possibly go wrong?

Interesting, what percentage of the stored energy will be used to keep the temp. at 500ºCover - say - 24 hours?

Please let there be a god...so that he can help us when molten salts, lead and antimony makes more environmental sense than water+gravity as an energy storage medium.

The round-trip AC-to-AC efficiency of large new pumped hydro plants is close to 85%, not 70% (and is that 70% DC-to-DC?). In other words, they have half the losses, or less, of the liquid battery approach. They don't run at high temperature or degrade over time. They last for 60+ years. They also can be built without hillsides (see gravitypower.net). Prof. Sadoway seems to produce a lot of hot air along with his hot battery.

Funny thing about digging a big hole you get a big pile of dirt. There are very few places where you cant find enough height difference to set up a pump storage hydroelectric system.

I don't get all the hate going on here - the thing being presented is that they've been able to reduce the operating temperature of these batteries. So you're all complaining that 500 is too hot - where were you when they were operating at 700? Industrial processes manage temperatures like these all the time, but reducing it makes for lower energy consumption and safer operation, which all sounds good to me. Representing this as a bad thing seems backwards.
Practical changes like these can make a huge difference - when 'high temperature' (not room temperature) superconductors were discovered it meant that superconductors could be made using liquid nitrogen instead of liquid helium, that alone representing a 100-fold decrease in cost.

& lower costs since competitive?

The reaction itself produces some heat, so if you make the batteries large enough, you don't need to add heat any more once the reaction is going. Batteries have no moving parts. All you have to do is put the components in a good casing and it is pretty hard for anything to go wrong. They can be placed anywhere, put in quickly, and require much less up front investment than a water pumping system. I saw Sadoway's talk on TED and thought it was great. These batteries could make a big difference.