Now we\'re talkin\' - I\'m already in the market for this one! MIT!

Why can\'t they just use hydrogen fuel cells, large amount of gas can be stored with a high energy to weight ratio. Use the power source that recharges these batteries for electrolysis and you have overcome all the problems.

^^ The electrolysis fuel cell cycle efficiency is \"maybe 40%\" (per the Strategic Electrochemistry Research Center). Lithium-ion batteries have a 97-99% charge/discharge efficiency.
True, H2 energy to weight ratio is high, but energy to volume ratio is as low as it gets.
Since the energy source is electricity anyway, it makes more sense to store it in a battery at ~98% efficiency than to utilize a 40% efficient process that involves a lot of additional equipment.

Hydrogen is difficult to store and transport.

Why not fuel cells? Electrolysis is only about 60% efficient and fuel cells are maybe 70% efficient.
LiOn battery tech is already in the 90\'s % in charge/discharge efficiency.

Khan:
I see two advantages over hydrogen. One is that this battery can easily be recharged directly within the battery with electricity, as well as pumped fuel. While it is true that hydrogen can be obtained by electrolysis, the process of producing, capturing, compressing, and storing it is a bit more complicated. The second is that hydrogen is explosive. Also, as a gas, it requires airtight, pressurized containers to transport and such. Just my quick take on this.
I especially like the idea of being able to store energy from wind and solar with this. Since \"the wind doesn\'t always blow and the sun doesn\'t always shine\" is the biggest arguement against renewables, this could be a major step forward.

So much effort to make a more expensive, less effective replacement for a practical, safe, and cleaner energy source.

Khan,
Hydrogen has low energy density. Whether in super-high pressure tanks, liquid form or in adsorption storage media like metal hydrides or carbon nanotubes, it doesn\'t begin to approach something like gasoline in terms of either volume or weight.

Slowburn, what is this magical practical safe and clean energy source?

They are simply trying to avoid using the most abundant energy source in the world. Water. Water is highly available, renewable, and virtually inexhaustible. No wonder every researcher that gets \"Paid\" is looking for a way to provide energy that is patentable, commercially monopolistic, and difficult for the average person to reproduce. They want the be the only kids on the block that can make it. Thus, the cycle of power and greed continues. We will never see any technology go mainstream that cannot be greedily controlled. ie: Nickola Tesla - Wireless electricity. They didn\'t have the technology to put a faucet on it, so guess what? It never happened.