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http://en.m.wikipedia.org/wiki/Direct_borohydride_fuel_cell
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Back in 2002 or so I remember reading about the use of borohydride as a battery. You can check the wiki to see why it wasn't practical.
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http://www.greencarcongress.com/2014/03/20140305-quant.html
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It appears that some folks in Liechtenstein have found a chemical compound that works better:
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According to nanoFLOWCELL, its flow battery has a specific energy of about 5-times that of a Li-ion battery (600 Wh/kg compared to ~120 Wh/kg). The company attributes the performance of the nanoFLOWCELL to the characteristics of its newly-developed, and unspecified, electrolytic fluids, made up of metallic salts at very high concentration. Slightly more specifically, the company says that a large increase in the number of charge carriers in the electrolyte fluid within the nanoFLOWCELL significantly increased its performance compared to conventional redox flow-cells (about 5x the specific energy and several orders of magnitude more specific power).
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This is very exciting if it doesn't have any serious problems. (like 1% recharge efficiency)
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