Sodium ion might be a good option in the future. Energy density will probably never be as good as lithium ion, but cost is significantly lower, and it has the g

Sodium ion might be a good option in the future. Energy density will probably never be as good as lithium ion, but cost is significantly lower, and it has the great advantage of being similar enough to lithium ion that it can be built with only small modifications to existing lithium ion manufacturing equipment. http://spectrum.ieee.org/energywise/energy/renewables/a-first-prototype-of-a-sodiumion-rechargeable-battery http://spectrum.ieee.org/energywise/energy/renewables/a-firs...