Sodium-ion batteries aren't necessarily the dead end you might think. Faradion has been working on portable batteries w/ decent energy density (~150 Wh/kg), ex

Sodium-ion batteries aren't necessarily the dead end you might think. Faradion has been working on portable batteries w/ decent energy density (~150 Wh/kg), excellent charge-discharge performance (93% capacity after 1000 cycles) and competitive costs: http://www.faradion.co.uk/about/news/2015/05/489/ http://www.faradion.co.uk/about/news/2015/05/489/ Recent formulations for sodium rechargeable batteries are room-temperature and non-flammable: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4525335/ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4525335/ Here's a a short article progress being made getting Na-ion batteries into an 18650 form-factor (their current prototype has an energy density of about 90 Wh/kg): https://news.cnrs.fr/articles/a-battery-revolution-in-motion https://news.cnrs.fr/articles/a-battery-revolution-in-motion While energy density is lower than Li-ion, there's probably a good argument for longevity, safety, and material costs.