Lithium batteries a reality - why not sodium, pottasium batteries?

Lithium batteries seem to be mainstream now, offering high energy density, high output current capability.


Stepping down the periodic table there are alkali metals with greater electronegativity than Lithium: sodium and potassium, etc. 


So why not sodium and potassium batteries. Can't these offer higher energy density, higher current output? Is the reactivity just so great that the chemistry can't be engineered to provide a practical battery? Or are the safety issues just too great?




The [sodium–sulfur battery](https://en.wikipedia.org/wiki/Sodium%E2%80%93sulfur_battery) is already in use for power leveling and other stationary applications, such as [a 1.2 MWh installation in WV, USA](http://www.energystorageexchange.org/projects/269). The [Ford Ecostar](https://en.wikipedia.org/wiki/Ford_Ecostar) did use a Na-S battery, but it did not have sufficient energy for distant travel (and some of the experimental batteries caught fire, but so do Li-ion cells on occasion. Oops!).


[Sodium-ion batteries](https://en.wikipedia.org/wiki/Sodium-ion_battery) are also in development. In the JACS, the article [*Na-ion batteries get closer to replacing Li-ion batteries*](http://phys.org/news/2015-03-na-ion-batteries-closer-li-ion.html) discusses some improvements in maximum current capacity and total energy storage.


That said, for *mobile use*, lithium has a higher energy/weight ratio than heavier metals, but for other use, sodium is less expensive and is readily available.


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There is relatively little research in sodium ion batteries, and as a result, breakthroughs in sodium battery technology are lacking. Recently, that has changed, and more scientists are looking into the potential of sodium ion batteries. However, the Na-ion battery's biggest drawback is its slow charge and discharge rates. This makes them impractical for everyday use. However, due to its lower cost, they are a cost-effective alternative to Li-ion batteries for applications in energy storage for wind and solar power.


With [more and more research](http://phys.org/news/2015-03-na-ion-batteries-closer-li-ion.html) in sodium-ion batteries, they may one day take over Li-ion batteries, and some companies such as [Aquion Energy](http://www.aquionenergy.com/) have already begun manufacturing consumer grade sodium-ion batteries.


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