Sodium Ion batteries safer?
I have to say I was curious about the claim that sodium ion batteries would be safer than lithium ion batteries.
Sodium and lithium are both alkali metals along with potassium, rubidium, caesium and francium. As you go down that group starting with Li, through Na, K, Rb and Cs, the metals get progressively more reactive. Nobody has seen francium in the wild, it has quite a short half life of just 22 minutes. Alkali metals are characterised by having just one electron in their outer shell. Atoms like to have 8 so alkali metals give up the electron easily, halogens with 7 electrons in the outer shell pick them up very easily.
The bigger the atom, the more that outer shell is shielded from the nucleus by other electron shells. For lithium with just 1 inner shell that means the nucleus keeps a hold on that electron making it a bit harder to remove, therefore less reactive. At the other end, Fluorine has just the 1 inner shell meaning it is very good at ripping electrons away from other atoms. In other words, the halogens on the right of the periodic table get less reactive as you go down the group while the alkali metals on the left get more reactive.
This increaing reactivity and decreasing electronegativity is a feature of the increasing ease of getting that one outer electron off the atom, the bigger the atom, the easier it is to oxidise (oxidation is the loss of an electron). Li has an electronegativity of 0.98, Na is 0.93.
In a fire, water makes lithium relase hydrogen and burn, it makes sodium release hydrogen and burn it just does it faster.