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Sodium + water is, unfortunately, violently flammable.

Add water to elemental sodium, and you get heat, hydrogen gas and sodium hydroxide.

I'm not sure how it's held in the proposed batteries, but isn't pure Sodium violently explosive when in contact with water?

Batteries aren't pure sodium. Pure sodium bonded with something (eg: chlorine) is less explosive.

Any concentration of energy (gasoline, batteries, compressed air, etc) is always a potential bomb if it is released too quickly.

The safety problem with Li-ion batteries is the electrolyte is a flammable organic solvent. It is unrelated to the amount of energy stored in the battery under normal operation.