If safety is a concern, why not flow storage? I see enough reports of Li battery related unpleasantness to be slightly concerned on this front, and for fixed l

If safety is a concern, why not flow storage? I see enough reports of Li battery related unpleasantness to be slightly concerned on this front, and for fixed location storage I'm less concerned with density and more so with maintenance and life span. TFA talks about (favourable) temperature tolerance of Na, at least at the low end, I didn't see high end figures.