To give the answer its the cyanide part... which I knew already when looking at the title, without having much clue about underlying chemistry.

Good luck selling batteries full of some variant of cyanide to general public, i can almost imagine it working till people hear that keyword.

Somewhat true, but consumers are buying vast quantities of lithium batteries and carrying them in their pockets. Those things are eager to catch fire and emit hydrofluoric acid.

You can have safe battery chemistry or useful battery chemistry. (Not an expert, open to correction.)

Yeah, just like no one would ever buy a lead-acid battery.

I don't think many people are concerned about their batteries being safe for ingestion.

The public is not going to buy a flow battery anyway.

Or perhaps they will, but only when public is taken in the context as in public utility or public city. But individuals are not going to buy a flow battery, it is a chemical plant.

Have to abbreviate it as FCN. Cyanide is ~320 times more toxic than ferrocyanide.

There are plenty of battery customers besides the usual suspects