I don't know what chemistry exactly these cells are using, but in sodium-ion batteries, prussian blue analogs as they are called are common anode materials. Overcharging these cells can lead to a release of hydrogen cyanide gas, notoriously known as Zyklon B.
It has damped my enthusiasm for perusing it as a potential future home energy storage solution.
It has damped my enthusiasm for perusing it as a potential future home energy storage solution.