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.

Just wait until you find out about hydrogen sulfide from overcharged car batteries.

Also, I think HCN can be scrubbed by adding a special absorptive cap onto the battery.