RE: Iron/saltwater battery tech15 Oct 2021 17:51
This was Alfa's response elsewhere two days ago (three different messages but you'll get the drift)
The main difference is that any flow battery that is not an all Vanadium flow battery has different electrolyte on either sides of the battery - so if there is any diffusion through the thin membrane that separates the two liquids you will get irreversible reaction between the electrolytes and immediate loss of energy storage capacity. Pinholes and large scale holes in the membrane will give you even more trouble.
In the VRFB it is the same chemical on both sides, the electrolyte can exist in 4 different states - allowing for different charge and discharge states on both sides of the battery, but with only ever the same chemical. Mixing of the two sides of the battery only yields a short term release of energy not a long term chemical change in the electrolyte
In the case of the Iron flow battery it is actually a negatively Chlorine atom that has to cross from one side of the battery, through the membrane and into the other side of the battery, instead of a tiny positively ionised Hydrogen ion (a proton). This means that membrane diffusivity is a much harder ask for the Iron battery, and furthermore there is actually metallic iron deposited inside the battery during operation - the risk of dendrite production from any such pure metal deposition process is of course one of the major challenges for Lithium-ion battery operation.