@danielfp248 I have just read your excellent info here and in the blog as I was seriously looking at this chemistry as an alternative to short lived Lithium cells. The ups and downs, the great promise and the final (I totally see your point) surrender. I know part of your goal was to do at least as good as Lithium (or maybe at least lead-acid?). Which got me to thinking --- for a static application, bulk and weight is a lesser issue than long term stability. I'm wondering if the cells weren't pushed as hard (less current, narrower voltage from charge to discharge) and perhaps less concentrated electrolyte... would ZnBr be able to last 1000 cycles or more?
I'm helping my sibling with an off-grid system. It's frustrating knowing that some days with good sun half of the solar power goes to waste. This could be fixed with doubling the Lithium battery size (not cheap, especially when considering limited life). Or, use the excess power to charge some alternative storage (ZnBr?), which then would charge the primary Lithium bank as power is used from it after dark. In this case perfect energy efficiency isn't hugely important as the power would have been lost otherwise (and any shortfall is made up with an LP genset). Using an alternative battery in this case removes a lot of the restrictions --- we can control the charge and discharge rate independent of the house's need.
It is true that lowering the current density immediately reduces or removes a lot of the issues. However, the biggest issue lower current doesn't solve, is the self-discharge of the battery. At low current densities, it will take longer to charge the battery, which gives more opportunity for bromine to migrate from cathode to anode and discharge the device. You would need small separation distance between cathode and anode to be able to charge below 1.9V at a reasonable current density (taking say 12 hours to charge the batteries) which makes the issue worse.
Adding sequestering agents will increase the internal resistance of the battery, which will add to your problems and costs. Especially because sequestering agents that are soluble enough in concentrated ZnBr2 are expensive (TBABr or TPABr do not work well because of lower solubility).
There is not much point in lowering the concentration of electrolyte, as that just makes the battery more inefficient.
It is also false that Zn-Br batteries do not degrade, hydrogen evolution reactions in the anode eventually lead to electrolyte degradation to the point where chemical regeneration of the electrolyte using HBr becomes necessary. This is even true if you were able to charge with no over-potential as the hydrogen evolving reactions are significant a this potential anyway. It isn't trivial to overcome these reactions.
Honestly, there is no demonstrated DIY solution - I am not aware of any solution that has been properly characterized and showed to work as intended - that offers you reasonable energy storage capabilities for lower cost than lithium ion batteries, even at low current densities and low energy/power densities. If you go this route, you are bound to spend a lot of time and effort to end up with something that is likely to be dangerous, not standardized and with unknown performance characteristics. Also, it is likely to last way less than 1000 cycles. Taking these systems apart is also no picnic, dealing with large amounts of nasty bromine is a possibility.
For your siblings build, I would go with a commercial solution.
Everyone likes to say it is super easy to get 60Wh/L with Zn-Br and get +10K cycle life, truth is, it is really not. Obviously I would love to be proven wrong by anyone who has actual charge/discharge cycling data of a DIY system. So far, a lot of people have talked the talk, no one seems to have walked the walk with any success.
