My adventures building a Zinc-Bromine battery

Adam007

New Member
Very much enjoying reading this, thanks for sharing. I'm currently working on a recycled plastic case for a minimal architecture battery from my workshop waste, using old ABS from my edge banding machines and a mold. Also some old carbon and fibreglass sheets I have lying around may come in handy. I'm hoping a surfboard/racing car part manufacture background using carbons/epoxy/fillers/foams may pay dividends when it comes to the final layups!
Keep up the good work.
 

danielfp248

Solar Addict
I am now running the exact same setup described in #200 in order to confirm stability at higher cycle numbers. Battery holding really well so far after 14 cycles. Energy density is 26 Wh/L. This is confirming that the elimination of Fe impurities increases both voltaic and energy efficiencies and improves battery stability.

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danielfp248

Solar Addict
Very much enjoying reading this, thanks for sharing. I'm currently working on a recycled plastic case for a minimal architecture battery from my workshop waste, using old ABS from my edge banding machines and a mold. Also some old carbon and fibreglass sheets I have lying around may come in handy. I'm hoping a surfboard/racing car part manufacture background using carbons/epoxy/fillers/foams may pay dividends when it comes to the final layups!
Keep up the good work.

Thanks for your support! How are you planning to build the battery? What geometry, cathode/anode materials, electrolyte characteristics, etc? Are you going to cover the ABS in a carbon material to prevent its reaction with Bromine? Let me know :)
 

danielfp248

Solar Addict
The test that was started in #203 continues. After 25 cycles the battery is still behaving fine. So far no signs of dendrites in the curves although there is some slight deterioration of the voltaic efficiency. I will continue this test till the battery either fails or loses 20% of its max capacity. So far the battery has lost around ~3%. A drop of stored charge to 10.8 mAh would imply a loss of 20%.

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danielfp248

Solar Addict
Battery from #203, still going strong at 50 cycles. Deterioration has been really small, energy efficiency has dropped but now stabilized and Coulombic efficiency has remained in the 87-90% range. Stored charge has also remained pretty consistent.

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danielfp248

Solar Addict
After cycling this battery 75 times, we start to see some important deterioration in the energy efficiency and stored charge. Voltaic efficiency has dropped quite substantially as well. Charging cycles are now taking the battery past the 2.1V mark, where there is expected to be substantial hydrogen evolution. This failure is however not due to dendrites but probably due to some phenomenon related with the TMPhABr, if the TMPhABr is decomposing with time, then it will not be a viable choice for a sequestering agent and we'll need to think about something else. I am tempted to open the battery now and examine how it has evolved.

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danielfp248

Solar Addict
So a Zn-Br battery without a sequestering agent will deteriorate faster, basically because it is being over-charged more if you try to charge it to 15mAh since elemental Br diffusion is much more efficient. You can see the significant deterioration of the charge potential. It becomes significantly higher with time as the battery becomes electrolyte deficient at the cathode due to the constant escape of Br from it with each charge cycle. Most probably this battery should not be charged above 8mAh. You can see that the charges stored are way lower compared to the TMPhABr cell tested in #203.

This all shows why a minimal architecture Zn-Br design is limited to a low energy density of around 7Wh/L if you want to use the battery without significantly deteriorating it with time.

However the discharge potential is significantly more stable, which does show that there are some problems when using TMPhABr as a sequestering agent. With these two devices characterized I will now try a battery with a GFE-1 cathode pretreated with a 10% TBABr solution, to see how it compares to these two batteries.

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danielfp248

Solar Addict
A battery made with a GFE-1 cathode pretreated with 10% TBABr (soaked and then air-dried as I do with TMPhABr) cannot even be charged to 15mAh due to its very high resistance. The very low solubility of the TBABr in the electrolyte causes the coating to be extremely inactive and basically an insulator over the cathode's conductive surfaces. This means that charging starts at 2V and goes to 2.9V at 5mAh, effectively making TBABr a bad choice as a sequestering agent in this configuration.
 

danielfp248

Solar Addict
I'm going to be moving out of the US within the next two weeks, so I will sadly be unable to perform any experiments till February :) However I will be happy to answer anyone's questions about Zn-Br batteries or any of my previous research.
 

Eiskuh

New Member
Hi All.

Wow, interesting article. I am also working on a static ZBr2 battery which will be used in a UPS at home as we have plenty of planned power outages in SA :( Still waiting for my KBr order before starting putting my theoretical knowlege into practical experiments.

In my online "research" I came across some interesting ideas that might help improve the DIY ZnBr2 cells.

1) According to a Chinese paper, adding a magnet to the anode side helps reduce dendrites as the zinc dendrites would have to counter the magnetic field. OK, crude method to help reduce dendrites... but might be helpfull to someone with lots of magnets .

2) In flow batteries a high velocity of electrolyte circulating around the anode helps reduce dendrites. As I see the use of a separator in this post, it might be of interest on a larger battery where one could circulate the electrolyte around the anode without disturbing the bromide from settling.

3) A 3:1 surface ratio between anode:cathode helps reduce dendrites. This came from a paper investigating anode/cathode ratio for flow batteries with same material anode and cathode. The confrimed theory is that the larger surface helps spread the zinc deposition on the anode across a larger surface making each spike less effective to grow. A 4:1 ratio did not have added benefits, thus 3:1 was considered an optimal ratio.
=> My battery design will thus include a split cathode in 3 independent "sectors" and only use 1 sector during charging whereas all 3 cathode sectors are used during discharge. In addition, should a dendrite reach that cathode sector the charging can be switched to the next cathode sector. In addition, (see 4) a targeted negative charge pulse can be used to break up the dendrites that reach the cathode.

4) Coming from the electronics side, the charge logic has been a main focus of my interest. Robert Murray mentioned an important keyword for charging the ZnBr2 batteries: electroplating. Dendrites & rough surfaces are a big issue in electroplating as well. To combat the rough surface during electroplating, the quality electroplating controllers use pulses instead of uninterrupted current supply and every x pulses (10th?) are negative. Simplified, the positive pulses accelerate the ions towards the Zn anode and during the pulse break the ions can float around & spread evenly at the anode surface instead of concentrate towards the closest dendrite spike. The negative pulse targets the dendrite itself as that is the path with the lease resistance between anode/cathode.
=> I think the effect of the charge controller for ZnBr2 batteries regarding dendrites are underestimated. My presumption is that following the electroplating charge logic will help reduce dendrites more than additives in the solution. That doesn't exclude combining charge controll and additives to even enhance the dendrite reduction.

Anyway, though I'd share this.

Mike
 
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