Current commercial Zn-Br flow batteries have specific energies in the 34.4–54 W·h/kg region, with most companies being at the lower end of this range. In order for a static Zn-Br battery to be better than its current industrial counter-parts it would ideally improve on this specific energy while reducing the costs of production substantially.

My current tests using carbon cloth cathodes, Zinc anodes, fiberglass separators and Zinc Bromide electrolytes in the 0.25-0.5M range with a TBABr sequestering agent present at concentrations of around 0.1-0.2M have shown an ability to store around 0.5mAh with a weight of around 0.250g per total cell (no packaging material), which would give the cells a specific energy of around 3 W·h/kg, which is one order of magnitude lower than current commercial Zn-Br flow batteries.

In terms of weight, I have been using a 0.2mm thick Zinc anode that is quite thicker than what would be strictly necessary for the battery, the anode thickness can be changed to 0.02mm Zinc foil (10x less mass) which would reduce the total amount of mass by more than 70%. The anode mass is currently around 180mg, so lowering this to 18mg would take the current specific energy to around 9 W·h/kg (since there is no expected loss in the current battery configuration from using a thinner Zn anode).

This improvement is still not enough, we need to increase the capacity by at least 4-6x which means increasing the amount of Zinc Bromide in the battery to at least the 1.5-2M range and increasing the amount of energy injected/extracted to at least 2.0-3.0 mAh for this battery. This means that TBABr is not going to work, reason why my tests are now going to move to using TMPhABr or TPABr. These new sequestering agents also have lower molecular weights, so they are bound to be significantly more “atom efficient” compared to TBABr. The end batteries right now contain around 50uL of electrolyte – I put 100uL but half is “pushed out” when Swagelok cells are closed (this is determined by weighting the dry and final battery cell) – so theoretically a 2-4M Zinc Bromide solution should offer a capacity of around 2.7-5.2 mAh but we are unlikely to be able to extract this amount because of the conductivity of the solution becoming lower as we plate Zn and oxidize bromide to perbromide in the cathode.

The current energy efficiency of the battery is still too low (max has been 60% in most cases) so the hope is that the higher Zinc Bromide concentration, coupled with the new sequestering agents, will help increase this efficiency to the 70-80% region while also helping improve maintain Coulombic efficiencies above 95%. The energy efficiency of current Zn-Br flow batteries is mostly below the 80% mark, so anything above this number would be highly desirable.

If the above mentioned sequestering agents can achieve these efficiencies at these concentrations then we would be able to reach specific energies of around 45 W·h/kg for the cells I’m constructing. If we can achieve energy efficiencies above 90% – already seen in published research using TPABr – this would already put them at a significantly more competitive place relative to current Zn-Br technology.

Currently Li-ion cells are in the 100-265 W·h/kg range, so this technology could only compete if significantly higher zinc bromide concentrations – in the order of 10M – can be achieved, while retaining a functional sequestering agent or if we can add a supporting electrolyte that enables the extraction of most of the zinc bromide without lowering the efficiency of the battery (although that electrolyte adds some weight). It is much more likely that a technology like this would compete in battery life and USD/kWh terms. Li-ion technology right now is at around 200 USD/kWh while a technology like Zn-Br in static cells could start at a fifth of this price. The life of a static Zn-Br battery with a viable sequestering agent is also expected to be significantly longer (>10,000 cycles) so that would also help it compete with Li-ion (with Lithium Iron Phosphate batteries surviving for around 2000 cycles when fully discharged on each cycle).

During this past week I have been experimenting and thinking more about Zn-Br batteries and how a real practical battery would look like (how it would be built and what its characteristics would be like). Let’s imagine we have found a complexing agent that can be used in highly concentrated ZnBr₂ solutions. What would a prototype battery look like and how much would it cost?

The first thing we need to consider is the geometry to build such a battery. Single-cell batteries for Zn-Br chemistry are impractical due to the limits that would impose on current density – and it’s not the 19th century – so the ideal battery would probably follow a configuration similar to modern lead-acid batteries, where multiple cells are put together to achieve better results. The simplest way to do this is to stack materials next to each other within a box, then flood the box with the desired electrolyte solution.

In a 101mm x 54mm x 44mm project box you could fit a volume of around 237mL. If we decide to use a very porous carbon felt electrode – which I have experience with – with titanium current collectors, glass fiber separators and zinc anodes, we would create a cell configuration like the one showed above. This would occupy the entire cell with either separator, current collector, anode or cathode material. Given that the materials used take little real volume, as they are either very porous or very thin, I’m going to assume the solution volume we will fit will be equal to 200mL, which is realistic given the characteristics of the materials.

If we use a 2M ZnBr₂ solution, this would give a maximum theoretical energy of 40Wh. If the cells are all connected in parallel, this would give us a battery with a voltage of 1.85 at a capacity of 21.6 Ah. The battery would be charged at a constant current of around 2.85A, although depending on the actual kinetics this might need to go down to even 285mA. In the above design you actually have only 6 cells that are each equal to 2 normal cells connected in parallel – as they share a current collector in the cathode – so connecting these 6 in series would give you a voltage of 11.1V with an expected charging current of 475mA.

The main caveat of the above design is that it uses a 2M ZnBr₂ solution, assuming we can find a complexing agent that forms an insoluble perbromide that can be in the initial formulation at a concentration equal to at least the same as the ZnBr₂ then this should be no problem. After a lot of research about the solubility of perbromides and organic ammonium salts I believe this might be possible using trimethylphenylammonium bromide, but such a complexing agent has never been tried! The 200 mL of solution used here would use 90.07g of ZnBr₂ and 86.45g of TMPB.

Note that this configuration would certainly not work without a complexing agent that precipitates the tribromide formed. Without it the bromine would pool at the bottom and discharge the cell – in a horizontal configuration – or just sink and discharge the cell in a vertical configuration.

Using all the materials above, the cost of building such a prototype would be in the order of probably 120 USD. Probably around 200 USD after you add shipping for everything. In reality this cell is also unlikely to yield 40Wh and will most likely be in the vicinity of 20Wh if everything works as expected.

It is also important to note that an ABS project box like the one above is a risky first-choice, given that ABS can adversely react with elemental bromine, so a PTFE project box would – although much more expensive – be a safer choice for a prototype. By the time I build something like this, I hope I have already established that TMPB forms insoluble enough perbromide salts under my much more controlled Swagelok cell conditions.

Note that I am still far away from executing something like this! Currently I am even far away from testing a TMPB cell, but I wanted to write this blog post to condense all this theoretical research and serve as a referring point for me or others in the future.