Zinc Bromine Batteries: Making higher purity zinc bromide from readily available salts

Although Zinc Bromide is a readily available commodity chemical in some parts of the world (readily available in bulk quantities or for businesses), it is difficult to source in places like the US due to a lack of retail applications to justify its sale to the general public. Small amounts can be bought from sources on ebay/amazon, but the cost can often be above 1 USD/gram with similar costs from Alibaba when buying small quantities (<10kg). For people wanting to do small/medium scale experimentation on Zn-Br batteries, it is often impractical to buy in the necessary quantities, so a low cost synthesis of Zinc Bromide from readily available chemicals is desirable.

Crude Zinc Bromide produced through the process described in this article before being put into the desiccator.

The cheapest solution I have found comes from the use of readily available Zinc Sulfate and Sodium Bromide. Both of these salts are available in high purity at very low retail prices (<10 USD/kg even when buying sub kilogram quantities). Since Sodium Sulfate is significantly less soluble than Zinc Bromide, preparing a very concentrated solution containing both salts leads to the precipitation of Sodium Sulfate with Zinc Bromide remaining in solution. The problem with this approach is that the solution always contains a substantial amount of Sodium Sulfate and the many separation/concentration/crystallization steps involved to obtain higher purity ZnBr2 make it a rather impractical approach if higher purity Zinc Bromide is required or if you want to achieve the result quickly.

My solution to this problem is to use a readily available organic solvent – rubbing alcohol – to help speed up the process and achieve better results. Here’s a summary of the synthesis for a small amount:

  1. In a 50mL tall beaker add ~20mL of distilled water and dissolve 5.5g of Zinc Sulfate Monohydrate and 6.1g of Sodium Bromide. Note that an excess of the Zinc Sulfate is used, as it is the least soluble of the reagents.
  2. Stir the mix until everything is fully dissolved. All the formed salts should be soluble enough to dissolve in this amount of water at 25C (77F)
  3. To this solution, add ~20mL of rubbing alcohol.
  4. Stir the mix for a couple of minutes.
  5. Let the solution set until everything separates. Three distinct layers should form (one solid, two liquid). The Sodium Sulfate will precipitate and a bottom aqueous layer will form while a top alcohol layer will remain.
  6. Decant the top alcohol layer and collect it. This layer contains no salts and can be used for future batches.
  7. Transfer the bottom aqueous phase to another container. The solid precipitate is normally well formed enough as to allow for transferring the bottom aqueous layer to another container without the need for filtering. The remaining solid is sodium sulfate plus the excess zinc sulfate.
  8. The aqueous layer transferred contains all the Zinc Bromide. You can heat and evaporate most of the water, but bear in mind that the Zinc Bromide will hold very tightly to it and might start to sizzle and “erupt” aggressively as you heat more and more (it’s similar to when water splashes into hot oil). Since the aqueous layer contains some alcohol make sure you do this in an open or very well ventilated space. It is better to reduce the volume until the point where the ZnBr2 starts to crystallize and then transfer it to a desiccator so that it can finish its drying process. A container with anhydrous Calcium Chloride or anhydrous Magnesium Sulfate can do the job.

The above described process – using rubbing alcohol – has the advantage of producing the Zinc Bromide quickly, without the need to perform successive steps of cooling/decanting/filtering/crystallizing, etc. Since both Sodium Sulfate and Zinc Sulfate are almost completely insoluble in alcohol containing solutions – while Zinc Bromide is not – this leads to a significantly faster and more satisfactory synthesis from readily available chemicals. Using a slight excess of zinc sulfate is recommended to avoid the presence of Sodium Bromide in the final solution.

The only tricky part is taking the Zinc Bromide out of the final solution. I prefer not to heat it till it’s completely dry, since the sizzling and “eruptions” of the Zinc Bromide can be pretty aggressive as it tries to hold dearly to every milligram of water it can manage to. Besides, if you heat it to dryness it will quickly become wet again as it cools unless you immediately put it inside a desiccator. It is therefore preferable to put this in a desiccator as soon as Zinc Bromide crystals start to appear and let the drying agent get all the water out of the Zinc Bromide. Note that a desiccator doesn’t need to be anything fancy, some air-tight tupperware you fill with a good enough drying agent can do (the drying agent needs to be more hygroscopic than Zinc Bromide).

I haven’t scaled this process up – as I only work at very small scales – so I don’t know what problems could occur at larger scales. Since it involves alcohol I would advice working at a small scale to see if this process might fit your needs and to be careful and follow all safety precautions.

Zinc Bromine Batteries: Trying a configuration without a solid separator

During my quest to build and characterize a zinc-bromine battery, I have mainly focused on the use of non-woven fiberglass separators between carbon electrodes in order to hold the electrolyte in the device. This is because some of the research papers with the most promising results for these batteries use this type of separator. Having a solid separator makes the battery easier to construct and easily translates to the construction of both coin-cell and pouch devices, which are the two classic prototype devices used in the modern battery industry. Moreover, a solid separator substantially increases the mechanical stability of the cell and reduced the influence of gravity on the device, making it less susceptible to changes in orientation. The image below details the best structure I have achieved so far for this type of device.

Most successful structure so far for the batteries I have built in my Swagelok cell. Note the cell was never measured in any preferred orientation, the cell was placed horizontally for measuring.

However, one big problem of having a solid separator has been the existence of edge effects at the border of the separator. Batteries have grown Zinc dendrites around the edges of the fiberglass separator, despite my best efforts to attempt to avoid them. While using a 20% PEG-200 concentration and increasing the concentration of Zinc Bromide to 3M helped to largely eliminate dendrite formation, none of these or other modifications were able to fully prevent the problem.

Dendrites formed within a solid separator are quite problematic, because they cannot be fixed and lead to extremely limited battery lifetime. Even if the battery is fully shorted to try to eliminate these dendrites, the mechanical damage to the separator caused by the dendrite structure is permanent, unless more expensive self-healing separators are used. Furthermore, a dendrite can partially react with perbromide coming from the cathode and be cut “half-way” effectively leaving some Zinc stranded in the middle of the battery which is only going to be slowly removed by reactions with diffusing bromine or perbromides.

New battery cell structure I am trying. Note that the batteries are going to be tested in this exact orientation, as the lack of a solid separator means that bromine will accumulate in the bottom electrode, so this electrode has to be the cathode.

For the reasons highlighted above, I have decided to try separator-less batteries in order to see if these batteries can effectively avoid the dendrite problem while still retaining or improving on the coulombic and energy efficiency values I have achieved so far. To do this I have used a 2mm piece of PVC shrink tubing as a separator, cutting it so that it forms a 2mm x 35mm strip that can go around the internal diameter of my Swagelok cell (0.5 inch diameter) and prevent the upper electrode from contacting the bottom electrode. The space is then filled with electrolyte and the cell is closed, with the spacer preventing any additional compression of the cell.

The biggest advantage of this configuration is that it can be easily maintained, since the cell can be opened and the electrodes can be easily changed or cleaned. The lack of a solid spacer also means that any Zinc dendrites that form will either dissolve as they touch the lower electrode or fall and just discharge the battery, while they will never be “stranded” in the middle of the battery. Large amounts of dendrites might still lead to battery shorts, but given that I am using 20% PEG-200 and there are no longer any separator related edge effects, I hope this will stop being a huge problem.

Coulombic and energy efficiency as a function of the charge/discharge cycle for first battery under the above configuration
Charge/discharge curve for last cycle in the previous image. Charge/discharge is done at 10mA.

The first prototype battery put together in this configuration – 3M ZnBr2 + 20% PEG-200 solution – has been able to survive 6 charge/discharge cycles, charging to 10mAh and discharging to 0.5V, both at 10mA. The battery has an energy density of 21.6 Wh/L, given its current geometry (all battery components) but I am hopeful this energy density can be pushed closer to 40 Wh/L after I confirm that dendrite formation is not an issue under this electrolyte conditions. Previous batteries in a fiberglass separator were able to sustain 10-20 cycles under these conditions before showing important shorting issues due to dendrite formation, so we will see how far this prototype battery can go before disaster strikes.

Zinc Bromine: Pushing energy density beyond 40 Wh/L

For Zinc-Bromine batteries, energy density is a key characteristic since these batteries are bound to be used under circumstances where the specific energy (Wh/kg) is not as relevant as the amount of space taken by the batteries to store a given amount of energy (like utility level energy storage). Given this fact, I wanted to explore how hard I could push the capacity of a Zn-Br battery to try to maximize its energy density.

I built a cell with a GFE-1 cathode pretreated with a 50% TMPhABr solution, used 16 layers of fiberglass separator (total cell height 0.53cm, total area 1.29cm2, volume 0.68mL), used a 3M ZnBr2 + 20% PEG-200 solution to minimize dendrites as much as a I could. I then tried to charge the cell to 30mAh, see what sort of efficiencies I could get. I used a current of 5mA since the 20% PEG content and highly loaded GFE-1 cathode both substantially increased the internal resistance of the device.

The battery was charged to 30mAh, then discharged to 0.5V, both charge and discharge were performed at 5mA. CE=81.08%, EE=62.35%.

Given the results shown above, I was able to achieve an effective stored charge of 18.7mAh, which gives the cell an energy density of 43.76 Wh/L. This puts the battery above the values that are achieved for commercial Zinc-Bromine flow batteries (5.7–39 W·h/L). However not everything was as good as I thought, as the battery shorted during the second cycle due to the formation of Zinc dendrites. I was however very puzzled by the presence of Zinc dendrites at a 20% PEG-200 concentration, so I decided to open up the battery and peel the layers to see what was going on.

As you can see in the image below, zinc dendrites form predominantly across the first 3 layers of fiberglass separator, which means that the PEG-200 was indeed effective at preventing dendrite formation from advancing too much through the battery (without PEG-200 you would see a significant presence of dendrites all the way to the cathode). However there were some Zinc dendrites forming predominantly close to the edge of the battery and these progressed all the way to the cathode material, although it is very hard to see their presence without magnification within the last couple of layers.

Layers of Zinc-Bromine battery after the battery was shorted by Zinc dendrites.

The fact that I am using a Zinc anode that is cut from a 0.2mm sheet with a perforator might have something to do with it, as the Zinc is bound to be extremely sharp at the edges – therefore high surface area – due to the cutting process. This is the perfect spot for the formation of dendrites and – due to the smaller amount of electrolyte at these points – could easily lead to the formation of dendrites moving through the battery, which is what we have observed. This also happened at a point where the Zn anode was particularly sharply cut, which further reinforces this hypothesis.

In order to see if the Zinc anode and the way it’s cut has a lot to do with this fact I have decided to repeat the above experiment using the graphite electrode as anode – without the presence of any Zinc anode – which should show if zinc dendrites are able to form all the way to the cathode in the presence of large concentrations of PEG-200. If Zinc dendrites do not form in this case, I will move to the use of graphite for the anode material from now on.

Zinc Bromine Batteries: Trying to improve energy efficiency above 80%

My experiments using carbon cloth cathodes have helped me construct some decent static Zinc-Bromine batteries. In particular, the CC4 carbon cathodes have been very flexible and have been used throughout most of my experiments. The last experiment I did, with a CC4 cathode previously soaked in a 50% solution of TMPhABr solution and then air dried, have shown a CE=91% with an EE=70% at a charge/discharge current of 5mA, charging to 3000 uAh and discharging to 0.5V.

Coulombic and energy efficiencies as a function of the number of cycles for a pre-treated CC4 cathode (soaked in 50% TMPhABr and air dried) with a 3M solution ZnBr2. Charged to 3000uAh at 5mA, discharged to 0.5V.

Charge/discharge curves for all the cycles in the first figure.

Coupling these cathodes with a ZnBr2 3M solution with 10% PEG200 has allowed me to achieve specific power values in the region of 30-40 Wh/kg – total weight of cell – with more than 40 charge/discharge cycles (see above), without the formation of any Zinc dendrites (which would short batteries after only 10-20 cycles at this charge density in previous battery tests). Higher PEG200 concentrations cause significant increases in the internal resistance of the cell while lower concentrations (<5%) are just not effective at preventing Zinc dendrite formation when using metallic Zinc anodes.

Despite the good results, I have yet to achieve high energy efficiencies, mainly due to a couple of problems. The first is that significant bromine diffusion is happening due to a lot of bromine being formed at the surface of the CC4 cathode without enough presence of TMPhABr to capture it and the second, that the internal resistance of the cell was still significantly high, owing to the significant resistance of the CC4 cathode being used.

First charge/discharge curve for a GFE-1 felt electrode, pre-treated with 10% TMPhABr. Charged/discharged at 5mA, charged to 3000uAh, discharged to 0.5V.

In order to attempt to solve these problems, I have decided to change to a carbon cathode that is both significantly more conductive and possesses a significantly higher surface area compared to CC4. My choice material being the GFE-1 carbon felt. For the first test I have soaked a piece of cathode in 10% TMPhABr and air-dried it before use.

You can see the first charge/discharge curve ever produced in this configuration above. The charging potential is already significantly lower than that of the CC4 electrode and the discharge potential significantly higher, both signs of a markedly lower internal resistance. For this first cycle the Coulombic efficiency was 79% while the energy efficiency was 72%. We’ll see if the CE and EE of this battery improves as its cycled and whether or not this cathode leads to more stable cycling than the CC4 cloth electrodes!

Zinc Bromine Batteries: Solid TMPhABr layers are not the answer

My latest efforts to build higher capacity Zinc-Bromine batteries, have focused on the use of solid TMPhABr layers, because the solubility of TMPhABr is very low in the presence of high concentrations of ZnBr2 (2-4M). The idea by doing this was to provide a relatively stable source of TMPhA+ cations that could be taken to the cathode and be used to form an insoluble perbromide as bromide is reduced to elemental bromine and then sequestered by the quaternary ammonium salt.

Evident formation of perbromide oustide the cathode material due to movement of elemental Bromine to the TMPhABr solid layer.

However, the solubility of TMPhABr is too low for this and what happens is that the cathode mainly generates elemental Bromine, which then flows through the battery and is converted – outside the cathode – into TMPhABr3 as it reaches the TMPhABr solid layers. What happens is that the perbromide is fixed outside the cathode, and only the portion that is in contact with the cathode is ever able to be reduced to contribute to the battery current during the discharge phase while the part that is far away from the cathode becomes “dead capacity” and is never able to be regenerated again.

This is evident by looking at disassembled batteries – see image above – where the yellow/orange perbromide is present across the battery separator, showing that elemental bromine was produced, migrated, reacted with the organic ammonium salt to form the perbromide and was then unable to be recovered because of its distance from the cathode. This is also showed by the loss in both energy and Coulombic efficiencies for batteries that use this solid layer at higher ZnBr2 concentrations, compared with the cells that used fully dissolved ZnBr2 0.5M + TMPhABr 0.25M. The Coulombic efficiency drops from >95% to <80% while the energy efficiency drops from >80% to <70%.

New cell structure proposed, using a cathode material that has been soaked in a 50% w/v solution of TMPhABr.

The best way to implement this solid TMPhABr strategy might actually be to introduce this solid within the structure of the cathode material (see proposed structure above). For this I have prepared a 50% w/v solution of TMPhABr (it is extremely soluble in distilled water), immersed two CC4 cathodes into it and I am now waiting for these to dry. Once they are dry I will be able to place them within batteries and run an experiment – without any solid layer – to see if this actually improves the results.

Zinc Bromine Batteries: Problems at higher capacities with TMPhABr

As you saw on my previous post, I was able to generate pretty decent results with TMPhABr when using Zinc Bromide solutions at 0.5M with an addition of 0.25M of this quaternary ammonium salt. However it is pretty clear that at this concentration of Zinc Bromide the specific energy is too low, so I subsequently tried to reach higher efficiencies by trying higher concentrations of Zinc Bromide with a solid layer of TMPhABr (since at >0.5M of ZnBr2 the solubility drops too much). My experiments were done with the cell configuration showed below. The electrolyte also contained 1% of PEG-200 in order to prevent dendrite formation.

Battery structure for tests shown below.
Charge/discharge curves charging to 2000 uAh at 2mA and discharging to 0.5V at this same current. Last value was CE=86.41% and an EE=68.74%. This electrolyte contained a 2M solution of Zinc Bromide.

These experiments were quite successful, with a Coulombic efficiency of 86.41% and an energy efficiency of 68.74%. The capacity of these devices was increased by 4x over my previous experiments at 0.5M of ZnBr2 showing that the solid layer of TMPhABr does work in order to generate insoluble perbromides within the battery. However the battery performance did start to degrade at around the 10th cycle, so I stopped cycling the above battery to see if I could get better behavior at even higher Zinc Bromide concentrations since the increase in ZnBr2 concentration did show a reduction in the internal resistance of the battery.

Charge/discharge curves for a 3M Zinc Bromide electrolye, where an attempt was made to charge to 5000 uAh and discharge to 0.5V at a current density of 5mA. Highest CE=74.71%, EE=55.76%

The attempt to use higher concentrations at higher current densities were not very successful. Although the capacity was increased to around 10x of my initial battery, the problem was that both the Coulombic and energy efficiencies dropped to unacceptable levels. The charging voltage also saw substantial climbs – reaching almost 2V – which probably created a lot of unwanted reactions. The worst problem was however the zinc dendrite formation, which became apparent after I tried cycles at lower capacity and current density for the same cell. You can see below that at the fourth cycle the charge voltage drops suddenly and then the discharge is extremely inefficient. This is because dendrites have pierced the separator effectively shorting the battery.

Curves where I attempted to charge to 2000 uAh and discharge to 0.5V at 2 mA.

This dendrite issue is one of the most important problems in Zinc-Bromine batteries – both flow and static – and one of the reasons why rechargeable Zinc chemistries have not been massively adopted thus far. If the above batteries are to be practical, I need to find a setup that provides both high capacity – which means a 3M ZnBr2 electrolyte – with the elimination of Zinc dendrites. The addition of PEG-200 helps, but it is clearly not enough to eliminate this issue. Upon opening the above battery, it was evident that dendrites had completely pierced through the entire separator and shorted the electrodes.

One hypothesis I have is that local formation of Zinc dendrites should be hindered by high local TMPhABr concentrations (since they do not form when high amounts of this are dissolved) so a potential solution is to create another solid layer of the TMPhABr next to the Zinc anode (as shown below). I am currently testing the battery configuration shown below to evaluate this hypothesis.

Current testing configuration to attempt to remove Zinc dendrites by a much higher local concentration of TMPhABr close to the Zn anode.
Curve for the above cell charged to 3000 uAh and discharged to 0.5V at 2mA. CE=76.51%, EE=61.01%

Another issue that has been pointed out to be is the absence of additional support electrolyte, so I am planning to test ammonium sulfate at 2M to see how this modifies the behavior of my batteries at these higher capacities. Ammonium ions will turn my battery more acidic, so I am expecting some losses in Coulombic efficiency at higher current densities from a more favorable hydrogen evolution potential.

Zinc Bromine Batteries: Going for high capacity with TMPhABr

The initial tests using TMPhABr have been a complete success. A battery made with 0.5M ZnBr2 + 0.25M TMPhABr charged to 500 uAh and discharged to 0.5V was able to achieve stability past 100 charge/discharge cycles at 2mA and more than 100 charge/discharge cycles at 5mA. There was a significant drop in energy efficiency when going to higher current densities (from 75% at 1mA to 66% at 2mA) but overall the Coulombic efficiency remained high through the entire testing, at values greater than 90% and in some cycles greater than 95%. This was also all using a CC4 carbon cloth cathode, which means I made no effort to optimize the cathode at all. The cell showed a difference of around 50mg between the dry state and discharged wet state, meaning that overall it contained around 30-40uL of solution (I haven’t measured the density of the ZnBr2+TMPhABr so I don’t have an exact answer).

RE: My adventures building a Zinc-Bromine battery
100 charge/discharge cycles at 2mA. Charged to 500 uAh and discharged to 0.5V.
70 charge/discharge cycles at 5mA. Charged to 500 uAh and discharged to 0.5V.

These results are extremely encouraging because they show that the TMPhABr is a way better behaved sequestering agent for bromide relative to TBABr. Most notably the tests also show a lack of performance degradation from Zinc dendrite formation, which was a big problem in the TBABr experiments. The charge/discharge curves are also significantly better behaved with a much longer and more stable “discharge plateau” which implies more stable electrochemical performance. There is also a complete absence of rare shoulders or spikes in the curve, which hint that important additional electrochemical processes are absent.

The CE and EE of the cell are always significantly lower when running the first few cycles, indicating that the formation of some surfaces or species is necessary for the cell to reach peak performance. This is likely due to the need for TMPhABr3-friendly sites to form, as the Br oxidation side is expected to be the rate limited process in this type of device. Since I’m using a Zinc anode, the formation of Zn nucleation sites is not expected to be significantly difficult.

A sample charge/discharge curve measured at 5mA. Notice the long discharge voltage plateau.

The biggest issue right now is that a cell like the above has a really low specific energy (around 2.8 Wh/kg), so a very substantial increase is required to make the above cell viable. I suggested some modifications in one of my last posts but it is clear that a cell with a ZnBr2 concentration lower than 2M is simply not going to be able to provide an adequate density. Given the solubility limitations of TMPhABr, we are unlikely to be able to achieve this using just a mixed solution of this sequestering agent and Zinc bromide.

My idea to solve this problem is to include a layer of solid sequestering agent in the battery and use a saturated solution of TMPhABr in 2M ZnBr2 as an electrolyte. The TMPhABr won’t be dissolved right away, but it will be slowly transported by the Zinc Bromide solution as TMPhABr3 is deposited in the cathode of the cell. Hopefully the process reverses when the cell is discharged and we’re able to get a cell that can successfully charge/discharge at high densities without the need for all the TMPhABr to remain in solution.

Suggested cell structure using a starting solid layer of sequestering agent

I expect that a cell like this will have way longer stabilization time – as the TMPhABr migrates through the cell and forms a stable structure in the cathode, hopefully without dramatically hindering its functionality. I also hope that the much higher ZnBr2 concentration won’t increase the formation of Zn dendrites or that the formation of these dendrites will be curtailed by the presence of a TMPhABr solid layer at some point.

The above cell design is now in testing, so we should see if we can achieve charge/discharge cycles to 2000 uAh!

Zinc Bromine Batteries: First tests using TMPhABr

As I’ve mentioned in previous posts, tetrabutylammonium bromide (TBABr) is not a very good sequestering agent for static Zn-Br batteries due to its very low solubility in Zinc Bromide solutions. To solve this problem, I have decided to test trimethylphenylammonium bromide (TMPhABr) as a potential replacement, since this salt also forms and insoluble perbromide but – due to its significantly higher polarity and lower molecular weight – should be significantly more soluble than TBABr. I ordered it from Alibaba around one week ago and recently got it delivered.

Picture of the TMPhABr I got from China

My initial tests with it involved testing its solubility in Zinc Bromide solutions. The solubility of TMPhABr in pure water is not indicated clearly anywhere, but I assumed its solubility would be similar to that of trimethylbenzylammonium bromide (TMBABr) or tetrapropylammonium (TPABr) bromide, both which have solubilities of around 10% by mass in water at 25C. My initial tests have confirmed this suspicion with solutions at 10% by mass being easy to prepare at 20-25C. I didn’t try to prepare more concentrated pure solutions as my objective is to judge its solubility in the presence of Zinc Bromide.

The first test I performed to evaluate this was a 0.25M solution of Zinc Bromide which was able to dissolve 0.12M of TMPhABr with no problems. I then increased the amount of ZnBr2 to 0.5M – which is what the authors of the Chinese paper using ZnBr2+TPABr use – and I was able to dissolve 0.25M of TMPhABr without issues. With this result I know I will be able to at least reproduce similar experimental conditions to those achieved by the Chinese researchers, something that I could never do with TBABr due to the solubility issues mentioned before.

To test how far I could take this I then attempted to prepare a 1M solution of Zinc Bromide and see if I could get 1M of TMPhAbr to go with it. Sadly at this point the concentration of TMPhABr is already too high – would be close to 10% by weight of the solution – so it was actually not possible to get to this point. This means that the practical limit of this battery will be to have around 0.25M of TMPhABr dissolved, which is probably a realistic limit for most quaternary ammonium salts since we are unlikely to get an effective sequestering agent – not electrochemically active and with no effect on pH – with a molar mass significantly lower than that of TMPhABr at a similar price point.

First two charge/discharge curves measured (at 2mA constant current). Battery was charged to 500 uAh and then discharged to 0.5V. First curve, CE=68%, EE=57%. Second curve, CE=79%, EE = 66%.

I then used this 0.5M ZnBr2 + 0.25M TMPhABr solution to create the first battery. This battery had a diameter of 0.5 inches and was built within my Swagelok cell. I used a 0.2mm thick Zinc anode followed by 8 layers of fiberglass separator and a CC4 carbon electrode. I also made sure to sand the graphite electrodes in the Swagelok cell to make sure their exposed surface was pristine. I put 50uL of the electrolyte on the cell but I won’t know how much ended up in the separator until I open the cell after testing and weight the wet components.

The graph above shows the first – to the best of my knowledge, the first ever public – charge/discharge curves of a static Zn-Br cell prepared using TMPhABr as a sequestering agent. It is very interesting to note that the shape of the discharge curve improved immensely moving from TBABr, showing that this battery is significantly better behaved. Although the CE and EE of this first curve were particularly low, the CE of the second curve measured already showed an increase of the CE to 79% and EE 66%. I will keep cycling the battery and will show you how the CE and EE change as a function of the number of cycles. Exciting times!

Zinc Bromine Batteries: What would be realistically required?

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.

An image of one of my current Zn-Br battery cells. The cell has a diameter of 0.5 inches and is placed inside a Swagelok cell with graphite electrodes for measuring

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).

Zinc Bromine Batteries: Can we just put solid TBABr in there?

I have mentioned how the usability of TBABr in Zn-Br batteries is limited due to the poor solubility of TBABr in the presence of large concentrations of zinc bromide. In my experiments the most concentrated solution I was able to get was around 0.1M ZnBr2 + 0.1M TBABr. This is problematic since we aren’t going to achieve high specific energy or power values with an electrolyte that is this dilute in terms of Zn concentration. However, what if we put a suspension into the cell as the electrolyte?

Image of the TBABr + ZnBr2 suspension prepared

When the cell is charging, the concentration of TBABr in the electrolyte will go down as TBABr3 precipitates out of solution. However, if there is extra TBABr within the cell, that solid will dissolve to replace the TBABr that precipitated. When the cell is discharged, the process will reverse, TBABr3 will redissolve and some TBABr will precipitate again as it is pushed out of solution by the perbromide that needs to go back into solution. The conductvity of the solution should be less affected, because it will only be reduced as a function of the loss of ZnBr2, without an actual loss of TBABr. The problem of course, is that there will be some solid TBABr in the cell, which is likely to increase the series resistance of the cell (because the solid salt is not a conductor).

How do we achieve this? To do this I first put 0.720g of ZnBr2 into a 10mL volumetric flask, then dissolved that into 1mL of distilled water. I then added as much 1M TBABr solution as needed to fill the volumetric flask to 10mL. The total concentration of ZnBr2 is around 0.33M but a lot of “solid” precipitates out of solution, forming a high viscosity phase with the consistency of honey that is made almost entirely out of TBABr. If we agitate the flask, this phase gets suspended into solution quite easily, forming a cloudy suspension (see image above).

Evolution of CE and EE as a function of the cycle number. The cell was charged to 500 uAh and discharged to 0.5V, both at 1mA.

I then built a battery within my graphite electrode Swagelok cell using a zinc anode, 8 layers of fiber glass separator and a carbon felt cathode. I then added 100uL of the above prepared suspension right after agitating the flask vigorously, allowing the material to wick through the cell for a minute before closing the Swagelok cell.

I have since started doing charge/discharge cycles of this cell with very interesting results (see above). The cell initially had relatively low coulombic efficiency (CE) and energy efficiency (EE) values, but these started improving as the cell was cycled. My hypothesis is that – per my previous explanation – the solid is first randomly distributed within the cell but gets organized and deposited within the cathode as the number of charge/discharge cycles increases. I believe this greatly improves the formation of the TBABr3 within the cathode and prevents the solubilization of the perbromide, which reduces self-discharge and therefore increases the cell’s efficiency.

All charge/discharge curves for the cell up until now.

I believe we can see some experimental evidence for this hypothesis as we see a “shoulder” emerge at the start of the charge phase as the number of cycles increases. I think this is consistent with a significant amount of TBABr deposited close to the cathode interface after discharge, which creates a higher resistance to current flow that subsides as the TBABr3 starts forming and this TBABr dissolves back into solution. This is of course an interpretation based on very limited information and I would be thrilled to know what any of you think about the evolution of the charge/discharge curves and what you believe they are telling us. I will continue cycling this cell during the next 2-3 days, to see how the cell stabilizes and whether the CE and EE start going down after.

With that said, it seems pretty clear that TBABr by itself is not going to be an adequate sequestering agent. I will be trying to use PEG200 to increase its solubility – as discussed in some of my previous posts – but I also already ordered TMPhABr (trimethylphenylammonium bromide) as I believe this will be a way better sequestering agent for these devices.