Zinc Bromine Batteries: Understanding the huge gap between theoretical and real energy densities

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The theoretical energy density of Zn-Br batteries is quite astounding. We can calculate this value for a given concentration of electrolyte by calculating the amount of ZnBr2 in one liter and then using Faraday’s constant and the expected output voltage (1.85V). Considering that each Zn atom is able to transfer two charges, we are left with this equation:

Theoretical Energy Density (Wh/L) = 1.85 * Molar Concentration * 2 * 96485.3329 / 3600

If we graph the theoretical energy density as a function of the concentration we get the plot below:

Theoretical capacity of Zn-Br batteries as a function of the ZnBr2 concentration.

Notice that if we could use all the Zinc in a solution, it should be pretty straightforward to get 50 Wh/L from a solution containing only 0.5M of ZnBr2. However, in practice most static and flow batteries use ZnBr2 solutions in a 2-3M concentration range and are able to only extract around 35-45Wh/L. Why aren’t we able to extract the 150+ Wh/L that is promised by the plot above at these concentrations?

The reason has to do with the conductivity of water, the migration of ions and the voltaic losses suffered by the battery. As we extract ions from an aqueous solution of ZnBr2, the electrical resistance will increase, as the solution becomes less conductive. This means that the voltage required to inject charges increases from the theoretical minimum – 1.85V – towards much higher values. As the solution becomes depleted of Zinc and Bromide ions it will also take much longer for ions to move across the battery and reach the electrode where they can be deposited, leading to further voltaic losses. If the battery is charged or discharged at higher currents, the above processes will be even more inefficient – as side reactions will happen – which will lead to even less efficiency.

Adding supporting electrolytes can help with the conductivity problem, but it will still not help a lot since the supporting electrolyte will lack either Zn or Br ions – or both – which will still limit the battery charging by how quickly these ions can migrate. In the end we are left with the need to use ZnBr2 itself as a supporting electrolyte, forcing us to create batteries that use only a fraction of the potential energy in the battery – around 10-30% at most – in order to prevent all these problems from being important.

Charging/Discharging a static ZnBr2 battery at a 0.5 ZnBr2 concentration

The plot above shows you what happens experimentally when we run a battery with 0.5M ZnBr2 saturated with TMPhABr (to sequester Bromide ions in the cathode) with a GFE-1 cathode pretreated with a 10% TMPhABr solution (saturated, then dried in air). In practice when we charge the battery to 5 mAh at 5mA we see a big increase in the charging voltage – because of the above mentioned voltaic loses – and the discharge curve shows our average voltage is only around 1.45V due to the higher resistance of the solution when we start the discharge process. In the end the energy density is only around 8.5 Wh/L, around one fifth of the capacity that we expected. The energy and coulombic efficiency values of the battery are also pretty low, as a lot of charges and energy are lost in the charging process.

The propensity for additional side reactions also means that there is going to be degradation in the organic materials and hydrogen evolution, both which will also increase the charging potential more and more with each cycle and lead to very premature death of the battery.

Because of all the above reasons, practical batteries are expected to have ZnBr2 concentrations of at least 2-3M and, even at these concentrations, it is unlikely for the energy density values to exceed 45-50 Wh/L. It might be possible to reach higher densities at higher concentrations but then we are left with a lot of additional problems, including higher dendrite formation and lower solubility of many elemental bromine sequestering agents, both things that are likely to kill the battery, making it unlikely that it will reach the expected higher density. Because of the above, a battery in the 2-3M ZnBr2 range that can substantially avoid dendrites and self-discharge is most likely the best that can be achieved with Zn-Br technology, either in a flow or static configuration.

Zinc Bromine Batteries: Dendrites, adhesion and failure

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This past week I did not post any new results for Zn-Br batteries. This is because I started to face significant reproducibility issues in my spacer based batteries with no separator. The image below shows you some of the typical curves I was getting from my batteries using PEG-200 containing solutions at a ZnBr2 concentration of 3M with different NaCl or NaBr additions. The battery started just fine – with CE values close to 90% – but fell sharply thereafter, with big increases in series resistance follower by large losses.

Screenshot taken from the measuring software I am using. This is cycling a battery to only 1mAh of capacity, the battery resistance starts to get higher and eventually fails very aggressively.

After a lot of investigation, the problem seems to be the adhesion of the Zn deposits to the anode’s graphite electrode. Even though the anode is always polished before every battery, the Zn deposits sometimes just “fall off” and – since there is no separator – that Zn falls to the cathode and is thereby lost and simply reacts slowly with the bromine. This was confirmed by moving again to a Zn metallic anode (0.2mm thickness) which didn’t show the above problems, as you can see in the curves below.

Although relatively normal CE and EE values were achieved for this battery configuration, dendrite formation was evident, both in the charge/discharge curves and after taking the battery apart (where dendrites were quite large). It is clear, both from NaCl and NaBr experiments, that additions of these supporting electrolytes contributes heavily to dendrite formation. It also seems pretty clear that going from 1% PEG-200 to 6% PEG-200 or higher doesn’t help enough with dendrite formation – they still form, even if a bit slower – but the heavy increase in series resistance is not worth the trade-off. If you try to add more PEG-200 and reduce the series resistance with NaBr or NaCl, then you just get the dendrites again.

3M ZnBr2 battery with a 1.7M NaCl and 6% PEG-200 addition. Charged to 15mAh at 15mA, discharged to 0.5V. Zinc anode (0.2mm) and GFE-1 cathode pretreated with 10% TMPhABr.

From these experiments, it is now pretty clear why commercial ZnBr2 batteries do not use PEG-200 as an additive – at least in very large quantities – it might work to suppress formation of Zinc dendrites at lower ZnBr2 concentration (<1M) but at the concentrations required for energy density values greater than 30-40 Wh/L it just doesn’t seem to work well enough. Furthermore, while PEG-200 can be used with little effect in highly conductive KOH solutions that are used in some Zn chemistries (like Zn/Mn oxide batteries) it just doesn’t work when the electrolyte’s conductivity is significantly lower, such as is the case with ZnBr2 solutions.

All hope is not lost though. While PEG-200 by itself might not be able to prevent dendrites in this configuration, it is possible that low concentrations of PEG-200 plus other additives might have a synergistic enough effect to help us alleviate the problem. One such potential case is with the use of PEG-200 and Tween-20, which at 0.5% each, have shown to be both quite effective and synergistic at reducing Zinc dendrites. The experimentation continues!

Zinc Bromine Batteries: PEG-200, bubbles and over-potential

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In my latest separator-free cells that use a PTFE o-ring spacer, I am now testing some additives to reduce dendrites and increase the life of the cells. A popular additive – PEG-200 – has proved not to be viable at a concentration of 20% due to large losses in the cell’s voltaic efficiency, moreover PEG-200 at a concentration of 1% offers little protection against dendrite formation. This last experiment tried a PEG-200 concentration of 6%, coupled with a small amount of NaCl to attempt to increase the conductivity and compensate for the loss caused by PEG-200.

Charge to 25mAh at 15mA, discharging to 0.5V. Electrolyte contains 0.1M NaCl, 3.0M ZnBr2, 6% PEG-200

Above you can see the charge/discharge curve measured for this device. Compared to my previous devices the Coulombic and energy efficiencies have dropped significantly, with the most dramatic drop being in the energy efficiency. This value has dropped more than 10% relative with previous devices using a 1% PEG-200 concentration at the same zinc bromide concentration.

A device with this energy efficiency will not be viable, so I saw no need to cycle the battery multiple times. However to answer the question of whether zinc dendrites are formed or not, I then charged the cell a second time to 25mAh and opened the device, taking the picture of the anode shown below (I apologize if it seems out of focus, it wasn’t very easy for me to focus on such a small amount of space with my camera).

Graphite anode after one charge/discharge cycle and a subsequent charge cycle to 25mAh.

The picture above shows some interesting results. First, it was evident that there was absolutely no zinc dendrite formation, the plating was very crystalline and the electrode was flat with no protruding dendrites. Previous cells that had dendrite related failures show very tall dendrites that can easily be seen with the naked eye, even after only a few cycles. However you can also see several big and medium holes in the electrode where absolutely no zinc was deposited, this was caused by “air bubbles” trapped when the Swagelok cell is closed, which I haven’t been able to find a method to consistently remove. These bubbles remove so much of the surface area of the electrode that they can be responsible for significant losses in voltaic efficiency. Pre-wetting the electrode seems to be a viable method to ameliorate the issue but isn’t perfect.

In order to see if a cell like this can be viable, I am now testing a 6% PEG-200, 3M ZnBr2, 1.7M NaCl electrolyte, which should dramatically reduce the voltaic losses caused by the PEG-200 by increasing the conductivity of the electrolyte. Stay tuned for these results.

Zinc Bromine Battery: Teflon o-ring separators, capacity and PEG-200

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In my previous post, I described my first tests of separator-less batteries using a PVC spacer. This turned out not to be a very good idea, due to the reactivity of PVC with bromine. Although the battery was able to run for 20+ cycles successfully, a lot of noise started to happen within the measurements. After opening up the battery, it was evident that the separator had degraded (it turned from black, to a whitish gray color). Due to this reactivity I decided to change my plans to work with Teflon o-rings as spacers (which I bought here). These are of the exact diameter I needed. Given the height of the spacers, I decided to use 3 (total height of 5.2mm) in order to match the same height of the previous cells I was building using fiberglass separators. This gives a total battery volume of around 0.68mL, counting the volume of the separators.

Battery built using PTFE spacers, GFE-1 cathode pretreated with 10% TMPhABr + 3M ZnBr2 + 1% PEG-200. Charged to 15mAh at 15mA, discharged to 0.5V.

The results for the first battery tested using this configuration is shown above. The energy density of this battery is around 30Wh/L and I was able to cycle it at this current density and charge capacity for 25 cycles without running into any problems or instabilities. At this point I decided to test what the maximum capacity of the battery could be, by charging the battery until the potential reached 2.1V.

With this test I was able to charge the battery to a capacity of around 60Wh/L, but this capacity usage is not sustainable given that the battery completely died on the next cycle due to the formation of a large amount of Zinc dendrites. This means that the usable capacity under this amount of 3M ZnBr2 electrolyte is likely to be around 75% of this value – given what we know from published research and patents – which should be at least 25mAh.

Maximum capacity test charging the battery until potential reached 2.1V at 15mA. We were able to recover 26.56 mAh discharging to 0.5V.
Charge/discharge curves of a battery with the same configuration charging to 25mAh at 15mA, discharging to 0.5V. Notice the instabilities on discharge.
Evolution of efficiency variables as a function of the cycle for the battery shown on the previous image.

After building another battery and charging to 25mAh – taking the energy density to ~45 Wh/L – there were substantial instability issues appearing on the discharge curves after 7 cycles. I believe these instabilities are due to Zn dendrites that fall from the anode into the cathode, temporarily killing the discharge potential of the device until the Zn dendrite is dissolved. These instabilities are correlated with loses in both the Coulombic and energy efficiency values of the battery, deteriorating the performance as a function of time.

Due to the above issues, it seems important to try to reduce dendrites to prevent problems at these capacities. I decided to try a PEG-200 additions at 20% to see what would happen. With this configuration, a 20% PEG-200 addition generated too much voltaic loses because of the huge increase in internal resistance. Even when charging/discharging to only 1mAh, the necessary potential was already above 2.15V, with the energy efficiency dropping below the 35% mark. You can see one such cycle in the image below.

Battery built with a 20% PEG-200, 3M ZnBr2 electrolyte. Otherwise identical to other batteries shown in this post.

Because of the above results, it is clear that a PEG-200 addition is likely going to need to be below the 10% mark in order to be viable. I have since prepared an electrolyte comprised of 3M ZnBr2, 6% PEG-200 and 0.1M NaCl in order to see what the behavior is when trying to charge to these higher capacity values. Up until now charge potentials at 15mA are higher than for the 1% PEG-200 cells, but low enough (2-2.1V) to prevent heavy voltaic loses. We’ll see what sort of efficiencies and Zinc deposits we can get with this electrolyte configuration.

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

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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.