Monthly Archives: November 2020

Zinc Bromine Batteries: Increases in resistance when using TW20+PEG-200 containing solutions

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As I showed in my last article on Zn-Br batteries using a Tween20+PEG-200 electrolyte, the batteries had no issues with dendrite production on the zinc anode but faced substantial deterioration of both the charging and discharging voltages as a function of time. I repeated the experiment using a Zinc anode to see if this phenomenon was consistent. The tests below were carried out for a battery that used a GFE-1 cathode pretreated with 10% TMPhABr and an electrolyte containing 1% PEG-200, 1% Tween 20 and 3M ZnBr2. The cell used a 0.2mm Zinc anode and graphite electrodes within the Swagelok cell.

Charge/discharge curves, charged to 15mAh at 15mA, discharged to 0.5V.
CE and EE as a function of the cycle. The cell was run for 8 cycles.

As you can see in the images within this post, although the CE and EE values of the battery remain fairly constant through the test, the mean charge potential increases and the mean discharge potential decreases as a function of the cycle number. This is showing that there is an overall increase in the internal resistance of the cell as a function of time, which appears to be linear and doesn’t seem to become smaller with time. Other cells that were charged to even larger numbers of cycles continued to show this deterioration, up to the point where the EE and CE of the cell started to decay and the cell started to fail.

This behavior points to some irreversible process happening within the device that is fundamentally affecting internal resistance. This has to be either a surface modification of the electrodes or an irreversible loss of conductivity in the solution. The first can happen due to bromide/perbromide interactions in the cathode, zinc oxide deposits in the anode caused by a local increase in pH due to hydrogen evolution or even the appearance of some insoluble polymers in the cathode due to electrochemical reactions of the additives.

Mean charge potential as a function of the cycle number
Mean discharge potential as a function of the cycle number

There is some evidence that cathode structure does deteriorate with time under the cell conditions. There is a significant amount of graphite that can be wiped off the Swagelok cell electrode after opening up the cells – while the graphite electrode was sanded and completely clean upon cell fabrication – which might point to the electrode itself or the GFE-1 cathode material degrading. I am currently running an experiment using a titanium electrode with a GFE-1 cathode to rule this out.

About the stability of the additives and sequestering agent, it might be worth it to carry out some cyclic-voltammetry (CV) experiments to investigate the stability of the organic additives being used to see if any of them can fundamentally degrade under these conditions. The TMPhABr could be especially susceptible, given its aromatic amine character. If the sequestering agent can deteriorate under these conditions, then we might be unable to use it as an effective agent and we might need to resort back to TBABr or TPABr. The Tween 20 might also be electrochemically vulnerable, so this additive also needs to be studied in this manner.

Zinc Bromine Batteries: What about non-aqueous solvents?

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As my avid reader Giancarlo pointed out in the comment section a few posts back, many of the big problems of the Zn-Br battery system seem to be caused by the use of an aqueous electrolyte. Hydrogen evolution and voltaic losses due to the use of insoluble or immiscible bromine sequestering agents are some of the biggest issues that are inevitably related with water. Changing to a non-aqueous solvent can help solve some problems, although some others are created.

An organic solvent to replace water in Zn-Br batteries would need to be aprotic and to allow for the creation of substantially conductive solutions using ZnBr2. The issue with these organic solvents is that they are also extremely friendly to bromine, most of them being infinitely miscible with elemental bromine. This means that a battery built using these highly polar, aprotic solvents, would discharge significantly faster, as bromine would be significantly more likely to migrate to the anode. Sequestering agents would not be usable as these agents and their tribromides are incredibly soluble in these solvents as well.

propionitrile - Wikidata
Model representation of propionitrile, a highly polar and aprotic organic solvent.

However, this property can be exploited to solve part of the problems of the Zn-Br battery, at least the part dealing with voltaic losses related with the sequestering agents in water. This paper from 1988 shows how propionitrile can be used within a battery to sequester bromine and prevent its migration through the cell. In this device, an aqueous anolyte is used with an organic catholyte to trap bromine near the cathode. Since bromine has such a high affinity for propionitrile it will tend to stay in the organic section of the device, preventing movements towards the aqueous layer and providing more efficient confining and higher conductivity compared with some common sequestering agents. This is the only paper I could find that discusses the testing of Zn-Br batteries with a non-aqueous solvent that is not an ionic liquid. There are however, no papers I could find where the anolyte is replaced by an ionic liquid as well.

Propionitrile – and nitriles in general – are nasty solvents though. They are significantly toxic and we wouldn’t want to use them for DIY home batteries due to these issues, especially if we’re going to be experimenting and having close contact with their solutions. For this reason I will avoid their use and will instead use the non-toxic aprotic polar solvent, propylene carbonate. This solvent can also be bought pretty easily on the internet (I got mine here).

Propylene Carbonate - What's in This?
Model representation of a molecule of propylene carbonate. This is a low toxicity, highly polar, aprotic solvent.

Propylene carbonate can form highly conductive solutions with some salts, it is reasonable to predict that both the quaternary ammonium salt I have (TMPhABr and TBABr) will be soluble in it, as well as ZnBr2. I have no idea about whether Br2 or perbromides are soluble in it though, but it is reasonable to expect Br2 to be highly soluble in it because of how other, similar solvents, behave. There is some precedent for a battery fully constructed with a metal bromide using entirely propylene carbonate, see this paper, so it might actually be possible to use propylene carbonate by itself, although this is probably not possible without a suitable membrane separator.

We know the diffusion coefficient of Br2 in propylene carbonate to be 3.41×10−6 cm2 s−1 (1) while the coefficient in water is 1.18×10−5 cm2 s−1 (2), this is in part because of the higher viscosity of the organic solvent. This means that diffusion of Br2 in propylene carbonate will be more than 5x slower, which might be enough to create a functional battery with limited self-discharge, especially if this coefficient can be reduced further with the addition of the quaternary ammonium salts.

Even if the creation of a battery using entirely propylene carbonate is not possible, it might be the case that a highly concentrated TMPhABr or TBABr solution in this solvent will be immiscible with a 3M solution of ZnBr2 in water. If this is the case then a split approach with a bottom organic solvent and a top aqueous solvent might be possible, although this will depend largely on what the final density values actually are.

In any case, I have now ordered some propylene carbonate and will be carrying out some tests with it during the next couple of weeks.

Zinc Bromine Batteries: PEG-200 plus Tween 20 to eliminate dendrites, first public results ever!

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In the past I have discussed zinc dendrites as one of the most important issues to deal with when creating Zn-Br batteries. While the effect of dendrites can be attenuated by using tall cells with large distances between the electrodes, these setups create high electric resistance that greatly diminishes energy efficiency. A high energy efficiency Zn-Br battery will therefore have the ability to reduce or eliminate zinc dendrites, such that a large number of cycles can be achieved without shorting the battery.

I have studied the use of PEG-200 quite extensively within this blog and although PEG-200 does reduce the formation of zinc dendrites, it also increases the internal resistance of the battery, to the point where the voltaic losses become unacceptable. At useful energy density values (>30 Wh/L) and acceptable charging currents (>10 mA/cm2), the maximum PEG-200 concentration for a 3M ZnBr2 solution is therefore restricted to around 1-3%.

Battery configuration used in the experiments discussed in this post.

Looking at other potential low cost solutions to eliminate zinc dendrites, this article using PEG and Tween 20 in alkaline batteries drew my attention. Although the article used PEG-600, it is reasonable to expect a similar effect with PEG-200, given that this has also been shown to reduce Zinc dendrites in alkaline batteries in multiple publications. It is particularly interesting that they can achieve this with a 0.5% PEG-600 + 0.5% Tween 20 solution, as this would be of practical use within Zn-Br batteries.

To investigate this, I bought some USP grade Tween 20. It is a very safe , non-ionic surfactant commonly used commonly for cosmetics. I then prepared a solution using ~1% PEG-200 and ~1% Tween-20 with 3M ZnBr2. I then assembled a battery as shown above. Note that although I have been using a separator-less setup during the last couple of weeks, I decided to try a fiber-glass based separator setup first, since this setup in the past suffered from dendrites at the edges that I believe might have been caused by surface tension issues with the solution. This problem is likely to be solved by the use of the Tween 20.

First eleven cycles, lighter plots are earlier cycles. Final CE and EE values shown. Charging was done to 15mAh at 15mA, discharge was done to 0.5V.
Coulombic and Energy efficiency evolution as a function of the charge/discharge cycle number.

The solution was easy to prepare and the Tween 20 did not generate any solubility issues. The assembly of the cell was quite interesting, while a 3M ZnBr2 solution (with or without PEG-200) normally takes around a minute to fully wick into the fiberglass separator and the GFE-1 cathode, this time the wicking was almost instantaneous, probably thanks to the use of the Tween 20, that greatly reduced the surface tension of the mixture.

The cycling of the cell is going on without any issues. After around 3 cycles, the magnitude of changes in the shape of the curves and capacity started to become smaller and smaller, with the battery currently settling at a CE ~ 92% and an EE ~ 68%. The total amount of charge extracted is around 13.8 mAh with an average potential of 1.46V, putting the energy density of the battery at this current density at around 30 Wh/L. I am so far amazed at the stability of this battery configuration with few aberrations showing in the charge/discharge curves and no signs of dendrites (so far). The above are the first ever published results – as far as I know – for a Zn-Br battery containing both PEG-200 and Tween 20.

An important early sign of dendrites is a decrease in the charging potential with time – as the zinc dendrites effectively enhance the surface area of the anode before shorting the battery – an effect that I haven’t observed after 11 cycles. Although still too soon, the above results are certainly encouraging, hopefully the synergistic effect between PEG and Tween 20 applies to the Zn-Br system as well.

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!