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

4 Replies

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

2 Replies

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

3 Replies

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

6 Replies

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.