Zinc Bromine Batteries: First results ever using Propylene Carbonate

Earlier this month, I wrote an article about the use of non-aqueous solvents in Zn-Br batteries. The only published result I could find was an article dealing with Zn-Br flow batteries using propionitrile as the catholyte solvent but I wanted to avoid the use of propionitrile due to its toxicity and scarcity (hard to find/buy for an individual in the US). However I thought propylene carbonate (PC) could be a suitable replacement, so I bought some to test whether this was true or not.

The first experiments I carried out were to figure out whether PC could be used as the sole solvent within the battery. Sadly the solubility of ZnBr2 is not high enough – at most in the 0.5-1M range at 20C – and the conductivity of these ZnBr2 solutions was also not high enough, with very noisy charge/discharge curves with very high charge voltages that retained almost no charge at all.

The solubility of both TMPhABr and TBABr in PC is better, although TBABr is by far the most soluble. With TBABr I was able to achieve saturated solutions with almost 50% of TBABr, giving them a very decent amount of conductivity. Sadly this wasn’t enough to make PC usable as a single electrolyte though, as the bad behavior of the charge/discharge was also apparent when using this as the sole solvent.

Charge/discharge curves for a cell built with a 1% Tween20 + 1% PEG 200 + 1.5M ZnBr2 electrolyte with a GFE-1 cathode fully saturated with a 50% TBABr in PC solution.
Evolution of CE and EE values for the curves shown before.

The idea then came to use this concentrated PC TBABr solution to saturate the GFE-1 cathode and use this in an inverted cell. It is interesting that although PC is infinitely miscible with water, a 50% solution of PC TBABr is actually not miscible with a 1.5M ZnBr2 solution in water. This is because the affinity of TBABr for PC is much higher than that of ZnBr2 and the affinity of ZnBr2 for water is also significantly higher as well.

This experiment was better behaved with actually measurable charge/discharge curves. I did 4 curves charging/discharging to 15mAh at 15mA – the results are shown above – with the best CE and EE values being 61% and 25% respectively. The charging voltages do show that the internal resistance is significantly higher than when using water so there is likely a lot more of hydrogen evolution at the anode. The generation of elemental bromine at the cathode is also probably significantly slower, given the much higher viscosity and lower conductivity of the PC electrolyte.

Given the higher charge density used, I thought It might be the case that the PC electrolyte is just not able to support as high of a current density as the normal aqueous electrolyte and therefore a much lower charge density needs to be used to use this successfully. I am going to be evaluating this hypothesis within my next few tests.

Zinc Bromine Batteries: Why an inverted configuration is likely more practical

Static Zn-Br batteries have gained popularity during the past 4 years due to their apparent simplicity and their theoretical ability to last thousands of cycles without deterioration at a very low cost (although this is often extrapolated from Zn-Br flow batteries). The particular configuration that has become popular among some researchers and DIY enthusiasts is the one shown below, where the Zn-Br battery is assembled with the cathode at the bottom of the battery and either a graphite or Zinc metal anode at the top, with the electrolyte in between both of them.

Battery configuration that has been popularized during the past few years.

In this configuration for the battery, elemental Bromine (or sequestered bromine if any sequestering agents are used) will tend to accumulate at the bottom. This happens because Bromine is both denser than water and attracted to the carbon felt electrode. As bromine is insoluble in water and prefers to interact with the felt. The Zinc is then deposited at the top electrode and the cell only appears to be limited by the self-discharge of the process, caused by Bromine diffusion from cathode to anode.

However, experimentally – as you can see extensively in my work in this blog – at practical current densities (>15 mA/cm2), even in devices with anode/cathode distances of only 2-3mm, there is a substantial evolution of hydrogen in the anode due to the overpotential required to overcome the internal resistance of the device. This is true for ZnBr2 concentrations of 1.5-3M. This means that a lot of hydrogen is produced, which is then either trapped against the anode – reducing its surface – or then leaves the device at the expense of making the solution more alkaline, both processes which inevitably kill the device as a function of time. Trying to increase the conductivity further also leads to other, worse problems, such as robust Zinc dendrite formation.

Inverted device configuration with the carbon felt cathode at the top of the device.

To deal with the above means that you either need to periodically replace the electrolyte or treat it in some manner. This might not be economical if additives like sequestering agents are used but it is definitely not desirable as you will have to deal with a lot of left-over bromine containing solution. A potential solution might be to replenish the battery by adding HBr – in an analogous way to how you replenish the sulfuric acid in a lead acid battery, but this solution is not likely to be very practical due to the makeup of this battery. This is because adding excess HBr makes the hydrogen evolution problem much worse, so careful titration of the solution with the HBr is required in order to arrive at just the right pH, very impractical for users.

A more permanent solution is to use an inverted architecture, where the cathode is placed at the top and the anode at the bottom. Any hydrogen gas created then reacts with the solution and cathode, regenerating the electrolyte in the process. This sadly decreases the Coulombic and Energy efficiency value of the device, because Bromine diffusion is substantially aided by its tendency to sink into a water solution. Experimentally the CE drops from around 90% to 70% and the EE from 70% to 50% at the same current densities (see here). However this is likely where we need to start if we really want a Zn-Br architecture that can be used for a long amount of cycles in practice.

This fundamental problem was also recognized by the author’s of the Princeton minimal architecture paper. In the image above – taken from the publicly available supporting information of the paper – they also show how this sort of inverted architecture would work. They see even worse decreases in CE and EE as I have found experimentally.

In order to make this architecture viable we would need to increase the affinity of the Br2 for the felt and prevent it from sinking into the water solution, a very challenging proposition but one we can work towards if we start from a solid base. Soaking the felt in an organic phase that is conductive enough is the first things I am going to try to get to this goal.

Zinc Bromine Batteries: Why internal resistance was increasing

For the past week I have been trying to find the reason why the internal resistance of my batteries seems to be increasing linearly as a function of the cycle number. I sought to evaluate several different hypothesis, starting with the potential deterioration of the graphite cathode as a cause for this. Using titanium as a cathode generated similar results though plus a substantial deterioration of the cathode as a function of time. The graph below shows you a Titanium cathode and how it was pitted aggressively by the oxidative conditions of the cathode side of the battery.

A certified Ti-6Al-4V 0.5 cathode after a few cycles in a Zn-Br battery using a 1% Tween 20, 1% PEG-200, 3M ZnBr2 electrolyte. Black spots are actually holes in the titanium electrode.

The above results showed that deterioration of the graphite cathode was not the main cause of the increase in internal resistance, plus, it also showed that titanium – at least in this form – is not a suitable replacement for the graphite electrode in the Swagelok cell.

One interesting thing about the internal resistance problem was that opening up the cell, taking out the anode and putting the anode back in, seemed to regenerate the device to close to its initial conditions. This pointed to the process that was causing the deterioration to be somewhat reversible, at least in that it depended on the state of the device and was therefore likely not caused by the deposition of some insoluble film in the cathode or the unwanted precipitation of something within the electrolyte.

Thinking about this, I realized that a significant amount of hydrogen was being generated inside the battery at these potentials, which, given the flat anode geometry, was unable to escape the device. This trapped H2 gas, against the anode created zones of high resistance which accumulated with time up to the point where they basically prevented the electrode from working correctly at all.

To test this hypothesis I assembled an inverted geometry cell with a GFE-1 cathode treated with 10% TMPhABr on top and a graphite anode at the bottom. The cell used no solid separator, but a spacer made from 3 PTFE o-rings. Sadly I was running out of ZnBr2 and made a mistake in the solution preparation as well, so the electrolyte used is quite strange with a concentration of 1.5M ZnBr2 + 5% PEG-200 + 5% Tween 20. I meant to prepare 0.5% but made a mistake in my calculations.

Test cell using an inverted geometry
EE and CE as a function of time for an inverted cell geometry
Evolution of charge and discharge potentials relative to mean charge or discharge potential of the first cycle

As you can see in the curves above, an inverted device appears to behave in a completely opposite way to my previous devices with larger cycle numbers. The charge and discharge potentials seem to be evolving favorably as a function of time, with the charge potential decreasing and the discharge potential increasing. This means that the internal resistance of the device is actually becoming lower and lower as time goes on. The hydrogen gas is no longer a problem since, as a low density gas, it will tend to go up the cell, into the electrolyte and cathode, where it reacts with perbromides or elemental bromine in solution which regenerates the chemistry (as the hydrobromic acid formed then diffuses and reacts with any ZnO formed in the anode).

The Coulombic and Energy efficiencies of this device are quite low though, which is likely a consequence of the bigger amount of PEG-200 and Tween-20 used, plus a lower concentration of ZnBr2. I will be reproducing this setup with a freshly and properly prepared electrolyte as soon as I get new ZnBr2 this week but I thought these results were interesting enough to share with you. I will also continue cycling the above device to see if it eventually fails due to dendrites or some other reason.

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

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?

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!

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

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

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

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

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.