Zinc Bromine Batteries: Problems at higher capacities with TMPhABr

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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

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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

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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?

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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?

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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.