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Towards a practical, high efficiency, high capacity DIY Zinc-Iodine battery

I’ve done a lot of experiments during the last two years around the construction of Zinc halide batteries, in particular, Zn-Br and Zn-I batteries. So far, I have been unable to find a battery setup that provides high energy and coulombic efficiencies, high capacity and high stability.

Zinc-Bromine batteries suffers from problems related with Br2 diffusion, zinc dendrites, hydrogen evolution and other problems that arise when you try to resolve these issues. Trying to add complexing agents, increase electrode distance or add physical barriers, will often increase coulombic efficiency at the expense of energy efficiency, such that batteries with low self-discharge or high capacity will tend to have extremely low energy efficiency values (often below 20%). Having a battery where you need to put 20kWh in to take 2kWh out is just not a viable strategy for most people. Hydrogen evolution reactions are also inevitable with Zn-Br batteries, and these irreversibly destroy the electrolyte as a function of time.

Zn-Br batteries might be easy to show in online videos and tout as a great chemistry with lots of possibilities, but the reality of constructing a >60Wh/L static Zn-Br battery with a long cycle life shows that this is extremely hard to achieve. This is why you will find no one showing proper testing – charge/discharge curves with CE and EE numbers for many cycles – of such DIY static batteries. You can read other posts in my blog and this forum thread, for more information about my journey in Zn-Br batteries.

The above were the reasons why – after doing a lot of experiments to get to know these batteries quantitatively – I decided to move to the Zinc-Iodine chemistry.

Image showing the reactions in a Zinc-Iodine battery. Note that in a static battery, the separator transports all ions, not only Zn2+ ions. This image was taken from here.

Zinc-Iodine batteries do not suffer from hydrogen evolution issues – due to the lower potential needed to charge the battery – but they also have strong problems dealing with I2 migration, especially due to the very iodide rich electrolyte, which generated a lot of readily soluble triiodide (I3). Although many solutions to these problems have been tried and published in peer reviewed journals, most are extremely hard to apply in practice and out of reach for someone with limited equipment and resources.

However, there is some hope. A paper published in 2019 got around the problem of the triiodide ion by using a “water in salt” approach (WiS). That is, they create a very highly concentrated solution of zinc chloride and potassium iodide, where water is a very minor component by mass. In this case, they add around 2g of ZnCl2 and 0.8g of KI to only 1mL of distilled water. In such a concentrated salt solution, iodide prefers to bond to Zn to form complexes, as the formation of the triiodide ions is strongly disfavored by the environment. This is proven extensively in the paper by the use of Raman spectroscopy, supported by computational chemistry calculations.

Thankfully, both ZnCl2 and KI are readily available chemicals plus, the electrode and separator material used by the researchers are also easy to get. The cathode is made out of carbon paper, the anode is zinc metal and the separator is cellulose filter paper. I purchased the salts for around 4 USD/kg (ZnCl2) and 40 USD/kg (KI).

Note that ZnCl2 is extremely hygroscopic, so it needs to be kept under airtight conditions. Solutions of ZnCl2 are also extremely acidic (pH < 1) so you need to be very careful when handling ZnCl2 solutions that are this concentrated. Zinc chloride also heats solutions aggressively when dissolved.

The results of trying to reproduce their research were quite astonishing. I prepared the electrolyte as published and used my Swagelok cell for testing. I used a Whatman 42 filter paper as separator and an HCB-1071 carbon cloth as cathode (note the variety kit has the 15mil, which is the one I used). For the anode, I used the surface of the graphite electrode of my Swagelok cell. In total, the thickness of my battery was just 350 microns (0.0350cm).

First cycle of my WiS cell. The device was charged to 1.35V and discharged to 0.6V at a constant current of 5mA. The CE of this device is 72.5% with an EE of 64.69%. The device has an area of 1.29cm2 and a thickness of 0.0350cm.

The above image shows you the results from the first charge/discharge cycle of this device. The capacity in this first cycle was 1.77mAh. Given the volume of this battery, the energy density is currently at 58Wh/L, the best energy density I have been able to create for any device.

I am now going to study the cycling characteristics of these devices further. I want to determine how the CE and EE change as a function of time and how the capacity changes as well. Taking this device apart confirms that elemental iodine is deposited on the carbon cloth and no appreciable triiodide – which has an orange color – is formed. This is also confirmed by the massive jump in CE and EE in these devices.

The above chemistry finally points to a path for a DIY battery that can be easy to build with readily available chemicals and materials. My hope is that this can lead to practical capacities in the >50Wh/L, with reasonable costs and hopefully low self-discharge and a long cycle life.

Zinc Bromine Batteries: A view and way forward

Zinc bromine batteries are a very interesting battery chemistry that goes back at least a hundred years (see here). These batteries are quite especial in that the battery is assembled in a completely discharged state, where both electrodes in the battery are relatively inert and all the charging of the battery is done by reducing/oxidizing materials in the liquid electrolyte into the electrodes. These batteries have the potential of high capacities with the use of very cheap materials, while having a safety profile significantly better than that of regular lead acid or lithium ion.

The battery reduces (plates) zinc into the negative electrode of the battery and oxidizes bromide to elemental bromine in the positive electrode of the battery. These reactions allow the battery to have a relatively high theoretical potential but also implies that you get elemental bromine – a highly reactive liquid – in the anode of the battery. This is one of the main complications and reasons why these batteries have never been very successful in commercial applications. The bromine is not entirely insoluble in the electrolyte – which is generally water – and can therefore migrate to the negative telectrode to react with the zinc, causing a self-discharging of the battery that is extremely detrimental to long term storage in these systems.

The minimum architecture Zn-Br battery built by Princeton

The above is why these systems have mostly been exploited as flow-batteries, because if you can take the bromine produced and just move it away from the zinc deposit, you can effectively ensure that the battery charge is preserved as a function of time. This inevitably complicates battery construction substantially, but does allow for some practical applications of the technology.

During the past few years there has been a renewed interest in this technology, first due to some Princeton articles that talked about these batteries in the context of a “minimum architecture” battery (see here) but this actually just reproduced some of the earlier results of this technology and does not provide substantial solutions to the problems associated with self-discharge in these systems. They however point to the fact that these problems can actually be relatively inconsequential if the batteries are used for mass storage and the actual time the energy needs to be stored for is low. The fact of the matter is that the self-discharge reaction in these batteries is not destructive for the battery, just a waste of energy, contrary to other battery chemistries where self-discharge can generate problematic issues that kill the batteries themselves. The point being, if you need to store the energy for a short time and you can waste some, this is your chemistry.

However some people realized that the problem of self-discharge could be reduced substantially by using some substances that capture the Bromine produced into insoluble tribromide salts. This is what some smart people from China did (see here) using TPA (tetrapropylammonium bromide). The introduction of this chemical into the battery chemistry allows tetrapropylammonium tribromide to form in the positive electrode of the battery – instead of liquid bromine – which substantially prevents the reaction with the zinc on the other electrode as these salt is significantly more insoluble than the bromine.

Some results of the Chinese paper mentioned above, which was published in July 2020.

The image above, which contains some of the results in their paper, shows that Zn-Br technology, when used in this manner, has the potential to have specific energy and power values that can rival even modern lithium ion technology. Furthermore, the cost of all of the materials involved here is relatively low, although these Chinese experiments did use a fancier carbon electron for their positive electrode.

Looking at what the Chinese did, I wondered if it would be possible to create a battery that reproduces their results, with some materials I could find online. I found a carbon felt from ceramaterials with high conductivity and surface area, you can buy an A4 sized sample for less than 50 USD and coupled this with some conductive HDPE, a copper mesh and copper tape. Since zinc can be plated onto carbon felt as well (see here) I decided to go with a construction mechanism where both of my battery electrodes will be constructed from the same material. For the separator I decided to go with a fiberglass separator, the best one I could find for the purpose was this tissue. In order to enclose the battery I decided to go with some shrink tubing, since I could easily seal the ends with a little bit of heat.

You can see my fully assembled 1 cm squared electrodes. I left a bit uncovered on the one to the left so that you can see all the layers.

The electrodes are made by heat pressing HDPE to the carbon felt using a regular iron (with some wax paper to prevent it from sticking to the iron) then heat pressing the copper mesh onto the HDPE, then putting the copper tape on the mesh and finally pressing another layer of HDPE to seal all the components in. The resistance measured between the copper and the felt in these electrodes should be lower than 1 ohm, meaning you should be easily able to measure continuity between them.

To build a battery two of these electrodes are put together with 4 layers of the fiber glass between them – it’s pretty large pore so using 1 or 2 layers often caused shorting issues between the carbon felt electrodes – and then placing that inside the shrink tubing. I then sealed the sides of the tubing using a regular iron to ensure everything was enclosed.

The next step is to prepare the solution, for which I used a class A 10mL volumetric flask and prepared a 0.5M solution of Zinc Bromide and tetrabutylammonium bromide (TBAB). Notice that I used tetrabutyl, not tetrapropyl, since this is more readily available. Both of these substances were bought on ebay or amazon, although they are more expensive than they would if you bought them in bulk quantities. The solution is then injected into the tubing using a needle and the hole is then sealed using regular electrical tape (usually I open to holes to allow air to go out of one). The above solution had some solubility problems (the TBAB wouldn’t go into solution at this concentration) so some of my future experiments will test out different concentrations.

Finished battery
Cross-section of a battery cut through the middle after assembly

I have already started testing some of these batteries and look forward to sharing some of their actual electrochemical results within a future post. As a sneak peak I can tell you that the coulombic efficiency of these cells is greater than 92%. But you will see much more, including charge/discharge curves and other tests, really soon!