Monthly Archives: October 2021

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

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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.

My learning curve with Zinc-Iodine batteries

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In July 2021, I wrote about my goal to reproduce the results of a paper using a high surface area carbon, loaded with elemental iodine, to create a practical Zinc-Iodine battery.

This was not successful because my material of choice – a GFE-1 cathode – did not absorb any iodine when placed in an iodine chamber at 55C for 10 minutes. This is because the cathode material is likely already passivated, as it has been in air for more than a year now, so activation would likely be necessary to get it to load with Iodine. Performing this process is outside my current technical possibilities (no ovens or vacuum chambers available).

Best results I obtained for a ZnI2 2.5M cell. The electrolyte was also saturated with NaCl. This used a Zn anode, GFE-1 cathode and 10 layers of fiberglass separator.

Instead of continuing down this path, I decided to create a battery using Zinc Iodide. This chemical is hard to get though, so I synthesized it from metallic Zinc and elemental Iodine. With the 2.5M solution ZnI2 I created – concentration determined by its density – I then proceeded to create batteries in my Swagelok cell configuration. The best result I got, where energy density was around 29 Wh/L, are shown above.

Despite the above, I was never able to achieve high CE or EE values, with the values for my batteries at >20Wh/L capacities being around 55% for CE and 35% for EE. Low coulombic efficiency happened because of diffusion of elemental iodine away from the cathode – through the formation and diffusion of I3– – as evidenced by the strong coloring of the fiber glass separators when taking the batteries apart. In a similar fashion to Zn-Br batteries, the elemental halogen diffuses and this inevitably lowers the CE, EE and increases the self-discharge of the battery. Even the high surface GFE-1 carbon felt, is just uncapable of holding to the halogen on its own.

Besides making most properties of the batteries worse, the diffusion eventually kills the battery, as so much reagent is lost per cycle that at some point the battery is simply unable to properly function, as any generated zinc is consumed by Iodine that reaches the anode.

I have tried several things so far to eliminate the problem. Thicker carbon material, more dilute ZnI2, TBABr as a complexing agent, none of these have worked, since I3– forms very efficiently under any excess of I– in the solution, which is inevitable as when the reaction starts there is ample iodide in solution. Adding TBAB does precipitate the TBAI3 salt, but this greatly increases the series resistance of the battery.

Is there anyway to stop the I2 from becoming I3– and migrating? Can we somehow plate elemental I2 at the cathode and avoid self-discharge and losses in CE and EE? Stay tuned for more research results.