My adventures building a Zinc-Iodine battery

danielfp248

Battery researcher
It is interesting to note that as the cycle number has increased the CE has reached higher and higher values, some cycles reaching very close to 100%. This does mean the electrochemical process is becoming more stable as a function of time and it is something not observed in the original WiS paper, where the CE is consistently around 90%.
 

danielfp248

Battery researcher
I have now run the battery built in #31 for 116 cycles. The results are below:

1633722546633.png
1633722561005.png
1633722581658.png
Contrary to what the original WiS paper shows, there has been no increase at all in the capacity after the initial 20-50 cycle decline. The discharge capacity at constant voltage charging has been dropping logarithmically from the start up until cycle 116. However, the battery is NOT dead! 🤓

I am now going to stop this test and try charging to constant capacity (3mAh) and see how the CE and EE do.
 
Last edited:

danielfp248

Battery researcher
Charging the battery to 3mAh can be done with a final charging potential below 1.35V (from charging to 0.9mAh to 3.0mAh there was only a 50mV increase in charging potential). Charging these batteries to constant potential might be a mistake - as the charging potential required might increase slightly as a function of cycle number - so charging to constant charging capacity might be a better approach (if the CE and EE can remain high and the battery does not die with cycling).

First cycle of this charging to constant charging capacity is shown below. CE= 98.95%, EE=82.21%, Discharge Capacity = 2.87mAh, Mean Discharge Potential = 1.2310V, Energy Density = 31.48 Wh/L

1633724167152.png
This shows that the battery did not lose its ability to store charge at all, it just had a small increase in internal resistance after 116 charging cycles to constant voltage (1.3V).

I am continuing to cycle it charging to constant charging capacity (at 3mAh) we'll see if it can keep the discharge capacity, CE and EE from dropping and if it doesn't get damaged as a function of time.

Also note, the above graph is perhaps one of the best charge/discharge curves I have ever posted!! 🤓 I can't believe I got a CE close to 100% and an EE > 80% with a >30 Wh/L capacity with a Zn-I battery :eek:
 
Last edited:

danielfp248

Battery researcher
I ran the battery for 46 cycles of charge to constant capacity at 3mAh at 15mA:

1633778233075.png1633778245519.png
1633778259319.png1633778587502.png
Capacity held for around 30 cycles, then there started to be some evident loss of capacity. It seems that the battery cannot be reliably charged/discharged at 15mA, internal resistance starts to increase too much which caused potentials to get abnormally high to reach 3mAh, which probably started damaging the battery.

Before disassembling this battery I have short circuited the battery for 5 min (to ensure complete discharge) I am then going to try cycling to 30mAh at 10mA, see if I see any significant difference.
 

danielfp248

Battery researcher
That cycling inevitably damaged the battery built in #31. I therefore built another one from scratch using the same process. Two layers of fiberglass, a CC6P cathode and the WiS solution. I am going to charge it to 1.3V and discharge to 0.5V but this time I am going to do so at 5mA, as the higher current seems to have played an important part in damaging the battery before.
 

danielfp248

Battery researcher
This is going to take a while 🤓 Doing the first charging cycle at 5mA to 1.3V, the battery charged all the way to around 9.5mAh. If I am able to discharge to 9mAh (which would be around 95% CE) at an average discharge voltage of 1.1V, this would give a total energy density of 88Wh/L. I will post the first curve in a few hours, when the first cycle is done.

Per the article, it seems the kinetics of the WiS approach are a bit limited, so it will take a lot longer to get cycling results. However, I am intrigued to see how this affects the stability of the battery. I suspect it will take around a week to get the first 50 cycles.
 

danielfp248

Battery researcher
I ran this battery for 7 cycles at 5mA, charging to 1.3V and discharging to 0.5V. I got similar deterioration to what we had seen before, although measured capacities were significantly larger, because of the lower charging current.

1633870695399.png1633870728227.png
1633870741125.png1633870805490.png
Given the constant drop in discharge capacity that is matched with decreases in mean discharge potential and increases in mean charging potential, it seems that the battery gets damaged by increases in internal resistance. This process is not driven by higher current, as the process is happening here, as well as when I charged at 3x this current level.
 

danielfp248

Battery researcher
The idea of the WiS (water-in-salt) electrolyte is to diminish the ability of water to dissolve triiodide (I3-) by lowering water activity, forcing water to interact with Zn ions and making ZnIxClxOHx complexes more stable than I3- under this environment. If iodide is forced to coordinate with Zn instead of forming bonds with elemental iodine, then the iodine plating should be stable and the battery much more reversible. This works, given the high Coulombic efficiencies we get, but the lack of stability suggests that there are some processes that are unfavorable that still remain.

I think these processes might be related with water activity. The current electrolyte is a solution that is 15m ZnCl2 and 5m KI. The original paper also tested a 20m ZnCl2 5m KI electrolyte, which had much worse results, as the viscosity of the mixture increases drastically as you add more ZnCl2. However, you can add another chloride salt - which does not allow for coordination chemistry - to reduce the water activity without increasing viscosity too much.

I have now created a 15m ZnCl2, 5m KI, 5m NaCl electrolyte and will be testing it to see if this in fact increases the stability of the battery. This is the highest NaCl concentration I could get, as adding more NaCl (>7m) causes a lot of the Zn salts to precipitate.

Note: Concentrations here and in the paper are given as molal concentrantion (lower case m) which means moles of solute per volume of solvent. This is not to be confused with molar concentrations (capital M) which means moles of solute per volume of solution.
 
Last edited:

danielfp248

Battery researcher
For these tests, I built a battery using the 15m ZnCl2, 5m KI, 5m NaCl WiS electrolyte using a CC6P cathode and two layers of fiberglass. I decided to charge the battery to just 1mAh of fixed charge capacity, at 10mA. This is in order to test cycling stability quickly as I have always been getting degradation as a function of cycle number, even at lower capacities.

1633873430214.png1633873440764.png
1633873451085.png 1633873466015.png

I am very excited to say that after 10 cycles, the capacity has been steady and the charge/discharge voltages have actually improved as a function of time. The coulombic and energy efficiency values did decrease a little bit compared to previous experiments without NaCl in the WiS electrolyte, but stability has increased tremendously. I am going to cycle 50 times to this capacity, if stability remains good, I will proceed to cycle at higher capacities (4mAh) and see what we get 🤓
 

danielfp248

Battery researcher
I cycled the device 21 times, charging to 1mAh, discharging to 0.5V, at 10mA, the results show it is very stable :cool: , even gaining capacity up until the last cycle. Moreover, it is improving significantly on CE and EE values (final values are CE=91.12%, EE=74.91%). I have now stopped the test and started testing to 4mAh at 10mA. Let's see if it is still stable up to higher capacity!

1633880745981.png1633880759240.png
1633880776688.png1633880789354.png

Note, there are no published results of devices with this exact electrolyte. These are completely new research results 🤓
 

danielfp248

Battery researcher
I ran one single cycle but it had some issues, gave CE>100 and discharge capacity > 4mAh, even though I had charged it to 4mAh. The battery probably had some charge remaining from the previous short cycles that ran to 1mAh. I shorted the battery for 5 min to completely discharge it and started the test again. Charging to 4mAh at 10mA.
 
Last edited:

danielfp248

Battery researcher
First 5 cycles to 4mAh, for the battery built in #51:

1633897420621.png1633897434774.png
1633897447348.png1633897460591.png

As you can see, up until now we have great cycling stability, with increasing capacity for the first 5 cycles and increasing CE and EE values. This behavior matches the results that we got when charging to only 1mAh. Maximum energy density at this moment is 35.35Wh/L. This is in great contrast to previous test using the original WiS electrolyte from the paper, where at this point our capacity had already dropped drastically. The modified WiS electrolyte seems to be paying off 🤓
 

danielfp248

Battery researcher
Right after I posted the above, the battery died as a cause of a short. It does seem 10mA is too high of a current for this electrolyte. I just built another battery with the same electrolyte and will try charging/discharging at only 5mA to 4mAh. Results will take significantly longer, so I'll post them probably next weekend.

Also worth noting that I saw absolutely no evident dendrites when I opened up the battery, Zinc plating seemed quite homogeneous and the separator had no grey marks, which are evident when dendrites grow through it. It might be that the plating got thick enough to contact a stranded conductive carbon fiber. The CC6P cathode has a lot of fibers (especially around the places where the circle is punched from the cloth) and it is easy for one of those to be within easy rich with just two layers of fiberglass separator.

The next battery is therefore going to be a CC4P carbon electrode (which is thinner at 250 microns and is cut more cleanly), with 3 separator layers instead of just 2.
 
Last edited:

curiouscarbon

Science Penguin
Amazing research!

Very impressive coulombic efficiency measures.
This thread is helping me learn about fundamental physics a bit as I attempt to follow and read along. Thank you!

A random query: do you have any thoughts on the topic of transducer based agitation of cells being studied with relationship to the chemistry dynamics of the cell? e.g. piezo or otherwise. i suppose this sort of approach is maybe pointless because how could a large pack vibrate. something about crystals being broken up or scattered by random motion. kind of like shaking a box of ping pong balls to pack them better. although packing better might cause more resistance evolution. anyways digressing!

Just kind of randomly wondered. Sorry if it’s OT.
 

danielfp248

Battery researcher
I also wanted to point your attention to one of the previous results. When I forced charging to 3mAh on a battery with the original ZnCl2+KI electrolyte (#45).

1633904637574.png
Notice how in cycles where the charging potential went very high - above 1.6V - we actually got some discharge at the 1.6V level. This points to the formation of some I+ in the cell that, at least for a little bit, was reversibly discharged. Recent research shows that it is possible to have this process in a WiS ZnCl2 electrolyte (see here) although they used LiCl2 and Acetonitrile to achieve good stability and reversibility. In 30m ZnCl2, the process wasn't very stable.

If I achieve good stability in the current test using a CC4P cathode with the new ZnCl2+KI+NaCl electrolyte, I wonder if I might be able to achieve the 4 electron process and charge to 1.8V. Since this electrolyte has much lower water activity it should in fact lead to much more stable I+ formation. Sadly, the authors of the nature paper mentioned above didn't do any ZnCl2+LiCl tests in the absence of acetonitrile, so no one seems to have tested something close.
 

danielfp248

Battery researcher
Amazing research!

Very impressive coulombic efficiency measures.
This thread is helping me learn about fundamental physics a bit as I attempt to follow and read along. Thank you!

A random query: do you have any thoughts on the topic of transducer based agitation of cells being studied with relationship to the chemistry dynamics of the cell? e.g. piezo or otherwise. i suppose this sort of approach is maybe pointless because how could a large pack vibrate. something about crystals being broken up or scattered by random motion. kind of like shaking a box of ping pong balls to pack them better. although packing better might cause more resistance evolution. anyways digressing!

Just kind of randomly wondered. Sorry if it’s OT.

Thanks for your reply and likes! I'm glad you like to follow my progress.

About your "random query", sadly I am unfamiliar with the topic you mention, so I cannot help answer your question.
 
Top