My adventures building a Zn-MnO2 battery

danielfp248

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After looking at Zn-halide chemistries and running into big road blocks with both Zn-I and Zn-Br batteries, I have decided to explore the world of Zn-MnO2 batteries. Especially, mildly acidic rechargeable Zn-MnO2 chemistries. These batteries use very earth abundant elements and very low cost salts. This chemistry is also very benign to build at home compared with halide containing chemistries, which generate elemental Iodine and bromine, both potentially nasty substances.

I will be posting the results of my experiments using the same link as before (https://www.dropbox.com/sh/67hdgpm5lijt5s1/AACS0gjDSYHZny_tLSfXXmfza?dl=0). As with previous experiments, I will be using my Swagelok cell. Experiments in this chemistry start at Exp23.
 
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danielfp248

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The first battery I am trying to replicate comes from this research paper (https://onlinelibrary.wiley.com/doi/full/10.1002/sstr.202100119). The authors use a 1M ZnSO4 and 1M MnSO4 solution with 0.2M acetic acid. Using a carbon cloth cathode and a Zn anode. They however charge at constant voltage to some predefined capacity, something that I haven't programmed yet. I will therefore just charge to fixed capacity. They also only discharge to 0.8V, but I've found that discharging to 0V seems to be preferred in many Zn-MnO2 chemistry papers, as you are able to fully dissolve any formed Mn oxides.

My first try (Exp23) gave rather interesting results. The battery was charged to 0.5mAh, discharged to 0V, both at 5 mA/cm2. The battery used a 0.2M acetic acid electrolyte with 2m MnSO4 and 2m ZnSO4 (molal, not molar concentration).

1638474795956.png

Note that the battery failed due to dendrites after 155 cycles, but battery discharge capacity remained stable, with increasing Coulombic efficiency through the run. This experiment showed the chemistry in this configuration has rather low energy efficiency (40%) and CE that reached almost 94%. Given the configuration in this battery, the energy density was around 5Wh/L. For starters I'm not very interested in energy density but I want to see if I can achieve something that is stable, before I build on it to improve the results.

Up until now my voltage efficiency sucks (charged at a mean voltage of 2.23V to discharge at 1.12V), so this is definitely one of the main places to make gains.

Upon taking the battery apart, there were sulfate crystals everywhere, so this is likely way too concentrated. The next experiment (Exp24) uses a more dilute electrolyte, at 1m ZnSO4 and 1m MnSO4, probably just a bit more dilute than what they used in the paper. Note that I'm not preparing exact molar concentrations (moles of solute per volume of final solution), as this is more involved (as I need to use volumetric flasks, dilute well in smaller volume, etc). For highly concentrated solutions, molal concentrations (moles of solute per weight of solvent) are just much easier to handle.
 
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danielfp248

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I don't like sulfates very much, in my experience, the presence of sulfate leads to easy dendrite formation. This is because some Zn sulfate hydroxide salts tend to deposit on the anode and the passivation of anode surface area leads to much higher electric field density in the areas that aren't passivated, which tends to create dendrites.

However, I only have Mn sulfate, so I prepared a 2m Mn sulfate + 4m NaCl solution. I then cooled it to -16C, which precipitated most of the Na sulfate from the solution, leaving only MnCl2 in solution. A little bit of sulfate remains (around 0.1M) but most of the sulfate is removed. I then used this prepared MnCl2 solution to mix a new electrolyte that is approximately 2m ZnCl2 + 2mMnCl2, with 0.2M acetic acid.

I am now testing that in Exp25.
 
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danielfp248

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Exp25 showed substantial improvements, basically no large changes in capacity up until failure due to dendrites, better CE (~92%) and EE (~50%). However, dendrites were mainly present on Zn anode edges and some crystals - presumably Na sulfate - were also present at electrode edges. The electrolyte also showed these crystals so it seems they just precipitated further with time. I am therefore repeating the experiment after filtering the electrolyte.

Exp25.png
 

danielfp248

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Exp26, game me 81 cycles, again, failure due to dendrites. Stability however remained quite high. Note that I also increased current density to 10mA/cm2, with little drop in the CE and EE of the battery. Capacity remained stable until the dendrite related failure.

Exp26.png
I am now running this exact same experiment (which is Exp27) but with 6 separators, to see if this is enough to prevent all shorts due to dendrite formation.

After I get the results of this experiment (which is 2m MnCl2 + 2m ZnCl2) I am going to run tests with 2m MnCl2 + 4m ZnCl2 and 2m MnCl2 + 8M ZnCl2, to see the effect that more highly concentrated ZnCl2 has on the performance and dendrites. Note that all of these electrolytes are prepared in 20% vinegar / 80% distilled water. I already prepared the solutions and had to filter them as significant amount of precipitation happened (probably Na2SO4) upon mixing.

I also ordered some USP grade calcium chloride (which is cheap), so that I can prepare much purer MnCl2 solutions as I can precipitate all the sulfate as Ca sulfate from MnSO4 solutions. I will then repeat the above tests with this much lower Na and sulfate approach.

I also reordered carbon papers (as I ran out). Performance for Zn-I batteries was similar with all the papers I tried before (except the Toray paper which was impossible to wet, even though it only has 1% wet proofing). I therefore made the order below:

P50 - 20cm x 20cmSize:
20cm x 20cm
× 1$28.00
Spectracarb 2050A-0850 - 20cm x 20cmSize:
20cm x 20cm
× 1$40.00
EP40 - 20cm x 20cmSize:
20cm x 20cm
× 1$28.00

The Spectracarb worked really well and I will try the P50 and EP40 papers (which are only substrate with no catalysts or PTFE treatments), which I hope will also work well. Working with carbon paper is way easier than working with carbon cloth, as it can be punched very easily to get the proper electrode form and it doesn't short the batteries due to stray fibers or things like that.
 
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danielfp248

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I have also taken the data from this paper (https://pubs.acs.org/doi/10.1021/je00044a025) to create a way to calculate the molal concentration of a MnCl2 solution from its density. I will use this to accurately estimate the molality - moles of substance per kg of solvent - of my final MnCl2 solutions when I prepare them again from CaCl2+MnSO4. The data and equation is below in case anyone wants to do this procedure. This relationship should remain linear for even much higher concentrations (2-4m).

1639067830601.png
 
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danielfp248

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Exp27 is marking a new record for stability, so far 301 cycles and counting. Virtually no deterioration in capacity so far. CE at 96%, EE at 51.4%. Energy density is quite low due to the thicker separator, currently around 3Wh/L. If it remains stable we can then think about cycling to higher capacity and removing separator layers. This battery will be cycled to failure so that we can judge how stable the configuration in Exp27 actually is.

1638789721079.png
 

danielfp248

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Dendrite related failure around cycle 348. I ran it for a total of 358, there was some recovery but the proximity of catastrophic dendrite failure was imminent.

1638805513929.png

Next experiment (Exp28) will look at the same configuration with a 4m ZnCl2 + 2m MnCl2 electrolyte.
 

svetz

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Seems like dendrite repelling separators would allow a greater energy density. I'm thinking of the snowflake video, where you might be able to do something at the seperator so that it's more than a "wall" waiting for the dendrite to pierce ... it becomes an anti-formation barrier where they encourage the dendrite to turn 120 degrees or something?
 

danielfp248

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Seems like dendrite repelling separators would allow a greater energy density. I'm thinking of the snowflake video, where you might be able to do something at the seperator so that it's more than a "wall" waiting for the dendrite to pierce ... it becomes an anti-formation barrier where they encourage the dendrite to turn 120 degrees or something?

The problem is that zinc dendrites have a Young's modulus often greater than 100 GPa. This means they have a ton of tensile strength and will break through almost anything you put in their way. Bear in mind you still need ions to go through, so you still need some porosity or ion exchange. This physical blocking of dendrites hasn't been very successful in Zn batteries due to the above reason. It has been tried, but none of the approaches I've read can be implemented in an easy manner.

There are some interesting approaches, for example using a separator that is covered by metallic Sn through sputtering on one side, such that the separator is able to sort of "passivate" dendrites when they reach it. This was a big deal (reason why it's published in nature https://www.nature.com/articles/s41467-021-23352-0). Sadly I cannot easily do this at my house.

Another potential approach is to do weird geometries involving backplating (see here https://www.nature.com/articles/ncomms11801). Batteries of this nature are however really hard to test - I have no idea how I would assemble this in a Swagelok cell - and suffer from too much additional series resistance due to the additional path ions need to travel. I tried it once on a beaker with Zn-Br batteries and my charging potential went to 3.5V.
 
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svetz

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This physical blocking of dendrites hasn't been very successful in Zn batteries due to the above reason. It has been tried, but none of the approaches I've read can be implemented in an easy manner.
I didn't mean it as a physically strong resisting penetration or increasing distance as when you went to 6 layers.

Dendrites probably grow one atom at a time, probably following the same rules as the snowflakes. That the dendrites beeline for the other electrode currently says conditions and crystal geometry favor that growth to occur that way.

I meant it as a dendrite "DNA alteration". Something in the separator or solution alters the "skeletal growth/shape" of the dendrite. If it inhibits "straight-line" growth or causes their shape to change such that they don't reach out towards the far electrode or the growth points to be redirected back rather than through then its a win. For example, if there was something in solution that latched on the vertex such that the dendrites went sideways than outwards or like the flat edge in the snowflake video causing no growth in that direction.
 

danielfp248

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I didn't mean it as a physically strong resisting penetration or increasing distance as when you went to 6 layers.

Dendrites probably grow one atom at a time, probably following the same rules as the snowflakes. That the dendrites beeline for the other electrode currently says conditions and crystal geometry favor that growth to occur that way.

I meant it as a dendrite "DNA alteration". Something in the separator or solution alters the "skeletal growth/shape" of the dendrite. If it inhibits "straight-line" growth or causes their shape to change such that they don't reach out towards the far electrode or the growth points to be redirected back rather than through then its a win. For example, if there was something in solution that latched on the vertex such that the dendrites went sideways than outwards or like the flat edge in the snowflake video causing no growth in that direction.

The Sn approach I mentioned does something like this, when the dendrite reaches the Sn layer, it is no longer thermodynamically favorable for the dendrite to grow.

Dendrites grow mainly because there is a gradient of Zn ion concentration going from the electrode to the bulk of the solution. As Zn ions are plated very fast at the electrode surface, they are depleted quickly, anything that protrudes into the solution requires shorter diffusion of Zn ions, so they get plated there faster. The more a dendrite grows, the faster access it has to the bulk concentration of ions in solution and the more surface area it has to branch and spread. It is a vicious cycle that magnifies the smallest protrusions in a zinc surface, growing them all the way to the cathode, shorting the battery.

There are some approaches to prevent this with additives, surfactants like polysorbate, laureate and cetrimonium, will in theory preferably cling to dendrites instead of the electrode surface, passivating them and preventing dendrite growth to increase. The problem is that they also affect the chemistry of the solution and, I've noticed, can encourage dendrites in some cases (when I put cetrimonium chloride in my devices, they form dendrites much faster, meaning cetrimonium chloride must adhere to the Zn surface instead of the dendrites in my particular case).

There are also approaches that modify the anode, for example by plating it with Cu or Sn through exposure to Sn or Cu solutions.

Using CuSO4 plating solution - https://www.mdpi.com/2079-4991/11/3/764
Using SnCl4 plating solution - https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.201906803

These lead to the formation of more stable metal alloys that do not favor dendrite structures as the energy for plating on them is lower than in pure Zn dendrites.

I have tried these two approaches with no success.

My dendrite issues notably happen much faster than normal Zn dendrite related failures in the literature and do seem to be located directly around my separator edges, it might be a feature of my fiberglass separator architecture that facilitates their growth around these points. I might need to get a new fiberglass separator with different porosity to see if this is the case.

I can also try a separator-less approach using teflon rings, to see if this is the case.
 

danielfp248

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There was a dendrite failure after 83 cycles. The results looked really good up until that point:

1638880727429.png
 

svetz

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Are dendrites formed during charging or discharging?
Wondering if the cell is held at the unfavorable state for dendrites longer if it would break them down and give more cycles.
 

danielfp248

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Are dendrites formed during charging or discharging?
Wondering if the cell is held at the unfavorable state for dendrites longer if it would break them down and give more cycles.

Dendrites are always formed during charging, which is when zinc is deposited. However, they aren't fully dissolved when discharging - since Zn plated on the electrode is more favorably dissolved - so they increase in size with cycling. If you held the device at 0V for a long time you could probably dissolve all dendrites. The strategy of putting a battery through a shunt resistance to dissolve all dendrites and increase cycle life is common, but not one I'm interested in.

As I mentioned, the dendrite failures I am getting are premature compared to what is normally expected in the literature and they happen directly on the edges everytime, so it makes me think I am missing something fundamental with my setup. The separator or the fact that I often put the cells down horizontally (while everybody usually has them in vertical holders) are both differences worth exploring. The fact that dendrites always seem to happen on only one side, point to the fact that they might be related with electrolyte pooling when I lay the cell on its side. Exp30 - the exact same as 28 but holding the cell vertically - seeks to see if this makes any difference.

Exp29 used a paper separator and failed after 82 cycles as well.
 
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