Last week I posted on the need to understand the history of battery development and how this will influence the future of batteries. We conclude today with Part 2 of this series.
Chapter 3: If it is sparingly soluble, then lets talk.
The lead acid battery is the first rechargeable battery ever made. Its endurance over 150 years is a testimony to its robustness (or to the fact that battery researchers can’t seem to find anything better even after 150 years. It is all a point of view!).
The lead acid battery undergoes what is called dissolution-precipitation. This is the mechanism by which charge/discharge occurs in the battery. Basically, you dissolve the compound in solution and then it precipitates out.
The behavior of the lead-acid battery is remarkably similar to that of the zinc electrode in a Zn-manganese oxide alkaline battery or for that matter to the lithium thionyl chloride battery.
But if the lithium thionyl chloride is not a rechargeable battery and the zinc-manganese oxide is not a rechargeable battery, then why is the lead acid a rechargeable battery?
This is because the lead sulfate is soluble in sulfuric acid (which is the electrolyte) unlike the lithium chloride. But it’s not as soluble as the zinc oxide in potassium hydroxide.
Its solubility is not too much, nor too little. It’s just right! It’s referred to as a sparingly soluble salt.
This feature of having sparing solubility is critical in making a battery that undergoes dissolution-precipitation recharge.
This occurs because the reactants and the products are right next to each other. This means that when you go in reverse, there is a high probability that things go back to the same place where they came from. Not having something move around is a great way to prevent shape change.
Once you understand that solubility is key you begin to understand the decades that were spent on trying to change the solubility of zinc oxide in electrolyte using various techniques. And you begin to start thinking about ways to encapsulate the zinc. And you begin to wonder if you should never let the zinc precipitate as zinc oxide and if you should just keep it as zincate by, say, flowing it.
All these perfectly valid ideas start to make a lot of sense. What you can’t answer is if these ideas will succeed in solving the fundamental problem with the zinc electrode.
But we should remember that in general, the lead-acid is not the greatest battery in the world when it comes to recharging. Think sulfation. Remember the blog post on battery rules where I Haiku-ed my way to better battery life? Remember that sulfation occurs in the discharged state.
The reason for this is also fundamentally connected to this dissolution-precipitation mechanism. On the one hand, this mechanism allows you to make a good rechargeable battery, on the other hand, it also causes it to die in time.
Moral of this story: If you want good rechargebility, dissolution-precipitation is not a good idea, although we may be able to live with it.
Chapter 4: And you thought electroplating was easy.
Electroplating has been a gift that has been giving for decades. Probably the last big development was the via-hole plating of copper for making semiconductor chip interconnects.
In general, plating something uniformly is not easy, but it’s not an unsolvable problem either. We do have a lot of smoothly plated stuff all over the place.
This is until you try plating lithium (and a few other metals, including zinc). Plating lithium is sort of important because this would be the charging reaction if you want to make your watch battery a rechargeable battery or if you want to make a Li-sulfur or Li-air battery rechargeable.
People spent much of the 2-3 decades of the last century trying to make a rechargeable lithium (watch) battery. The last time I check, I was asked to buy a new watch battery and not try to recharge it.
This is because, in the case of lithium, the plating results in dendrites and lead to shorting of the battery.
The reason for this starts with surface inhomogeneities that lead to nucleation of the deposition process in one spot, after which ohmic and transport effects lead to further amplification of this inhomogeneity.
That complicated paragraph is tying to tell you that it plates out like a needle sticking out of the electrode. The needle can puncture through the separator and short to the cathode. As I keep mentioning in these blog posts, shorting a battery is not a good idea. Really, it is not.
Same problem happens in the zinc electrode. Zinc wants to plate out as a dendrite instead of a smooth surface. Same reasons as above.
Every electrochemist that learns of this issue immediately thinks of 10 things to try that could solve the problem. Turns out all 10 ideas probably don’t work.
There have been, literarily, thousands of studies on trying to solve this issue. The most promising appears to be using a separator that is hard and prevents the dendrite from growing.
But as of today, we do not have a method to prevent lithium dendrites at room temperature and give us good power capability. It’s a problem that is still around.
The moral of this story: If your battery requires you to plate out a metal, it is probably going to be an issue achieving good rechargebility.
And if you want to make a rechargeable Li-air or Li-S electrode, getting the lithium to recharge is, I’m pretty confident, a pre-requisite.
Epilogue: Rules to live by.
So how do we make a rechargeable Li-S and Li-air (or zinc-air) battery?
If I knew that I would not be writing blog posts!
But we need to beat three things that history has taught us:
1. Avoid electrodes that require a plating reaction.
2. If you have a product that is highly soluble, you are in trouble
3. If you don’t have any solubility, its worse
One can avoid all this by finding systems where no structural changes happen. Thus were born systems like Ni-MH, Li-ion, and Ni-hydrogen. These systems have their own problems, but atleast we are starting with something that has certain inherent advantage. I will elaborate on these problems in the very near future when I delve into the present-day developments in batteries.
But suffice to say, if you want to make the battery of the future, then you have to beat the three issues listed above.
Along the way, you may make the batteries of the past also work.
Venkat
