When I was around 13 years old I remember learning about primary batteries in school. This was about the time when I had also encounter lead-acid batteries and had realized that the car starter battery is being charged periodically. I began to wonder why some batteries could not be recharged while other could. Thus began my fascination with batteries.
The above story is probably not true. But it’s etched in my memory probably because it provides me with a romanticized notion of why I work on batteries today. In reality, I was probably contemplating something more useful like “what is the point of learning math?”.
Nonetheless, the question posed is an interesting one in that if we want to understand how to make a rechargeable battery that lasts a long time over many cycles, we first need to understand why some batteries do not recharge.
The development of batteries is deeply rooted in this understanding (atleast one hopes it is and that it isn’t a series of accidents!). Many of the ideas that people are proposing today for better batteries are in some ways trying to beat the fundamental limitations that prevent recharging of these primary batteries.
If you don’t want to make the mistakes of the past, you better understand the past. This post is an attempt to do just that.
The topic I’m discussing is technical. I’ve tried my best to make it accessible. In doing so, I lose some technical accuracy; but this is the price of making it understandable.
This blog post is written in the style of a novel. The story of rechargeability can be written in a very linear fashion which steps through time to illustrate the rules one after another. But this would make it easy to understand. Where is the fun in that!
So, I’ve borrowed a page from Quentin Tarantino’s movie Pulp Fiction and scrambled the narrative. Each storyline is distinct, and they are all interrelated. If the stories were arranged chronologically as the systems developed we would go Chapter 3, 2, 4, 1, Prologue, Epilogue. I personally think the scrambled version reads better.
This week I will start with the Prologue and Chapters 1 and 2. Next week I will finish with Chapters 3 and 4 and the Epilogue.
Fortunately, these stories are easier to follow when compared to Pulp Fiction.
And there is no swearing or violence.
Prologue: Do I smell sulfur in the air?
There has been a lot of interest over the last few years on two systems that have been around for years. These batteries are sold as going “beyond lithium-ion”. These systems use lithium metal as the negative electrode and air (rather, the oxygen in the air) or sulfur as the positive electrode. Some consider these systems to the Holy Grail of battery research.
The promise that these battery chemistries hold is enormous. In the case of lithium-air, the gravimetric energy is an order of magnitude higher than today’s batteries. Think cars that can drive 500+ miles, cell phones that actually last all day, laptops that can last the complete transatlantic flight... The volumetric energy is not that great, but hey, let us focus on the positives, shall we.
But there is a hitch (there always is).
In the case of lithium-air, it turns out that recharging lithium metal is a problem. And the oxygen in the air reacts to form a compound that is basically pretty insoluble.
In the case of sulfur, the lithium metal continues to be a problem. And the sulfur electrode undergoes a series of reactions giving products that are either highly soluble, or insoluble. All the products are a real pain to deal with and history shows us that dealing with them may not be for the faint of heart.
If we are to succeed in getting these batteries to work, we first need to understand why similar concepts have failed in the past.
Chapter 1: A business plan that can’t fail- Take cheese; make milk.
You may remember my previous blog post where I alluded to the lithium thionyl chloride battery.
This chemistry is used for missile applications. In this battery lithium dissolves from Li metal on the negative electrode and the resulting lithium ions combine with thionly chloride (SOCl2) to form lithium chloride (LiCl) in the positive electrode.
This is the process that occurs on discharge and it works pretty well. If you want to recharge this battery, you will need to take the lithium chloride and convert is into thionyl chloride. This is where the problem occurs.
Lithium chloride is an insoluble solid. Once you form it, it pretty much sits around and starts clogging up the whole cell. Converting lithium chloride back to thionyl chloride is like trying to find a Quentin Tarantino movie with no violence- its not hard to find; its impossible to find!
Think of this as taking a pot full of hot milk and adding lemon juice to it. The milk curdles and you get cheese. There is pretty much no way to get back to milk and lemon juice from cheese (as far as I know). Same story with lithium chloride.
If, on the other hand, the lithium chloride were soluble in the electrolyte and dissociated in the solution, one can think of trying to recharge the system. However, because lithium chloride is not soluble, there is no way to recharge the battery.
Moral of the story: If the product of your reaction leads to an insoluble product, in some batteries, recharging can be a problem.
The caveat “in some batteries”, is kind of important, but that is not for this post.
Something similar happens in the lithium carbon monofluoride system (Li-CFx). In this system, much like the thionly chloride system above, the lithium dissolves and reacts with the carbon monofluoride to form lithium fluoride and carbon; both solids. Trying to go back to the starting chemicals by reacting the lithium fluoride with the carbon is pretty much impossible. It’s the cheese to milk problem.
So solubility is something we ought to be paying attention to when it comes to recharging.
But what if the product of the reaction is highly soluble (think salt in water)? This must be the best thing possible for recharging, would it not?
Turns out that this is not true either. This would be the story of the zinc electrode. As in the zinc-manganese oxide alkaline battery (the Energizer-bunny battery).
Chapter 2: If only the Dead Sea weren’t as salty.
The alkaline battery is actually a rechargeable battery. It’s just that it is not a very good one. Some of you may remember that there were rechargeable alkaline batteries in the market that promised a few cycles. These all appear to be gone from the market. I guess no one wants to carry around a charger.
The positive electrode (MnO2) in these batteries can recharge thousands of times if you are careful to avoid certain phases. I personally believe that this chemistry is worth revisiting.
It’s the zinc electrode that is the problem. The reaction involves zinc electrochemically reacting and forming a complex called zincate. As the concentration of zincate increases, it precipitates out as zinc oxide. Think of this as adding salt into water. You can only add so much salt before the solution gets saturated and any more salt you add stays as crystals.
This is the simple version.
The whole reaction scheme is actually pretty complicated. Turns out that five other reactions can happen in this battery depending on various conditions. Some years ago, I was involved in modeling of the zinc-MnO2 battery and it is by far the most complicated battery chemistry there is. Lithium batteries are so much easier to understand with compared to this system.
The zinc oxide has pretty high solubility in the electrolyte (potassium hydroxide). What this means is that zinc oxide easily goes into solution to form zincate in the electrolyte. One would think this is great for recharging and it is. But it turns out that when you try to deposit the zinc from the zincate, it does not deposit in the same place where it dissolved.
This leads to the structure of the zinc electrode changing continuously as you charge and discharge the battery repeatedly. This is referred to as shape change. Shape change leads to the zinc depositing as a solid mass in the one part of the electrode. And this solid mass is hard to react. Slowly, as the solid mass gets more pronounced, the zinc electrode fades.
Reduce the solubility of zinc oxide and you can decrease the shape change and increase the cycle life. There is some wonderful data that shows that. This is like somehow making salt less soluble in water. Turns out that in reality this approach also kills the power of the battery and so this is not a real solution.
The reason why the shape change happens is a bit complicated, and probably not relevant at this stage. What does matter is that, in some batteries, very high solubility can be a bad thing and can decrease the ability to recharge.
Moral of the story: High solubility may not necessarily be a good thing for recharging.
So we have a problem. Neither no solubility nor high solubility appears to be a good idea. So how do we make a rechargeable battery?
We shall look at that in the next chapter. Next week.
Now… isn’t that a nice tease! Stay tuned.