A tech-savvy friend of mine recently told me that to him, batteries are “magic”. He waved his fingers a little when he said that. This made me smile. And then it made me think. Do people want to believe in magic? Are some curious about the underlying truths? Can I explain it to them in a way they can easily understand and never forget?
This post is for those of you who are curious.
If you internalize this post, you will know more about lithium-ion batteries than 99.9% of the earth’s population. I know this to be true because my readership is substantially low.
Let's begin.
You know that the term ‘lithium-ion battery’ refers to all the awesome batteries you rely on every day. They really are amazing. They belong to a class of "electrochemical energy storage" devices. Sometimes they’re referred to as just lithium batteries, which bugs me because that might imply (incorrectly) that lithium metal is in the battery, which is not the case in our rechargeable Li-ion batteries. One day maybe…
Lithium ions (AKA Li-ions, Li+) are in your battery. An ion is a charged species dissolved in something, like Na+ and Cl- are when you dissolve table salt in water. Every positive thing has a corresponding negative thing (ain’t that a bitch... but hey, it's just a law in our universe), so along with Li+ there is a PF6- ion, also dissolved in the liquid. It’s actually kind of important that the negatively charged ion here is hexafluorophosphate (say that out loud - I bet you can). It’s used not just because it (LiPF6) dissolves in battery liquid quite easily or just because it’s kinda cheap. It’s used because the PF6- reacts to protect one of the other battery components, the aluminum metal current collectors. Crazy, right! No one realizes it’s in there doing that, but it is and it’s very necessary. Small magic.
So Li+ is floating around in the battery liquid, which is called the electrolyte. An electrolyte is a solvent (like water) and a salt (like table salt), but in Li-ion batteries it’s NOT water. This is pretty crucial. Instead the solvent is based on carbon compounds and we call the ensuing electrolyte an ‘organic electrolyte’. Organic electrolytes are just a lot more stable than water in the voltage range that Li-ion batteries operate within. If the electrolyte is not stable it will degrade and that’s not good for battery performance.
Whoa wait, back up – what’s a voltage? Excellent question. Like seriously, excellent, excellent question. It’s a number, right? But WTF does it mean? Well if anyone ever asks you, just say that it’s the difference in potential of two things. What’s a potential? This is getting deep. Here goes. There’s something about each thing that’s special to it based on what it is made of. Let’s call that something its ‘potential’. If I have two identical baseballs in my hand, their potential is the same because they are the same. Diving deeper and deeper down we know that there small differences in the baseballs, maybe one is a little dirtier than the other, so the potentials are not exactly the same, but super close. But essentially the voltage between the two baseballs would be zero. They are the same thing. No potential difference. But if I have one baseball and one tennis ball they ARE different because they are made up of different things. They have different potentials (potentials to do something… anything).
The potential of two different electrodes in a battery are what matter now and that “something” is the ability to let Li+ creep on inside them. Those electrodes are called the anode and the cathode. Imagine one baseball (the anode) and one tennis ball (the cathode). And 'potential' is now defined as the energy (vs. a standard) that Li+ reacts with (creeps inside of) each type of ball. The standard we use is the energy at which Li+ becomes lithium metal (gains an electron, reduces). This doesn’t happen in a battery, but we call this potential zero. Now the game is to shuttle Li+ between two electrodes with the voltage between them being as large as possible. This gives you batteries with high energy. Maybe this figure helps:
How are we doing? Any questions?
The energy of a battery is also related to how many lithium ions you can fit into the baseball and the tennis ball. That is called an electrode’s 'capacity'. Hundreds of balls that are the exact same size might all have different capacities simply because of what the balls are made of. Some are stubborn bitches and some are rather permeable. A battery’s energy goes up (as does its usefulness to you) when the voltage between the anode and the cathode is large and when the capacity to store Li+ in them is high. So what are these special balls anyway?
Before we get there, one thing to know is that lithium ions are loathe to become lithium metal. They HATE it. They would rather be positive ions more than anything. No one hates lithium more than lithium ions do. It’s a fact. So instead of using lithium metal we use a thin layer of graphite (like in pencils) as anodes in Li-ion batteries. The potential that Li+ goes inside of graphite (that’s right, it just moseys on inside the pencil, dropped off by the liquid that carries it around) is rather close to that of lithium metal. We’re tricking Li+ to be as happy as possible sitting in a state (inside a material) that is rather metallic, like lithium metal, but not as mercenary to the lithium ions. See, in graphite, the Li+ gets to stay kinda positively charged. The graphite handles the negative charge, and all is well. Very cool.
When the lithium ions are inside the graphite, that’s when the battery is considered “charged”. Li-ions prefer to be at high potential, where their precious positive charges are surrounded by much cooler company in the battery cathode. Imagine you’re in a room with people you hate, but the next room over has a bunch of your friends in it. You get all agitated and want to leave the crappy room like mad. This is how the Li-ions are feeling when they are with the graphite. They are charged UP and will fight to get out (although they don't get out without permission - that would be called 'self-discharge'). When we use our batteries, discharging them, we release the Li+ from personal hell in the anode, letting them join their friends in the cathode.
Join me next week for part 2!
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