15 Mar 2021

How does a battery work?

Our mastery of electricity is one of the defining achievements of humanity and yet few of us have more than a very basic understanding of what electricity is. Here I look at one crucial aspect of our relationship with electricity: the battery. 

Electricity is one of the fundamental forces which bind the universe: these days it's combined with magnetism to form one force, electromagnetism, and one of the key guiding principles of this force is the attraction of opposites. Put two magnets side by side and they will push each other away, as if by magic. But put a magnet next to iron or steel and it will stick to it like a limpet. Similarly, electricity works by having positives and negatives. 



This reflects what happens down at sub-atomic level where the elements that make up atoms exhibit small electrical charges. Protons at the centre are said to be positive, whilst electrons fizzing around the outer parts of the atom are said to be negative. These opposites attract each other and most atoms stay in a nice, stable balance with an equal number of protons and electrons. However, not all electrons are stable and some can easily be pulled away to other types of atom. Electricty's main job is to hold atoms together yet, until around 220 years ago, all we saw of it was the occasional lightning bolt. 


But many proton-electron relationships are far from stable and that is the reason why atoms don't stay single but go out and form bonds with other kinds of atoms. Electrons have a habit of breaking away from the mother protons and finding another home.


Every element is defined by the number of protons and electrons it is made up of. I am deliberately ignoring the mysterious neutron here, which clusters around the core next to the proton, but isn't subject to this electric tension between protons and electrons. Hydrogen atoms have one proton balanced with one electron, helium two of each, lithium three and so on as you work your way up the periodic table. 


As the proton/electron numbers increase, we get into the world of metals like copper, iron and zinc, and we start to find that the bond between protons and electrons begins to get a little more exotic. Copper’s atomic number is 29, but one of its outer electrons (the so-called valent electron) can easily be persuaded to leave the orbit of the mother proton and head off someplace more attractive. If you can create a really attractive destination for these loosely aligned electrons, you are halfway to building a battery. Get the conditions just right and you can create a flow of electrons one way and a counter-flow of newly configured atoms, known as ions, the other way to balance out the matter. Chemists refer to this as a redox reaction.


It's all about balancing these charges. As the negatively charge electron leaves its home (the anode), what's left behind is an atom minus an electron. A copper atom is no longer electrically neutral, it's now positive because its now got 29 protons and only 28 electrons. Conversely, the atom where the electron has gone to (the cathode) now becomes a negatively charged ion, because it's gained a negatively charged electron. These two new ions (the anode and the cathode) now become attracted to each other and want to form a bond, a new compound material. The key to batteries working is to seat the anode and the cathode in a salt bath known as an electrolyte, which enables the newly charged ions to move into and meet. 


In a closed state — i.e a battery at rest — the electrolyte is configured in such a way to stop a redox reaction taking place as the anode and cathode are kept apart and the pull to swap electrons isn't strong enough. But fix a good conductor like a copper wire between the anode and cathode and the valent electrons in the anode start queuing up to escape the mother atom and head off to the delights of what the cathode has to offer. And as this happens, the electrolyte itself now become a pathway for the newly configured ions to meet up in. 


The two materials chosen to be the anode and the cathode have to be carefully chosen. There are lots of metals out there which exhibit what is called electrical potential (or voltage), so that in the right configuration electrons will flow from one to the other. And there are lots of materials which can be used to make up the electrolyte. Batteries have come a long way since Alessandro Volta demonstrated the world's first battery in 1800, and the development of batteries continues to grow apace. One of the most remarkable developments is the discovery/invention of the rechargeable battery which reverses the redox reaction by putting an external electric current through the battery. 


But all electric batteries have the same three common features. Two electrodes:  the anode (always given the - sign)which gives up its electrons and the cathode (or + sign) which receives them, plus an electrolyte substance which sits between them and acts to both stop reaction happening when the battery is off and enables the exchange of ions when the battery terminals are connected. 



You want a metaphor? Here’s a one for you, based around teenagers heading out for the local Wetherspoons. Home is safe but boring. They seek excitement and the lure of meeting friends and cheap drinks is the voltage required to get the interaction going. Off they head, along the copper wire, paying a bus fare to get there. Wetherspons is empty but stacked with cheap booze. There is a significant electrical potential between these two locations. The teenagers will only flow one way (until closing time, when the process can be reversed – that’s the beauty of the rechargeable battery).  

 

There is a parallel reaction that goes on simultaneously. The atoms left behind by the deserting electrons are no longer electrically neutral: they are anxious parents worried about the loss of their children. Similarly, the manager at Wetherspoons, having been more than willing to take the teenagers off the parent's hands, starts to get worried about the responsibility of looking after them. The more teenagers turn up, the more worried the manager gets. Both the parents and the Wetherspoon managers have ceased to be normal and stable  and star behaving like ions, keen to make contact with the other party to find out if everything is OK. The teenagers meanwhile are oblivious to all this anxiety and are busy getting rat-arsed.


The metaphor breaks down here, because if what happens in a battery was reflected in a teenager's Friday night out, the anxious parents would end up coming into town themselves in their own cars (through the electrolyte) and would meet up with the Wetherspoons staff outside the pub, together with their children, and they would all choose to live together happily ever after in the streets somewhere between home and the pub. I don't think that's what happens in real life, but it does in a battery. 


However, the rechargeable battery is a different story. In this instance, everyone goes back to where they were when they started. The children go home, the pub is restocked with booze and another day dawns, ready for a re-run. 





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