Rechargeable Batteries
Rechargeable batteries build on our understanding of electrochemistry to create a battery that can be charged and discharged repeatedly many times before needing to be replaced.
Before tackling rechargeable batteries, it is a good idea to go through the battery basics lesson.
In this lesson we will take a closer look at the structure of rechargeable batteries and how they work.
It's time to add some caveats!
As of this writing, the field of rechargeable batteries is under a great deal of study and it seems like new developments are being announced every day.
But currently, by far the most popular commercially available rechargeable batteries are lithium-ion, so for this lesson we will focus on them in our examples.
If and when a new technology takes precedence I'm sure I will come back and change this...
Structure
The basic components that make up the structure of a rechargeable battery are similar to those of a basic, non-rechargeable battery.
But there are nuanced differences.
Rechargeable batteries have five main components:
- A cathode electrode
- An anode electrode
- A separator
- An electrolyte
- Current collectors
The cathode is the positive electrode. This electrode supplies the both the electrons and the positive ions required for current flow.
In lithium-ion batteries, the cathode is made of a metal oxide.
This metal oxide is in turn made of p-type material, such as nickel oxide. The nickel oxide bonds with the lithium atom's valence electron in order to balance its charge.
This creates a new molecule: lithium nickel oxide.
The anode is the negative electrode. It stores electrons when the battery is charged and supplies them to the load when the battery is in use.
Graphite is commonly used to make the anode. This is because it has a stable crystal structure, made of carbon atoms, which can act as a storage container for lithium atoms.
The lithium atoms can remain balanced within the crystal structure, without actually chemically reacting with the carbon atoms.
The separator ensures that the electrodes don't touch each other, as this would short out the battery, but it is porous and allows positive ions to flow freely across it.
The electrolyte is a medium in which everything else is embedded. It conducts positive ions from one electrode to the other but does not conduct electrons.
The electrolyte is made up of a solution of lithium as well as different solvents.
The lithium in the electrolyte dissolves to form lithium ions which attract the solvent molecules to them.
The solvent molecules don't actually interact with the lithium ions, but instead form a shell around them. This is known as a solvation shell.
These solvation shells help the lithium ions disperse evenly throughout the electrolyte.
Current collectors are the connection between the battery electrodes and the external circuit. These do conduct electrons.
The collectors are made of different alloys which are chosen based on how well they work with each electrode.
There is another important component created by a chemical reaction within lithium-ion batteries; the SEI, or solid electrolyte interface.
The SEI is a layer of film that coats the anode when lithium ions, with their solvation shells, come into contact with the graphite for the first time.
The exact thickness and makeup of the SEI has a dramatic effect on the properties of a battery and great deal of research is ongoing on just how this works.
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