Definition
Like a game of catch between two walls, tiny particles travel back and forth between two rooms to collect electricity and send it out. It's the rechargeable battery at the heart of smartphones and electric cars. Unlike a throwaway AA battery, you can plug it in and refill it hundreds of times.
Lithium Bounces Between Two Rooms Like a Ping-Pong Ball
When you plug your phone into a charger, a busy moving day begins inside the battery. Lithium particles that were sitting quietly in the cathode (the positive side) get a push from the charger and march across to the opposite side, the anode (the negative side). The anode is a temporary room where lithium particles, now full of energy, pile up and wait.
When you watch videos or play games, they move the other way. The lithium particles stuck in the anode head back to the cathode, their original home. As they go, they push electrons out into the wire, which lights up your screen and makes your speaker play.
Because the lithium rocks back and forth between the two rooms like a rocking chair, scientists call this the "rocking-chair principle." The battery isn't a power plant that makes new electricity. Lithium is a tiny worker that acts like a shuttle bus, carrying energy from place to place.
Why Lithium, of All Things?
There are plenty of metals you could build a battery from. But for a phone you carry every day, or an electric car that has to move a heavy body, weight is the biggest problem. Lithium is light enough to float on water. It's the lightest metal element on Earth.
Lithium also has an eager personality: it loves to throw its electrons out. It's as small and light as a speck of dust, yet very strong at pushing electrons away. So it can pack much more electrical energy into the same weight than other metals can.
To be a bit more precise, lithium is one of the elements most likely to lose an electron and become an ion (an atom with an electric charge). Its "energy density," the amount of energy you can store per unit of weight, is outstanding. That is what made the portable electronics revolution possible.
Why Does a Battery Wear Out After Long Use?
A new phone easily lasts all day. After about two years, though, the battery gauge visibly drops faster. The reason is that lithium particles travel between the two rooms hundreds of times. Over that time, the paths they use to go in and out get warped, and gunk builds up thickly on the surface.
Once the paths are damaged, more and more lithium particles get stuck on the road and can't get home. As fewer workers are left to carry electricity between the rooms, the battery's total storage capacity shrinks along with them. That's the real reason a battery's life gets shorter.
Also, the liquid inside the battery (the electrolyte) can catch fire easily if the battery takes a hard hit or gets hot. So scientists are working hard on next-generation batteries that replace the liquid with a solid that doesn't burn, to remove the fire risk.
Common misconceptions
You should drain your phone battery all the way to 0% before charging it so it lasts longer.
Unlike the nickel batteries of the past, a lithium-ion battery can be damaged inside if you run it all the way down, which actually shortens its life. That's why it's said to be better to use it in the middle range, rather than draining it completely or keeping it at 100% all the time.
Where you meet it
When you plug your phone into a charger, lithium particles move to the anode and store energy.
When an electric car drives down the road, lithium particles return to the cathode and spin the motor.
In one sentence
A lithium-ion battery is a device in which light but powerful lithium particles shuttle between the cathode and the anode like a bus, storing electricity and releasing it.