Eli5: What exactly is happening, or changing, in a lithium battery when it “wears out” to cause it to have a diminished capacity?

Question

When you read about it, you’ll learn that the average lithium battery can take about 300-500 charge cycles before its considered worn out. What exactly changes inside the battery for it to reach a worn out status?

In: 217

Answer 1

Ever hear of electroplating? It’s what we use to do things like plate gold or other metals onto a cheaper metal. We do this all the time for things like for USB plugs. The wire contacts tend to be plated with gold or other corrosive resistant conductive metal.

Chemical batteries work kinda similar to this. You have 2 conductive materials, the + and – sides of the battery. The battery chemistry is basically like the chemical bath used for electroplating.

As you discharge the battery, usually some metal will start to “plate” (modern batteries actually use a sponge-like electrode that absorbs the metal) on one of the electrodes. When charging, this gets reversed and the plated metal dissolves into the electrolyte. In lithium batteries this is the lithium in the electrolyte. The specific type of lithium battery will have different makeups for the electrolyte, but lithium is usually the metal that gets “plated”.

The problem is that over time this process isn’t perfect. The chemicals in the electrolyte eventually wears down. When you fully charge/discharge lithium batteries some of the lithium winds up “stuck” forming spikes of lithium metal that don’t really dissolve back into the electrolyte when you recharge it. Less free lithium in the electrolyte means that it can hold less charge. This is the main reason why lithium batteries wear down. We haven’t really figured out a good way to reverse this buildup yet, although there is a lot of research into it and some promising results in recent years.

Eventually, after enough time, the spikes might create a connection between the electrodes forming a short circuit. At this point the battery is dead and can be very dangerous to recharge because the short circuit just causes the battery to heat up and explode instead of reversing the chemical reaction. Lithium batteries compared to other chemistries (like lead acid batteries) are especially prone to this kind of failure.