It's a chemical reaction - there's a gradual degradation in the anode/cathode as a reaction with the electrolyte. There's environmental factors to it too - tem

It's a chemical reaction - there's a gradual degradation in the anode/cathode as a reaction with the electrolyte. There's environmental factors to it too - temperature has an effect, and if you can control the temperature over time, you can reduce the effect. This lecture (1h, 13m) goes into it in detail, including how it can be prevented, ways of testing, and more: https://www.youtube.com/watch?v=9qi03QawZEk https://www.youtube.com/watch?v=9qi03QawZEk