There are more mobile phones in the world than there are people. Nearly all of them are powered by rechargeable lithium-ion batteries, which are the single most important component enabling the portable electronics revolution of the past few decades. None of those devices would be attractive to users if they didn’t have enough power to last at least several hours, without being particularly heavy.
Why do batteries die? And, why can they only be recharged so many times before they won’t hold a useful amount of charge? My young son asked me about that years ago when his battery-powered toy car stopped moving, wondering about what he called an “everlasting battery.” And this same question has probably crossed the mind of every cellphone user trying to send one last text before the screen blinks off.