Why do phone batteries lose capacity as they age?

Almost everyone has noticed it. The phone you bought two years ago lasted a full day on one charge, and now it barely makes it to the afternoon. You did not break it, you have not changed how you use it, and yet the battery just does not hold as much as it used to. This slow decline is not a defect or a software trick. It is the unavoidable chemistry of the battery itself.

Phones use lithium-ion batteries. Inside each cell, lithium ions move back and forth between two electrodes through a liquid electrolyte. When you charge the phone, ions are pushed to one side. When you use the phone, they flow back, and that flow is what powers the device. As long as the ions can keep moving freely, the battery works as designed.

From the very first charge, tiny side reactions take place alongside the useful ones. A thin layer called the solid electrolyte interphase grows on the surface of one of the electrodes. Each time the battery is charged and discharged, this layer grows a little, locking up a small number of lithium ions and a little electrolyte forever. Active material on the electrodes can also crack or lose contact over time. Once lithium and material are trapped, they are no longer available to carry charge, so the battery can simply hold less.

Not every battery ages at the same speed. Heat is the biggest accelerator. A phone left in a hot car, or pushed hard while charging in direct sunlight, will degrade faster than one kept cool. Sitting for long periods at a very high charge level also speeds up the chemical aging. This is why a phone that spends its life plugged in at one hundred percent in a warm room often shows more wear than one used gently and kept in milder conditions.

The result is the everyday experience of an aging phone. The screen-on time gets shorter, the battery percentage drops faster, and the phone may switch off earlier than its reading suggests. Nothing has been removed from the device. The chemistry inside simply has fewer mobile lithium ions left to move, so it cannot deliver the same amount of energy it once did.

This article was written using information from Wikipedia.