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Rechargeable lithium-ion batteries gradually lose the ability to hold a full charge as they age. Each time the battery is charged and discharged, small chemical changes build up inside it, and a thin layer slowly forms on the electrodes that traps some of the lithium. Because that trapped lithium can no longer move freely to carry energy, the battery's usable capacity falls, and after hundreds of cycles a phone may run for noticeably less time on a single charge.

As a lithium-ion battery is repeatedly charged, a layer that builds up on the electrodes —trapsreleasescools some of the lithium, which causes the battery's storage —capacityvoltageweight to decline over time.