The longer you own a portable gadget – whether it’s a laptop, a smartphone or an MP3 player – the shorter its battery life becomes. One reason for this is the chemical reaction that powers rechargeable lithium-ion batteries.
Every time that you charge the cell, an electrical current pushes lithium ions from the battery’s cathode (made of lithium cobalt oxide) to a graphite anode. When you turn on your device, the lithium ions flow in the opposite direction. With each charge/recharge cycle, the cathode material ‘degrades’ slightly, meaning its internal crystalline structure is altered so that some of the lithium ions refuse to make the trip to the anode. As more lithium ions get tied down, the battery loses capacity. High temperatures also speed up a chemical reaction that degrades the graphite anode. As a precaution, try not to use or store your device in temperatures above 35 degrees Celsius (95 degrees Fahrenheit).
Answered by Dave Roos