Every time you recharge your lithium-ion batteries, their storage capacity decreases just a little bit. That is why your mobile gadgets won't stay on nearly as long as they did even a year ago. But thanks to research by the US Department of Energy, we finally know why exactly that occurs and, more importantly, how to stop it from happening.
As Lithium-ion batteries discharge, the lithium ions (Li+) carry an electrical charge from the anode to the cathode across a non-aqueous electrolyte. This is what powers your phone. It is not a perfectly repeatable system though, and each time the lithium ions move through the battery, they cause minute changes to the electrodes' physical structures. This is what eventually kills your battery's capacity.