There are actually electrochemical cells which do change mass as they charge and discharge, metal-air batteries. In these, oxygen from the air is the cathode used to reduce the metal anode. But mass gain goes the opposite direction: the batteries are actually heaviest when discharged.

Various metals can be used. Iron is cheap, whilst aluminium-air cells actually offer some of the best performance in kWh/kg of available materials, exceeding that of LiON cells.

https://en.wikipedia.org/wiki/Metal%E2%80%93air_electrochemi...