The principle that energy cannot be created nor destroyed is known as the law of conservation of energy. In the case of a cell phone battery, the energy it contains does not disappear; it is transformed from one form to another.
When a cell phone battery gets discharged, the energy stored in the battery is converted from chemical energy into electrical energy, which powers the phone's components. As the battery delivers electrical energy to the phone, the chemical reactions inside the battery occur, depleting the reactants and converting them into different chemical compounds.
During the discharge process, the battery's anode and cathode materials react with the electrolyte, releasing electrons and allowing them to flow through the external circuit (i.e., the phone's components). This flow of electrons is the electric current that powers the phone.
As the chemical reactions proceed, the battery's active materials undergo changes in their composition. Eventually, the battery reaches a point where the reactants have been significantly consumed, leading to a decrease in the battery's voltage and capacity. This is when the phone starts to lose power and needs to be recharged.
So, the energy of the cell phone battery does not vanish but rather undergoes a transformation from chemical energy to electrical energy, powering the phone's operation until the chemical reactants inside the battery are depleted, and it needs to be recharged to restore its energy storage capacity.