Designation of the anode and cathode in a rechargeable cell are defined during the discharge process. The anode always refers to the negative electrode and the cathode always refers to the positive electrode, even though the reverse is actually true during charging, where the anode becomes the cathode and the cathode becomes the anode by definition of the terms anode and cathode. Common battery lingo maintains the anode cathode designations derived from the discharge process be applied when both discharging and charging in order to avoid confusion.
During discharge, the negative anode electrode is oxidized (loss of electrons is oxidation) and it is the source of electrons, while the positive cathode electrode is reduced (gain of electrons is reduction) and it is the receiver of electrons. Each electrode depends upon the other electrode to maintain a balance of flow of electrons. The number of electrons provided by the anode must equal the number of electrons received by the cathode. Electrode materials are often described by their mAh/g capacity ratings, from which the amount of each material required for the construction of a balanced cell can be calculated.
During discharge, the number of electrons transferred in the external electric circuit from the anode to the cathode equals the number of ions (positive or negative atoms/molecules) that must be transferred by the cell’s internal electrolyte. The electrolyte is ionically conductive, but electronically non-conductive. The ionically conductive electrolyte completes the electro-chemical circuit by carrying only ions between the active cathode and anode materials. The electrode-electrolyte-electrode interfaces are where all the real action occurs within the cell, and these two interfaces determine much of the cells characteristics and features such as cell voltage, capacity, power capability, cycle life, calendar life, self discharge, temperature effects, safety, and more.
During charging, the anode and cathode reactions are reversed by forcing electrons to flow opposite in direction than they flowed during discharge. The charger must apply a voltage across the cells’ terminals that is higher in potential than the open circuit cell voltage in order to generate electron flow back into the anode from the cathode, electro-chemically reversing the chemical reaction that took place during the discharge phase. During charging the electrolyte must also reverse function and shuttle ions back from the cathode to the anode.