>>11223892Batteries convert chemical energy directly to electrical energy. In many cases, the electrical energy released is the difference in the cohesive [14] or bond energies of the metals, oxides, or molecules undergoing the electrochemical reaction.[3] For instance, energy can be stored in Zn or Li, which are high-energy metals because they are not stabilized by d-electron bonding, unlike transition metals. Batteries are designed such that the energetically favorable redox reaction can occur only if electrons move through the external part of the circuit.
A battery consists of some number of voltaic cells. Each cell consists of two half-cells connected in series by a conductive electrolyte containing metal cations. One half-cell includes electrolyte and the negative electrode, the electrode to which anions (negatively charged ions) migrate; the other half-cell includes electrolyte and the positive electrode, to which cations (positively charged ions) migrate. Cations are reduced (electrons are added) at the cathode, while metal atoms are oxidized (electrons are removed) at the anode.[15] Some cells use different electrolytes for each half-cell; then a separator is used to prevent mixing of the electrolytes while allowing ions to flow between half-cells to complete the electrical circuit.
Each half-cell has an electromotive force (emf, measured in volts) relative to a standard. The net emf of the cell is the difference between the emfs of its half-cells.[16] Thus, if the electrodes have emfs {\displaystyle {\mathcal {E}}_{1}}\mathcal{E}_1 and {\displaystyle {\mathcal {E}}_{2}}\mathcal{E}_2, then the net emf is {\displaystyle {\mathcal {E}}_{2}-{\mathcal {E}}_{1}}\mathcal{E}_{2}-\mathcal{E}_{1}; in other words, the net emf is the difference between the reduction potentials of the half-reactions.[17]
