Batteries, storehouses for electrical energy “on demand,” range in size from large house‐sized batteries for utility storage, cubic meter‐sized batteries for automotive starting, lighting, and ignition, down to tablet‐sized batteries for hearing aids and paper‐thin batteries for memory protection in electronic devices. In bulk chemical reactions, an oxidizer (electron acceptor) and fuel (electron donor) react to form products resulting in direct electron transfer and the release or absorption of energy as heat. By special arrangements of reactants in batteries, it is possible to control the rate of reaction and to accomplish the direct release of chemical energy in the form of electricity on demand, without intermediate processes. The electrons involved in the chemical reactions are transferred from the active materials undergoing oxidation to the oxidizing agent by means of an external circuit. The passage of electrons through this external circuit generates an electric current, providing a direct means for energy utilization without going through heat as an intermediate step. The three main types of batteries are primary, secondary, and reserve. A primary battery is used or discharged once and discarded. Secondary or rechargeable batteries can be discharged, recharged, and used again. Reserve batteries are normally special constructions of primary battery systems that store the electrolyte apart from the electrodes, until put into use. They are designed for long‐term storage before use. The U.S. primary battery market is usually divided according to the chemical system used in the batteries, whereas the secondary battery market is usually divided according to usage. The lead–acid battery accounts for over 85% of the secondary battery market. Batteries are miniature chemical reactors that convert chemical energy into electrical energy on demand. The voltage is unique for each group of reactants comprising the battery system. The amount of electricity produced is determined by the total amount of materials involved in the reaction. The voltage may be thought of as an intensity factor. Reversible processes yield the maximum output. In irreversible processes, a portion of the useful work or energy is used to help carry out the reaction. Electrolytes are a key component of electrochemical cells and batteries. Electrolytes are formed by dissolving an ionogen into a solvent. When salts are dissolved in a solvent such as water the salt dissociates into ions through the action of the dielectric, water. Strong electrolytes, ie, salts of strong acids and bases, are completely dissociated in solution into positive and negative ions. The electrolyte also provides the physical separation of the positive and negative electrodes needed for electrochemical cell operation. Battery electrolytes are concentrated solutions of strong electrolytes and the Debye‐Hückel theory of dilute solutions is only an approximation. Each electrolyte is stable only within certain voltage ranges. Exceeding these limits results in decomposition. The stable range depends on the solvent, electrolyte composition, and purity level. In aqueous systems, hydrogen and oxygen form if the voltage limit is exceeded. In the nonaqueous organic solvent‐based systems used for lithium batteries, exceeding the voltage limit can result in polymerization or decomposition of the solvent system. It is especially important to remove traces of water from the nonaqueous electrolytes as water can catalyze the electrolytic decomposition of the organic solvent. One solid electrolyte, lithium iodide, LiI, has found application in heart‐pacer batteries even though it has a fairly low conductivity. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode–solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equilibrium. On the electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. Electrically, the electrical double layer may be viewed as a capacitor with the charges separated by a distance of the order of molecular dimensions. To be useful in battery applications reactions must occur at a reasonable rate. The rate or ability of battery electrodes to produce current is determined by the kinetic processes of electrode operations, not by thermodynamics, which describes the characteristics of reactions at equilibrium when the forward and reverse reaction rates are equal. Electrochemical reaction kinetics follow the same general considerations as those of bulk chemical reactions. The velocity of electrode reactions is controlled by the charge‐transfer rate of the electrode process, or by the velocity of the approach of the reactants, to the reaction site. The movement or transport of reactants to and from the reaction site at the electrode interface is a common feature of all electrode reactions. Most battery electrodes are porous structures in which an interconnected matrix of solid particles, consisting of both nonconductive and electronically conductive materials, is filled with electrolyte. Most battery electrode structures do not have a well‐defined planar surface but have a complex surface extending throughout the volume of the porous electrode. When a battery produces current, the sites of current production are not uniformly distributed on the electrodes. Two types of current distribution, primary and secondary, can be distinguished. The primary distribution is related to the current production based on the geometric surface area of the battery construction. Secondary current distribution is related to current production sites inside the porous electrode itself. Most practical battery constructions have nonuniform current distribution across the surface of the electrodes. Cell geometry, such as tab/terminal positioning and battery configuration, strongly influence primary current distribution. The monopolar construction is most common. Several electrodes of the same polarity may be connected in parallel to increase capacity. Current‐producing reactions occur at the electrode surface, they also occur at considerable depth below the surface in porous electrodes. Porous electrodes offer enhanced performance through increased surface area for the electrode reaction and through increased mass‐transfer rates from shorter diffusion path lengths. Models to describe and predict porous electrode performance in the lead–acid battery system have been developed. The positive electrode in a battery system is most often a metal oxide, but it may also be a metal sulfide or halide. Generally, these materials are relatively poor electrical conductors. Only a few reactions have the characteristics requisite for use in commercial batteries. A set of criteria can be established to characterize reactions suitable for battery development. The principal features necessary for battery reactions are mechanical and chemical stability, ie, the reactants or active masses and cell components must be stable over time (5 years or more) in the operating environment and must reform in their original condition on recharge; energy content, ie, the reactants must have sufficient energy content to provide a useful voltage and current level; power density, ie, the reactants must be capable of reacting at rates sufficient to deliver useful rates of electricity; temperature range, ie, the reactants must be able to maintain energy, power, and stability over a normal operating environment; safety, ie, the battery must be safe in the normal operating environment as well as under mild abusive conditions; cost, ie, and the reactants and the materials of construction should be inexpensive and in good supply.
