In a chemical battery, chemical energy is directly converted into electrical energy as a result of spontaneous oxidation and reduction reactions within the battery. These reactions occur at the two electrodes. The negative electrode active material consists of a reducing agent with a relatively negative potential and stable in the electrolyte, such as active metals like zinc, cadmium, and lead, and hydrogen or hydrocarbons.
The positive electrode active material consists of an oxidant with a relatively positive potential that is stable in the electrolyte, such as metal oxides like manganese dioxide, lead dioxide, and nickel oxide, oxygen or air, halogens and their salts, and oxyacids and their salts. The electrolyte is a material with good ionic conductivity, such as aqueous solutions of acids, bases, and salts, non-aqueous solutions of organic or inorganic substances, molten salts, or solid electrolytes. When the external circuit is disconnected, although there is a potential difference (open-circuit voltage) between the two electrodes, there is no current, and the chemical energy stored in the battery is not converted into electrical energy.
When the external circuit is closed, current flows through it due to the potential difference between the two electrodes. Simultaneously, inside the battery, since there are no free electrons in the electrolyte, charge transfer is inevitably accompanied by oxidation or reduction reactions at the interface between the active materials at the electrodes and the electrolyte, as well as the migration of reactants and products. Charge transfer within the electrolyte is also accomplished by ion migration. Therefore, normal charge and mass transfer processes inside the battery are necessary conditions for ensuring normal power output. During charging, the direction of electrical and mass transfer processes inside the battery is exactly opposite to that during discharging; the electrode reactions must be reversible to ensure the normal progress of the reverse mass and electrical transfer processes.
Therefore, reversible electrode reactions are a necessary condition for the construction of a storage battery. G is the Gibbs free energy increment (J); F is the Faraday constant = 96500 coulombs = 26.8 ampere-hours; n is the equivalence number of the battery reaction.
This is the fundamental thermodynamic relationship between the battery's electromotive force and the battery reaction, and also the basic thermodynamic equation for calculating the battery's energy conversion efficiency. In reality, when current flows through the electrodes, the electrode potential deviates from the thermodynamic equilibrium electrode potential; this phenomenon is called polarization. The higher the current density (the current passing through a unit electrode area), the more severe the polarization.
Polarization is a significant cause of energy loss in batteries. Due to the different electrolyte materials used in lithium-ion batteries, they are classified into polymer lithium-ion batteries and liquid lithium-ion batteries. Polymer lithium-ion batteries and liquid lithium-ion batteries use the same positive and negative electrode materials and have similar working principles, but their electrolytes differ. Polymer lithium batteries are lightweight, have high energy storage capacity, good discharge performance, can be manufactured in various shapes, and have a longer lifespan. The electrolyte in polymer lithium batteries is a solid electrolyte. Compared to the liquid electrolyte in liquid lithium-ion batteries, polymer lithium batteries can be shaped in various ways to improve their specific capacity.
