Working principle of energy storage batteries
An energy storage battery is a device that converts and stores energy between electrical energy and chemical energy. During discharge, chemical energy is directly converted into electrical energy; during charging, electrical energy is converted back into chemical energy for storage. The positive and negative electrodes in the battery are made of different materials. When the same electrolyte is inserted, both electrodes will establish their own electrode potentials, as shown by the ABCD broken line in Figure 1-4 (the space between the dashed line and the electrodes represents the formed electric double layer). The difference in equilibrium electrode potential between the positive and negative electrodes constitutes the battery's electromotive force (EMF) E.
Figure 14 Schematic diagram of the working principle of energy storage battery
When the positive and negative electrodes are connected to an external load, the positive electrode material gains electrons and undergoes a reduction reaction, producing cathodic polarization, thus lowering the positive electrode potential; the negative electrode material loses electrons and undergoes an oxidation reaction, producing anodic polarization, thus raising the negative electrode potential. For the external circuit, electrons flow from the negative electrode to the positive electrode, therefore the current direction is from the positive electrode to the negative electrode. In the electrolyte, charge transfer occurs through ion movement, thus the current direction in the internal circuit is from the negative electrode to the positive electrode. In the discharge state, the battery potential distribution is shown by the broken lines A'B'C'D' in Figure 1-4. The entire process forms a complete closed loop, allowing the oxidation and reduction reactions at the electrodes to continue continuously, thus ensuring a continuous current flow within the closed loop. When the battery is working, the electrochemical reactions that generate electrical energy at the electrodes are called flow-generating reactions, and the substances participating in these reactions are called active materials.
The charging process of a battery is essentially the reverse of its discharging process. During charging, oxidation occurs at the positive electrode, while reduction occurs at the negative electrode; simultaneously, the migration direction of ions in the electrolyte is opposite to that during discharge, and an external power source exceeding the battery's open-circuit voltage is required to drive this chemical conversion process, as shown by the broken lines A"B"C"D" in Figure 1-4.
To facilitate the direct conversion of chemical energy into electrical energy, the redox process occurring within an energy storage battery differs fundamentally from conventional redox reactions. In a battery, the processes of losing electrons (oxidation) and gaining electrons (reduction) must be separated into different regions. Furthermore, electrons must flow through an external circuit when active components participate in the reaction. These two key elements distinguish the redox mechanism within a battery from ordinary chemical redox reactions and the micro-cell reactions in electrochemical corrosion phenomena.
Composition of energy storage batteries
A basic energy storage battery should contain four fundamental components: electrodes, electrolyte, separator, and battery casing.
electrode
Electrodes, as key components of a battery, are divided into positive and negative electrodes, mainly composed of active materials and a conductive framework. Among them, the active materials generate electrical energy through chemical reactions during battery discharge, and are the main factor determining battery performance. Active materials are mostly solid, but can also exist as liquids or gases.
Active materials have a decisive influence on the overall performance of a battery, and therefore generally have the following performance requirements: ① The positive electrode material should have a high potential, while the negative electrode material needs to maintain a low potential, in order to ensure that the battery can generate a large electromotive force; ② Active materials must have good electrochemical reactivity, that is, they should easily participate in redox processes; ③ Active components need to have high specific capacity by weight and volume; ④ Active materials need to have excellent chemical stability in electrolyte solutions, and the self-dissolution rate should be as low as possible; ⑤ Active materials should have high electronic conductivity; ⑥ From the perspective of economics and sustainable development, ideal active materials should be resources that are abundant and inexpensive on Earth; ⑦ Active materials should also be harmless to human health and the natural environment.
Meeting all the above standards for a specific active material is quite challenging; therefore, a comprehensive consideration is necessary when selecting an active material. Currently, the most widely used cathode materials are metal oxides, such as lead dioxide, manganese dioxide, and nickel oxide, as well as oxygen from the air. For anode materials, a range of chemically reactive metals are preferred, such as zinc, lead, cadmium, iron, lithium, and sodium.
The function of the conductive framework is to connect the active material to the external circuit and ensure a balanced current distribution. It also supports the active material. An ideal conductive framework should possess excellent mechanical strength, high chemical stability, low resistivity, and good processability.
electrolytes
The primary function of the electrolyte is to ensure effective ion conduction between the positive and negative electrodes, undertaking the task of ion transport. In some cases, it may also participate in electrochemical reactions. For the electrolyte used in a battery, its performance should meet the following requirements: ① It should possess good chemical stability to prevent significant electrochemical reactions at the interface between the electrolyte and the active material during storage, thereby reducing battery self-discharge; ② It should have high electrical conductivity. The electrolyte composition varies among different types of batteries, and typically, aqueous solutions of acids, alkalis, or salts with excellent conductivity are chosen as the electrolyte. However, some new power technologies may use novel materials such as organic solvent electrolytes, molten salt electrolytes, or solid electrolytes.
isolation
A separator, also known as a membrane or partition, is placed between the positive and negative electrodes of a battery. Its main function is to prevent direct contact between the electrodes, which could lead to a short circuit. The basic performance requirements for separators include: ① being a good electronic insulator to prevent internal short circuits; ② having low resistance to ion migration in the electrolyte, thereby reducing the internal resistance of the entire device and significantly reducing energy loss under high-current discharge conditions; ③ possessing good chemical stability, withstanding electrolyte corrosion and the redox reactions of electrode active materials; ④ having sufficient mechanical strength and bending resistance to effectively block dendrite growth and prevent tiny active particles from penetrating the membrane;⑤Considering economic factors, it should be readily available and inexpensive.
Common separator materials include cotton paper, pulp paper, microporous plastics, microporous rubber, hydrated cellulose, nylon cloth, and glass fiber, etc.
Battery casing
The battery casing, also known as the battery container, is the only type of battery in existing energy storage batteries where the zinc electrode also serves as the casing. In contrast, other battery types tend to use specific materials for external encapsulation rather than the active material itself. An ideal battery casing should possess excellent mechanical properties, withstand vibration and shock, remain stable under extreme temperature conditions, and resist corrosion from the electrolyte. In practice, materials such as metals, plastics, and hard rubber are widely used as battery casings due to their respective advantages.