A lithium-ion battery is a rechargeable battery that uses two materials capable of reversibly inserting and deintercalating lithium ions as the positive and negative electrodes, respectively. During charging, lithium atoms in the positive electrode ionize into lithium ions and free electrons.
These lithium ions then migrate to the negative electrode and combine with the free electrons there to form lithium atoms. Conversely, during discharge, lithium atoms at the negative electrode reionize into lithium ions and free electrons, and then recombine at the positive electrode to form lithium atoms again. Therefore, throughout the entire charge-discharge cycle, lithium always exists in ionic form and never in metallic lithium form, which is why this type of battery is called a lithium-ion battery. Based on different designs, the most common lithium-ion batteries on the market are pouch cells, cylindrical cells, and button cells. The positive electrode material of lithium-ion batteries is usually a lithium-rich compound, such as LiCoO2, LiFePO4, LiNiO, and LiMnzO2. The negative electrode material of lithium-ion batteries is mostly based on graphite. Regarding the electrolyte, solutions of specific lithium salts (such as LiPF6, LiASF6, and LiCIO4) dissolved in organic solvents are generally used. These organic solvents can be ethylene carbonate (EC) or diethyl carbonate (DEC), etc. When the battery is in a charge-discharge cycle, lithium ions migrate back and forth between the positive and negative electrodes. This process is figuratively described as a "rocking chair battery," as shown in the diagram below.
Taking lithium-ion batteries with lithium cobalt oxide (LiCoO2) as the positive electrode and graphite as the negative electrode as an example, its electrochemical expression is: (-)C I LiPF(EC+ DEC) / LiCoO2(+).
Positive electrode reaction:
Negative electrode reaction:5.2
Overall reaction:5.3
In essence, a lithium-ion battery can be viewed as a device that operates based on differences in lithium-ion concentration. During charging, lithium ions are released from the positive electrode material, pass through the electrolyte, and embed into the negative electrode material, making the negative electrode lithium-rich while the positive electrode relatively lithium-poor. Simultaneously, to maintain the charge balance of the entire system, corresponding electrons are supplied to the carbon-based negative electrode through an external circuit. Conversely, during discharge, lithium ions migrate from the negative electrode to the positive electrode, causing the positive electrode to become lithium-rich. Normally, during normal charge-discharge cycles, lithium ions repeatedly embed and extract between the layered carbon materials and oxides. This process primarily manifests as changes in interlayer spacing and does not disrupt the basic crystal structure of the materials. Therefore, from the perspective of reaction reversibility, the chemical changes within a lithium-ion battery are considered a highly ideal reversible process. A lithium-ion battery comprises four basic components: electrodes, electrolyte, separator, and casing. The electrodes are the core component of a lithium-ion battery, consisting of active materials, conductive agents, binders, and current collectors. Active materials (or electrode materials) are the electrode materials in lithium-ion batteries that release electrical energy through electrochemical reactions during charging and discharging. They determine the electrochemical performance and basic characteristics of lithium-ion batteries. Active materials include positive electrode materials and negative electrode materials. Positive electrode materials are mainly powdered composite metal compounds with relatively high potentials (relative to lithium metal electrodes), such as LiCoO₂, LiMnO₄, LiNi₁-x-Co:MnyO₂, LiCo:Ni-O₂, and LiFePO₂.
Negative electrode materials include carbon materials, alloy materials, and metal oxide materials. Currently, the main positive and negative electrode materials for lithium-ion batteries widely used in portable devices are LiCoO₂ and graphite, respectively. In addition, conductive agents (such as acetylene black) are usually added during electrode fabrication to improve the conductivity of the positive and negative electrode materials to better meet the practical application needs of lithium-ion batteries. To ensure the granular positive and negative electrode materials and conductive agents adhere firmly to the current collector, a binder is typically added. Common binders are classified as oil-based and water-based. Oil-based binders mainly include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), while water-based binders are mainly carboxymethyl cellulose/styrene-butadiene rubber (CMC/SBR). The main function of the current collector is to conduct electrons from the active material and distribute the current evenly, while also supporting the active material. Current collectors are generally required to have high mechanical strength, good chemical stability, and high conductivity. The current collector for the positive electrode is aluminum foil, and the current collector for the negative electrode is copper foil.
The role of the electrolyte is to conduct lithium ions between the positive and negative electrodes. The choice of electrolyte largely determines the battery's working principle and affects its specific energy, safety performance, cycle performance, rate performance, low-temperature performance, and storage performance. Currently, commercially available lithium-ion batteries mainly use non-aqueous electrolyte systems, which include organic solvents and conductive lithium salts. The organic solvent is the main component of the electrolyte and is closely related to its performance; it is typically a mixture of organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and methyl ethyl carbonate. The conductive lithium salt provides the lithium ions that are transported between the positive and negative electrodes and is composed of inorganic or organic anions and lithium ions. Currently, the main commercially available conductive lithium salt is LiPF6. In the new era, to improve the electrochemical performance of lithium-ion batteries and achieve certain special functions, functional additives, such as flame retardants, are often added to the electrolyte.
In lithium-ion battery design, the separator is located between the positive and negative electrodes, and its main function is to prevent direct contact between the two electrodes, thus avoiding short circuits. Simultaneously, the unique microporous structure of this material allows lithium ions to pass through freely.The separator plays a crucial role in the battery's storage capacity, cycle life, and overall safety; therefore, using a high-quality separator can significantly improve the overall electrochemical performance of the battery. Currently, the most widely used separator types on the market are high-strength films made of polyolefins, including porous products made of polypropylene and polyethylene, as well as products manufactured through copolymerization of propylene and ethylene or using polyethylene homopolymer alone. The use of non-aqueous solvents in lithium-ion batteries leads to lower lithium-ion conductivity, thus requiring a large electrode area. Furthermore, the use of a spiral-wound structure during battery assembly means that battery performance depends not only on the electrodes themselves but also on the binders used in battery manufacturing. These binders must ensure the uniformity and safety of the active materials during electrode fabrication, effectively bond the active materials to the current collector, facilitate the formation of a protective SEI (solid-clectrolyte interphase) film on the graphite anode, maintain sufficient thermal stability during drying, and be effectively wetted by the electrolyte.
The outer casing is the container of a lithium-ion battery. Commonly used casings include steel casings, aluminum casings, and aluminum-plastic composite casings. Typically, the casing is required to withstand changes in high and low temperatures and corrosion from the electrolyte.