Working principle of lithium-ion batteries
A lithium-ion battery is a rechargeable battery that relies heavily on the movement of lithium ions between the positive and negative electrodes to function. During charging and discharging, Li+ ions repeatedly insert and extract between the two electrodes: during charging, Li+ ions extract from the positive electrode, pass through the electrolyte, and insert into the negative electrode, leaving the negative electrode in a lithium-rich state; the reverse occurs during discharging.
Lithium-ion batteries discharge through a chemical oxidation-reduction reaction. During discharge, which is when we use the battery and consume its power, lithium ions are inserted into the positive electrode and deposited into the negative electrode. Charging is the reverse process. This gain and loss of ions creates a voltage, resulting in a charge on the battery.
During each charge-discharge cycle, lithium ions (Li) act as carriers of electrical energy, repeatedly moving back and forth between the positive and negative electrodes. They react chemically with the positive and negative electrode materials, converting chemical energy into electrical energy and achieving charge transfer. This is the basic principle of a lithium-ion battery. Since the electrolyte and separator are electron insulators, there is no movement of electrons between the positive and negative electrodes during this cycle; they only participate in the chemical reactions at the electrodes.
Reasons for lithium-ion battery capacity decay
Lithium-ion batteries are the fastest-growing type of rechargeable battery after nickel-cadmium and nickel-metal hydride batteries. The application of lithium-ion batteries largely depends on the stability of their charge-discharge cycles. Like other rechargeable batteries, capacity decay during cycling is unavoidable for lithium-ion batteries.
Structural changes in cathode materials
Cathode materials are a crucial component of lithium-ion batteries. When a lithium-ion battery is removed from the cathode, the metal elements must be oxidized to a high oxidation state to maintain the material's electrical neutrality, resulting in a compositional shift. This shift can easily lead to phase transfer and changes in bulk structure. Phase transitions in electrode materials can cause changes in lattice parameters and lattice mismatch. The resulting induced stress causes grain breakage and crack propagation, leading to mechanical damage to the material's structure and consequently, a decline in electrochemical performance.
Negative electrode material structure
Commonly used negative electrode materials for lithium-ion batteries include carbon materials and lithium titanate. This article analyzes graphite as a typical negative electrode. The capacity decay of lithium-ion batteries first occurs during the formation stage, when an SEI (Sediment-Insulated Plate) forms on the negative electrode surface, consuming some lithium ions. As lithium-ion batteries are used, changes in the graphite structure also contribute to a decrease in battery capacity.
Although the morphology and structure of graphite are maintained, the half-width at half-maximum (WHM) of its (002) crystal plane increases, resulting in a smaller grain size in the c-axis direction. The change in crystal structure leads to cracks in the carbon material, which in turn damages the SEI film on the negative electrode surface and promotes the repair of the SEI film. The excessive growth of the SEI film consumes active lithium, thus causing irreversible capacity decay of the lithium-ion battery.
Oxidative decomposition and interfacial reactions of electrolyte
The properties of the electrolyte significantly affect the specific capacity, lifespan, rate charge/discharge performance, operating temperature range, and safety performance of lithium-ion batteries. The electrolyte mainly consists of three parts: solvent, electrolyte, and additives. The decomposition of both the solvent and the electrolyte leads to capacity loss in lithium-ion batteries. Electrolyte decomposition and side reactions are important factors in lithium-ion battery capacity decay. Regardless of the positive and negative electrode materials or manufacturing process used, with the cycling of lithium-ion batteries, electrolyte decomposition and interfacial reactions between the electrolyte and the positive and negative electrode materials will cause capacity decay.
Positive overcharge reaction
When the ratio of positive electrode active material to negative electrode active material is too low, positive electrode overcharging is likely to occur. The capacity loss caused by positive electrode overcharging in lithium-ion batteries is mainly due to the presence of electrochemically inert materials (such as Co3O4, Mn2O3, etc.), which disrupts the capacity balance between electrodes, and the capacity loss is irreversible.
Electrode instability
During charging, the unstable positive electrode active material reacts with the electrolyte, causing a decrease in capacity. Factors affecting the instability of positive electrode materials include structural defects, carbon black content, and excessively high charging potential, among which structural defects are the most significant.
