The positive electrode material is an important source of lithium-ion batteries. When the lithium-ion battery is removed from the positive electrode, in order to maintain the neutral state of the material, the metal element will inevitably be oxidized to a high oxidation state, which is accompanied by the change of composition. The transition of components easily leads to phase transition and change of bulk phase structure. The phase transition of the electrode material can cause changes in lattice parameters and lattice mismatch. The induced stress caused by the induced stress causes the breakage of the grains and the propagation of cracks, resulting in mechanical damage to the structure of the material, resulting in the degradation of electrochemical performance.
2. Negative electrode material structure
Commonly used anode materials for lithium-ion batteries include carbon materials, lithium titanate, etc. This article analyzes typical anode graphite. The capacity attenuation of lithium-ion batteries first occurs in the formation stage, in which SEI is formed on the surface of the negative electrode, consuming part of the lithium ions. As lithium-ion batteries are used, changes in the graphite structure can also cause battery capacity to drop.
Although the morphology and structure of graphite is maintained, the width at half maximum of its (002) crystal plane becomes larger, 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 destroys the SEI on the surface of the negative electrode. film and promote the repair of SEI film, the overgrowth of SEI film consumes active lithium, thus causing irreversible capacity fading of Li-ion batteries.
3. Oxidative decomposition and interfacial reaction of electrolyte
The properties of the electrolyte significantly affect the specific capacity, life, rate charge-discharge performance, operating temperature range, and safety performance of lithium-ion batteries. The electrolyte mainly includes three parts: solvent, electrolyte and additive. The decomposition of the solvent and the decomposition of the electrolyte will lead to the loss of the capacity of the lithium-ion battery. The decomposition and side reactions of the electrolyte are important factors for the capacity attenuation of lithium-ion batteries. No matter what kind of positive and negative materials and processes are used, with the recycling of lithium-ion batteries, the decomposition of the electrolyte and the occurrence of the interaction with the positive and negative materials The interfacial reaction will cause the capacity decay.
4. Positive electrode overcharge reaction
When the ratio of the positive electrode active material to the negative electrode active material is too low, overcharging of the positive electrode is likely to occur. The capacity loss caused by overcharge of the positive electrode of lithium-ion batteries is mainly due to the appearance of electrochemically inert substances (such as Co3O4, Mn2O3, etc.), which destroys the capacity balance between electrodes, and the capacity loss is irreversible.
5. Electrode instability
The positive active material is unstable during charging and reacts with the electrolyte to reduce the capacity. The factors affecting the instability of the positive electrode material are the structural defects of the positive electrode material, the content of carbon black, and the high charging potential, among which the structural defects of the positive electrode material are the most important factors.
As the usable area of a lithium-ion battery shrinks, the amount of energy that can be filled decreases, and the charging time gradually shortens. In most cases, Li-ion battery capacity decays linearly due to cycling and aging.
6. Storage temperature
The charge-discharge cycle is not the only reason for the capacity decay of Li-ion batteries. A fully charged Li-ion battery stored at 40°C (104F) for one year without use will cause a 35% capacity loss.