How is SEI formed?
When a lithium-ion battery is charged and discharged for the first time, a small amount of polar aprotic solvent in the electrolyte undergoes a reduction reaction after obtaining some electrons, and combines with lithium ions to form an interface film with a thickness of about 100-120nm. This film is SEI. SEI is usually formed at the solid-liquid interface between the electrode material and the electrolyte.
When the lithium-ion battery starts to charge and discharge, lithium ions are extracted from the active material of the positive electrode, enter the electrolyte, penetrate the diaphragm, and then enter the electrolyte, and finally are embedded in the layered gap of the negative carbon material, and the lithium ions complete a complete deintercalation behavior. At this time, electrons exit from the positive electrode along the outer end loop and enter the negative electrode carbon material. An oxidation-reduction reaction occurs between the electrons, the solvent in the electrolyte and the lithium ions. After the solvent molecules receive the electrons, they combine with the lithium ions to form SEI and generate H2, CO, CH2=CH2 and other gases. As the thickness of the SEI increases, until the electrons cannot penetrate, a passivation layer is formed, which inhibits the continuation of the redox reaction.
What are the ingredients of SEI?
The thickness of SEI is about 100-120nm, and its composition varies with the composition of the electrolyte. It is generally composed of Li2O, LiF, LiCl, Li2CO3, LiCO2-R, alkoxides and non-conductive polymers. It is a multilayer structure and is close to the electrolyte. One side is porous, and the side close to the electrode is dense.
SEI's impact on lithium batteries?
The function of SEI should be analyzed from its own characteristics. Its characteristics are: ① SEI is an interface layer between the electrode material and the electrolyte, which separates the two. ②It has the characteristics of a solid electrolyte; ③Li+ can pass smoothly (a good conductor of lithium ions), but electrons cannot pass.
SEI has an important impact on the performance of carbon anode lithium-ion batteries.
First, SEI is completed between the first charge and discharge, which is accompanied by the consumption of some lithium ions. The consumption of lithium ions results in an increase in the irreversible capacity of the battery, which reduces the charge and discharge efficiency of the electrode material;
Second, the SEI film is insoluble in organic solvents and can exist stably in organic electrolyte solutions. PC exists in part of the electrolyte, and PC is easily co-embedded in the negative electrode material to cause damage to the electrode material. If a suitable additive can be added to the electrolyte to promote the formation of SEI, it can effectively prevent the co-intercalation of solvent molecules and avoid the problem of solvent The damage caused by molecular co-embedding to the electrode material greatly improves the cycle performance and service life of the electrode.
Third, SEI allows lithium ions to pass through but prohibits electrons from passing through. On the one hand, it ensures the continuation of the rocking chair-style charge and discharge cycle, and on the other hand prevents the further consumption of lithium ions and increases the battery life.
What factors affect the formation of SEI?
The formation of SEI is mainly affected by factors such as electrolyte formation (Li salt, solvent, admixtures, etc.), chemical formation (first charge and discharge), current size, and temperature.
1. The influence of electrolyte composition. The difference of Li salt and solvent composition leads to the different composition of SEI, and the stability of the product is also different.
2. The influence of the formation current. When the chemical conversion charging current is large, the high-potential inorganic components are formed first, followed by the insertion of lithium ions, and finally the organic components are formed. When the formation current is small, the organic components of the SEI film begin to form soon.
3. The SEI formed by the formation of lithium-ion batteries at -20°C has good compactness and low impedance, which is very conducive to the service life of the battery. Too high temperature will reduce the stability of SEI and affect battery cycle life.
In addition, the thickness of SEI is also affected by the type of anode material.
The reaction of SEI under thermal runaway of lithium battery:
SEI consists of two layers of materials, the main component of the inner layer is Li2CO3, and the main component of the outer layer is lithium alkyl carbonate such as (CH2OCOLi) 2 and so on. When the internal temperature of the battery is 80-120°C, the outer layer gradually decomposes and releases heat to generate gas. The reaction equation is as follows. In the SEI pyrolysis reaction, the reaction temperature and exothermic heat are related to the type of lithium salt, the composition of the solvent, the active material of the negative electrode, and the number of battery cycles.
(CH2OCOLi)2→Li2CO3+CH2=CH2+1/2O2+CO2
Li+(CH2OCOLi)2→2Li2CO3+CH2=CH2
Does the positive electrode have SEI?
The latest research shows that a film is also formed on the solid-liquid interface between the positive electrode material and the electrolyte, and the thickness of the film is much thinner than the negative SEI film, about 1-2 nanometers. Due to the high potential of the positive electrode material, the reduction product of the organic electrolyte is very unstable, while the inorganic product such as LiF can exist stably and become the main component of the positive electrode SEI film.
