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What is SEI? The impact on lithium batteries is so great!

SEI English full name is Solid Electrolyte Interphase, which is commonly known as solid electrolyte interface film.

 How is SEI formed?

When the 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 a part of electrons, and reacts with lithium ions to form an interface film having a thickness of about 100-120 nm. This film is SEI. The SEI is usually formed at a solid-liquid phase interface between the electrode material and the electrolyte.

When the lithium ion battery begins to charge and discharge, lithium ions are removed from the positive active material, enter the electrolyte and penetrate the membrane and then enter the electrolyte, and finally embedded in the layered void of the negative carbon material, and the lithium ion completes a complete deintercalation behavior. At this time, electrons exit from the positive electrode along the outer end circuit and enter the negative carbon material. Oxidation-reduction reaction occurs between the solvent and the lithium ion in the electron and the electrolyte, and the solvent molecule receives the electron and combines with the lithium ion to form SEI and generates a gas such as H2, CO, CH2=CH2. As the thickness of the SEI increases, until the electrons cannot penetrate, a passivation layer is formed, which suppresses the continuation of the redox reaction.

 What is the composition of SEI?

The thickness of SEI is about 100-120 nm, and its composition varies with electrolyte composition. It is generally composed of Li2O, LiF, LiCl, Li2CO3, LiCO2-R, alkoxide and non-conductive polymer. It is a multi-layer structure close to the electrolyte. One side is porous and the side close to the electrode is dense.

What is the impact of SEI on lithium batteries?

The role of SEI is to be analyzed from its own characteristics. It is characterized by: 1SEI is an interfacial layer between the electrode material and the electrolyte, separating the two. 2 has the characteristics of a solid electrolyte; 3Li+ can pass smoothly (good conductor of lithium ion), but electrons cannot pass.

SEI has an important influence on the performance of carbon negative lithium ion batteries.

First, the SEI is completed between the first charge and discharge, and the consumption of lithium ions is accompanied by the consumption of lithium ions. The lithium ion is consumed to increase the irreversible capacity of the battery, thereby reducing the charge and discharge efficiency of the electrode material. Second, the SEI film has an organic solvent. Insoluble, stable in organic electrolyte solution. PC exists in some electrolytes, PC is easy to be embedded in the anode material to cause damage to the electrode material, and if a suitable admixture can be added to the electrolyte to promote the formation of SEI, the co-insertion of solvent molecules can be effectively prevented, and the solvent is avoided. The molecular co-embedding causes damage to the electrode material, thereby greatly improving the cycle performance and service life of the electrode. Third, SEI allows lithium ions to pass through and prohibits the passage of electrons. On the one hand, it ensures the continuous charging and discharging cycle of the rocking chair, on the other hand, it hinders the further consumption of lithium ions and improves the service life of the battery.

 What factors are affected by the formation of SEI?

The formation of SEI is mainly affected by factors such as electrolyte formation (Li salt, solvent, admixture, etc.), formation (first charge and discharge) current, temperature, and the like.

First, the influence of electrolyte composition. The difference in Li salt and solvent composition results in different SEI components, and the stability of the product is different.

Second, the impact of the formation of current. When the charging current is large, the high-potential inorganic component is formed first, followed by the insertion of lithium ions, and finally the formation of organic components. When the formation current is small, the organic component of the SEI film begins to form rapidly.

Third, the lithium-ion battery formed at -20 ° C formed SEI compactness and low impedance, very beneficial to the battery life. Excessive temperatures can reduce the stability of the SEI and affect the cycle life of the battery.
In addition, the thickness of the SEI is also affected by the type of negative electrode material.

The reaction of SEI in the thermal runaway of lithium battery:

The SEI is composed of two layers of materials, the inner layer of which is mainly composed of Li2CO3, and the outer layer of which is mainly composed of lithium alkyl carbonate such as (CH2OCOLi)2. When the internal temperature of the battery is 80-120 ° C, the outer layer gradually decomposes, releasing heat to generate gas, and the reaction equation is as follows. In the SEI pyrolysis reaction, the reaction temperature and exotherm are related to the type of lithium salt, the composition of the solvent, the active material of the negative electrode, and the number of cycles of the battery.

(CH2OCOLi)2→ Li2CO3+CH2=CH2+1/2O2+CO2
Li+ (CH2OCOLi)2→ 2Li2CO3+CH2=CH2
 
Is there a SEI for the positive electrode?

Recent studies have shown that there is also a film formation at the solid-liquid interface between the positive electrode material and the electrolyte, and the thickness of the film is much thinner than that of the negative SEI film, which is about 1-2 nm. Since the potential of the positive electrode material is high, the reduction product of the organic electrolyte is unstable, and the inorganic product such as LiF can be stably present and becomes a main component of the positive SEI film.

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