How does SEI film affect the performance of lithium battery?

SEI overview

During charge and discharge of lithium ion batteries for the first time, a small amount of polar aprotic solvents in electrolyte occurs reduction reaction after get some electronic. Combined with lithium ion, the reaction generated a thickness of about 100-120 nm of interfacial film. The film is the SEI.SEI is usually formed in the solid liquid interface between the electrode materials and electrolyte.

When the lithium ion battery started charging and discharging lithium ion emerge from the positive living material, electrolyte membrane and then into the electrolyte, and then embedded into the layered space of anode carbon materials, lithium ion to complete a full release behavior. At this point, the electrons from the anode loop out along the outer end, into the anode carbon materials.

In electronic, electrolyte solvent and REDOX reaction between lithium ion, solvent molecules with lithium ions to form after receive an SEI and generate H2, CO and CH2 = CH2 gases. With SEI thickness increases, until the electrons cannot penetrate, the passivation layer is formed, inhibits the continuation of REDOX reaction.

What is the composition of the SEI?

SEI thickness of about 100-120 nm, its composition varies with the electrolyte composition, generally by Li2O, LiF, LiCl, Li2CO3, LiCO2 -r, alcohol, salt and the conductive polymer, is a multilayer structure, the side near the electrolyte is porous, the side near the electrode is dense.

The influence of SEI to the lithium battery

SEI role will come from its own characteristics were analyzed, and its features are: (1) the SEI is one of the electrode materials and electrolyte interface layer, the two separated.(2) has the characteristics of the solid electrolyte;(3) Li + can smoothly through the excellent conductors (lithium), and the electron is unable to pass.

SEI on carbon anode of lithium ion battery performance has an important influence.

First, SEI completed between the first charge and discharge, formed with part of the consumption of lithium ion, lithium ion is consumed is caused by the battery the increase of the irreversible capacity, reduces the charge and discharge efficiency of electrode materials.

Second, SEI film with organic solvent insoluble in organic electrolyte solution can be stable. Part of PC were found to exist in the electrolyte, anode materials were embedded PC easily damage to the electrode material, and if can add proper additive in the electrolyte prompted SEI formation, can effectively prevent the solvent molecules were embedded, avoided because of the damage caused by solvent molecules were embedded to the electrode material, and thus greatly improve the cycle performance and service life of the electrode.

Third, SEI allowed to ban electronic by lithium ion by, on the one hand, to ensure the rocking-chair recharge cycles of continuous, hindered the further consumption of lithium ions on the other hand, improve the service life of the battery.

The formation of SEI is influenced by what factors

SEI formed mainly by the electrolyte into Li salt, solvent, additive, etc.), into (the first charge and discharge current size, the influence of factors such as temperature.

One, the influence of electrolyte composition. Li salt, solvent composition is different, lead to different composition of the SEI, the stability of the product is different.

Second, the influence of forming current. Charging current is larger, high potential inorganic composition form first, second lithium ion insertion, the last is the formation of organic compounds. Forming current is small, the SEI film of organic compounds was quickly began to form.

Three, lithium ion battery under - 20 ℃ to formation of the SEI density is very good and has a low impedance, very beneficial to the service life of batteries. Too high temperature will reduce the stability of the SEI, influence the battery cycle life.

In addition, the thickness of the SEI is also influenced by types of anode materials.

SEI in lithium battery under the thermal runaway reaction

SEI consists of two layers of material, lining was mostly Li2CO3, while its outer main ingredient is alkyl lithium carbonate, such as (CH2OCOLi) 2, etc. When the battery internal temperature is 80-120 ℃, outer gradually decompose, gives off heat generated gas, reaction equations are as follows.

In SEI pyrolysis reaction, the reaction temperature and heat release and lithium species, solvent composition, the cathode material and cell cycles.

(CH2OCOLi)2→Li2CO3+CH2=CH2+1/2O2+CO2

Li+(CH2OCOLi)2→2Li2CO3+CH2=CH2

The anode have SEI?

, according to a new study in the positive electrode materials and electrolyte on the solid-liquid interface of membrane formation, the thickness of the membrane is thinner than negative SEI a lot, about 1 to 2 nanometres. Due to the high potential anode materials, organic electrolytic reduction product is not very stable, and inorganic product such as stable LiF can exist, as the main composition of the positive of SEI.

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