Briefly describe the SEI film of lithium ion battery

During the first discharge of the battery from the lithium ion battery, the electrode material reacts with the electrolyte at the solid-liquid phase interface to form a passivation layer covering the surface of the electrode material. The passivation layer is an interfacial layer characterized by a solid electrolyte, which is an excellent conductor of lithium ions, and lithium ions can be freely embedded and extracted through the passivation layer, so the passivation film is called "solid electrolyte interface" (SEI film).

The SEI film performance impact

1. The formation of the SEI film consumes part of lithium ion, the charge-discharge irreversible capacity increase for the first time, reduces the charge and discharge efficiency of electrode materials.

2. The SEI film with organic solvent insoluble in organic electrolyte solution can be stable, and solvent molecules cannot pass the passivation membrane, which can effectively prevent the solvent molecules were embedded, avoided by solvent molecules were embedded to the electrode material damage, and thus greatly improve the cycle performance and service life of the electrode.

The SEI film formation and the adverse event loss capacity difference

1. After multiple charge and discharge, the surface of the graphite negative electrode tends to form a layer of SEI film, which prevents the interaction between the electrolyte and the graphite negative electrode. However, when the temperature rises, the SEI film undergoes a decomposition reaction, causing an irreversible reaction between the electrolyte and the surface of the negative electrode, resulting in irreversible capacity formation and heat generation, which further raises the temperature.

2. The temperature, solvent and electrolyte will react, gives off heat.

The influence of environment temperature on the SEI film

The use and storage temperature has a large effect on the SEI film, which affects the battery life. The stability of the SEI film at high temperatures is not only affected by temperature but also by the state of charge SOC of the battery. A high temperature of more than 45 degrees in the 100% SOC state will destroy the uniformity of the SEI film, resulting in an increase in electrode impedance and a decrease in cycle performance. When the battery was stored at 70 degrees at SOC9%, the SEI film on the negative electrode surface disappeared.

The influence of voltage on the SEI film

Graphite anode system, the formation of the SEI film depends on the battery voltage. When the battery voltage of 3.0 V, the SEI film began to form, until 3.8 V, this stage mainly generate Li2CO3. There is also a small amount of LiF and CH3OCO2Li generated, finally arrived at 4.2 V is mainly salt electrolyte decomposition. So the outermost layers of SEI compounds mainly for LiF.

The impedance of the SEI film is also different under different states of charge. The impedance of the SEI film when the negative electrode is fully charged is higher than the discharge state, which is caused by the change in the volume of the negative electrode during lithium insertion and delithiation.

When the battery is overcharged, excess Li+ has no negative electrode material for embedding, and part of Li+ will be reduced to metal lithium on the surface of the negative electrode, which may cause short circuit, and cause irreversible changes in the structure of the positive active material and decomposition of the electrolyte. A large amount of gas is generated, and a large amount of heat is released, so that the battery temperature and the internal pressure are increased, and there are hidden dangers such as explosion and burning. When the battery is over-discharged, Li+ in the SEI film on the surface of the negative electrode is completely removed, and the SEI film is broken. When the battery is recharged and discharged again, the stability and compactness of the re-formed SEI film may be deteriorated, and the amount of Li+ is required to be large, thereby causing a decrease in discharge capacity and charge and discharge efficiency.

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