The effect of moisture on the life of 18650 lithium battery is related to the compactness and uniformity of SEI film. What is SEI film: (in liquid lithium)
During the first charge and discharge of the ion battery, the electrode material reacts with the electrolyte at the solid-liquid interface to form a layer covering the electrode
Passivation layer on material surface. This passivation layer is an interface layer, which has the characteristics of solid electrolyte. It is an electronic insulator, but it is the advantage of Li +
A good conductor, Li+, can be embedded and released freely through the passivation layer. Therefore, the passive film is called "solid electrolyte boundary film" (solid).
Electronic interface (SEI membrane for short)
When the SEI film is uniform and dense, it is difficult for lithium ions to be embedded into the negative plate and occupy the Li + insertion vacancy, so there is little capacity attenuation. And
On the contrary, when the SEI film is not dense and uniform as a whole, the Li + intercalation vacancy is relatively easy to be occupied by the electrolyte solvent. Li2C03（
Lithium carbonate) is the most important component to form uniform and dense SEI film. In the electrolyte solvent system, when the moisture content exceeds the normal value, it will
As a result, the SEI film is not dense and uneven locally, and the attenuation of 18650 lithium battery life is accelerated. The battery will expand, which is important because of the SEI film
The presence of generated water decomposes LiPF6 to form HF gas.
The water sources in the 18650 lithium battery manufacturing process are basically divided into the following situations:
1、 Water source of workshop:
a. Moisture in the air is generally measured by relative humidity. In different temperatures and weather, there are great differences, which can be seen on rainy days in summer
To reach 90%, the snow days in winter are about 30%, the sunny days in summer are about 50%, and the sunny days in winter are about 20%.
b. Moisture in the human body
c. The moisture content of cleaning accessories such as packaging materials (especially cartons), paper towels and rags carried by materials is very high
d. Water seepage of equipment and facilities
2、 18650 water source of lithium battery raw materials:
a. Positive and negative materials
Most of the positive and negative active materials are micron or nano particles, which are easy to absorb water molecules in the air, especially ternary materials
The specific surface area of the material is too large, and the material surface is very easy to absorb water and react.
In the solvent structure of the electrolyte, there are carbonyl groups with high electronegativity and metastable double bonds, which are easy to form with polar H2O molecules
The higher the temperature, the faster the reaction. Moreover, the solute lithium salt of the electrolyte is also easy to absorb water and interact with the electrolyte
Diaphragm paper is also a porous plastic film, and its water absorption is also great. Since water generally does not react chemically with the diaphragm
It should be noted that it can also be basically eliminated by baking, so the diaphragm is generally rarely subject to strict moisture control.
Summary of the influence of moisture on the life of 18650 lithium battery: the moisture content in raw materials is an important source of moisture in 18650 lithium battery
First, the higher the humidity of the production environment, the more moisture in the air the 18650 lithium battery material absorbs. Therefore, it is necessary to control the production environment
Humidity, the recommended humidity control ladder of 18650 lithium ion battery production workshop is (for reference only, and the humidity control is based on each lithium battery company
The company will operate according to the actual situation):
Workshop with relative humidity ≤ 30% (such as mixing, coating head, tail, etc.)
Workshop with relative humidity ≤ 20% (such as rolling, film making, baking, etc.)
Workshop with relative humidity ≤ 10% (such as lamination, winding, assembly, etc.)
Dew point temperature ≤ - 45 ℃ workshop (such as cell baking, liquid injection, sealing, etc.)
Even if it is controlled according to the above humidity gradient, the residence time of the process shall be controlled.