Effect of moisture on life of 18650 Lithium battery
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2023/05/26 11:29:47
The influence of moisture on the life of 18650 lithium battery is related to the density and uniformity of SEI film. SEI film: (In the first and second charge and discharge process of liquid lithium ion battery, the electrode material and electrolyte react at the solid-liquid phase interface to form a passivation layer covering the surface of the electrode material. This passivation layer is a kind of interface layer, with the characteristics of solidelectrolyte, is an electronic insulator but a good conductor of Li+, Li+ can be freely embedded and removed through the passivation layer, so the passivation layer is called "solidelectrolyte interface" (SEI membrane for short). When the SEI film is uniformly dense, it is difficult for lithium ions to be embedded into the negative electrode film and occupy the Li+ embedding vacancy, so the capacity attenuation is small. On the contrary, when the overall SEI film is not dense and homogeneous, the Li+ embedding vacancy is relatively easy to be occupied by the electrolyte solvent. Li2C03 (lithium carbonate) is the most important component in the formation of uniform and dense SEI film. In the electrolyte solvent system, when the moisture content exceeds the normal value, it will lead to the local non-dense and non-uniform SEI film, resulting in accelerated attenuation of 18650 lithium battery life. The cell will expand because the presence of water after SEI film formation causes LiPF6 to decompose into HF gas.
The water sources in the process of 18650 lithium battery are basically divided into the following situations:
First, workshop water source:
a, the moisture in the air, generally measured by relative humidity. In different temperatures and weather, there is a big difference, in summer rainy days can reach 90%, in winter snow days about 30%, in summer sunny days about 50%, in winter sunny days about 20%.
B. The moisture in the human body
c. The moisture content of packing materials (especially cartons), paper towels and rags and other cleaning accessories is very high
d. Water seepage of equipment and facilities
Two, 18650 lithium battery raw material water source:
A. Positive and negative electrode materials
Positive and negative active substances are mostly micron or nano-sized particles, which are easy to absorb water molecules in the air during the production process, especially ternary materials, whose specific surface area is large, which are easy to absorb water and react on the material surface.
b. Electrolyte
In the solvent structure of the electrolyte, there are carbonyl groups with high electronegativity and metastable double bonds, which are easy to form complexes or react with polar H2O molecules to generate corresponding alcohols, and the higher the temperature, the faster the reaction. And the electrolyte solute lithium salt is also easy to absorb water and with
Water reaction.
C. Diaphragm
Diaphragm paper is also a kind of porous plastic film, its water absorption is also great. Due to the water does not generally occur with the diaphragm chemical reaction, through baking can also be basically eliminated, therefore, diaphragm generally rarely strict moisture control. Summary: The moisture content in raw materials is one of the important sources of moisture in 18650 lithium battery. The higher the humidity in the production environment, the more moisture the 18650 lithium battery material absorbs in the air. Therefore, it is necessary to control the humidity of the production environment. The recommended humidity control ladder of the 18650 lithium-ion battery production workshop is (for reference only, humidity control shall be operated according to the actual situation of each lithium battery company) :
Relative humidity ≦30% workshop (e.g. mixing, coating machine head, tail, etc.)
Relative humidity ≦20% workshop (such as roll pressing, filmmaking, baking, etc.)
Relative humidity ≦10% workshop (e.g. lamination, winding, assembly, etc.)
Dew point temperature ≦-45℃ workshop (such as cell baking, filling, sealing, etc.)
Even in accordance with the above humidity gradient control, also to control the process residence time.
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