Effect of moisture on 18650 lithium battery life
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2023/08/22 16:49:50
The effect of moisture on the life of 18650 lithium battery is related to the density and uniformity of SEI film. What is SEI film: (In the first and second charge and discharge process of the liquid lithium-ion battery, the electrode material and the electrolyte react at the solid-liquid phase interface to form a passivation layer covering the surface of the electrode material. This passivation layer is an interface layer, with the characteristics of a solidelectrolyte, is an electronic insulator but a good conductor of Li+, Li+ can be freely embedded and detached through the passivation layer, so this passivation layer is called "solidelectrolyte interface" (SEI) film.) When the SEI film is uniform and 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 very small. On the contrary, when the 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 of the formation of uniform dense SEI film, electrolyte solvent system, when the water content exceeds the normal value, it will lead to the local undense SEI film, uneven lead to 18650 lithium battery life decay accelerated. The battery will expand because the presence of water after the SEI film is formed causes LiPF6 to decompose into HF gas.
The water source in the 18650 lithium battery process is 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 the summer rainy days can reach 90%, in the winter snow days is about 30%, in the summer sunny about 50%, in the winter sunny about 20%.
b, the human body appears water
c, the water packaging materials (special cartons), paper towels and rags and other cleaning accessories are very high water content
D. Water seepage of equipment and facilities
Second, 18650 lithium battery raw material water source:
a, positive and negative electrode materials
Positive and negative active substances are mostly micron or nanometer particles, which are easy to absorb water molecules in the air during the production process, especially ternary materials, whose specific surface area is larger, and it is easy to absorb water and react on the surface of the material.
b, electrolyte
There are large electronegative carbonyl groups and metastable double bonds in the solvent structure of the electrolyte, which are easy to form complexes with polar H2O molecules or react to produce corresponding alcohols, and 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
Water reaction.
c, diaphragm
Diaphragm paper is also a porous plastic film, and its water absorption is also very large. Since water generally does not react chemically with the diaphragm, it can also be basically eliminated by baking, so the diaphragm is rarely strictly controlled. The moisture content in the raw material is one of the important sources of moisture in the 18650 lithium battery, the higher the humidity of 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, and the recommended humidity control ladder of the 18650 lithium-ion battery production workshop is (for reference only, the humidity control is operated according to the actual situation of the lithium battery company) :
Relative humidity ≦30% workshop (such as mixing, coating head, tail, etc.)
Relative humidity ≦20% workshop (such as rolling, filmmaking, baking, etc.)
Relative humidity ≦10% workshop (such as lamination, winding, assembly, etc.)
Dew point temperature ≦-45℃ workshop (such as cell baking, liquid injection, sealing, etc.)
Even in accordance with the above humidity gradient control, it is necessary to control the residence time of the process.
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