The influence of moisture on the lifespan of 18650 lithium batteries.
The effect of moisture on the lifespan of 18650 lithium batteries is related to the density and uniformity of the SEI film. What is SEI film When the SEI film is uniformly dense, lithium ions are difficult to embed into the negative electrode, occupying Li+insertion vacancies, resulting in minimal capacity decay. On the contrary, when the SEI film is not dense or uniform as a whole, it is relatively easy for Li+insertion vacancies to be occupied by electrolyte solvents. Li2C03 (lithium carbonate) is the most important component for forming a uniform and dense SEI film. In the electrolyte solvent system, when the moisture content exceeds the normal value, it can cause local non density and non-uniformity of the SEI film, leading to accelerated degradation of the lifespan of 18650 lithium batteries. The important reason for battery expansion is that the presence of water after SEI film formation causes LiPF6 to decompose and generate HF gas.
The sources of moisture in the 18650 lithium battery manufacturing process are basically divided into the following situations:
1、 Workshop moisture source:
a、 The moisture in the air is generally measured by relative humidity. There are significant differences in temperature and weather, with rainy days in summer reaching up to 90%, snowy days in winter around 30%, sunny days in summer around 50%, and sunny days in winter around 20%.
b、 Water that appears in the human body
c、 The moisture content of packaging materials (especially cardboard boxes), tissues, and cleaning materials such as rags carried by materials is high
d、 Equipment and facilities seepage
2、 18650
lithium battery raw material moisture source:
a、 Positive and negative electrode materials
Most of the positive and negative active substances are micro - or nano sized particles, which are prone to absorbing water molecules in the air during the production process. Especially for ternary materials, their specific surface area is relatively large, and the material surface is highly prone to absorbing moisture and reacting.
b、 electrolyte
The solvent structure of the electrolyte contains highly electronegative carbonyl groups and metastable double bonds, which can easily form complexes or react with polar H2O molecules to generate corresponding alcohols. Moreover, the higher the temperature, the faster the reaction. Moreover, the solute lithium salt in the electrolyte is also prone to absorbing water and reacting with
Water reaction.
c、 the diaphragm
Diaphragm paper is also a porous plastic film with high water absorption. Due to the fact that moisture generally does not react chemically with the diaphragm and can be effectively eliminated through baking, strict moisture control is rarely applied to the diaphragm. Summary of the impact of moisture on the lifespan of 18650 lithium batteries: The moisture content in raw materials is one of the important sources of moisture in 18650 lithium batteries. The higher the humidity in the production environment, the more moisture the 18650 lithium battery material absorbs from the air. So it is necessary to control the humidity of the production environment. The recommended humidity control steps for the 18650 lithium-ion battery production workshop are (for reference only, humidity control should be operated according to the actual situation of each lithium battery company):
Workshop with relative humidity ≤ 30% (such as mixing, coating machine head, tail, etc.)
Workshop with relative humidity ≤ 20% (such as rolling, slicing, 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 controlled according to the above humidity gradient, the residence time of the process should still be controlled.
