From：CTECHI GROUP Limited Release time：2019-03-21
Overview：With the rapid development of the electric vehicle industry in the past two years, the quality requirements for lithium-ion batteries, especially power lithium-ion batteries, have been significantly improved, while the lithium-ion process moisture control level, which has a significant impact on battery performance, is more stringent. This article from the impact of moisture on lithium-ion batteries.
The effect of moisture on lithium-ion batteries
With the rapid development of the electric vehicle industry in the past two years, the quality requirements for lithium-ion batteries, especially power lithium-ion batteries, have been significantly improved, while the lithium-ion process moisture control level, which has a significant impact on battery performance, is more stringent. This article from the impact of moisture on lithium-ion batteries.
First, the impact of moisture on lithium-ion batteries
The interior of a lithium-ion battery is a relatively complex chemical system. The reaction processes and results of these chemical systems are closely related to moisture. The loss of control or coarsening of water leads to the presence of excessive moisture in the battery, which not only leads to the decomposition of the electrolyte lithium salt, but also has a bad influence on the film formation and stability of the positive and negative materials, resulting in the electrochemical characteristics of the lithium ion battery. Such as capacity, internal resistance, product characteristics will produce more obvious deterioration.
The aforementioned membrane, that is, the solid electrolyte interface membrane, is a layer of selectively permeable membrane that allows Li+ to pass freely and the electrolyte molecules to be impermeable. The composition of the electrolyte and the trace amount of the additive have a significant influence on the potential formed by the SEI film, the degree of compactness, the irreversible capacity loss of the battery, and the internal resistance of the battery. As a trace component in the electrolyte, water has a certain influence on the formation of SEI film of lithium ion battery and battery performance. The main performance is that the battery capacity becomes smaller, the discharge time becomes shorter, the internal resistance increases, the circulation capacity decreases, and the battery expands. Therefore, in the production process of the lithium ion battery, the humidity of the environment, the positive and negative materials, and the electrolyte must be strictly controlled. Water content.
Effect of moisture on internal resistance of lithium battery
As the moisture of the battery increases, the internal resistance increases. The main factors that cause the difference in internal resistance of the battery are as follows.
1. The difference in SEI film leads to the difference in internal resistance of the battery. In the electrolyte solvent system, a trace amount of water can form a SEI film which is mainly composed of Li2CO3, has good stability and is uniform and dense, and has a small internal resistance.
2. When the moisture content is more than the required content of the SEI film formed, POF3 and LiF precipitates are formed on the surface of the SEI film, resulting in an increase in internal resistance of the battery.
Effect of moisture on battery cycle capacity attenuation
The battery capacity decay gradually decreases as the moisture content increases. This is related to the degree of compactness and uniformity of the SEI film. When the SEI film is uniformly dense, the electrolyte solvent is not easily embedded in the negative electrode, occupying the Li+ insertion vacancy, and thus the capacity attenuation is small. In contrast, when the local portion of the SEI film is not dense and uneven, it is relatively easy for the Li+ insertion vacancy to be occupied by the electrolyte solvent. Li2C03 is the most important component for forming a uniform and dense SEI film. When the moisture content is excessive, the locality of the SEI film is not dense and uneven, so the capacity attenuation increases.
The battery expands mainly because the presence of water after the formation of the SEI film causes LiPF6 to decompose to form HF gas.