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The influence of moisture on the attenuation of battery cycle capacity

Influence of moisture:



The Solid-Electrolyte Interface (SEI) membrane is a selective permeable membrane that allows Li+ to permeate freely, but electrolyte molecules cannot permeate. The composition of the electrolyte and trace additives have a significant impact on the potential of the SEI film formation, 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 and battery performance of lithium-ion batteries. The important performance is that the battery capacity becomes smaller, the discharge time becomes shorter, the internal resistance increases, the cycle capacity decreases, and the battery swells. Therefore, in the production process of lithium-ion batteries, the humidity of the environment, the positive and negative materials, and the electrolyte must be strictly controlled. The water content.



The influence of moisture on discharge capacity:



The first discharge capacity of the battery decreases with the increase of moisture in the battery. Under the condition of a constant lithium source, the change of the battery's first discharge capacity is mainly restricted by two important factors.



1. The formation of SEI film consumes part of Li and causes irreversible capacity loss. The lithium alkyl carbonate generated during the single-electron reduction process can also react with trace water in the electrolyte, 2ROC02Li+H2O→LieC03+C02+2ROH, when After CO2 is generated, a new chemical reaction occurs on the surface of the negative electrode at low potential 2C02+2Li++2e→LieC03+CO.



2. After the SEI film is formed, under the condition that H2O is still present, H2O will promote the decomposition of LiPF6 in the electrolyte and shorten the battery discharge time, LiPF6→LiF+PFs, PFs+H2O→POF3+2HF.



The influence of moisture on the internal resistance of lithium-ion batteries:



With the increase of battery moisture, the internal resistance is on the rise. There are two important factors for the difference in battery internal resistance.



1. The difference in SEI film leads to the difference in battery internal resistance. In the electrolyte solvent system, a trace amount of water can form an SEI film with Li2CO3 as the main component, which is stable, uniform and dense, and its internal resistance is small.



2. When the moisture content is more than the required content of the system to form the SEI film, POF3 and LiF will be precipitated on the surface of the SEI film, resulting in increased internal resistance of the battery.



The influence of moisture on the attenuation of battery cycle capacity:



The battery capacity decay gradually decreases as the moisture content increases. This is related to the density and uniformity of the SEI film. When the SEI film is uniform and dense, the electrolyte solvent is not easy to embed in the negative electrode, occupying the Li ten intercalation vacancies, so the capacity attenuation is small. On the contrary, when the part of the SEI film is not dense and uneven, it is relatively easy for the Li intercalated vacancies to be occupied by the electrolyte solvent. LieC03 is the most important component to form a uniform and dense SEI film. In the electrolyte solvent system, when the moisture content is excessive, the SEI film will not be dense and uniform locally, so the capacity attenuation increases.



The main reason that the battery swells is because the presence of water after the formation of the SEI film decomposes LiPF6 to generate HF gas.

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