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Influence of moisture on battery cycle capacity attenuation

2021-11-09
Influence of moisture on battery cycle capacity attenuation

Effects of moisture:
Solid-electrolyte interface (SEI) film is a selective membrane that allows Li+ to pass through freely while electrolyte molecules cannot. The composition of electrolyte and trace additives have significant effects on the potential, density, irreversible capacity loss and internal resistance of SEI film. As a trace component of electrolyte, water has a certain influence on the formation of SEI film and the performance of lithium ion battery. The important performance is that the battery capacity becomes smaller, the discharge time becomes shorter, the internal resistance increases, the circulation capacity attenuates, the battery expands and so on. Therefore, in the production process of lithium ion battery, the humidity of the environment and the water content of the anode and cathode materials and electrolyte must be strictly controlled.



Influence of moisture on discharge capacity:
The initial discharge capacity of the battery decreases with the increase of water content in the battery. Under the condition of constant lithium source, the change of battery's first discharge capacity is restricted by two important factors.

1. The formation of SEI film consumes part of Li 10, resulting in irreversible capacity loss. Lithium alkyl carbonate generated in the process of one-electron reduction can also react with trace water in the electrolyte, 2ROC02Li+H2O→LieC03+C02+2ROH. Eg. A new chemical reaction takes place.

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

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

1. The difference of SEI film leads to the difference of battery internal resistance. In the electrolyte solvent system, trace water can form Li2CO3 based SEI film with good stability and uniform density, and its internal resistance is small.

Influence of moisture on battery cycle capacity attenuation:
The attenuation of battery capacity decreases with the increase of water content. This is related to the density and uniformity of SEI film. When the SEI film is uniform and dense, the electrolyte solvent is not easy to embed into the negative electrode, occupying the Li ten insertion vacancy, so the capacity attenuation is very small. On the contrary, when the SEI film is locally incompact and inhomogeneous, the Li ten insertion vacancy is relatively easily occupied by the electrolyte solvent. LieC03 is the most important component in the formation of uniform and dense SEI film. In the electrolyte solvent system, excessive water content will lead to local non-density and non-uniformity of SEI film, so the capacity attenuation is increased.

The expansion of the battery is important because LiPF6 is decomposed into HF gas due to the presence of water after SEI film is generated.

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