Effect of moisture.
The Solid-ElectrolyteInterface (SEI) membrane is a selectively permeable membrane that allows Li+ to pass freely, while electrolyte molecules cannot pass through. The composition of the electrolyte and trace additives have significant effects on the potential of the SEI membrane, the degree of densification, the irreversible capacity loss of the cell, and the internal resistance of the cell. Water, as a trace component in the electrolyte, has a certain influence on the formation of SEI film and battery performance of Li-ion batteries. It is important to show that the battery capacity becomes smaller, the discharge time becomes shorter, the internal resistance increases, the cycle capacity decays, and the battery swells. Therefore, in the process of making lithium-ion batteries, the humidity of the environment and the water content of the positive and negative materials and electrolyte must be strictly controlled.
Effect of moisture on discharge capacity.
The first discharge capacity of the battery decreases with the addition of moisture in the battery. Under the condition of constant lithium source, the change of the first discharge capacity of the battery is importantly governed by 2 important factors.
1, SEI film formation consumes part of Li ten, resulting in irreversible capacity loss, single electron reduction process generated alkyl lithium carbonate can also react with trace water in the electrolyte, 2ROC02Li + H2O → LieC03 + C02 + 2ROH, when the generation of C02, the negative surface at low potential, a new chemical reaction occurs 2C02 + 2Li + + 2e → LieC03+CO.
2, after the SEI film is formed, under the condition that H2O is still present, H2O will prompt the decomposition of LiPF6 in the electrolyte, which will shorten the battery discharge time, LiPF6→ LiF+PFs, PFs+H2O→ POF3+2HF.
Effect of moisture on the internal resistance of Li-ion battery.
With the addition of water to the battery, the internal resistance tends to rise. There are 2 important factors for the difference of internal resistance of the battery as follows.
1. The difference of SEI film leads to the difference of internal resistance of the battery. In the electrolyte solvent system, traces of water can form a Li2CO3-based, stable, uniform and dense SEI film, which has a smaller internal resistance.
2. When the water content is more than that required for the system to form SEI film, POF3 and LiF precipitates are generated on the surface of SEI film, resulting in new internal resistance of the battery.
The effect of moisture on the decay of battery capacity.
Battery capacity decay decreases with the addition of moisture content. This is related to the denseness and uniformity of the SEI film. When the SEI film is uniform and dense, the electrolyte solvent is not easily embedded in the negative electrode and occupies the empty space for Li10 embedding, so the capacity decay is small. LieC03 is the most important component to form uniform and dense SEI film, and when the water content in the electrolyte solvent system is too much, it will lead to the local non-dense and non-uniform SEI film, so the capacity decay will be added.
The cell will swell because the presence of water after the SEI film generation causes the decomposition of LiPF6 to generate HF gas.
