Lithium ion battery is a more complex chemical system. The reaction process and results of these chemical systems are closely related to water. While the water is out of control or rough control, resulting in the presence of the battery exceed the standard of water, not only can cause the decomposition of electrolyte, and the film forming material on the positive and negative pole and the stability of its bad effects, leading to the electrochemical properties of the lithium ion battery, such as capacity, internal resistance and the characteristics of the product will deteriorate obviously.
The membrane mentioned above, namely the Solid-Electrolyte Interface (SEI) membrane, is a selective permeation membrane which enables Li+ to permeate freely while the electrolyte molecules can not penetrate. The composition of the electrolyte and the trace additives have significant influence on the potential, the density, the irreversible capacity loss and the battery internal resistance of the SEI film. Water, as a trace component in the electrolyte, has a certain influence on the formation of SEI film and the performance of the battery. It is mainly manifested in the phenomenon of smaller battery capacity, shorter discharge time, increased internal resistance, cyclic capacity decay, and battery expansion. Therefore, in the process of making lithium ion battery, we must strictly control the humidity of the environment and the water content of positive and negative pole materials and electrolyte.
1. The effect of water on the discharge capacity
The initial discharge capacity of the battery decreases with the increase of the water content in the battery. Under the condition of constant lithium source, the change of the first discharge capacity of the battery is mainly controlled by 2 main factors, as follows:
（1）.SEI membrane, causing irreversible capacity loss, alkyl lithium carbonate single electron reduction process can also react with trace amounts of water in the electrolyte, 2ROCO2Li + H2O, Li2CO3 + CO2 + 2ROH, when generating CO2, the surface of the anode at low potential, a new chemical reaction 2CO2 +2e + 2Li+, Li2CO3 + CO.
（2）.SEI film, under the presence of H2O, H2O will promote the decomposition of LiPF6 in the electrolyte and shorten the discharge time of the battery.
2, the effect of water on the internal resistance of lithium battery
With the increase of the water content of the battery, the internal resistance is on the rise. There are 2 main factors to produce the difference of battery internal resistance.
the difference in the internal resistance of the battery caused by the difference of SEI membrane. In the solvent system of electrolyte, the trace water can form a SEI film with Li2CO3 as the main, good stability and uniform density, and its internal resistance is small.
When the water content is more than the required content of the SEI film, the POF3 and LiF precipitation on the surface of the SEI film are generated, which leads to the increase of the internal resistance of the battery.
3. The effect of water on the attenuation of battery cycle capacity
The capacity attenuation of battery gradually decreases with the increase of water content. This is related to the density and uniformity of the SEI film. When the SEI film is homogeneous and compact, the electrolyte solvent is not easy to embed into the negative electrode and occupies the Li+ space, so the capacity attenuation is very little. On the contrary, when the local SEI film is not dense and inhomogeneous, it is relatively easy for Li+ to embed the space by the electrolyte solvent. Li2C03 is the most important component to form uniform and dense SEI film. When the water content is too high, the SEI film will become local and dense.
The expansion of the battery is mainly due to the existence of water after the formation of the SEI membrane and the decomposition of the LiPF6 to the HF gas.
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