The humidity requirement during the production process of lithium-ion batteries is very high, mainly because uncontrolled or excessive moisture can have adverse effects on the electrolyte. The electrolyte, composed of lithium salts and organic solvents, serves as the medium for ion transport within the battery and ensures the advantages of high voltage and high energy density for lithium-ion batteries. Excessive moisture can lead to negative impacts:

1. Degradation of the electrolyte:

The electrolyte, consisting of lithium salts and organic solvents, facilitates the ion transport within the battery. It is crucial for the high voltage and high-energy advantages of lithium-ion batteries. During the battery electrolyte filling process, it is necessary to work in an environment with humidity less than 1%. After filling, the battery should be sealed promptly to prevent contact with the air. If the moisture content is too high, the electrolyte reacts with water and generates trace amounts of harmful gases, negatively affecting the filling environment. This can also affect the quality of the electrolyte itself, resulting in poor battery performance.

2. Reduction in battery capacity:

The initial discharge capacity of the battery decreases with an increase in moisture content. Excessive moisture can damage the effective components of the electrolyte and deplete lithium ions, causing irreversible chemical reactions at the negative electrode. Depletion of lithium ions leads to a decrease in battery energy.

3. Increase in internal resistance:

Increasing moisture in the battery results in an increase in internal resistance. A battery with low internal resistance allows for high current discharge and thus provides high power output. Conversely, high internal resistance prevents high current discharge, leading to lower power output. Excessive moisture can also affect the quality of the solid electrolyte interface (SEI) film in lithium batteries.

4.Excessive internal pressure in the battery

Exposure to moisture can react with the LiPF6 in the electrolyte, resulting in the production of harmful gases. When there is too much moisture, the internal pressure of the battery increases, causing it to undergo deformation. In the case of a cellphone battery, this could manifest as swelling of the casing. If the internal pressure continues to rise, the battery becomes dangerous and can potentially explode, causing electrolyte splatter and the risk of injury from battery fragments.

5. Battery leakage

The reaction between LiPF6 in the electrolyte and moisture not only generates gases but also produces hydrofluoric acid, which is highly corrosive. This acid can corrode the metal components inside the battery, leading to eventual leakage. If the battery leaks, its performance rapidly declines, and the electrolyte can also corrode the device of the user.

The electrolyte and electrode materials in the battery are sensitive to water. To ensure the quality of the battery, it is necessary to strictly control the moisture content in the workshop and glove box, especially during critical processes such as cell baking, electrolyte injection, and sealing, which should be carried out in an environment with less than 1% humidity to prevent moisture from entering the electrolyte. The fluctuation of humidity can be reflected by changes in dew point temperature. Generally, the dew point temperature should be controlled below -45°C, even drier if possible.

Using dew point temperature to indicate the concentration of water vapor at this level, as the corresponding relative humidity values are below 1%. Most instruments used for measuring relative humidity lack the required resolution and accuracy for meaningful measurements at this level, even if they are capable of converting displayed and output values to dew point temperatures. For example, at a dew point temperature of -50°C, increasing the temperature by 5°C reaches -45°C.