Influence of moisture content on battery performance

There are many factors that affect the performance of lithium batteries, such as the type of material, the compaction density of the positive and negative electrodes, moisture, coating surface density, and the amount of electrolyte. Among them, moisture has a vital influence on the performance of lithium-ion batteries. Moisture is a key factor that needs to be strictly controlled during the production process of lithium-ion batteries. Excessive moisture can not only cause the decomposition of lithium salts in the electrolyte, and cause damage to the positive and negative materials, The current collector has a certain corrosive and destructive effect, and it also leads to a decrease in the cycle performance and safety performance of the battery. However, trace amounts of moisture have important significance. The following describes the impact of moisture on the performance of lithium batteries.

1. The disadvantages of excessive moisture

In the production process of the ternary/graphite system battery, the preparation of the positive electrode slurry generally uses an oil-based dispersion system, using PVDF as the binder and NMP as the solvent. When PVDF encounters excessive moisture, it will form a gelatinous substance, resulting in poor fluidity and leveling of the slurry, which is not conducive to the coating of the slurry. Therefore, when preparing the slurry, attention must be paid to the moisture content of the raw materials, the working environment, and the introduction of moisture during the operation of the personnel.

In addition to having a huge impact on the preparation of lithium battery slurry, excessive moisture will cause the decomposition of the electrolyte. The response is as follows:

Hydrofluoric acid is a particularly corrosive acid, which can cause serious damage to the positive and negative electrode materials and current collectors of lithium batteries, and ultimately lead to battery safety problems.

2. The meaning of trace water

However, it is not that the less water is better in lithium batteries. As we all know, the solid electrolyte interface (commonly known as SEI membrane) is a selective permeable membrane that allows Li+ to pass freely, but electrolyte molecules cannot pass through. 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. Water, as a trace component in the electrolyte, has a certain impact on the formation of SEI film and battery performance of lithium-ion batteries.

Third, the impact of moisture on the performance of lithium batteries

In different material systems, moisture content has a great influence on battery performance. However, what remains unchanged is that moisture affects the first charge and discharge capacity, internal resistance, battery cycle life, and battery volume of lithium batteries. Take the lithium cobalt oxide/graphite system battery as an example.

1. Impact on the first charge and discharge capacity

Moisture is related to the formation of SEI in lithium batteries and will inevitably affect the first irreversible capacity loss of lithium batteries. As shown in Figure 1:

Figure 1. The influence of moisture content on battery charge and discharge capacity

When the moisture in the battery is less than 0.015%, the first discharge capacity of the battery meets the national standard and the change is small; when the moisture of the battery is within the range of 0.015% to 0.04%, the first discharge capacity of the battery decreases as the moisture in the battery increases. When the moisture content in the battery is less than 0.015%, the following reactions dominate.

The lithium alkyl carbonate generated by the single-electron reduction process can also react with trace water in the electrolyte to generate lithium carbonate.

2ROCO2 Li+H2O →Li2CO3 +CO2 +2ROH

When CO2 is generated, a new chemical reaction occurs on the negative electrode surface at low potential:

2CO2+2Li+ +2e →Li2CO3 +CO

It can be seen that an appropriate amount of water will help to form a stable, uniform and dense SEI film dominated by Li2CO3. When the SEI film completely covers the negative electrode, the irreversible reaction stops immediately.

When the moisture content is within the range of 0.0150% to 0.04%, the consumption of lithium ions increases, and the charge and discharge capacity of the battery decreases, which has a negative impact on it.

2. Impact on battery internal resistance

The internal resistance of the battery is one of the most important characteristic parameters of the battery. It is an important parameter that characterizes the cycle life and operating status of the battery, and is the main indicator of the difficulty of transporting ions and electrons in the battery. The internal resistance is mainly affected by factors such as battery materials, manufacturing processes, and battery structure. The smaller the internal resistance, the smaller the voltage occupied when the battery is discharged, and the more energy it can output. For batteries stored for a long time, the internal resistance will increase with the increase of storage time, and the performance of the battery will be greatly affected when the internal resistance exceeds a certain value. Since the moisture content has an impact on the quality of the SEI film in a lithium battery, the introduction affects the internal resistance of the battery.

In the EC/DMC/EMC electrolyte solvent system, a trace amount of water can form a Li2CO3-based, stable, uniform and dense SEI film with low internal resistance. When the moisture content is more than the required content of the system to form the SE I film, POF3 and LiF precipitate on the surface of the SEI film, resulting in an increase in the internal resistance of the battery. as shown in picture 2:

Figure 2. The influence of moisture content on battery internal resistance

3. Impact on battery capacity degradation

The moisture content affects the properties of the battery SEI film, such as uniformity and compactness. When the SE I film is uniform and dense, the electrolyte solvent is not easy to be embedded in the negative electrode, occupying the Li+ insertion vacancy, so there is little capacity degradation. On the contrary, when the part of the SEI film is not dense or uniform, it is relatively easy for the Li+ insertion vacancies to be occupied by the electrolyte solvent. Li2CO3 is the most important component to form a uniform and dense SEI film. In the EC /DMC /EMC electrolyte solvent system, an appropriate amount of water can promote the formation of SEI film based on Li2CO3. When the moisture content is sufficient or excessive, the SEI formed The denser and uniform the film, the smaller the probability of solvent embedding in the carbon anode. This is the reason why the battery capacity attenuation gradually decreases with the increase of moisture content when the moisture is in the range of 0.015% to 0.04%. When the moisture content is less than 0.015%, a denser SEI film is formed on the surface of the negative electrode to maintain the solvent embedding in an equilibrium state. Therefore, the battery capacity decay is maintained in a relatively stable state.

Figure 3. Influence of moisture content on battery capacity decay rate

4. Impact on battery thickness

Figure 4. Influence of moisture content on battery thickness

It can be seen from the figure that as the moisture content increases, the thickness of the battery also increases. When the moisture content is due to CO2, CO and other gases generated during the formation of the SEI film. And when the water is excessive, the excess water will continue to react with LiPF6 to produce HF gas. The internal gas production of the battery is related to the safety of the battery. In order to avoid serious flatulence problems in the battery, the water inside the battery must be strictly controlled.

references:

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[3] Zhang Hailin, He Xiangyun, Li Yan, et al. The influence of electrode moisture on the performance of lithium ion batteries［J］. Battery Industry, 2013, 18(1 /2): 44-46.