The Impact of Moisture in the Lithium Battery Manufacturing Process
In the manufacturing process of lithium-ion batteries, there are three crucial factors that must be strictly controlled: dust, metal particles, and moisture.
Failure to control dust and metal particles properly can directly lead to safety incidents such as internal short circuits, fires, and combustion in batteries. Similarly, if moisture is not effectively controlled, it can also pose a significant risk to battery performance, resulting in serious quality issues.
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Therefore, strict control of water content in key materials such as electrodes, separators, and electrolytes during the manufacturing process is of utmost importance and cannot be compromised.
The following will provide a detailed explanation from three perspectives: the harm of moisture to lithium batteries, the sources of moisture in the manufacturing process, and the control of moisture during the manufacturing process.
1. The Impact of Moisture on Lithium Batteries
1.1 Battery Swelling and Leakage
Excessive moisture content in lithium-ion batteries can lead to a chemical reaction with the lithium salt in the electrolyte, resulting in the formation of HF (hydrofluoric acid):
H2O + LiPF6 → POF3 + LiF + 2HF
Hydrofluoric acid (HF) is a highly corrosive acid that can severely damage battery performance:
– HF can corrode internal metal components, the battery casing, and seals, ultimately causing the battery to rupture and leak.
– HF can also deteriorate the Solid-Electrolyte-Interface (SEI) film inside the battery, reacting with the main components of the SEI film:
ROCO2Li + HF → ROCO2H + LiF
Li2CO3 + 2HF → H2CO3 + 2LiF
Finally, the formation of LiF precipitate inside the battery results in irreversible chemical reactions on the negative electrode, consuming active lithium ions, and reducing the battery’s energy.
When moisture is abundant, the generation of gas increases, leading to an increase in internal battery pressure. This, in turn, causes the battery to undergo mechanical stress, leading to dangerous battery swelling and leakage.
Most cases of battery swelling and causing device covers to pop open in mobile phones or digital electronic products on the market occur due to high moisture levels inside lithium batteries, resulting in gas production and expansion.
1.2 Increase in Internal Resistance of the Battery
Internal resistance is one of the most critical performance parameters of a battery. It serves as a primary indicator of the ease or difficulty with which ions and electrons move within the battery, directly impacting the battery’s cycle life and operational state.
The lower the internal resistance, the less voltage is consumed during battery discharge, resulting in higher energy output.
As water content increases, it leads to the formation of POF3 and LiF precipitates on the surface of the Solid-Electrolyte-Interface (SEI) film in the battery. This disrupts the density and uniformity of the SEI film, causing the internal resistance of the battery to gradually increase. Consequently, the battery’s discharge capacity continues to decrease.
1.3 Shortened Cycle Life
Excessive moisture content damages the battery’s SEI film, leading to a gradual increase in internal resistance.
As a result, the battery’s discharge capacity diminishes over time, and with each full charge, the battery’s usage time also becomes shorter. Naturally, the number of charge and discharge cycles the battery can go through (cycle life) decreases, ultimately shortening the battery’s overall lifespan.
2. Sources of Moisture in the Lithium Battery Production Process
The production process of lithium-ion batteries generally follows the steps below:
During the manufacturing process of lithium-ion batteries, the sources of moisture can be categorized as follows:
2.1 Moisture Introduced by Raw Materials
2.1.1 Positive and Negative Electrode Materials: Both positive and negative electrode active materials consist of particles at the micrometer and nanometer levels, making them highly susceptible to absorbing moisture from the air.
This is especially true for ternary or binary positive electrode materials with high nickel (Ni) content, which have a large surface area and readily absorb moisture, leading to chemical reactions.
After coating, if the storage environment has high humidity, the electrode surface coating can quickly absorb moisture from the air.
2.1.2 Electrolyte: The solvent components in the electrolyte undergo chemical reactions with water molecules. The solute lithium salts in the electrolyte are also prone to absorbing moisture and reacting chemically. Therefore, the electrolyte contains a certain amount of moisture.
If the electrolyte is stored for an extended period or subjected to high storage temperatures, the moisture content within the electrolyte can increase.
2.1.3 Separator: The separator is a porous plastic film (PP/PE material), and it also exhibits significant moisture absorption.
2.2 Moisture Introduced during Electrode Slurry Preparation
During the slurry preparation for the negative electrode, water is added and mixed with the raw materials before coating, which means the negative electrode itself contains moisture.
Even though there is heating and drying during the subsequent coating process, a significant portion of moisture remains adsorbed within the coating of the electrode.
2.3 Moisture in the Workshop Environment
2.3.1 Moisture in the Workshop Air
The moisture content in the air is generally measured by relative humidity.
Relative humidity varies significantly with different seasons and weather conditions. Spring and summer typically have higher air humidity (above 60%), while autumn and winter are drier with lower humidity (below 40%). Humidity is higher on rainy days and lower on sunny days.
Therefore, different air humidities result in varying levels of moisture in the air.
Moisture content /ppm
Water weight g/m3
2.3.2 Moisture Produced by Humans
This includes sweat from the human body, exhaled breath, and moisture from washing hands.
2.3.3 Moisture Introduced by Various Auxiliary Materials and Paper
Examples include moisture brought in by materials such as paper boxes, shredded fabric.
3. Control of Moisture in the Lithium Battery Production Process
3.1 Strict Control of Workshop Humidity
3.1.1 Electrode production workshop slurry mixing, relative humidity ≤ 10%.
3.1.2 Electrode production workshop coating (head and tail), roller dew point humidity ≤ -10°C DP.
3.1.3 Electrode production workshop slitting, relative humidity ≤ 10%.
3.1.4 Stacking, winding, assembly workshop, dew point humidity ≤ -35°C DP.
3.1.5 Cell filling, sealing, dew point humidity ≤ -45°C DP.
3.2 Strict Control of Moisture Introduced by Human and External Sources
3.2.1 Adherence to Work Procedures
- Entry into the drying room requires changing into specialized clothing, wearing a hat, changing shoes, and using a mask.
- Direct contact with electrodes and cells with bare hands is strictly prohibited.
3.2.2 Management of Moisture Introduced by Auxiliary Materials
- Bringing paper boxes into the drying room is strictly prohibited.
- Paper-based posters and signage inside the drying room must be sealed in plastic.
- Using water for cleaning the floor inside the drying room is prohibited.
3.3 Strict Control of Electrode Storage and Exposure Time
3.3.1 Management of Low Humidity Storage
- Electrodes after rolling and slitting must be stored in a low humidity environment (≤ -35°C DP) within 30 minutes.
- Electrodes that have been baked but not used for lamination, winding, or encapsulation must be vacuum-sealed (≤ -95kPa).
3.3.2 Management of Exposure Time
- After electrode baking, lamination, winding, sealing, cell filling, and sealing must be completed within 72 hours(workshop dew point humidity ≤ -35°C).
3.3.3 First-In-First-Out (FIFO) Management
- Electrode usage must adhere to the first-in-first-out principle, meaning batches should be used in the order they were baked.
3.4 Strict Control of Electrode and Separator Baking Processes
3.4.1 Electrodes and separators must be baked before use.
3.4.2 If electrodes and separators cannot be baked before lamination or winding, the cells must be baked before filling.
3.4.3 During the baking process of electrodes or cells, oven parameters (temperature, time, vacuum level) must be closely monitored.
3.4.4 Oven temperature and vacuum levels must be regularly calibrated to ensure accuracy.
3.5 Moisture Content Testing and Control
3.5.1 Electrodes, separators (or cells), and electrolytes must undergo moisture content testing and meet the specified criteria before filling.
3.5.2 Testing Method: Sampling as per regulations; measure using a Karl Fischer moisture tester.
3.5.3 Moisture Content Acceptance Criteria:
- Electrode moisture content ≤ 200 ppm (pre-control ≤ 150 ppm).
- Separator moisture content ≤ 600 ppm.
- Electrolyte moisture content ≤ 20 ppm.
To sum it up, in the manufacturing process of lithium-ion batteries, moisture control at various stages, including environmental humidity, storage and exposure time of electrodes, drying processes for electrodes and separators, electrolyte shelf life, and moisture content testing, is absolutely essential.
Once it goes out of control, it can lead to fatal defects in the performance of battery batches, with very serious consequences!
Hence, whether it’s management, production operators, or quality inspectors, it’s crucial to strengthen awareness of moisture control in batteries and always strictly adhere to the prescribed processes to ensure that battery moisture remains under control and in a qualified state.