The influence of moisture in the lithium battery manufacturing process

During the manufacturing process of lithium-ion batteries, there are three crucial items that must be strictly controlled: dust, metal particles, and moisture. If dust and metal particles are not properly controlled, it will directly lead to safety accidents such as internal short circuits and fires in the battery. If moisture is not effectively controlled, it will also cause significant harm to battery performance and lead to serious quality accidents! Therefore, it is crucial to strictly control the water content of main materials such as electrodes, separators, and electrolytes during the manufacturing process. There must be no relaxation and constant vigilance!
The following is a detailed explanation from three aspects: the harm of moisture to lithium batteries, the source of moisture during the manufacturing process, and the control of moisture during the manufacturing process.

1.  The harm of moisture to lithium batteries

(1) Battery swelling and leakage: If there is excessive moisture in lithium-ion batteries, it reacts chemically with the lithium salt in the electrolyte, generating HF:

H2O + LiPF6 → POF3 + LiF + 2HF

Hydrofluoric acid (HF) is a highly corrosive acid that can cause significant damage to battery performance:

HF corrodes the metal components, battery shell, and sealing within the battery, eventually leading to cracks, ruptures, and leakage.

HF also destroys the SEI (Solid-Electrolyte-Interface) film inside the battery by reacting with its main components:

ROCO2Li + HF → ROCO2H + LiF

Li2CO3 + 2HF → H2CO3 + 2LiF

Eventually, LiF precipitates form inside the battery, causing irreversible chemical reactions in the negative electrode that consume active lithium ions, thereby reducing the battery's energy capacity.

When there is a sufficient amount of moisture, more gas is generated, increasing the internal pressure of the battery. This can lead to deformation, swelling, and even leakage, posing a safety risk.

Many instances of battery swelling and cover popping encountered in mobile phones or digital electronic products on the market are often attributed to high moisture content and gas generation inside the lithium battery.

(2) Increased battery internal resistance:

Battery internal resistance is one of the most critical performance parameters, serving as a primary indicator of the ease with which ions and electrons can travel within the battery. It directly affects the battery's cycle life and operating state. A lower internal resistance means less voltage is consumed during discharge, resulting in higher energy output.

An increase in moisture content can lead to the formation of POF3 and LiF precipitates on the surface of the SEI film (Solid-Electrolyte-Interface). This degrades the density and uniformity of the SEI film, gradually increasing the battery's internal resistance and decreasing its discharge capacity.

(3) Shortened cycle life: Excessive moisture can damage the SEI film, leading to a gradual increase in internal resistance and a decrease in discharge capacity. Over time, the battery's usable time after each full charge shortens, and the number of normal charge-discharge cycles (or lifespan) decreases. This ultimately results in a shortened overall battery lifespan.

2.Sources of moisture in the production of lithium batteries 

During the manufacturing process of lithium batteries, the sources of moisture can be categorized into the following aspects: 

(1) Moisture introduced through raw materials

a. Positive and negative materials: Both positive and negative active materials are particles at the micrometer or nanometer scale, which are highly susceptible to absorbing moisture from the air. Especially for ternary or binary cathode materials with high nickel content, their specific surface area is relatively large, making their surfaces prone to absorbing moisture and undergoing chemical reactions. If the coated electrode sheets are stored in an environment with high humidity, the coating surface of the electrode sheets will also rapidly absorb moisture from the air. 

b. Electrolyte: The solvent component in the electrolyte reacts with water molecules, and the lithium salt solute in the electrolyte is also prone to absorbing moisture and undergoing chemical reactions. Therefore, there is a certain amount of water content in the electrolyte. If the electrolyte is stored for too long or stored at a high temperature, the water content within the electrolyte will increase. 

c. Separator: The separator is a porous plastic film (PP/PE material), which has significant water absorption capacity. 

(2) Moisture added during slurry preparation for electrode sheets

During the preparation of the negative electrode slurry, water is added and mixed with the raw materials before coating. Therefore, the negative electrode sheet itself contains water. Although there is heating and drying during the subsequent coating process, a considerable amount of water remains adsorbed within the coating layer of the electrode sheet. 

(3) Moisture in the workshop environment

a. Moisture in the workshop air: The moisture content in the air is generally measured by relative humidity. The relative humidity varies greatly depending on the season and weather conditions. During spring and summer, the air humidity is relatively high (above 60%), while in autumn and winter, the air is drier with lower humidity (below 40%). The air humidity is higher on rainy days and lower on sunny days. Therefore, the water content in the air differs based on the humidity: 

b. Water generated by humans (sweat, exhaled breath, water after washing hands)

c. Moisture brought in by various auxiliary materials and papers (cartons, rags, reports) 

Control of moisture during lithium battery production

(1) Strict control of the humidity in the production workshop

a The electrode production workshop for slurry mixing should maintain a relative humidity of ≦10%;

b The electrode production workshop for coating (machine head, tail), and rolling should have a dew point humidity of ≦-10℃ DP;

c The electrode production workshop for slitting should maintain a relative humidity of ≦10%;

d The stacking, winding, and assembly workshops should have a dew point humidity of ≦-35℃ DP;

e The electrolyte injection and sealing processes for the battery cells should have a dew point humidity of ≦-45℃ DP.

(2) Strict management of moisture brought into the workshop by humans and the outside environment

a Compliance with operational regulations:

-- Employees must change their clothes, wear hats, change shoes, and wear masks when entering the drying workshop;

-- It is prohibited to touch the electrode sheets and battery cells with bare hands;

b Management of moisture brought in by auxiliary materials:

-- It is strictly prohibited to bring cartons into the drying workshop;

-- Paper postings and signboards in the drying area must be laminated;

-- It is forbidden to mop the floor with water in the drying area.

(3) Strict control of the storage and exposure time of electrode sheets

a Management of low-humidity storage:

-- Electrode sheets after rolling and slitting must be stored in a low-humidity environment (≦-35℃ DP) within 30 minutes;

-- Electrode sheets that cannot be promptly processed into cells or wound after baking must be stored under vacuum (≦-95kpa);

b Management of exposure time:

-- After baking, the electrode sheets must be processed, wound, packaged, filled with electrolyte, and sealed within 72 hours (workshop dew point humidity ≦-35℃);

c First-in, first-out management:

-- The use of electrode sheets must follow the first-in, first-out rule, i.e., the earlier batches are used first; those that are baked first are used first.

(4) Strict control of the baking process for electrode sheets and separators

a Before use, electrode sheets and separators must be baked;

b If electrode sheets and separators cannot be baked before cell processing and winding, the cells must be baked before electrolyte injection;

c During the baking process of electrode sheets or cells, the oven parameters (temperature, time, vacuum) must be strictly monitored;

dThe oven temperature and vacuum must be regularly calibrated to ensure accuracy.

(5) Water content testing and control

a. The water content of electrode sheets, separators (or cells), and electrolyte must be tested and meet the standards before electrolyte injection;

b. Testing method: Sampling according to regulations; using Karl Fischer moisture tester for measurement;

c. Standards for acceptable water content:

-- Water content of electrode sheets ≦200ppm (pre-control ≦150ppm)

-- Water content of separators ≦600ppm

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