The "Breathing Technique" in Lithium Battery Baking: Decoding the Core Technology of Nitrogen Cycle

Why is nitrogen circulation introduced in the vacuum baking of lithium batteries? The seemingly safe inert gas actually hides the risk of condensation! Behind the efficiency improvement is the ultimate control of the "breathing rhythm" by precision technology - this article deciphers the game logic of nitrogen filling, dehumidification and risk prevention and control.

Is it necessary to introduce nitrogen circulation during vacuum baking of battery cells? When filling with nitrogen, the pressure inside the cavity will change. Will the moisture condense again and affect the baking effect?

1. The core purpose of nitrogen circulation in baking

Replaces residual moisture

After vacuuming, nitrogen is filled in to destroy the internal air pressure balance, forcing the moisture to be discharged with the nitrogen (such as the "intermittent breathing" process). Literature data shows that the effect improves the drying efficiency by about 30%. 

Maintain an inert environment

Nitrogen isolates oxygen, preventing oxidation of active materials or deterioration of electrolyte at high temperatures.

Auxiliary heat transfer and safe exhaust

During the atmospheric or slightly pressurized stage, hot nitrogen circulation enhances the uniformity of heat transfer.

2. Nitrogen filling pressure change and water condensation risk

Key Principles：Using Clapeyron equation to control temperature-pressure relationship 

Anti-condensation conditions：After nitrogen filling, if the chamber temperature is lower than the dew point temperature corresponding to the current pressure, water vapor will condense. However, in the actual process, nitrogen filling is mostly in the baking stage (temperature ≥ 80°C). At this time, even if the pressure rises to normal pressure (such as -0.085MPa → normal pressure after nitrogen filling), the saturated vapor pressure at the actual temperature is still much higher than the actual water partial pressure, and the vaporized water will not condense. 

Risk Aversion：Process step restrictions 

For example, the following process can be used in actual production: the system is cooled to the set temperature (heating is stopped) before being filled with 0-5°C cold nitrogen, and the pressure is reduced again by rapid vacuuming (maintaining pressure for 30-240s) to ensure that critical condensation conditions are not reached.

3. Why are different types of baking ovens not filled with nitrogen during the vacuum stage?

Wind circulation/tunnel furnace mode

Vacuum stage：Mainly rely on vacuum pump to continuously evacuate air (pressure ≤ 20Pa). At this time, filling with nitrogen to break the vacuum will reduce efficiency.

Nitrogen filling stage: Usually only used in the preheating or cooling process (such as Yang Zhiming’s pulsating preheating process), rather than the core vacuum drying period.

Contact Oven

The heat source directly contacts the battery cell, no gas heat transfer medium is required, and nitrogen filling is only used for breaking vacuum or cooling.

4. Nitrogen pressure and process adaptability

Baking method	Nitrogen use stage	Principles and advantages
Contact heating	Nitrogen filling only during cooling phase (normal or low temperature)	Avoid vacuum disturbance during heating phase
Hot air circulation	Hot nitrogen filling during preheating or vacuum breaking	Increase the heating rate and replace moisture
Tunnel continuous baking	Pulsating nitrogen filling to break vacuum during preheating stage	Staged dewatering (discharge some free water first)

Key conclusions

The essence of nitrogen fillingThe mass transfer process is accelerated by gas flow rather than simply increasing the pressure; the risk of condensation is avoided through precise design of temperature-pressure parameters.

Device SpecificityFactors that determine the difference in nitrogen usage: High-efficiency vacuum equipment does not require additional gas circulation, but complex structure cells (such as stacked cells/deep injection ports) require nitrogen-enhanced drying.

The core of process optimizationIt is a balance between drying efficiency, energy consumption and equipment cost, and nitrogen filling is not required in all scenarios (for example, efficient suction in a tunnel furnace can saturate and separate moisture).

The above content is all obtained from my daily work, communication, and literature reading. Due to my limited ability, there are inevitably omissions in the views expressed in the article. I welcome colleagues in the industry to actively communicate and make progress together!

References:

1. References Study on the mechanism of heat conduction and water evaporation during vacuum baking of lithium-ion battery cells, Guan Yuming

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