Negative Pressure Formation: Why It Matters for Large-Format Batteries

1. What is Negative Pressure Formation?

Negative pressure formation is a critical process in prismatic lithium battery manufacturing where cells undergo the initial charge in a sealed chamber with pressure below atmospheric levels (typically -50kPa to -90kPa). This controlled environment accelerates gas evolution during SEI formation and minimizes the risk of swelling defects while enhancing formation consistency. It is widely used in prismatic aluminum-shell cell production lines to manage gas expansion and ensure uniform electrochemical activation.

2. The Gas Evolution Challenge

During formation, gas generation occurs in three phases: initial bubble formation and adhesion, bubble coalescence and growth, and formation of stable gas escape channels. If gas is not efficiently removed, it can become trapped, causing internal pressure buildup, cell swelling, or delamination. Without negative pressure, gas remains inside the cell, causing swelling that deforms the cell and increases mechanical stress, leading to higher internal resistance and potential separator failure. Vacuum formation lithium battery technology solves this by continuously removing gases as they form, ensuring cell integrity.

3. Stepped Negative Pressure Pattern

The optimal approach to negative pressure formation is a stepped pattern (e.g., -30kPa → -80kPa). The stepped approach:

• Phase 1 – Lower negative pressure prevents premature bubble detachment and minimizes residuals.
• Phase 2 – Medium negative pressure promotes bubble growth and coalescence.
• Phase 3 – Higher negative pressure establishes stable gas escape channels, ensuring efficient gas removal.

Applying sudden high vacuum (>100kPa/min) causes a sharp internal pressure differential that can suck electrolyte into vacuum lines. Stepped designs limit ramp rates (<70kPa/min), balancing pressure differences and preventing electrolyte loss. Stepped negative pressure patterns also give the chamber and cell time to equalize pressure, providing stable power for gas extraction while extending vacuum pump life and protecting sensitive components.

4. Benefits of Negative Pressure Formation

Key advantages include: prevents cell swelling (the primary cause of cell distortion in aluminum-shell cells); improves SEI uniformity (more stable SEI due to consistent gas removal during the initial charge cycle); enhances cell consistency (identical pressure conditions for all cells ensure more uniform SEI formation and higher overall pack reliability); and reduces safety risks by minimizing trapped gas that can contribute to thermal runaway or hazardous failure modes.

5. Equipment for Prismatic Cell Negative Pressure Formation

Modern prismatic cell formation equipment integrates five key subsystems: sealed chamber (stainless steel or aluminum), vacuum system (vacuum pumps, valves, pressure sensors), formation power supply (constant current/constant voltage charging), data acquisition software (real-time monitoring), and integrated heating/cooling systems.

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