Lithium Battery Formation & Capacity Grading

Formation and capacity grading are two consecutive core processes after electrolyte injection and sealing of battery cells. Formation activates cells and forms passivation film; capacity grading performs inspection, sorting and matching, mandatory procedures for both PACK factories and cell manufacturers.

1. Formation

1.1 Core Purposes

1.    Form a uniform, dense SEI (Solid Electrolyte Interphase) passivation film on the anode surface to block continuous electrolyte decomposition;

2.    Complete the first lithium intercalation and activation of cathode and anode active materials;

3.    Exhaust gases (hydrogen, carbon dioxide, etc.) generated during initial charging to avoid swelling and bulging;

4.    Stabilize cell internal resistance and voltage plateau.

1.2 Standard Procedures (Pouch / Prismatic Aluminum Shell Cells)

1.    Open-pocket Pre-formation

Low current rate (0.02C~0.1C) low-voltage pre-charging; massive gas generation, followed by gas extraction, exhaust and final sealing.

2.    Main Formation Charging

Multi-stage CC (Constant Current) → CV (Constant Voltage) charging to full-charge voltage

·         LFP (Lithium Iron Phosphate): 3.65 V

·         NCM Ternary: 4.2 V

3.    High-Temperature Aging

Storage at 45~60°C for 12~48 hours to allow self-repair and densification of SEI film, greatly reducing subsequent self-discharge.

4.    Shallow discharge to transition to capacity grading.

Key Risk Factors

·         Excessive charging current: cracked SEI film, lithium plating, high internal resistance, poor cycle life;

·         Over-high temperature: excessive electrolyte decomposition, cell bulging, enlarged irreversible capacity loss;

·         Insufficient clamping pressure (pouch cells): electrode layer separation, uneven SEI thickness.

2. Capacity Grading

Conducted after qualified formation for capacity calibration and performance screening, equivalent to a full physical examination for cells.

2.1 Core Purposes

1.    Precisely measure the actual cell capacity and calculate initial coulombic efficiency;

2.    Collect internal resistance and static voltage under full / half / empty state, voltage drop data;

3.    Reject defective units: micro-short circuit, severe self-discharge, insufficient capacity, abnormal internal resistance;

4.    Sort cells by capacity, internal resistance and voltage difference, and match cells of identical grade for PACK assembly.

2.2 Standard Charge-Discharge Cycle for Grading

1.    CC-CV full charging to upper limit voltage;

2.    Rest and voltage stabilization;

3.    Constant-current discharge to cut-off voltage (2.5 V for LFP, 2.75 V for NCM), recorded discharge capacity = rated actual cell capacity;

4.    Re-test for 1~3 cycles to obtain stable values;

5.    Static storage at ambient temperature to measure voltage drop and judge self-discharge level.

Example Grading Standards

·         Grade A: Rated capacity met, minor internal resistance fluctuation, tiny 24-hour voltage drop;

·         Grade B: Slightly low capacity / elevated internal resistance, applied to low-speed energy storage, backup power supplies;

·         Scrap: Capacity below 80% of nominal value, micro-short circuit, excessive voltage drop, swelling or bulging.

3. Comparison: Formation vs Capacity Grading

4. Full Set of Production Line Equipment for Formation & Grading

1.    Formation machine: Equipped with high-temperature oven, clamping fixtures, multi-channel charge-discharge power supply;

2.    Vacuum sealing machine (for pouch cells only);

3.    Capacity grading cabinet: High-precision multi-channel charge-discharge test equipment;

4.    OCV resting cabinet: Measure static voltage drop to screen high self-discharge cells;

5.    Scanning code automatic sorting pipeline for graded cell tray loading.

5. Simplified Process Reference by Application

1.    High-capacity power cells (energy storage / EV): Slow formation + long-period high-temperature aging + two-cycle grading;

2.    Small consumer cells (power banks, digital devices): Simplified pre-formation, shortened aging time, single-cycle grading acceptable.