The Causes And Solutions of Black Spots On Lithium Battery Electrodes

Ⅰ. Characteristics of black spot phenomenon ‌

Appearance ‌ :

The black or dark gray spots that appear on the surface of the electrode sheet are mostly concentrated at the edge of the coating area or the winding interface.

2. The black spot area is accompanied by interlayer delamination of graphite and expansion of active substances, resulting in abnormal local thickness of the electrode sheet (with an increase of more than 85%).

Performance impact: ‌

Capacity attenuation (typical loss 5-10%), cycle life decreases by more than 30%;

2. Lithium plating in the black spot area poses a risk of thermal runaway, with local temperatures reaching over 80℃

Ⅱ. Analysis of Core Causes

Material defect: ‌

1. Excessive impurities in raw materials (such as residual rolling oil in copper foil) or agglomeration of conductive agents (particle size > 5μm) cause local failure of the conductive network;

2. Surface contamination of the substrate (dust, metal particles) hinders the wetting of the slurry, resulting in abnormal solvent evaporation during the drying process.

Process runaway: ‌

Poor dispersion of the coating slurry introduces air bubbles and forms pinhole defects.

2. The sudden change in the drying temperature gradient causes the surface skin to form too quickly, and the internal solvent retention triggers stress cracks.

3. Improper negative pressure control in the formation process (pressure fluctuation > 10%) accelerates the deposition of decomposition products of the electrolyte.

Interface reaction failure ‌

1. The HF produced by the decomposition of LiPF₆ in the electrolyte corrodes the graphite layer, causing partial rupture of the SEI film;

Insufficient lithium salt concentration or moisture intrusion (> 50ppm) can induce side reactions to generate high-impedance products such as LiF/Li₂O.

Ⅲ. Common solutions ‌

Process optimization measures: ‌

1. A closed-loop coating control system is adopted to maintain tension fluctuation ≤0.5% and match the drying temperature gradient (heating rate ≤3℃/min).

2. Optimize the formation negative pressure parameters (such as vacuum degree controlled within -90 to -95 kpa), and verify the process stability through the simulation of the blockage detection fixture

Material modification scheme: ‌

Increase the proportion of binder to 3-5% (such as PVDF) to inhibit slurry sedimentation and particle agglomeration;

2. The use of nano-composite current collectors (such as carbon-coated aluminum foil) reduces the interface contact resistance by more than 30%.

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Environmental control upgrade: ‌

The humidity in the workshop is controlled at no more than 30%, and the wetting Angle of the copper foil after plasma cleaning is no more than 20°.

2. Pre-lithiation treatment before storage reduces the loss of active lithium in the anode (capacity recovery rate increases by 7-9%).

Ⅳ. Testing and validation methods ‌

Microscopic analysis: ‌

1. SEM/EDS detection of the components in the black spot area (abnormal contents of O/F/P elements suggest electrolyte decomposition);

2.XRD analysis of the interlayer spacing of graphite (d002 > 0.344nm indicates structural failure).

Process validation tool: ‌

1. Use the formation blockage detection fixture to simulate the battery cell and collect the matching threshold conditions of the pressure and temperature curves;

2. High-temperature storage tests (55℃ for 7 days) are conducted to verify the rate of black spot expansion and screen out abnormal battery cells.