Causes and Solutions of Black Spots on Lithium Battery Electrodes

I. Characteristics of the Black Spot Phenomenon

Visual Appearance:

• Black or dark gray spots appearing on the surface of the electrode, mostly concentrated at the edges of the coating area or at the winding interface;
• The black spot regions are accompanied by graphite interlayer delamination and active material expansion, leading to abnormal local thickness (increase exceeding 85%).

Performance Impact:

• Capacity fade (typical loss of 5–10%), with a cycle life reduction of over 30%;
• Lithium plating in black spot areas increases the risk of thermal runaway, with localized temperatures reaching above 80°C.

II. Core Cause Analysis

Material Defects:

• Excessive impurities in raw materials (e.g., residual rolling oil on copper foil) or conductive agent agglomeration (particle size >5 μm), leading to localized failure of the conductive network;
• Contamination on the substrate surface (dust, metal particles) hindering slurry wetting, causing abnormal solvent evaporation during drying.

Process Deviations:

• Poor dispersion of coating slurry, introducing bubbles that form pinhole defects;
• Sudden changes in drying temperature gradients leading to rapid surface skinning, trapping internal solvents and causing stress cracks;
• Improper negative pressure control during formation (pressure fluctuation >10%), accelerating the deposition of electrolyte decomposition products.

Interfacial Reaction Failure:

• HF generated from LiPF₆ decomposition in the electrolyte corrodes the graphite layer, causing localized SEI film rupture;
• Insufficient lithium salt concentration or moisture ingress (>50 ppm), triggering side reactions that produce high-resistance byproducts such as LiF and Li₂O.

III. Common Solutions

Process Optimization Measures:

• Adopt a closed-loop coating control system to maintain tension fluctuations ≤0.5% and match drying temperature gradients (heating rate ≤3°C/min);
• Optimize formation negative pressure parameters (e.g., vacuum level controlled at -90 to -95 kPa) and verify process stability using blockage simulation tools.

Material Modification Solutions:

• Increase binder proportion to 3–5% (e.g., PVDF) to suppress slurry sedimentation and particle agglomeration;
• Use nano-composite current collectors (e.g., carbon-coated aluminum foil) to reduce interfacial contact resistance by over 30%.

Environmental Control Upgrades:

• Maintain workshop humidity ≤30%, with copper foil plasma cleaning achieving a wetting angle ≤20°;
• Pre-lithiation treatment before storage to reduce negative electrode active lithium loss (capacity recovery rate improved by 7–9%).

IV. Detection and Validation Methods

Microscopic Analysis:

• SEM/EDS to examine the composition of black spot areas (abnormal O/F/P content indicates electrolyte decomposition);
• XRD to analyze graphite interlayer spacing (d002 > 0.344 nm suggests structural damage).

Process Validation Tools:

• Use formation blockage simulation tools to test cells, collecting pressure and temperature curves to match threshold conditions;
• High-temperature storage testing (55°C/7 days) to verify black spot propagation rate and screen abnormal cells.