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Home / News / Industry News / Slurry Fineness Passes, Dispersion Looks Good — So Why Are There Particles on the Coated Surface?
Industry News
Functional Slurry Systems · Battery / Photovoltaic / Conductive
In lithium battery slurries, photovoltaic pastes, conductive slurries, and other functional material systems, a situation arises repeatedly: the slurry tests within fineness specification, the dispersion state looks good, and the material appears uniform — but after coating, the electrode or functional layer surface shows particles, raised spots, or surface roughness.
Passing Fineness Does Not Equal Stable Coating Quality
State of the slurry immediately after production:
Fineness test within specification
Dispersion state appears good
Appearance uniform and stable
These results confirm that initial processing was performed correctly. They do not confirm that the slurry will maintain this state through its full use cycle.
A passing fineness result does not mean coating will be particle-free. The fineness test captures a single moment — it cannot account for what happens to the slurry state during subsequent storage, transport, and coating operations.
Seven Reasons Particles Appear After Coating
1
Secondary Agglomeration During Storage
During static storage, particles continue to move, contact each other, and interact. If system stability is insufficient, originally dispersed particles may re-combine and form small agglomerates. These agglomerates then appear as surface particles during coating.
2
Dispersion State Affects Final Coating Quality
Maintaining good slurry stability requires uniform particle distribution, adequate dispersion stability, and an appropriate system structure. When particles lack sufficient protection, they may re-agglomerate during shear, transport, or static storage — reducing coating uniformity.
3
Coating Process Amplifies Small Defects
At the inspection stage, minor agglomerates may not be visible and fineness tests may not fully capture the actual dispersion state. Once the slurry is formed into a thin film during coating, the coating thickness decreases, particles become more concentrated at the surface, and defects become more pronounced — small particles that were not easily detected can become visible raised spots.
4
System Stability Affects Continuous Production
During continuous coating operation, the slurry is repeatedly circulated and transported. Piping and equipment introduce shear forces, and temperature and environment change continuously. These factors can gradually affect slurry state, causing particles to accumulate and impact coating quality.
5
High-Solid Systems More Susceptible
In high-solid-content slurries, solid particles are closer together, interactions are stronger, and maintaining system stability is more difficult. Compared to low-solid systems, secondary agglomeration after storage and coating particle formation are both more likely to occur.
6
Inspection Conditions Differ from Production Conditions
At the inspection stage, the sample volume is limited, the slurry has just been agitated, and the system is temporarily uniform. In actual production, the slurry experiences extended storage, state change during transport, and defect amplification during coating — so particle problems are only revealed progressively.
7
Fineness Test Cannot Detect All Re-Agglomeration
Micro-agglomerates formed during storage may be too small to clearly register on a standard fineness test, yet large enough to produce surface texture after coating. There is a gap between the detection threshold of common fineness instruments and the particle size at which coating surface defects become visible.
In Summary
Slurry fineness meeting specification but coating surfaces showing particles is fundamentally the result of secondary agglomeration, system stability change, and the amplification effect of the coating process acting together. When analysing this type of coating particle problem, attention needs to extend beyond grinding fineness to include a comprehensive assessment of slurry dispersion stability, storage state, and system changes during continuous coating operation.
Frequently Asked Questions
If the slurry is re-stirred before use, will the particle problem be eliminated?
If agglomeration is at an early stage and the agglomerates are loose, re-stirring can partially restore a uniform state. Once more compact agglomerates have formed after extended static storage — particularly in high-solid systems where particles have been under gravitational pressure — re-stirring may not fully restore the original dispersion quality, and may also introduce additional air or change consistency.
How can secondary agglomeration be detected before coating without waiting for coating results?
Tracking particle size distribution at multiple time points after production — immediately after mixing, after 24 hours, and after longer storage periods — and comparing the results is more sensitive than a single fineness check. An increasing particle size over storage time directly indicates secondary agglomeration is occurring.
Is this problem specific to lithium battery slurries, or does it apply to other functional material systems?
The same mechanisms — secondary agglomeration, system stability loss, and coating-process amplification — apply across lithium battery electrode slurries, photovoltaic pastes, conductive slurries, ceramic functional materials, and other systems where fine particles must remain stably dispersed through storage and continuous coating operation.
