How to Achieve Uniform Particle Size in Dry Grinding of NCM

Uniform particle size (narrow PSD, low span value) is a key quality target for dry‑ground NCM cathode powder. Poor particle uniformity manifests as wide span, coexistence of ultrafine fines and residual coarse agglomerates, which negatively impacts tap density, electrode coating consistency, impedance and battery cycle performance. Uniform particle size cannot be realized by single‑parameter adjustment; it requires coordinated control of precursor quality, calcination clinker status, jet‑milling parameters, air‑classifier performance, closed‑loop circulation and stable feeding. This article systematically introduces technical approaches to obtain uniform particle‑size distribution during NCM dry grinding.

1. Core constraints for particle uniformity in dry grinding

Dry grinding for NCM targets de‑agglomeration of sintered lumps rather than crushing intact secondary spherical particles. Non‑uniform PSD mainly comes from three sources:

1. Upstream variation: uneven precursor particle size and hard sintered agglomerates generated in calcination. Hard fused lumps cannot be fully broken, co‑existing with fine fragments.
2. Grinding imbalance: partial secondary particles are over‑ground into ultrafine fines, while some agglomerates remain unbroken.
3. Classification deterioration: particle interference, poor flow‑field, turbine wear cause incomplete separation of coarse and fine fractions.

Important note: Jet mill and air classifier can optimize PSD, yet cannot fully compensate severe defects from precursor and calcination.

2. Upstream process control: foundation of uniform particle size

2.1 Stabilize precursor particle consistency

• Maintain stable co‑precipitation parameters including pH, ammonia concentration, temperature, stirring speed and feeding rate, suppress abnormal secondary nucleation.
• Adopt seed‑assisted crystallization to obtain narrow‑span precursor. Eliminate excessively large precursor particles.
• Standardize aging, filtering and drying processes to reduce irreversible hard agglomerates of precursor.
• Perform PSD inspection for each batch of precursor; avoid feeding heavily out‑of‑spec precursor into calcination.

2.2 Optimize calcination to reduce hard sintered lumps

• Lock heating curve, peak sintering temperature and holding time. Prevent local over‑sintering which produces hard‑to‑break fused lumps.
• Standardize saggar loading thickness and lithium‑source mixing homogeneity to guarantee consistent sintering state across materials.
• Inspect calcined clinker before grinding. If the proportion of hard agglomerates rises, adjust calcination instead of raising grinding intensity blindly. Massive hard lumps will create mixed coarse‑fine powder even under optimal grinding‑classification settings.

3. Jet mill parameter tuning for dry grinding uniformity

3.1 Appropriate grinding pressure

Select the minimal grinding pressure that meets Dmax requirement.

• Too low pressure: many agglomerates remain unbroken → coarse fraction retained, PSD broadens.
• Excessively high pressure: intact secondary particles shatter to generate large amounts of ultrafine fines → D10 drops sharply, span increases and uniformity deteriorates.
• For each NCM grade, complete pressure gradient test to find the optimal pressure window for de‑agglomeration without over‑grinding. High‑nickel materials adopt moderate pressure.

3.2 Stabilize feed rate and gas‑solid ratio

Over‑feeding causes high particle concentration inside grinding and classification zones, triggering particle‑interference effect. Coarse agglomerates are easily entrained into finished powder, destroying particle uniformity.

• Use gravimetric screw feeder with anti‑bridging structure to keep stable mass feed rate. Avoid lump surging.
• Operate within reasonable gas‑solid ratio window. Reduce feed load when classification sharpness declines, rather than simply increasing classifier speed.
• Avoid extremely low feed rate: prolonged residence time leads to partial over‑grinding and generates excessive ultrafine components.

4. Air classifier optimization for narrow particle‑size distribution

Classifier is the most critical unit to separate coarse and fine fractions and improve particle uniformity.

4.1 Matching of turbine speed and total airflow

Turbine wheel speed defines cut‑point; airflow determines drag force. These two parameters must be adjusted synchronously instead of changing one single variable.

• Excessively high wheel speed: massive qualified medium‑size powder circulates back, extended residence time causes partial over‑grinding and broadens PSD.
• Too high airflow: coarse bypass occurs, coarse particles leak into finished products.
• For narrow‑PSD requirement, properly reduce total airflow to improve classification sharpness under matched wheel speed.

4.2 Secondary air and guide vane adjustment

Reasonable secondary‑air volume eliminates local turbulence in classification chamber, reduces fine powder mixed in returning coarse fraction, improves separation effect between coarse and fine components, and lowers span value.

4.3 Two‑stage series classification for high‑uniformity demand

For high‑nickel NCM requiring strict particle uniformity:

• First‑stage: remove super‑large coarse killer particles.
• Second‑stage: precisely tune D50 and further narrow PSD span.
  Two‑stage classification greatly improves particle uniformity compared with single‑stage classifier.

4.4 Maintain classifier mechanical condition

• Keep turbine wheel, guide vanes and inner lining in full ceramic or non‑metal material to avoid metal contamination.
• Replace worn turbine blades on schedule. Blade abrasion distorts flow‑field, reduces separation precision and brings random coarse leakage, which cannot be offset by parameter adjustment.
• Regularly clean volute and return pipeline to eliminate powder dead‑zone accumulation. Prevent old agglomerates mixing into finished powder.

5. Closed‑loop circulation management

• Control circulation ratio within reasonable range. Excessively high circulation extends particle residence time inside the loop; part of secondary particles are crushed into ultrafine fines, deteriorating particle uniformity.
• Regularly test PSD of circulating coarse powder. If large‑size hard sintered lumps continuously exist, trace back to calcination process rather than increasing circulation ratio.

6. Environment and quality monitoring

• Control dew‑point in grinding system. High‑nickel NCM absorbs moisture and forms temporary soft agglomerates, which falsely widen measured PSD. Maintain low‑humidity nitrogen‑protected environment.
• Test complete PSD indicators: D10, D50, D97, Dmax and span value. Span = (D90‑D10)/D50; smaller span represents better particle uniformity.
• Deploy online PSD monitoring at finished‑product outlet, set alarm thresholds for span and Dmax to catch uniformity drift in time.
• Use SEM for auxiliary verification: observe secondary‑particle integrity and check for residual agglomerates or crushed fine fragments.

7. Common troubleshooting for poor particle uniformity

Achieving uniform particle size for dry‑ground NCM relies on full‑process control. Stable precursor and well‑controlled calcination avoid hard‑agglomerate sources. Jet‑mill grinding pressure and feed rate are tuned to realize sufficient de‑agglomeration while avoiding over‑grinding. Optimized air classifier, preferably two‑stage series grading, separates coarse and fine fractions sharply. Reasonable circulation management, anti‑moisture environment and complete PSD monitoring guarantee narrow‑span, uniform NCM powder, supporting consistent electrode processing and electrochemical performance of lithium‑ion batteries.