Spherical secondary‑spherical NCM particles are originally formed in co‑precipitation precursor and retained after calcination. Grinding cannot turn irregular fragments into perfect spheres. The core objective of jet‑mill dry grinding is de‑agglomeration: break sintered hard necks between bonded spherical particles, while preserving intact secondary‑sphere morphology and avoiding particle fracture into angular fragments. Improper high‑energy grinding shatters spheres and produces irregular debris, even if D50 meets specification. This article describes process principles, equipment selection, parameter control, upstream preconditions and quality verification to retain spherical morphology during NCM dry grinding.
1. Core principle
- Sphericity originates from spherical hydroxide precursor, formed during co‑precipitation. Calcination converts hydroxide into layered oxide while retaining spherical secondary‑particle shape.
- After calcination, many spherical NCM particles stick together via sintered necks to form hard agglomerates. Jet‑milling separates these connected spheres by breaking only sintered necks, without crushing the secondary spheres themselves.
- Risk: Excessive collision energy breaks secondary spheres into irregular angular fragments; these cannot be reshaped back into spheres by further grinding.
Critical rule: Grinding preserves existing sphericity; grinding cannot reshape irregular particles into spheres.
2. Upstream preconditions (must be satisfied before grinding)
2.1 Precursor quality
- Produce dense, well‑spherical hydroxide precursor via stable co‑precipitation (pH, ammonia concentration, temperature, stirring speed). Irregular precursor yields irregular NCM; no grinding can fix poor sphericity.
- Reduce irreversible hard agglomerates in precursor via controlled aging, filtration and drying.
2.2 Calcined clinker quality
- Optimize sintering curve: heating rate, peak temperature, holding time, pure‑oxygen atmosphere. Obtain mechanically robust secondary spheres with strong internal grain‑boundary bonding.
- Avoid severe local over‑sintering which generates huge fused hard lumps. Such large lumps require very high grinding energy; that energy will destroy normal spherical particles before lumps are fully dispersed.
- Avoid under‑sintered clinker: weak secondary particles fracture easily even under moderate jet‑mill collision.
3. Equipment selection for retaining spherical morphology
Fluidized‑bed opposed‑jet mill (fluid‑bed jet mill) is the mainstream industrial choice for NCM cathode materials.
- Working mechanism: particle‑to‑particle collision (“material‑against‑material”), no grinding media contact. Stress concentrates on weak sintered necks of agglomerates; intact secondary spheres survive collision.
- Avoid high‑energy impact mills, hammer mills or ball mills: they produce heavy shear and compression, readily shatter spherical secondary particles into fragments.
- All powder‑contact components adopt ceramic lining to prevent metal contamination. Equip integrated high‑precision turbine classifier.
- Closed‑loop nitrogen inert‑gas system, low dew‑point environment to avoid moisture‑induced agglomeration.
4. Key jet‑mill parameter settings to keep spherical shape
4.1 Grinding pressure (collision energy control)
- Apply minimum effective grinding pressure: just enough to break sintered necks of hard agglomerates.
- For high‑nickel NCM typical operating range: 0.5–0.8 MPa. Do not use excessive pressure margin.
- Too high pressure: secondary spheres crack and break into irregular fragments; BET rises sharply, many angular particles appear under SEM.
- Too low pressure: agglomerates are not fully dispersed; large bonded particle clusters remain in finished powder.
4.2 Feed rate and gas‑solid ratio
- Use loss‑in‑weight gravimetric feeder for stable continuous feeding, avoid hopper bridging and material surging.
- Prevent extremely low feed rate: low particle concentration causes violent single‑particle collision, secondary spheres fracture easily.
- Prevent severe over‑feeding: particle‑interference effect reduces de‑agglomeration efficiency, residual agglomerates pass into finished product.
4.3 Classifier tuning to reduce over‑circulation
- Follow the principle of minimum sufficient classifier speed to meet D50 and Dmax targets.
- Excessively high rotor speed sends large amount of medium‑size spherical particles back into grinding chamber. Repeated collisions gradually wear and break secondary spheres.
- Synchronously match classifier speed and total system airflow. Optimize secondary‑air volume to disperse soft electrostatic agglomerates by shear force without destroying real spheres.
- Two‑stage classification is recommended for high‑nickel grades: intercept oversized bonded agglomerates in first stage; fine‑tune PSD in second stage, reduce single‑pass circulation load.
4.4 Control closed‑loop circulation ratio
- Keep circulation ratio within reasonable limits. High circulation means particles pass grinding chamber repeatedly, accumulating impact damage and gradually fragmenting spherical particles.
- Periodically sample circulating coarse return stream for SEM inspection. If large numbers of broken spheres appear, reduce grinding pressure or classifier speed.
4.5 Electrostatic elimination
Install reliable grounding for mill chamber, classifier and pipelines. Static‑driven soft agglomerates make multiple spheres stick together; these look like large irregular particles, but are dispersible soft clusters rather than permanently fractured particles.
5. Operation and quality verification
- SEM inspection is mandatory: Laser PSD (D50/D97/Dmax) cannot detect broken secondary spheres. Under SEM:
- Good result: mostly complete spherical secondary particles; few fragments; rare bonded agglomerates.
- Poor result: abundant angular fragments, cracked spheres (over‑grinding).
- Monitor BET value: sharp BET increase indicates massive sphere fracture.
- Standardize sampling operation for high‑nickel NCM, minimize air exposure to avoid moisture‑caused false agglomeration misleading morphology judgment.
- Prevent equipment powder buildup: periodically clean volute and return pipelines. Peeling‑off powder lumps behave like irregular large particles.
6. Common troubleshooting
| SEM Observation | Root Cause | Countermeasure |
|---|---|---|
| Many broken, angular non‑spherical fragments | Excessive grinding pressure; too low feed rate; excessive classifier speed and heavy circulation | Lower grinding pressure; raise stable feed rate; reduce classifier rotor speed with airflow matching |
| Clusters of multiple spheres bonded together (hard agglomerates) | Calcination over‑sintering; insufficient grinding energy | Optimize upstream calcination; moderately raise grinding pressure; two‑stage classification |
| Loose clusters where individual spheres remain intact (soft agglomerates) | Static electricity or moisture absorption | Improve static grounding; lower system dew‑point; optimize secondary‑air shear |
| Poor inherent sphericity of all particles | Defective precursor sphericity | Adjust co‑precipitation process; grinding cannot reshape these particles |
Grinding cannot create spherical NCM particles from irregular raw material. Sphericity must be built at precursor co‑precipitation and preserved during calcination. The jet‑mill dry‑grinding process only breaks sintered necks between agglomerated spheres, while protecting secondary‑particle integrity. Adopt fluidized‑bed opposed‑jet mill with minimal effective grinding pressure, stable gravimetric feeding, controlled classifier speed and circulation ratio. Combine SEM morphology inspection together with PSD and BET monitoring. Only full‑chain control can deliver finished NCM powder with well‑retained spherical secondary‑particle morphology, especially critical for high‑nickel EV‑grade cathode materials.
