How to Achieve Bimodal Particle Size Distribution for NCM

Bimodal‑PSD NCM contains two discrete particle‑size populations: coarse secondary particles and fine secondary particles. Fine particles fill voids among coarse spheres to raise tap density and electrode compaction density, improving volumetric energy density and rate performance. Standard single‑loop jet mill with integrated turbine classifier naturally produces narrow unimodal distribution; it cannot directly generate bimodal PSD in one‑pass grinding. Industrial production mainly adopts dry powder blending (most widely used); alternative routes include precursor co‑synthesis and multi‑stage classification‑grinding. Critical risks to avoid: over‑grinding, broken secondary spheres, poor mixing homogeneity and uncontrolled BET increase.

1. Typical target specification for bimodal NCM

• Coarse fraction: D50 = 8‑13 μm (intact polycrystalline secondary spheres)
• Fine fraction: D50 = 2‑5 μm (complete secondary particles, not crushed primary fragments)
• Optimal particle‑size ratio (coarse / fine): approx 3‑5 : 1; mass ratio commonly 70:30 ~ 80:20 (coarse : fine)
• Requirement: Both coarse and fine fractions keep intact spherical secondary‑particle morphology; avoid angular fragments generated by over‑grinding.
• Warning: Bimodal PSD is two populations of complete secondary particles, not a mixture of intact spheres plus over‑ground fine debris.

2. Three technical routes to realize bimodal NCM

Route 1: Independent production of coarse powder & fine powder + dry blending (Mainstream industrial solution)

Process steps:

1. Produce coarse‑size NCM powder: Gentle jet‑mill de‑agglomeration, set classifier for coarse cut‑point; remove oversized hard agglomerates, preserve complete large secondary spheres.
2. Independently produce fine‑size NCM powder: Use separate jet‑mill / classification line to obtain fine secondary‑particle powder. Adopt gentle grinding; strictly prevent smashing secondary spheres into primary crystallites. Monitor BET and SEM; fine fraction must still retain spherical morphology.
3. Accurately weigh coarse and fine fractions according target mass ratio.
4. High‑efficiency low‑shear dry powder mixer (3D mixer / conical mixer): Low‑speed mixing to realize homogeneous blending. Avoid high‑shear mixing which fractures particles.
5. After blending: Verify full PSD curve by laser diffraction; use SEM to check uniform distribution of coarse‑fine particles; test tap density, BET, residual lithium.

Advantages:

• Fully flexible ratio adjustment; independent quality control for coarse / fine fractions.
• No modification to original calcination process.

Key risks & controls:

• Poor mixing uniformity → local unimodal region inside electrode. Control mixing time, avoid over‑mixing.
• Fine fraction over‑grinding → sharp BET rise, high side‑reaction. Fine powder production must follow gentle‑grinding principles, control circulation ratio.

Route 2: Precursor co‑precipitation & one‑step calcination (material synthesis route)

Synthesize bimodal precursor: co‑exist large and small spherical hydroxide precursor particles, then calcine in one batch to get bimodal NCM powder。

Advantage: Powder is inherently bimodal without post‑processing blending.

Limitations:

• Precise control of precursor particle‑size ratio is difficult; batch‑to‑batch consistency hard to stabilize.
• Not flexible for fast ratio adjustment; mainly used for special custom‑grade products.

Route 3: Multi‑stage external classification + partial re‑grinding (process‑int‑situ route)

Single jet‑mill cannot produce bimodal in one loop, but multi‑stage external classification system can realize continuous split‑stream production:

1. Feed calcined NCM into primary classifier: Separate feed into three streams: oversize, target coarse fraction, fine fraction.
2. Oversize returns to jet‑mill for gentle de‑agglomeration.
3. Collect coarse fraction; send part of material to second‑stage classifier / gentle grinding unit to produce fine secondary‑particle fraction.
4. Online proportion control, then inline blending to obtain bimodal finished powder.

Limitations: Complex equipment configuration; strict process control required to prevent fine‑fraction over‑grinding. Mostly used for large‑scale continuous production lines.

❌ Common misunderstanding: Adjust single‑jet‑mill internal classifier parameters to directly get bimodal PSD. Single‑rotor closed‑loop jet mill is designed for narrow unimodal output. Improper parameter setting only produces wide unimodal PSD containing over‑ground debris, which is not true bimodal distribution.

3. Critical process‑control points for bimodal NCM

3.1 Fine‑fraction quality control (most critical risk point)

The fine fraction must be intact secondary spheres, not broken fragments from over‑grinding:

1. When producing fine powder, apply minimum‑effective grinding pressure; strictly control circulation ratio.
2. Multi‑indicator monitoring: PSD(D50, D10), BET, SEM morphology. Sharp BET increase indicates secondary‑particle fracture, such fine fraction cannot be used for bimodal formulation.
3. Distinguish: True bimodal (two spherical populations) vs defective wide PSD (mixture of spheres + angular broken debris).

3.2 Ratio & particle‑size matching

• Size‑ratio between coarse and fine: fine D50 should be approx 1/3 ~1/5 of coarse D50, for optimal void‑filling effect.
• Mass ratio: typical window coarse : fine = 70:30 ~80:20. Too much fine powder causes BET to rise sharply, degrading cycle and gas performance.

3.3 Mixing process control

1. Adopt low‑shear 3D or double‑cone mixer. Avoid high‑speed agitator which smashes particles.
2. Optimize mixing time: insufficient time leads to segregation; excessive mixing may induce particle damage and static agglomeration.
3. After mixing, sample multiple positions of mixing container for PSD test to evaluate mixing homogeneity.

3.4 Prevent segregation during subsequent conveying & packaging

Coarse‑fine density difference may cause powder segregation during pneumatic conveying, hopper storage.

• Minimize free‑fall height; adopt soft‑transport; avoid violent pneumatic conveying.
• After packaging, re‑sample and test PSD to confirm bimodal feature remains.

4. Verification indicators for qualified bimodal NCM

1. Laser particle‑size curve: Two obvious independent peaks, no excessive ultrafine tail.
2. SEM observation: Both coarse and fine particles are intact spherical secondary particles, low proportion of angular fragments.
3. BET value: Moderate, no obvious rise compared with monomodal baseline.
4. Tap density: Higher than monomodal reference sample.
5. Electrochemical verification: Slurry stability, coating quality, compaction density, cycle performance.

5. Typical failure modes and troubleshooting

For NCM cathode materials, the most reliable industrial method for bimodal particle‑size distribution is separately manufacturing qualified coarse‑size and fine‑size intact secondary‑particle fractions followed by precise low‑shear dry blending. A single standard closed‑loop jet mill cannot produce true bimodal PSD in one pass. The core risk is over‑grinding of fine‑particle fraction into primary‑crystallite fragments. Always validate bimodal quality combining laser PSD, SEM and BET, rather than relying solely on particle‑size curve.