How to Reduce Dmax in NCM Powder Grinding

Dmax (often represented by D99/D100) for NCM cathode powder refers to the maximum particle size, mainly from sinter‑bonded hard agglomerates formed during high‑temperature lithiation. Oversized particles risk separator puncture and micro‑short circuits in lithium‑ion cells. Reducing Dmax cannot rely solely on harder grinding; excessive milling will generate massive ultra‑fines, raise residual lithium, and deteriorate moisture‑resistance of high‑nickel NCM. The solution combines de‑agglomeration, classification, upstream sintering adjustment, and process parameter tuning.

1. Optimize pre‑grinding feed condition (upstream source control)

1. Suppress sinter‑bonded hard agglomerates in sintering process

• Apply two‑step sintering profile; avoid over‑sintering (too high temperature or over‑long holding time), which causes adjacent secondary spheres to fuse together to form giant hard agglomerates.
• Improve homogeneity of lithium‑salt mixing. Local lithium enrichment accelerates inter‑particle sinter bonding and increases Dmax.
• Screen sintered clumps before feeding to remove large bulk sintered cake manually / by coarse jaw crushing, prevent oversized bulk from entering fine grinding loop.

Important: If feed contains large hard sintered agglomerates, grinding alone can hardly eliminate Dmax completely.

2. Select proper grinding equipment & set grinding parameters

Goal: break sinter‑bonded agglomerates, while keeping intact NCM secondary spheres, avoid over‑grinding.

1. Fluidized‑bed jet mill (preferred for NCM)
   Operate under nitrogen inert atmosphere for high‑nickel grades.

• Increase grinding nozzle pressure moderately: higher jet energy improves breaking efficiency for hard agglomerates. Do not over‑increase pressure, otherwise abundant sub‑micron fines will be produced.
• Lower feed rate: extend material residence time inside grinding chamber for better agglomerate dissociation. Too‑fast feeding leads to incomplete breaking and residual coarse fraction.
• Keep stable material bed height inside jet mill.

2. Pin mill (for soft agglomerates only)
   Pin mill is less effective for hard sinter‑bonded agglomerates.

• Increase rotor speed within machine limit to enhance impact shear;
• Reduce feed throughput;
• Note: pin mill tends to produce more fine debris when tackling hard agglomerates.

Avoid ball milling for NCM: high mechanical force destroys secondary spherical structure, generates huge fines and introduces metal contamination.

3. Air classification: the most effective way to cut Dmax

Grinding breaks agglomerates; air classification removes remaining over‑size particles physically, which is the core measure to lower Dmax / D99.

1. Single‑stage turbine air classifier

• Raise classifier wheel rotational speed: reduces cut‑point, rejects more coarse particles. But excessively high speed will also lose qualified medium‑size fraction and raise fine‑powder ratio.
• Adjust system air volume: lower main air flow reduces maximum particle that can be carried over to finished product.
• Increase classifier feed concentration moderately for better separation efficiency.

2. Two‑stage / multi‑stage classification loop (industrial best practice for strict Dmax requirement)

• First‑stage classification: remove ultra‑coarse killer particles; coarse fraction returns back to grinding chamber for re‑processing.
• Second‑stage fine classification: trim PSD span and further compress D99/Dmax.
  Closed grinding‑classification loop circulates over‑size material rather than passing oversized particles to final product.

4. Critical process constraints & common pitfalls

1. Do not pursue Dmax reduction only by raising grinding intensity. Consequence:

• Sharp increase of D10 (massive ultra‑fine NCM particles)
• Higher residual lithium content, powder becomes more hygroscopic
• Tap density drops, electrode processability worsens

2. High‑nickel NCM (NCM811 etc.) must run grinding‑classification under inert N₂ protection to prevent surface lithia formation and material hydrolysis.
3. Online laser particle size monitoring (D99 / D100) at classifier outlet for closed‑loop parameter adjustment.
4. If Dmax still cannot meet target after parameter optimization: check upstream sintering, hard sintered agglomerates in feedstock are the root cause.

5. Quick troubleshooting checklist for high Dmax

To reduce Dmax of NCM powder: minimize sinter‑bonded hard agglomerates at sintering stage → mild de‑agglomeration grinding (do not over‑mill) → multi‑stage closed‑loop air classification to reject over‑size particles. Classification is more decisive than grinding for Dmax control.