How to Control Particle Size Distribution (PSD) of NCM Cathode Material

NCM particle‑size distribution determines tap density, electrode coating uniformity, lithium‑ion diffusion, compaction density and cell safety. PSD control runs through precursor co‑precipitation, lithiation‑sintering, post‑sinter crushing & air classification stages. Typical industrial target: polycrystalline NCM D50=8‑12 μm, D99＜35‑45 μm; single‑crystal NCM D50=3‑5 μm.

1. Precursor co‑precipitation (source control of PSD)

The PSD of spherical hydroxide precursor Ni‑Co‑Mn(OH)₂ is inherited largely by final NCM product.

1. Core reaction parameters tuning
   • pH value: Strictly stabilize 11.2‑11.6 for NCM523/622; slightly higher for high‑nickel NCM811. Fluctuation causes inconsistent nucleation and broad PSD.
   • Ammonia concentration (chelating agent): Adjust ammonia to regulate metal‑ion release rate. Higher ammonia yields larger secondary spherical particles; insufficient ammonia generates fine fragments and wide PSD.
   • Reaction temperature, stirring speed, solid content: Keep temperature 50‑58 ℃; match stirring intensity to avoid local supersaturation and abnormal fine/coarse particles.
2. Reactor configuration optimization
   Adopt multi‑stage series CSTR reactors: separate nucleation stage and particle‑growth stage. This reduces CV value of particle size and narrows PSD effectively. Seed‑feeding strategy stabilizes D50 and suppresses over‑nucleation of fine particles.
3. Slurry post‑treatment: Control filtering, washing and drying to prevent secondary hard‑agglomeration which brings coarse pseudo‑particles.

2. Lithiation mixing and sintering (adjust primary particle, preserve secondary‑sphere PSD)

1. Li‑salt mixing homogeneity: Uniform mixing of precursor with LiOH/Li₂CO₃ avoids local abnormal grain growth. Li‑excess ratio normally 1.03‑1.05.
2. Sintering temperature & holding time:
   Higher temperature / longer holding time increases primary‑crystal size; excessive sintering triggers inter‑sphere sintering bonding to form oversized agglomerates. Two‑step sintering (low‑temp decomposition + high‑temp lithiation) helps restrain abnormal grain growth and keep secondary‑particle size distribution inherited from precursor.

Note: Sintering cannot radically correct bad precursor PSD; it only fine‑tunes primary‑particle dimension.

3. Post‑sintering crushing and air classification (industrial final PSD tuning)

Sintered NCM contains soft agglomerates and a small number of over‑size “killer particles” that may pierce separator and induce micro‑short circuit.

1. Milling / de‑agglomeration
   • Jet mill (fluidized‑bed opposed‑jet mill, inert N₂ atmosphere preferred): Particle‑on‑particle collision, low contamination. Avoid over‑milling which produces excessive super‑fine powder (high residual alkali, severe moisture absorption).
   • Pin mill: Efficient for breaking soft agglomerates; adjust rotor speed and feeding rate to balance de‑agglomeration and over‑grinding risk.

   Critical principle: Disassemble agglomerates, do not violently smash intact NCM secondary spheres.

2. Dynamic air classification (most critical for narrow PSD)
   Integrated turbine air classifier removes oversized coarse particles and excess ultra‑fines simultaneously. Adjust classifier‑wheel rotating speed, system air flow and feed rate:

• Higher classifier‑wheel speed → finer cut‑point;
• Lower classifier‑wheel speed → coarser cut‑point.
  Multi‑stage classification further compresses PSD span and strictly caps D99 / maximum particle size to eliminate killer particles.

4. Key practical tips & common defects

1. PSD index should not only focus on D50; monitor D10, D90, D99 and span ((D90‑D10)/D50). Smaller span means narrower distribution.
2. High‑nickel NCM must adopt inert‑gas protection during milling & classification to suppress water‑induced surface alkali formation and material degradation.
3. If PSD is broad:
   • Root cause: unstable precursor co‑precipitation parameters;
   • Post‑remedy: optimize multi‑stage air classification; do not rely purely on intensified milling, which creates large amount of fines.
4. On‑line laser particle‑size monitoring at precursor outlet and finished‑powder classification outlet enables closed‑loop parameter adjustment for stable PSD batch‑to‑batch.

Stable narrow‑PSD NCM comes from narrow‑PSD spherical precursor → well‑controlled sintering without inter‑particle sinter‑bonding → mild de‑agglomeration + precise dynamic air classification to strip oversized particles and excess fines. Precursor quality is decisive; post‑processing only corrects limited deviation.