D50 and D90 are volume‑based particle‑size parameters from laser diffraction particle‑size test, widely used for NCM cathode powder specification. They represent different cumulative volume percentage points on particle‑size distribution curve. D50 reflects median particle size; D90 describes coarse‑side particle fraction. Even with identical D50, D90 can vary greatly, directly affecting Dmax, tap density, electrode processing and battery safety. This article clarifies definitions, physical meaning, production significance and practical examples for NCM materials.
1. Basic definition (volume‑based distribution)
- D50 (Median particle size)
Volume cumulative 50 % point.
50 % of total powder volume is smaller than this value; 50 % is larger than this value.
Commonly called “average particle size” in factory floor language.
- D90
Volume cumulative 90 % point.
90 % of total powder volume is smaller than this value; only 10 % volume fraction of particles are larger than D90.
Example: For polycrystalline high‑nickel NCM: D50 =8 μm, D90 =13 μm
Meaning:
- Half of powder volume is below 8 μm, half above 8 μm.
- 90 % volume is smaller than 13 μm; only 10 % volume are particles >13 μm.
Important: D90 is not Dmax. Dmax is the maximum detected particle size; D90 is the 90% cumulative point, still allows small quantity of even larger particles above D90.
2. Core difference
| Item | D50 | D90 |
|---|---|---|
| Cumulative percentage | 50 % | 90 % |
| What it represents | Median / central particle size | Coarse‑end feature of PSD |
| Sensitive to | Majority of medium‑sized particles | Coarse particles, agglomerates, hard lumps |
| Mainly controlled by | Classifier wheel speed | Grinding efficiency, agglomerate removal, upstream calcination |
| Indicator meaning | Main particle dimension of bulk powder | Degree of coarse tail in particle‑size distribution |
Key practical phenomenon:
Two batches can have same D50, but very different D90.
- Batch A: D50=8 μm，D90=12 μm → narrow distribution, few coarse particles
- Batch B: D50=8 μm，D90=18 μm → broad distribution, significant coarse tail / residual agglomerates
3. Influence on NCM powder and battery performance
D50 impact
- Determines main particle size level of bulk NCM powder.
- Affect lithium‑ion diffusion path, rate performance, tap density baseline.
- Primary set‑point adjusted by classifier speed during jet‑milling.
- High‑nickel polycrystalline EV‑grade typical target D50: 6‑10 μm.
D90 impact
D90 reflects coarse tail condition, closely linked to hard sintered agglomerates:
- Electrode coating quality: High D90 means more large particles/agglomerates, easily cause pin‑holes, scratch on copper‑coated foil during coating & calendaring.
- Safety risk: High D90 usually accompanied by higher Dmax. Oversized hard agglomerates are “killer particles”, may pierce separator and trigger internal short‑circuit.
- Tap density & compaction density: Too high D90 (many large porous agglomerates) reduces packing density.
- Even D50 is on target, high D90 indicates incomplete de‑agglomeration in grinding or poor calcination clinker quality.
For EV‑grade polycrystalline NCM: typical D90 target range: 12‑18 μm, together with strict Dmax limit (<20‑25 μm).
4. Process behaviour in jet‑milling production
- Adjusting classifier speed changes both D50 and D90 simultaneously:
- Higher classifier speed: D50 decreases, D90 also decreases; coarse particles are rejected back into loop.
- Lower classifier speed: D50 increases, D90 increases; more coarse particles pass into finished product.
- Special case: D50 acceptable but D90 too high
- Root causes: insufficient grinding energy cannot break sintered necks of hard agglomerates; upstream calcination generates large fused lumps.
- In this case, only tuning classifier speed cannot solve D90 problem. Need improve grinding pressure or fix calcination.
- Special case: D90 acceptable but D50 too fine
- Usually from over‑grinding: large amount ultrafine fines generated, D50 drops, D90 remains relatively normal; BET rises obviously.
5. Relation with span
Span = (D90‑D10)/D50, a common indicator for distribution width.
D50, D90 and D10 together calculate span.
NCM typical target span: 0.8‑1.2.
D50 is the median size describing bulk main particles; D90 characterizes coarse tail of particle‑size distribution. D50 reflects overall particle level, D90 reflects residual coarse particles and agglomerate condition. In NCM mass‑production quality control, only monitoring D50 is insufficient. D90 (plus D10 and Dmax) must be inspected together to judge de‑agglomeration effect, avoid hidden coarse‑particle safety risk even when D50 meets specification.
