Excessive fine particles (sub‑micron ultrafine fractions) in NCM cathode powder mainly originate from over‑grinding during jet‑milling dry processing. Abundant fines raise BET specific surface area, increase residual alkali content, aggravate moisture absorption, cause battery gas swelling, increase electrode impedance and accelerate capacity fading. Reducing fine particles cannot simply lower grinding intensity; it requires coordinated adjustment of jet‑mill parameters, air‑classifier operation, closed‑loop circulation, upstream clinker quality and process monitoring. This article analyzes fine‑particle generation mechanisms and systematic solutions to cut ultrafine fractions while maintaining qualified Dmax and particle‑size distribution.
1. Main sources of fine particles in NCM powder
- Jet‑mill over‑collision: Excess grinding energy fractures intact spherical secondary particles, generating large amounts of sub‑micron fragments. This is the primary source of fines.
- Excessive closed‑loop circulation: Long particle residence time inside grinding‑classification loop leads to repeated impact of the same particles.
- Improper parameter matching: Too low feed rate, mismatched classifier speed‑airflow setting amplify over‑grinding effect.
- Upstream defects: Weak secondary‑particle structure from precursor or calcination makes particles easy to be shattered under collision.
Important note: When reducing fine particles, Dmax / oversized agglomerates must remain under control. Simply cutting grinding intensity may cause coarse killer‑particle leakage and bring safety risks.
2. Optimize jet‑mill parameters to suppress fine‑particle generation
2.1 Lower grinding pressure appropriately
Grinding pressure controls particle collision kinetic energy.
- Reduce grinding pressure to the minimum value that can break most sintered agglomerates and satisfy Dmax specification. Avoid excessive pressure margin.
- High‑nickel NCM is more prone to produce fines; operate at moderate pressure window.
- Do not reduce pressure too much: insufficient pressure leaves hard agglomerates unbroken. Perform gradient pressure test to balance fine‑particle content and Dmax index.
2.2 Increase stable feed rate (optimize gas‑solid ratio)
Too low feed rate reduces particle crowding inside collision zone. Each particle gains excessive acceleration energy and suffers repeated impacts, producing abundant fines.
- Raise feed rate within allowable range of classification sharpness. Higher particle concentration buffers collision intensity and decreases probability of secondary‑particle fracture.
- Strictly adopt gravimetric feeding to prevent actual feed dropping accidentally caused by hopper bridging.
- Monitor classification performance after feed‑rate increase: ensure no coarse bypass or Dmax spike. If classification sharpness deteriorates, do not further lift feed rate.
3. Air classifier tuning to reject excess fine particles
The classifier cannot destroy fines, but can adjust cut‑point and reduce fine fraction in finished product by process matching.
3.3 Reduce classifier wheel speed properly
Excessively high turbine speed returns large‑volume medium‑size powder back to grinding loop. Extended residence time creates more ultrafine fines.
- Lower classifier speed moderately while keeping D97 and Dmax within specification. Reduce circulation volume of medium‑size particles, shorten average residence time.
- When reducing wheel speed, synchronously adjust total system airflow to avoid coarse particle leakage. Speed‑airflow must be matched, single‑parameter adjustment is not recommended.
3.2 Optimize secondary air and flow‑field
Reasonable secondary‑air setting improves separation efficiency, reduces over‑circulation of near‑cut‑point particles, and avoids repeated grinding of these particles into fines.
3.4 Avoid over‑circulation of coarse fraction
- Control circulation ratio. High circulation ratio means particles circulate many times inside the loop; secondary particles gradually wear and crack into fine fragments.
- Regularly test PSD of circulating coarse powder. If circulating material already contains lots of fine fragments, it is clear that over‑grinding has occurred; adjust grinding and classifier parameters immediately.
3.5 Two‑stage classification strategy
- First stage: remove oversized coarse agglomerates.
- Second stage: adjust cut‑point, strip partial ultrafine fines.
Two‑stage grading can better control fine‑particle content than single‑stage classifier for high‑nickel grades.
4. Upstream process improvement to decrease tendency of fine‑particle formation
4.1 Precursor quality control
Produce dense, compact secondary spherical precursor particles. Loose, fragile precursor secondary structures are easily crushed into fines even under moderate grinding energy. Keep stable co‑precipitation conditions to obtain dense precursor particles.
4.2 Optimize calcination process
Reasonable sintering temperature and holding‑time improve mechanical strength of NCM secondary particles.
- Under‑sintered clinker has weak bonding inside secondary particles and is easy to shatter into fines during impact.
- Avoid severe over‑sintering which generates hard fused lumps. Those lumps force operators to raise grinding pressure, indirectly producing large amounts of fines.
5. Operation, monitoring and troubleshooting
Key monitoring indicators when reducing fine particles:
- D10: D10 rises when fine‑particle fraction decreases;
- BET specific surface area: effective indirect indicator for ultrafine fines;
- D97, Dmax, span: guarantee no deterioration after adjustment.
| Phenomenon | Root Cause | Countermeasure |
|---|---|---|
| High fine content, low D10, high BET | Excessive grinding pressure; too low feed rate; too high classifier speed causing heavy circulation | Reduce grinding pressure; raise feed rate properly; lower classifier wheel speed with airflow matching |
| Fine particles decrease but Dmax exceeds limit | Grinding pressure too low; feed rate too high leading to coarse bypass | Slightly increase grinding pressure; reduce feed load; optimize classifier separation performance |
| Fine‑particle content fluctuates batch‑to‑batch | Unstable feed rate; nitrogen‑pressure oscillation; turbine blade wear | Stabilize gravimetric feeding; stabilize gas source; inspect and replace worn rotor |
| Cannot cut fines even after parameter adjustment | Secondary particles are inherently fragile from precursor/calcination | Optimize precursor density and calcination sintering state |
Fine particles in dry‑ground NCM are mainly generated by mechanical fracture of secondary particles in jet‑mill closed‑loop system. To reduce fines: adopt minimal feasible grinding pressure, appropriately increase feed rate to buffer collision energy, avoid excessive classifier speed and over‑circulation. Meanwhile improve precursor compactness and calcined particle strength. All adjustments must keep Dmax under control. Track D10 and BET together with conventional PSD indexes, so as to lower ultrafine‑fraction content while preventing oversized agglomerates, improving powder comprehensive performance for lithium‑ion cathode application.
