Does jet milling affect the crystal structure of LFP

Under proper operating conditions (target: deagglomeration only), jet milling does not destroy the bulk olivine crystal structure of LFP. X‑ray diffraction (XRD) still shows complete characteristic peaks, no new impurity phases, and lattice parameters remain basically unchanged. However, excessive high‑energy jet milling introduces surface lattice strain, surface defects, and may damage the carbon‑coating layer, even though the bulk crystal lattice remains intact.

1. Moderate jet milling (industrial deagglomeration scenario)

Jet mill works by particle‑to‑particle collision; gas expansion cools the grinding zone, thermal stress is very low, no crystal phase transformation occurs.

• Only weak‑bonded soft sintered agglomerates are separated; primary LFP grains retain original olivine crystal structure.
• XRD diffraction peaks are sharp, crystallinity is preserved.
• Main damage occurs mostly at particle surface: partial carbon‑coating stripping, rather than bulk lattice destruction.

This is the standard working window for post‑sinter LFP processing: use minimum jet pressure to dissociate agglomerates, cooperate with closed‑loop turbo classification.

2. Damage caused by over‑energetic jet milling (too high pressure, under‑feeding, excessive circulation)

Although the bulk crystal phase does not disappear, adverse micro‑structural changes appear:

1. Surface lattice strain & micro‑defects: High‑velocity impact creates dislocation and lattice distortion on particle surface; XRD peak shows slight broadening (Scherrer effect). These surface defects increase side‑reactions with electrolyte, raise self‑discharge.
2. Primary‑particle fracture: Hard LFP grains break into sharp fine fragments. Bulk crystal structure of each fragment is still olivine, but new high‑defect fresh surfaces are exposed. BET rises sharply, tap density drops.
3. Carbon‑coating layer peeling / partial removal: This is the most common practical problem in mass production. The conductive carbon network is damaged, leading to higher impedance, poorer low‑temperature and cycling performance, even if crystal phase is unchanged.
4. No new crystalline impurity phases are generated, unlike long‑time high‑energy ball milling which may cause more serious lattice amorphization.

Important distinction: Jet milling rarely damages bulk crystal lattice, but easily creates surface‑level micro‑defects and destroys surface carbon coating.

3. Comparison with ball milling

• Jet milling: Mainly surface strain, carbon‑coating damage; bulk olivine phase remains.
• Dry ball milling (long‑time): More serious lattice distortion, partial surface amorphization, obvious crystallinity reduction.

4. How to avoid crystal‑surface damage during jet milling LFP

1. Use minimum effective grinding pressure for deagglomeration. Do not blindly increase pressure for finer PSD.
2. Maintain stable, sufficient feed rate. Under‑feeding magnifies particle impact energy.
3. Adopt closed‑loop turbo classification: discharge qualified deagglomerated powder promptly, avoid repeated high‑energy circulation inside grinding chamber.
4. Pre‑break hard sintered lumps upstream. Hard chunks amplify impact force and fracture surrounding primary particles.
5. Joint quality monitoring:
   • XRD: Confirm olivine phase, check peak broadening.
   • BET specific surface area: Abnormal BET jump signals primary‑particle fracture and heavy surface damage.
   • SEM: Check particle edges for sharp broken fragments.
   • Electrochemical test: Monitor impedance, low‑temperature performance (indirect reflection of carbon‑coating integrity).

Proper‑parameter jet milling for LFP preserves bulk olivine crystal structure. It only breaks soft sintered agglomerates. Excessive jet‑milling energy will not destroy bulk crystal phase, but introduces surface lattice strain, micro‑defects and carbon‑coating stripping, degrading electrochemical performance. The core process principle is deagglomeration, not primary‑particle crushing.