Grinding (deagglomeration / milling‑classification) directly changes particle size distribution, specific surface area, particle morphology, carbon‑coating integrity, surface defects and impurity level of LFP powder. These physical properties further affect impedance, rate capability, cycle stability, self‑discharge and low‑temperature performance. Proper grinding optimizes powder processing; over‑grinding or insufficient grinding both degrade electrochemical behaviour.
1. Insufficient grinding (incomplete deagglomeration)
Sintered soft agglomerates are not fully disassembled.
- Physical changes: Large residual agglomerates exist in powder; actual D50 is coarse; tap density is low; uneven particle dispersion during slurry mixing.
- Electrochemical impacts
- Poor electrode homogeneity: agglomerates form isolated regions inside electrode sheets, incomplete contact with conductive agent.
- Increased polarization, poor rate performance. Lithium‑ion diffusion path inside large agglomerates becomes longer.
- Local non‑uniform compression during calendaring; electrode micro‑cracks easily appear.
- High internal resistance; actual discharge capacity cannot be fully exerted.
2. Proper grinding (target: deagglomeration only, keep primary particles intact)
Only weak inter‑particle agglomerates are broken; primary grains, carbon‑coating and bulk crystal structure are preserved.
- Physical changes: Optimized PSD, reasonable BET, good flowability and tap density; no massive fine fragments; intact carbon conductive network.
- Electrochemical benefits
- Uniform slurry dispersion, stable electrode processing performance.
- Appropriate specific surface area balances lithium‑ion diffusion and side‑reaction area.
- Low charge‑transfer impedance, good rate capability.
- Stable interface between LFP and electrolyte, long cycle life, low self‑discharge.
3. Over‑grinding (excessive impact energy, primary‑particle fracture)
Caused by too high jet pressure / ACM rotor speed, under‑feeding, excessive closed‑loop circulation. Bulk olivine crystal phase remains unchanged, but heavy surface damage occurs.
- Physical changes
- Primary particles fracture, produce large amount of sub‑micron fine fragments; BET rises sharply.
- Carbon‑coating layer partially stripped or damaged; local loss of conductive carbon network.
- High surface lattice strain and surface defects.
- Risk of increased metal pick‑up due to intensified equipment wear.
- Electrochemical degradation
- Larger powder‑electrolyte contact area accelerates side‑reactions. More solid‑electrolyte interphase (SEI) forms, increases irreversible capacity.
- Damaged carbon coating raises charge‑transfer impedance; high‑rate performance deteriorates.
- Surface lattice defects aggravate self‑discharge and gas generation inside cells.
- Excessively high BET makes slurry absorb more solvent; viscosity hard to control, deteriorating coating process.
- Metal impurities from accelerated liner wear cause internal micro‑short‑circuit risk.
4. Influence of classification effect
Poor sharp‑cut classification leads to wide PSD distribution or excessive ultrafine tail.
- Too many ultrafine particles: similar consequences as over‑grinding: high BET, more side‑reactions.
- Residual coarse agglomerates: same as insufficient deagglomeration, high polarization.
- Good turbo classification realizes narrow reasonable PSD, matches target BET and tap density for optimal electrochemical output.
5. Link between grinding‑process parameters and electrochemical indicators
| Grinding condition | Powder feature | Typical electrochemical performance |
|---|---|---|
| Under‑grinding | Residual agglomerates, low tap density | Low capacity utilization, poor rate capability |
| Optimal deagglomeration | Intact primary particles, target BET & PSD | High capacity, stable cycling, good rate performance |
| Over‑grinding | Particle fracture, high BET, partial carbon loss | High irreversible capacity, poor cycle, higher self‑discharge |
6. Practical process guidance for LFP dry‑grinding
- Core principle: deagglomerate soft agglomerates, avoid crushing primary LFP particles.
- Do not blindly pursue finer D97 by raising grinding energy. Finer powder does not equal better electrochemistry.
- Multi‑indicator joint control: PSD, BET, tap density, SEM morphology, metal impurity content. Do not rely only on particle‑size data.
- When electrochemical performance deteriorates after milling: check whether over‑grinding, carbon‑coating damage or metal contamination has occurred.
Grinding does not change the intrinsic theoretical capacity of LFP, but strongly affects practical electrochemical performance via powder physical characteristics. Insufficient deagglomeration causes poor dispersion and high polarization; over‑grinding brings surface defects, carbon‑coating damage and more electrolyte side‑reactions. The goal of dry processing is to achieve moderate deagglomeration with high‑precision closed‑loop classification to obtain powder matching battery‑grade requirements.
