Pitch (coal‑tar pitch / petroleum pitch) serves as binder for secondary granulation and carbon‑coating precursor for artificial graphite anode. Residual quinoline‑insoluble particles, ash, metallic impurities, moisture, sulfur and oxygen from oxidation will cause coating defects, gas generation, low initial coulombic efficiency, and battery safety risks. Impurity‑removal standards differ for raw crude pitch and final pretreated pitch for jet‑milling / high‑speed mixing. All indexes below apply to pretreated battery‑grade pitch after molten thermal filtration, devolatilization and drying, before jet milling and coating process.
Core Impurity Removal Standard (Finished Pretreated Pitch)
| Parameter | Standard Specification | Explanation |
|---|---|---|
| Quinoline‑Insoluble (QI) | ≤0.3 wt% | Hard QI particles are major solid impurity; removed by two‑stage molten thermal filtration; high QI creates pinhole defects in carbon coating layer |
| Ash content | ≤0.05 wt% | Ash mainly contains Fe, Ca, Na, Si, Al mineral oxides; strict limit to lower magnetic foreign matter |
| Moisture | ≤0.2 wt% | Remove adsorbed moisture via vacuum inert drying; moisture causes foaming during carbonization |
| Total sulfur | ≤0.4 wt% | High sulfur generates large gas volume during carbonization & graphitization, brings cell swelling risk |
| Oxygen content | ≤0.8 wt% | Minimize pre‑oxidation; whole pretreatment under nitrogen protection; excessive oxygen increases irreversible capacity loss |
| Magnetic foreign substances (Fe, Ni, Cr) | ≤25 ppm | From raw pitch ash and equipment wear; controlled by thermal filtration + magnetic separation; avoid separator piercing risk |
| Coking value | ≥45 wt% | Guarantee high carbon yield after carbonization |
| Oversize hard particles | 100 % pass 200‑mesh | No residual coarse impurity grit for jet‑milling feedstock |
Note: Raw crude pitch before purification normally has QI 3‑8 %, ash >0.2 %, moisture >1 %, cannot be directly used for anode production. Molten‑state thermal filtration is mandatory to reach above indexes; dry powder screening cannot remove embedded QI impurities.
Impurity‑Removal Process Corresponding to Each Index
- Quinoline‑Insoluble (QI) & ash removal: Two‑stage molten thermal filtration (100‑150 mesh pre‑filter + 250‑325 mesh ceramic‑metal sintered fine filter) at 170‑210 °C under nitrogen atmosphere. Filter differential‑pressure monitoring prevents impurity breakthrough.
- Moisture removal: Vacuum indirect drying, temperature ≤150 °C, oxygen <5 %, cool below 45 °C under inert atmosphere to prevent moisture re‑absorption.
- Light‑component & partial sulfur removal: Vacuum devolatilization at 220‑260 °C, remove low‑molecular volatile fractions; cannot fully remove bound sulfur, so low‑sulfur raw pitch is preferred.
- Magnetic metal impurity control: Non‑metallic lining for crushing and milling equipment; high‑intensity magnetic separation before jet‑milling feeding.
- Oxygen control: All melting, filtration, cooling, crushing, drying and storage under nitrogen‑blended low‑oxygen atmosphere to suppress pitch pre‑oxidation.
Grade differentiation: Binder pitch vs coating pitch
- Coating‑grade pitch (for surface carbon coating, strictest standard)
QI ≤0.3 %, ash ≤0.05 %, moisture ≤0.2 %, magnetic foreign matter ≤25 ppm. Used for jet‑milled fine pitch powder. - Binder‑grade pitch (for secondary granulation)
QI ≤0.5 %, ash ≤0.08 %, moisture ≤0.3 %. Slightly relaxed compared with coating pitch, still requires molten filtration.
Common non‑compliant risks caused by unqualified impurities
- QI＞0.3 %: Hard particles remain inside carbon coating, form micro‑defects, reduce initial coulomb efficiency.
- Ash＞0.05 %: High trace‑metal content, electrolyte decomposition, lithium dendrite risk.
- Moisture＞0.2 %: Foaming and crack formation in carbon‑coating layer during heating.
- High oxygen content: Extra irreversible lithium consumption, poor cycle retention.
Key test‑method notes
- QI test: Standard quinoline dissolution gravimetric method.
- Moisture test: Karl‑Fischer titration, do not adopt 105 °C constant‑weight method (light pitch fractions volatilize and produce false reading).
- Magnetic foreign matter: Digestion + ICP‑MS for Fe, Ni, Cr quantitative analysis.
