1. Core Definition: AQL Classification for Graphite Impurities
AQL (Acceptance Quality Limit) defines the maximum allowable number of defective samples per inspection batch, paired with defect severity grading (Critical / Major / Minor) and ppm impurity threshold limits from GB/T 24533-2019, GB/T 38887-2020 and IEC TS 62607-4-7 for lithium-ion spherical graphite anodes.
For battery graphite, impurity AQL has two layered systems:
- Impurity concentration limit (ppm grade standard) – fixed maximum impurity content by graphite product tier
- Statistical sampling AQL value (inspection pass/fail rule) – lot acceptance criteria for QC batch testing
2. Defect Severity Ranking for Graphite Impurities (Basis for AQL Setting)
| Defect Class | Risk Description | Covered Impurity Types | AQL General Setting Rule |
|---|---|---|---|
| Critical Defect (CR) | Triggers cell short circuit, thermal runaway, fire; violates battery safety specs | Total magnetic impurities (Fe+Ni+Cr+Co+Zn), oversized magnetic metal particles ≥10 μm, heavy metal toxic impurities (Pb, Cd, Hg) | AQL 0.01 (Zero tolerance; 0 defective samples allowed per batch) |
| Major Defect (MAJ) | Severe capacity decay, low first-cycle efficiency, obvious self-discharge; fails customer electrochemical standards | Single high Fe, Cu, Si, Al, high ash, excessive moisture, incomplete carbon coating metal residues | AQL 0.4 ~ 1.0 |
| Minor Defect (MIN) | Slight performance deviation, no safety risk; minor off-spec trace non-hazardous elements | Low-level Na, K, Ca, minor surface organic residues, trace sulfur | AQL 2.5 ~ 4.0 |
3. Tiered Impurity Concentration Limits (Matching AQL Zero/Critical Control)
Based on national standard GB/T 24533-2019, spherical graphite is split into Grade I (EV power battery), Grade II (energy storage), Grade III (consumer electronics) with strict magnetic impurity caps (core critical AQL control index):
3.1 Total Magnetic Impurities (Critical Defect, AQL 0.01 Zero Tolerance)
Total magnetic metals = Fe + Cr + Ni + Co + Zn (extracted via 6000–7000 Gs magnetic rod, tested by ICP-OES/MS)
| Graphite Grade | Application Scenario | Max Total Magnetic Impurity (ppm) | AQL Control Rule |
|---|---|---|---|
| Grade I (Premium EV) | New energy vehicle power cells, high-end coated spherical graphite | ≤0.1 ppm | AQL 0.01; any batch over limit = full batch rejection, rework magnetic removal |
| Grade II (Energy Storage) | Stationary energy storage, mid-tier consumer batteries | ≤0.5 ppm | AQL 0.4; max 1 defective sub-lot per full batch |
| Grade III (Consumer) | Digital 3C, low-power small lithium cells | ≤1.0 ppm | AQL 1.0; limited secondary release after double re-inspection |
3. Single Magnetic Metal Element Thresholds (Major Defect AQL 0.4–1.0)
| Element | Grade I Limit (ppm) | Grade II Limit (ppm) | Grade III Limit (ppm) | Defect Class | AQL |
|---|---|---|---|---|---|
| Fe (Iron) | ≤50 | ≤100 | ≤200 | Major | 0.4 |
| Cr / Ni / Co / Zn | ≤5 | ≤10 | ≤20 | Major | 0.4 |
| Cu, Mo, V | ≤5 | ≤10 | ≤15 | Major | 1.0 |
3. Non-Metallic & Ash Impurity Limits (Minor/Major Defects)
| Impurity Index | Grade I Limit | Defect Class | AQL |
|---|---|---|---|
| Fixed carbon purity | ≥99.95% | Major | 0.4 |
| Total ash content | ≤0.05% | Major | 0.4 |
| Moisture | ≤0.05 wt% | Minor | 2.5 |
| Si / Al silicate gangue | ≤30 ppm | Major | 1.0 |
| S (sulfur) | ≤50 ppm | Minor | 4.0 |
| Residual organic solvent | ≤100 ppm | Minor | 4.0 |
4. Standard Sampling AQL Levels for Graphite Batch Inspection (GB/T 24533-2019 Inspection Rules)
The standard adopts General Inspection Level II (GII) for normal batch testing; reduced level GI for finished warehouse stock, tightened GIII for incoming raw spheroidized graphite before coating.
4.1 Standard AQL Matrix for Factory QC Incoming/Finished Inspection
- Critical Impurity Items (Magnetic total, heavy metals Pb/Cd/Hg)
- AQL = 0.01 (Zero defect acceptance)
- Sampling rule: Double sampling upon first failure; double re-inspection failure = full batch reject
- Major Impurity Items (Single Fe, Cu, ash, carbon purity)
- AQL = 0.4 (Premium Grade I graphite)
- AQL = 1.0 (Grade II/III standard graphite)
- Minor Impurity Items (moisture, sulfur, alkali metals Na/K/Ca)
- AQL = 2.5 (power battery grade)
- AQL = 4.0 (consumer battery grade)
4.2 Special AQL for Online Production Line Spot Check
For hourly sampling from graphite spheroidizer/classifier discharge (online rapid magnetic rod screening):
- Magnetic visible metal particles >5 μm: AQL 0.01 critical trigger, immediate line alarm & material diversion
- Minor trace element drift: AQL 4.0, continue production with enhanced lab re-test frequency
5. International Industry Custom AQL for Top Battery OEMs
EV battery manufacturers (CATL, BYD, BTR) adopt stricter custom AQL than national standards for Grade I spherical graphite:
- Total magnetic impurities AQL 0.006 (target ≤0.05 ppm, far below GB 0.1 ppm cap)
- Single Fe impurity AQL 0.2, limit ≤20 ppm
- Any magnetic particle ≥10 μm observed under SEM: 100% batch rejection (AQL zero)
6. AQL Implementation Workflow for Graphite Impurity Testing
- Lot division: Each 5–10 ton graphite production batch as one inspection lot
- Sampling: GII general level, extract 5–10 representative sub-samples via rotary splitter
- Hierarchical testing:
- Rapid magnetic rod screening (critical AQL 0.01 first-pass filter)
- ICP-OES full elemental quantification for major/minor impurity AQL judgment
- SEM-EDS particle counting for oversized magnetic foreign body critical defect verification
- Disposition:
- Critical defect out of spec: 100% re-magnetic separation or scrap
- Major defect over AQL limit: Double re-sampling; second failure = downgrade to lower graphite grade
- Minor defect slight over limit: Conditional release with customer notification
Impurity AQL levels for battery-grade spherical graphite are built around magnetic impurity safety risk grading:
- Critical safety magnetic impurities use AQL 0.01 zero tolerance (Grade I EV graphite ≤0.1 ppm total magnetic metals)
- Major performance metal impurities apply AQL 0.4–1.0 by product tier
- Low-risk minor trace impurities adopt relaxed AQL 2.5–4.0
All AQL rules align with GB/T 24533-2019 and IEC TS 62607-4-7, forming the full-process impurity control standard for graphite milling and spheroidization production lines referenced on graphite-mill.com.
