How to Detect Magnetic Impurities in Battery Materials

Magnetic impurities (Fe, Ni, Co, Cr, Zn and their metal oxides) are critical hazardous contaminants in spherical graphite, silicon-carbon anodes, lithium iron phosphate, ternary cathode powders and other lithium-ion battery materials. Derived from mill liner abrasion, classifier blade wear, raw ore gangue and production line equipment corrosion, these micro metal particles pierce the SEI film during battery cycling, triggering severe self-discharge, capacity decay, internal short circuits and thermal runaway risks.

Based on graphite processing production standards from graphite-mill.com, combined with IEC TS 62607-4-7 and GB/T 24533-2019 industrial specifications, this article systematically introduces rapid on-site screening, quantitative laboratory testing, microscopic particle characterization and online real-time monitoring four categories of detection methods, covering full-process quality control for spherical graphite production lines.

1. Rapid Magnetic Rod Extraction Screening (On-Site QC First-Line Test)

Principle

Use high-strength PTFE-coated neodymium magnet rods (6000–7000 Gs magnetic field) to physically adsorb all ferromagnetic metal particles mixed in battery powder samples; separate captured magnetic impurities for visual rough judgment or follow-up quantitative analysis. This is the mandatory pre-screening step for graphite spheroidization and classification production lines.

Standard Operation Procedure

1. Sample preparation: Weigh 100–200 g representative spherical graphite powder with PTFE non-magnetic utensils; avoid metal sampling spoons to prevent secondary contamination.
2. Magnetic adsorption: Immerse sealed PTFE magnet rod fully into powder, stir clockwise and counterclockwise continuously for 5–10 min; repeat adsorption 3 times to capture all fine magnetic debris.
3. Impurity collection: Take out the magnet rod, wash adsorbed metal particles into a clean PTFE beaker with ultrapure water, scrape residual metal powder with plastic scraper (no metal tools).
4. Preliminary visual judgment: Observe the quantity of black/grey metal particles under white light; massive visible metal debris indicates serious equipment abrasion on grinding/spheroidizing mills.
5. Follow-up quantification: Dissolve collected magnetic impurities with dilute aqua regia (HCl:HNO₃=3:1) via heating, then send the clear solution to ICP-OES/ICP-MS for precise ppm-level measurement.

Advantages & Limitations

• Pros: Low cost, fast 10-min test, no complex instruments, suitable for hourly online sampling inspection on graphite milling lines.
• Cons: Cannot quantify trace metal content below ppm level; unable to distinguish single metal element concentrations.

2. Quantitative Elemental Analysis (Lab Precision Testing, Standard Reference Method)

This category delivers accurate mass concentration (mg/kg, ppm) of total magnetic metals (Fe, Ni, Co, Cr, Zn), complying with IEC TS 62607-4-7 international standard and Chinese graphite anode material standard GB/T 24533-2019. Two mainstream instrument solutions are widely adopted for spherical graphite inspection.

2.1 ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry)

Working Flow

1. Two pretreatment options:
   • Magnetic extraction digestion: Only digest magnetically adsorbed metal impurities (target magnetic substances only).
   • Full microwave digestion: Complete graphite powder digestion with mixed acid (HF+HNO₃) to release all total metal impurities, then separate magnetic metal data via calculation.
2. Heat digested solution to remove acid, dilute with 2% ultra-pure nitric acid to fixed volume.
3. Inject liquid sample into plasma torch; each metal element emits characteristic spectral lines, and the instrument converts light intensity to element concentration.

Key Parameters

• Detection limit: ≥0.02 ppm for magnetic metals; ideal for routine batch factory lab testing.
• Application: Mass production finished spherical graphite batch release inspection, magnetic removal equipment effect verification after magnetic separator.

2.2 ICP-MS (Inductively Coupled Plasma Mass Spectrometry)

Advantages over ICP-OES

Ultra-low detection limit (ppb level), capable of detecting ultra-trace magnetic metal impurities below 0.01 ppm, mandatory for high-end coated spherical graphite and silicon-carbon anode for automotive power batteries.

Standard Sample Digestion for Graphite

0.1–0.5 g graphite sample + mixed acid (HF+HNO₃+HClO₄) sealed in PTFE microwave tank; staged high-pressure microwave digestion to fully decompose graphite layered crystal structure, completely release embedded micro metal particles inside spherical graphite spheres. After acid removal and constant volume, multi-element quantitative analysis is performed via mass spectrum ion counting.

Comparison of Two Quantitative Instruments

3. Microscopic Visual & Particle Counting Detection (Single Magnetic Particle Characterization)

Quantify the size, quantity, distribution and embedding position of individual magnetic metal particles in spherical graphite powder, used for root cause analysis of abnormal magnetic impurity content.

3.1 Polarized Optical Microscope Counting (DB51/T 3235–2024 Standard)

1. Disperse graphite powder evenly on a clean glass slide, cover with cover slip.
2. Observe under polarized light: metal magnetic particles show bright reflective spots against dark graphite background; graphite matrix appears dark and non-reflective.
3. Count magnetic particles above 5 μm in multiple fixed visual fields, calculate particle number per gram of graphite sample.

• Feature: Fast low-cost offline spot check, directly judge oversized metal debris causing battery safety risks.

3. SEM-EDS Scanning Electron Microscopy with Energy Dispersive Spectroscopy

Core Detection Functions

1. High-magnification imaging to locate magnetic metal inclusions embedded inside cracked spherical graphite particles (generated from mill roller abrasion).
2. EDS point scanning & surface element mapping: Confirm element composition of single bright particles (Fe, Ni, Cr etc.) and visualize metal impurity distribution on graphite sphere surfaces.

Application Scenario

Troubleshooting abnormal high magnetic impurity values: identify whether metal impurities come from raw graphite ore or grinding/spheroidization equipment wear on graphite-mill production lines.

3. XRF X-Ray Fluorescence Spectroscopy

Rapid non-destructive bulk screening: Test total metal element content of graphite powder without acid digestion. Only used for rough pre-screening before precise ICP testing, cannot detect ppb-level trace magnetic impurities.

4. Online Continuous Magnetic Monitoring (Production Line Real-Time Control)

Installed on the discharge outlet of graphite spheroidizer, air classifier and magnetic iron remover for uninterrupted real-time detection without offline sampling.

1. Online magnetic flux sensors: Monitor magnetic particle concentration in flowing graphite powder slurry or dry powder; trigger automatic alarm once magnetic impurity threshold is exceeded.
2. Online laser particle + metal impurity integrated detector: Combined dynamic image analysis to simultaneously detect spherical graphite circularity and mixed magnetic metal particle quantity, automatically adjust mill gap and magnetic separator running parameters.

Production Value for Graphite Milling Lines

Real-time feedback avoids large batches of unqualified spherical graphite caused by sudden equipment liner abrasion, greatly reducing material scrap rate in continuous spheroidization production.

5. Auxiliary Supporting Detection Methods

5.1 SQUID Superconducting Quantum Interference Device Magnetometry

High-precision magnetic susceptibility testing, measure total magnetic susceptibility value of graphite powder, reflect overall ferromagnetic impurity content. Mainly used for laboratory research and high-purity nuclear-grade graphite testing, not for factory routine QC.

5. Electrochemical Half-Cell Verification (Indirect Risk Detection)

High magnetic impurity graphite exhibits obvious self-discharge and low first-cycle efficiency after cell assembly. Conduct charge-discharge cycle test to indirectly verify magnetic impurity hazards, matched with physical detection data to evaluate material electrochemical performance loss.

6. Standard Complete Testing Workflow for Spherical Graphite (Factory QC SOP)

1. On-site rapid screening: PTFE magnetic rod adsorption test for each production tank sample every hour.
2. Batch sampling daily: Send representative finished graphite powder to lab for microwave full digestion + ICP-OES total magnetic metal quantification.
3. Abnormal value troubleshooting: Once total magnetic impurity exceeds 50 ppm standard limit, use SEM-EDS to analyze impurity source and particle morphology.
4. Long-term production line control: Deploy online magnetic sensors at classifier discharge port for 24h continuous monitoring.

Magnetic impurity detection forms a three-level control system for spherical graphite production: rapid magnetic rod on-site screening for real-time line monitoring, ICP-OES/ICP-MS for accurate ppm quantitative certification, SEM-EDS/microscope for particle root cause analysis. Factories equipped with graphite grinding and spheroidization equipment from graphite-mill.com must integrate the above detection methods into full-process QC, timely adjust mill liner maintenance cycle and magnetic separator operating parameters, to strictly control magnetic metal content and guarantee lithium battery safety and cycle performance.