A Comprehensive Guide to the Control of Metal Magnetic Impurities in Lithium Iron Phosphate Factories:

From risk management to system implementation

In today's rapidly evolving new energy battery industry, the quality of Lithium Iron Phosphate (LFP), a core cathode material, directly determines the safety and service life of batteries. Metal magnetic impurities are precisely the "invisible killers" threatening LFP quality—they can not only cause cell short circuits and failures but may also trigger severe safety accidents such as electric vehicle fires.

According to data from China's National Emergency Management Department, the annual incidence rate of electric vehicle fire accidents nationwide is approximately 0.03%, higher than that of traditional fuel vehicles, with metal contaminants being a significant contributing factor.

Invisible fatal risks

1. How alarming is the hazard posed by foreign objects?

Metal impurities cause chain-reaction damage within batteries: during charging, active metals lose electrons to form ions. These ions migrate through the separator, combine with electrons, and deposit as dendrites, ultimately piercing the separator and causing cell micro-short circuits. The order of hazard severity is: Cu > Zn > Fe. Even more concerning is the extremely potent contamination potential of minute impurities. Data shows that at a specification of 30 magnetic particles per kilogram (Pcs/Kg), grinding a single 5mm diameter steel ball and dispersing it uniformly can contaminate up to 240 tons of material! In terms of cost impact, losses from failures induced by impurities escalate exponentially: 1 ton of defective powder results in a loss of 40,000 - 50,000 RMB, while losses at the cell PACK stage can soar to tens of millions of RMB.

2. Core control targets: Copper, zinc & magnetic metal particles

Control standards for copper, zinc, and metal particles are clearly defined for different raw materials, with requirements progressing in layers from short-term to long-term:

Short-term: Materials such as Lithium Carbonate, Glucose/Sucrose, and PEG require selective use. Copper color development test spots must be zero. Metal particle limits are: Glucose/Sucrose & PEG ≤ 100 Pcs/Kg; Lithium Carbonate ≤ 150 Pcs/Kg.

Long-term: Copper particles ≤ 8 Pcs/Kg (particles <5μm are not counted; particles 5~25μm are counted as 0.5; particles 25~50μm are counted as 1; particles >50μm are not permitted). Metal particle limits are tightened to: Glucose/Sucrose & PEG ≤ 50 Pcs/Kg; Lithium Carbonate ≤ 100 Pcs/Kg.

End-to-end control system: Comprehensive protection from source to terminal

The core philosophy of metal impurity control is "Prevention First, Source Elimination >> Interception & Rework", forming a closed-loop management system that covers all scenarios including factory design, production operations, and personnel management.

1. Foundational Infrastructure: Establishing the First Line of Defense through Plant Design & Utility Systems

The workshop buildings and utility systems are the starting point of control, requiring risk avoidance from the design stage.

Plant Design: Personnel and material passages are separate and dedicated, both equipped with air showers. Personnel passages are additionally outfitted with changing rooms and metal detection doors. Workshops are designed with a slightly positive pressure environment, minimizing the number of doors and windows. Office and production areas are independent. Infrastructure prioritizes the use of non-ferrous materials such as aluminum alloys and non-metallics.

Utility Systems: Water, electricity, and gas systems are housed in independent rooms. Control cabinets are physically isolated from production equipment. Water and gas supply lines utilize multi-stage filtration, with the final filter being made of non-metallic materials. Pipe connections employ clamp-type or cold-press methods to avoid wear and the generation of foreign particles.

2. Equipment management: Full lifecycle control from procurement to maintenance

Equipment is a significant source of foreign object introduction, making it essential to achieve the "Zero Copper/Zinc" control objective.

Procurement Requirements: Incorporate the "Equipment Cleanliness Technical Specification" into purchase contracts, explicitly stating the "Zero Copper/Zinc" requirement for equipment and components. Suppliers must provide material composition lists down to the smallest unit. Non-compliant items are to be rejected, with the right to recourse reserved.

Installation & Acceptance: Execute according to the process: "Equipment Specification Output → Procurement & Manufacturing → Supervision & Inspection → Incoming Acceptance → Installation → Cleanliness Acceptance." Deliver core documents such as the "Metal Foreign Object Map & FMEA" and "Foreign Object Control Plan & Checklist."

Maintenance Standards: Prior to maintenance, define the work level and required protective measures. Tools must be free of copper/zinc components and should be laser-marked for identification. Post-maintenance, clean the area using wet wipes in a unidirectional motion; blowing air is prohibited. Maintenance tasks involving copper require copper color development testing for acceptance. All tools must be demagnetized, and their completeness verified.

3. Material control: Full-chain screening from raw materials to finished products

Every stage of material flow must be strictly controlled to prevent the introduction of foreign objects.

Raw Material Monitoring: Suppliers of precursors, lithium salts, additives, etc., are required to establish a metal foreign object FMEA. Foreign object management must be a key item in the annual audit. Incoming materials must pass inspections such as copper color development tests and Jomesa testing. Auxiliary materials like packaging bags, sieves, and saggars also require vacuum cleaning and demagnetization protection before shipment.

Production Process Control: Implement checkpoint testing at key process stages including raw materials, slurry mixing, crushing, sieving & demagnetization, and finished products. Detection methods such as JMS, ICP, and electron microscopy are employed. DOE experiments are conducted to optimize sieving & demagnetization process parameters, ensuring demagnetization efficiency. Weekly monitoring of magnetic particle levels in sieve residues and high-magnetic materials is performed, coupled with reverse traceability.

Warehouse Management: Copper/zinc components are prohibited in warehouse infrastructure and tools. Ferrous (Fe-based) materials require protective measures. Logistics doors are equipped with buffer rooms and air showers. Windows are sealed. Regular demagnetization and vacuuming are performed. Materials are stored in closed/sealed containers throughout to avoid direct exposure to the environment.

4. Personnel and area management: Standardizing behaviors and zoning by risk level

Personnel and work areas are control points that are easily overlooked and require the establishment of standardized management procedures.

Personnel Management: Personnel in critical positions (e.g., feeding, maintenance, testing) must undergo training and assessment before authorization, with positive incentives implemented. Personnel entering or exiting production workshops must comply with the "'Zero Metal' attire" regulation: changing into designated work clothes, wearing non-metallic belts and shoes. Any tools brought in must be registered in a log, with quantities verified upon both entry and exit.

Area Zoning: Areas are classified into S, A, and B zones based on risk level. S-zones (e.g., feeding areas, packaging rooms—where materials are exposed) are subject to the strictest controls: personnel require demagnetization procedures, equipment must be sealed, and materials must be protected. Movement between different zones must adhere to corresponding protection requirements to avoid cross-contamination.

5. Environmental control: Continuous monitoring and dynamic optimization

Environmental cleanliness directly impacts material quality, necessitating the establishment of a routine monitoring mechanism.

Environmental Demagnetization: Workshop floors are cleaned per shift using demagnetization carts with a magnetic field strength ≥8000 Gauss. Equipment surfaces and corners undergo spot demagnetization using magnetic bars. Collected foreign particles are subjected to regular JOMESA testing for analysis and root cause tracing.

Dust Fall Monitoring: Monitoring covers the entire workshop and key process checkpoints, conducted monthly using methods such as color development agents and ICP analysis. Graded standards and an abnormal response plan are established to promptly investigate potential contamination sources like personnel, tools, and airflow.

6. Logistics and transportation control: Building a mobile clean barrier

Vehicle/Container Control: Use dedicated, cleaned vehicles. Before loading, perform thorough cleaning and demagnetization (residual magnetic field <1mT), ensuring interior walls are smooth and free from rust/corrosion.

Packaging Protection: Packaging must maintain complete seal throughout transit (e.g., double-sealed or lead-sealed). Use wear-resistant inner linings. Secure cargo within the vehicle to prevent friction and movement.

Loading/Unloading Specifications: Maintain cleanliness in loading/unloading areas. Personnel must wear non-metallic protective clothing. Tools (e.g., forklift tines) must be covered with non-metallic sleeves. Dragging or throwing of materials/packages is strictly prohibited.

In-Transit Monitoring: Plan clean transportation routes. Apply tamper-evident seals. Record key information (seal numbers, GPS tracks, etc.). Establish contingency plans for emergencies like vehicle changeovers en route.

Receiving Verification: Upon arrival, first verify seal integrity and inspect the container interior for abnormalities. Unloading may only proceed after passing inspection, achieving closed-loop traceability of transport information.

System support: A triple-layer guarantee through organization, technology, and culture

1. Organizational Support:

Progress is coordinated by a cross-functional management team. A Cross-functional Metal Foreign Object Management Team is established with high-level executive backing. This team reports directly to the General Manager and includes members from departments such as Quality, Production, Equipment, and Process Engineering. It is responsible for core tasks including system planning, supervision and acceptance during equipment fabrication, mechanism investigation, and traceability database development. Their authority and responsibilities are formalized through a company-level appointment letter.

2. Technical support: Building detection and traceability capabilities

Detection Methods: Equip the facility with instruments such as JOMESA, XRF (handheld X-ray fluorescence analyzers), ICP (Inductively Coupled Plasma spectrometers), and Scanning Electron Microscopes (SEM) to achieve precise detection of the quantity, composition, and size of metal particles. Rapid detection tools like copper chromogenic agents and magnetic rods are widely used for on-site acceptance checks.

Traceability Management: Establish a traceability database (recording the morphology, size, and composition of foreign objects through failure simulation) and a reverse traceability mechanism (comparing foreign objects found in finished products and sieve residues against the database). This system is enhanced by integrating AI vision and big data analytics to achieve automatic foreign object recognition with an accuracy rate exceeding 95%.

3. Cultural development:

Fostering full participation and continuous improvement training and assessment:

Develop training materials such as "Metal Foreign Objects: Essential Knowledge and Practices" and "Key Position Training Courseware." Establish an electronic question bank and implement pre-job and quarterly assessments. Third-party contractors and construction personnel are also required to undergo training and obtain certification.

Continuous Optimization: Set clear goals such as "Zero Copper/Zinc" and "Zero Large Metal Particles." Form dedicated task force teams. Drive the continuous upgrading of the management system through methods like layered process audits, quality reward/penalty systems, and the establishment of a lessons-learned database.

Effective metal foreign object control is a core corporate competitiveness

There are no shortcuts in the control of metal magnetic impurities within Lithium Iron Phosphate (LFP) factories. It must start at the design phase, run through the entire production process, and be implemented at every workstation and each procedure. This is not only an essential requirement for ensuring battery safety and reducing cost losses but also constitutes the core competitiveness for a company to establish itself in the new energy industry. Only by adhering to a mindset of "walking on thin ice"—treating the matter with utmost caution and reverence—and by consistently practicing the management philosophy of "prevention first, source control, whole-process monitoring, and continuous improvement," can a company achieve truly transparent, source-to-end process control of metal impurities, thereby solidifying the foundation for the safe development of the new energy battery industry.