Full Process Flow and Mass Production Standard SOP for High-Compaction-Density LFP via Two-Step Calcination

The two-step calcination process (two-step solid-state sintering) is currently the mainstream mass production process for high-compaction-density LFP used in high-end power batteries and energy storage. Leading domestic lithium battery material companies adopt this process for their fourth-generation and above LFP products. The core process logic of the two-step calcination is that the first calcination completes the synthesis of the main crystal phase and preliminary carbon coating, while the second calcination achieves grain densification, lattice defect repair, and the formation of a continuous carbon film. Compared with the single-step sintering process, the two-step calcination product exhibits significantly improved crystallinity, compaction density, and cycling stability, making it suitable for high-end energy storage and power lithium battery applications. This article comprehensively reviews the eight core process steps of mass production and specifies the standard control parameters for each step, for High-Compaction-Density LFP via Two-Step Calcination.

I. Two-Step Calcination Process Flow

Raw material precise batching → Wet sand milling and dispersion → Spray granulation drying → First pre-sintering (first calcination) → Crushing and classification of first-calcined material → Secondary modification mixing → Second calcination (second calcination) → Finished product post-processing (fine grinding, magnetic separation, sieving, homogenization, packaging)

II. Detailed Process Flow and Standard Parameter Control for Each Stage

(A) Raw Material Precise Batching

1. Mainstream raw material system for mass production

   • Iron source: Battery-grade iron phosphate dihydrate / anhydrous iron phosphate, purity ≥99.5%, Fe³⁺ proportion >99.9%, D50=2–5 μm, no heavy metal exceedances, no agglomerates or impurities.

   • Lithium source: High-purity battery-grade lithium carbonate, particle size D50=3–8 μm, moisture ≤0.1%.

   • Carbon source: Composite carbon source system. Main carbon sources are sucrose, glucose, PEG; auxiliary conductive carbon sources are carbon nanotubes (CNTs) and graphene. Total carbon addition is controlled at 4–6 wt%.

   • Dopants: Dedicated doping ions for high compaction (Ti⁴⁺, Zr⁴⁺, Nb⁵⁺, Mg²⁺), used to refine grains, repair lattice defects, and enhance ionic conductivity.

2. Standard batching parameters

   • Li/Fe molar ratio: 1.02–1.05:1. Slight lithium excess to compensate for lithium volatilization during sintering, preventing capacity fading due to lithium deficiency.

   • P/Fe molar ratio: Strictly 1:1 to ensure crystal phase purity.

   • Total molar proportion of metal dopant ions: 0.3%–1%. Excessive doping is prohibited to avoid impurity phase formation.

   • Batching accuracy: Weighing error for a single raw material ±0.1%; total error of the entire batching system <0.3%. Fully enclosed batching throughout to prevent dust and impurities.

(B) Wet Sand Milling and Dispersion

All raw materials + deionized water are fed into a temperature-controlled stirred tank for pre-mixing → multi-stage horizontal sand mill for circulation grinding → high-intensity magnetic impurity removal and filtration → sealed slurry storage tank for stabilization.Mass production standard parameters

• Slurry solid content: 75–85 wt%, balancing grinding efficiency and spray granulation sphericity.

• Grinding media: 0.3 mm + 0.8 mm high-purity zirconia beads, graded combination with extremely low wear rate to avoid introducing impurities.

• Grinding endpoint particle size: Slurry D50=0.35–0.45 μm, D99<1.2 μm, nanoparticles uniformly dispersed.

• Grinding time: 4–6 h, slurry temperature maintained ≤45 °C throughout to prevent premature thermal decomposition of the organic carbon source.

• Impurity control: Magnetic impurities in slurry ≤5 ppb, multi-gradient demagnetization to eliminate metal foreign matter.

(C) Spray Drying Granulation

A centrifugal spray drying tower is used to prepare spherical agglomerated precursor, a critical upstream step for high-compaction LFP.Standard process parameters

• Temperature control: Inlet air temperature 210–230 °C, outlet air temperature 85–100 °C, temperature fluctuation ≤±2 °C, to avoid hollow spheres, broken spheres, and agglomerated spheres.

• Equipment parameters: Atomizer disc rotation speed 12000–16000 rpm, feed flow rate fluctuation <±2%, ensuring granulation uniformity.

• Precursor product indicators: Moisture content ≤0.4 wt%, particle D50=8–15 μm, sphericity >0.85, apparent density 0.7–0.9 g/cm³.

• Environmental control: Oxygen content in exhaust gas inside the tower <1%, to prevent premature oxidation of the precursor.

(D) First Calcination (First Firing / Pre-sintering, Main Crystal Phase Synthesis)

Purpose: Complete the synthesis of the olivine LiFePO₄ main crystal phase, form a preliminary carbon coating layer, generate primary grains, and retain a slightly agglomerated structure to facilitate subsequent crushing and regrading, without pursuing ultimate crystallinity.Main reaction equation: 2FePO₄ + Li₂CO₃ + C → 2LiFePO₄ + CO₂↑ + CO↑Three-stage heating sintering parameters

• Preheating/dehydration stage: Room temperature → 350 °C, heating rate 3 °C/min, hold at constant temperature for 2 h, to remove residual moisture, decompose lithium carbonate, and pre-carbonize the carbon source.

• Low-temperature transition stage: 350 °C → 620 °C, heating rate 2 °C/min, hold for 1 h, initiate solid-state reaction, lithium ions intercalate into the lattice.

• Main sintering holding stage: 620–650 °C constant temperature, hold for 4–5 h, complete main phase synthesis, crystallinity controlled at 75%–85%.Atmosphere and cooling control

• Protective gas: 99.999% high-purity nitrogen, furnace pressure 50–100 Pa.

• Strict oxygen content control: Oxygen content in the furnace ≤50 ppm, to prevent oxidation of Fe²⁺ to Fe³⁺ which creates impurity phases.

• Nitrogen flow rate: 8–12 L/min·m³.

• Cooling regime: Uniform cooling, cooling rate ≤3 °C/min, discharge allowed only below 200 °C.Indicators of the first-calcined material: Slightly agglomerated primary LFP, D50=15–30 μm, tap density 1.2–1.35 g/cm³, residual unreacted raw materials <0.5%.

(E) Jet Milling and Classification of First-Calcined Material

Mechanical crushing of first-calcined blocks → high-purity nitrogen jet milling → turbo precision classification → sealed storage of bimodal powder.Mass production standard parameters

• Milling gas source: High-purity nitrogen, milling pressure 0.4–0.7 MPa.

• Classification parameters: Classifier wheel speed 2200–3000 rpm.

• Product particle size: D50=1.2–2.5 μm, standard bimodal distribution (large particles 1.8–2.5 μm, small particles 0.6–1.0 μm).

• Particle size uniformity: Span <1.2, no oversized particles >5 μm.

• Material indicators: Moisture content ≤0.3%, secondary in-line magnetic separation to further reduce magnetic impurities.

(F) Secondary Modification Mixing

A small amount of liquid carbon source is supplemented to build a uniform carbon precursor layer on the surface of the first-calcined particles, laying the foundation for the formation of a continuous, dense conductive carbon film during the second calcination, thereby improving the overall electronic conductivity of the material.Standard process parameters

• Equipment: High-speed three-dimensional mixer, using a combined process of dry mixing + trace wet spraying of carbon source.

• Carbon source supplementary amount: 0.5%–1.2 wt%. The final total carbon content of the material is locked at 2.5%–3.5 wt% (balancing conductivity and compaction density).

• Mixing time: 1.5–2.5 h, mixing uniformity RSD<3%.

• Post-treatment: After mixing, drying at 120 °C to constant weight, moisture content ≤0.3%.

(G) Second Calcination

The second calcination has no large-scale solid-state reaction; its core roles are atomic migration in the lattice, defect repair, grain regularization and densification, and the graphitization and formation of the carbon layer. It directly determines the material's cycling life, rate capability, and compaction density.Four-stage precise heating sintering parameters

• Solvent removal stage: Room temperature → 300 °C, heating rate 2.5 °C/min, hold 1.5 h, remove residual solvent and trace moisture.

• Carbon source carbonization stage: 300 °C → 550 °C, heating rate 2 °C/min, hold 2 h, organic carbon is completely converted into a solid conductive carbon layer.

• Lattice repair stage: 550 °C → 780 °C, heating rate 1.5 °C/min, hold 1 h, repair lattice defects from the first calcination and regularize grain structure.

• Final sintering constant temperature stage: 760–800 °C (industry mainstream constant temperature 780 °C), hold 6–8 h, grains grow uniformly to 120–180 nm, and the carbon layer becomes dense and continuous.Strict atmosphere and temperature control standards

• Protective atmosphere: High-purity nitrogen; for high-end products, a 1%–3% H₂/N₂ reducing gas mixture can be used to enhance oxidation prevention.

• Oxygen content control: Oxygen content in the furnace ≤30 ppm, slight positive pressure 80–120 Pa.

• Over-temperature control: Local temperature strictly forbidden to exceed 830 °C, to prevent grain coarsening and carbon layer burn-off leading to weight loss.

• Staged cooling: Cooling rate ≤1.5 °C/min above 500 °C, ≤3 °C/min below 500 °C, discharge below 200 °C.Key indicators of the second-calcined semi-finished product: Crystallinity ≥95%, lattice defect ratio <3%, pure olivine phase without impurity phases; grain size 150±20 nm; carbon coating layer thickness 20–50 nm, continuous without breakpoints; tap density ≥1.55 g/cm³.

(H) Finished Product Post-Processing (Finalization of Batch Consistency)

• Fine grinding and classification: Secondary jet milling to break up soft agglomerates, final product D50=1.5–3.0 μm, particle size span <1.1.

• Multi-stage magnetic separation: Gradient high-intensity magnetic separation to remove impurities, finished product magnetic foreign matter ≤3 ppb, meeting the highest standards for power batteries.

• Sieving: 325 mesh (45 μm) precision sieving to completely remove oversized particles.

• Batch homogenization: Large-capacity sealed silo mixing and homogenization to eliminate batch performance variation.

• Inspection and packaging: After all indicators pass inspection, vacuum moisture-proof packaging for warehousing.

  
  

III. Ultimate Mass Production Standards for LFP via Two-Step Calcination Process

• Compaction density: 2.60–2.70 g/cm³, high-end fourth-generation products stably ≥2.65 g/cm³.

• Capacity performance: 0.1C first discharge specific capacity 155–160 mAh/g, 1C rate capacity ≥145 mAh/g.

• Carbon content: 2.8%–3.4 wt%.

• Purity indicators: Fe²⁺ oxidation ratio <0.1%, no impurity phases.

• Cycling performance: Capacity retention ≥90% after 1000 cycles at 1C at room temperature.

  
  

IV. Core Process Advantages and Key Control Points

1. Process advantagesCompared to the single-step calcination process, compaction density increases by 5%–8%, cycle life increases by more than 10%, and grain uniformity, conductivity, and stability are greatly optimized, making it suitable for high-end power battery and large-scale energy storage applications. The only drawback is a production cycle of 18–24 h and approximately 30% higher overall energy consumption.

2. Critical production line control taboos

   • The first calcination temperature must not be too high to avoid premature grain growth that prevents subsequent densification.

   • The crushing step must ensure a bimodal particle size distribution, which is crucial for high compaction density.

   • Strict oxygen content control throughout both calcinations is mandatory; oxygen exceedance directly causes abnormal iron valence states and product scrap.

   • The constant temperature in the second calcination must not exceed 800 °C to prevent carbon layer damage and excessive grain coarsening.

3. Industry-standard equipment configurationSpray drying tower + pusher kiln (first calcination) + nitrogen jet mill + high-speed three-dimensional mixer + rotary kiln (second calcination) + a complete set of precision post-processing separation equipment.