I. Research Background and Significance
Lithium-ion batteries are widely used in power batteries, energy storage systems, and consumer electronics due to their advantages of high energy density, high power, long cycle life, and environmental friendliness. Among various cathode materials, Lithium Iron Phosphate (LiFePO₄ or LFP) has become a mainstream cathode material owing to its high safety, relatively low cost, and good structural stability. However, the processing performance of LFP directly affects electrode sheet quality and battery performance, with the crushing process being a key step in controlling material particle size and distribution. This paper systematically studies the effects of feed speed and grinding pressure on the particle size distribution, slurry characteristics, electrode sheet quality, and electrochemical performance of LFP materials, providing a basis for process optimization in industrial production.
II. Experimental Methods
LFP precursor was prepared using iron phosphate as the iron source via a carbothermal reduction method. The initial material, labeled LFP-0, was obtained after spray drying and high-temperature sintering. A QLM-2 type mulino a getto was used to process LFP-0 under different feed speeds (0.50 kg/h, 0.75 kg/h, 1.00 kg/h, 1.25 kg/h) and grinding pressures (15 m³/h, 18 m³/h, 21 m³/h, 24 m³/h), yielding multiple sample groups. Material physical properties were characterized using a particle size analyzer, scanning electron microscope (SEM), and powder resistivity meter. The samples were then made into electrode sheets and 6 Ah pouch cells for systematic testing of slurry fineness, viscosity, electrode sheet compaction density, and cell capacity and impedance.
III. Analysis of Initial Material Pre-Milling
The unmilled LFP-0 material consisted of spherical particles with a concentrated particle size distribution: D₅₀ was 16.3 μm, and Dmax exceeded 30μm. The corresponding slurry fineness reached 37–39
μm, exceeding the production line’s acceptable standard (≤35μm). The electrode sheet compaction density was only 2.17 g/cm³, lower than the requirements for power and energy storage batteries (≥2.40 g/cm³). Although the 0.1C discharge specific capacity reached 160.8 mAh/g, the poor processing performance indicated the necessity of the milling process.
IV. Impact of Feed Speed on LFP Performance
Keeping the grinding pressure constant at 21 m³/h, the influence of different feed speeds was studied:
1. Particle Size and Morphology: Lower feed speeds resulted in better milling efficiency. LFP-I50 (0.50 kg/h) particles were uniform with Dmax < 10 μm; LFP-I75 (0.75 kg/h) showed slight agglomeration with Dmax < 20 μm; whereas LFP-I100 and LFP-I125, due to excessively high feed speed, had insufficiently broken particles with Dmax close to 50 μm, similar to the initial material.
2. Slurry and Electrode Sheet Performance: As feed speed increased, slurry fineness increased significantly (from 21 μm to 42 μm), solid content slightly increased, and viscosity changed little. Electrode sheet compaction density decreased from 2.46 g/cm³ to 2.40 g/cm³. At high feed speeds (e.g., 1.25 kg/h), agglomerates, bubbles, and spots appeared on the electrode sheets, affecting appearance uniformity.
3. Electrochemical Performance: The 0.1C specific capacity for all samples exceeded 158 mAh/g, with minor differences. However, the electrochemical impedance (Rct) increased with higher feed speeds, indicating that excessively high feed speeds damage the carbon coating layer, increasing interfacial resistance.
V. Impact of Grinding Pressure on LFP Performance
At a constant feed speed of 0.75 kg/h, the influence of different grinding pressures was studied:
1. Particle Size and Morphology: At 15 m³/h pressure, particle breakage was insufficient, with Dmax > 10 μm; when pressure increased to 21 m³/h and above, Dmax decreased to below 20 μm; the vast majority of LFP-V24 (24 m³/h) particles were smaller than 2 μm, with a concentrated size distribution.
2. Processing Performance: At low pressure (15 m³/h), slurry fineness reached 42 μm, and obvious particle protrusions appeared on the electrode sheets; when pressure increased to 21 m³/h, fineness decreased to 33 μm, and electrode sheet appearance improved; at 24 m³/h, the sheets were smooth and defect-free, with compaction density rising to 2.46 g/cm³.
3. Electrochemical Behavior: All samples achieved capacities above 159 mAh/g. However, increasing the grinding pressure exacerbated carbon layer damage, leading to increased powder resistivity and battery Rct.
VI. Comprehensive Optimization and Conclusion
By balancing particle size control, processing performance, and electrochemical performance, the optimal process parameters were determined as: feed speed 0.75 kg/h, grinding pressure 21 m³/h. Under these conditions:
Dmax is controlled within 20 μm
Slurry fineness is ≤35 μm
Electrode sheet compaction density is ≥2.44 g/cm³
0.1C discharge specific capacity is ≥159 mAh/g
While avoiding electrode sheet appearance defects and excessively high impedance.
VII. Practical Application Suggestions
LFP manufacturers need to dynamically adjust the milling process based on particle size distribution, SEM morphology, and battery impedance spectra. This avoids damaging the carbon coating layer through over-milling or compromising processing performance due to insufficient milling. Equipment parameters should be reasonably selected to improve production efficiency while ensuring product processability and electrochemical performance.
VIII. Research Value
This study not only provides specific and feasible parameter windows for the jet milling process of LFP materials but also deepens the understanding of the relationship between particle size distribution and overall battery performance. It holds significant guiding importance for advancing the industrial application of LFP batteries.
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