Abstract:Lithium iron phosphate batteries have a large market share in the new energy vehicle industry and the energy storage industry. The low cost of iron resources is benifit to reduce the production cost of lithium iron phosphate and maintains its market share. In this paper, high-value battery-grade iron phosphate products were prepared, using iron-containing solid waste from mines, by-product sulfuric acid and sulfur dioxide gas from smelters as raw materials, through a process route of reductive acid leaching-impurity removal-synthesis of iron phosphate. The effects of various process parameters were investigated, and lithium iron phosphate cathode materials were prepared and tested, using the synthesized iron phosphate sample. The following main conclusions were obtained: in the reductive acid leaching process, when the sulfuric acid concentration was 10g/L, the liquid-to-solid ratio was 4∶1 (L/kg), the temperature was 75℃, the amount of SO2 gas (flow rate 80mL/min) was twice the theoretical amount, and the reaction time was 30min, the iron concentration in the acid leaching solution was 29.3g/L, the utilization rate of sulfur dioxide was 45.72%, and the iron leaching rate reached 81.21%. Lime had a good effect on the removal of Al and Cu. When the pH value was neutralized to 5.0 with lime, the Al concentration decreased from 191mg/L to 3.79mg/L, and the Cu concentration decreased from 8.60mg/L to below the detection limit. Under the conditions of phosphoric acid concentration was 0.03mol/L and aging time was 2 hours, the iron-to-phosphorus ratio of iron phosphate was approximately 0.97, and the physical and chemical indicators met the HG/T 4701—2021 standard for battery-grade iron phosphate. The aging mechanism of amorphous iron phosphate was speculated: under the combined action of phosphoric acid and heating, amorphous iron phosphate gradually dissolved, and at the same time, a large amount of sulfate ions wrapped by iron phosphate were released into the solution, causing the concentrations of iron ions and phosphate ions in the slurry to gradually increase. After reaching supersaturation, small particles of FePO4·2H2O crystals would slowly recrystallize, and the crystallization process would adjust the iron-to-phosphorus ratio towards the theoretical value of 1∶1. The charg and discharg performance of the lithium iron phosphate cathode material was tested. The initial discharge specific capacity of lithium iron phosphate was 160.02mAh/g at 0.1C, and the initial coulomb efficiency was 99.42%. After 200 cycles at 1C rate, the discharge specific capacity reached 147.2mAh/g, and the capacity retention rate was 99.73%, indicating excellent cycling performance.
