Study on the influence of Al in⁃situ doping on the regeneration performance during iron phosphate recovery

Currently， extensive research has been carried out on the recycling of spent lithium-ion batteries， with a primary emphasis on the recovery of valuable metal elements such as Li， Ni， Co， and Mn. The recycling of spent lithium iron phosphate batteries places more focus on the efficient extraction and recovery of Li， while Fe and P receive relatively less attention. The residues remaining after Li extraction are often discarded or landfilled， leading to substantial resource waste and environmental pollution. Thus， the recovery of iron phosphate after lithium extraction from spent lithium iron phosphate batteries is of great resource and environmental significance. This paper explores the in⁃situ doping regeneration of iron phosphate following the selective leaching of spent lithium iron phosphate， offering a novel concept for the efficient utilization of Fe and P in spent lithium iron phosphate. First， the selective leaching residue （iron phosphate/carbon） was pretreated via calcination at 600 °C in an O2 atmosphere to eliminate the residual carbon and obtain pure iron phosphate. The directional enrichment of Al and impurity removal were accomplished through cascade acid pickling of the iron phosphate. The study revealed that the impurity content of iron phosphate after two rounds of acid pickling was low， with the Al content reached 0.61%. As the number of pickling times increased continuously， the Al content exhibited a slow downward trendency. Second， the Al-enriched iron phosphate was mixed with a lithium source （lithium carbonate） and a carbon source （glucose）， and then regenerated into an Al in⁃situ doped lithium iron phosphate cathode material through carbothermal reduction. It was discovered that Al in⁃situ doping could effectively enhance the high-rate electrochemical performance of the regenerated lithium iron phosphate cathode material. The results demonstrate that the in⁃situ doping effect of Al in lithium iron phosphate （R-LAFP-2H） regenerated from twice-acid-pickled iron phosphate （R-AFP-2H） is optimal. XPS testing indicated that the Al/Al3+ value in the material is 0.09， suggesting that a large quantity of Al3+ has successfully completed the doping process.Additionally， through SEM and TEM observations， it can be seen that R-LAFP-2H has the most favorable particle morphology and carbon coating layer. This is because the pickling process can facilitate the redistribution of iron phosphate precursor particles； however， excessive pickling will cause the degradation of iron phosphate microparticles， and the broken particles are easily coated with an overly thick carbon layer during the regeneration process. Meanwhile， R-LAFP-2H displayed the best electrochemical performance， with a discharge capacity of up to 109.57 mAh/g at 5 C and a capacity retention rate of up to 95.14% after 800 cycles.