Research On The Preparation Of Battery-grade LiFePO₄ By Recycling Waste Lithium Iron Phosphate Battery

Aiming at the problems of low value and high pollution in the existing lithium iron phosphate battery recycling process,this paper innovatively proposed a reduction acid leaching-precipitation-solid phase regeneration method to recover waste lithium iron phosphate batteries.Compared with traditional oxidative acid leaching,this method reduced iron to ferrous iron in the leaching stage,and then prepared ferrous phosphate precursor by precipitation method and mixed it with lithium phosphate to sinter and regenerate Li Fe PO₄ cathode material.This method avoided the problems of incomplete reduction of ferric iron and low purity of the regenerated lithium iron phosphate during the sintering process,and simultaneously reduces the sintering cost and improves the economic benefit.Firstly,the H₃PO₄+C₆H₈O₆ reduction was used to acid to leach waste lithium iron phosphate cathode powder.According to the possible phases and the effective equilibrium reaction in the leaching system,the E-p H diagram of the Li-Fe-P-H₂O system was drawn,and the stable region of each phase was determined,which provided a theoretical basis for the experiment.The effects of phosphoric acid concentration,leaching time,leaching temperature and solid-liquid ratio on the leaching rate of Li⁺and Fe²⁺were systematically studied.The optimal leaching conditions were determined as follows:C₆H₈O₆ solution with a volume ratio of 10%（1mol/L）as reducing agent,2 mol/L phosphoric acid concentration,leaching time of 120 min,leaching temperature of 70 ^oC,and solid-liquid ratio of70 g/L.Under this condition,the leaching rates of Li⁺and Fe²⁺were 98.15%and 98.10%,respectively.Kinetic analysis found that the chemical reaction model equation 1-（1-η）^1/3=kt,as the reaction rate model of reducing acid leaching could well describe the leaching kinetics of the valuable metals Li⁺and Fe²⁺,so the leaching process was controlled by interface chemical reaction.Secondly,the p H value of the leaching solution was controlled by ammonia water,and the Fe₃（PO₄）₂·8H₂O precursor aggregated with primary flakes was precipitated.The results showed that when the p H=6.5,Fe₃（PO₄）₂·8H₂O had the best performance in terms of phase structure and microscopic morphology.At this time,the iron precipitation rate was reached 98.2%,and the iron recovery rate in whole leaching-precipitation was 96.3%.The precipitation was mixed with lithium phosphate to regenerated the Li Fe PO₄ cathode material by sintering.Under the optimized condition,the particle distribution of the material is the most uniform.The first discharge specific capacity was 141.0 m Ah/g at 0.1C,and the first discharge specific capacity was 108.7 m Ah/g at 1C,with the capacity retention rate of 96.9%after 200 cycles,showing relatively excellent electrochemical performance.