Research On Repair And Modification Of Spent LiFePO₄ Cathode Material Based On Direct Regeneration Strategy

The energy transformation driven by the "Net Zero" goal has accelerated the rapid development of the Li-ion battery(LIB)in the transportation and thermal power industries.As retired batteries enter an "explosive period" in the next ten years,strengthening the recycling of key materials(especially cathode materials)has become an urgent need for the development of resource recycling in the LIB industry.At present,the conventional recycling strategy based on pyrometallurgy/hydrometallurgy has many drawbacks,such as a large amount of energy consumption,input of toxic reagents,generation of harmful substances,loss of valuable materials,and emission of greenhouse gases(GHGs).It is urgent to develop novel strategies for environmentallyfriendly and efficient sustainable resource recycling.In this paper,the retired power LIB cathode material LFP is taken as the research object,and the key crux of its capacity fading is precisely grasped.The direct regeneration strategy based on relithiation is used as the core repair method,and the composite or coating of advanced carbon materials is used as the optimization modification method,with the main goals of maximizing resource utilization and minimizing input/emissions,two novel strategies were designed and proposed,and the structure-activity relationship between the structure,morphology and electrochemical performance of LFP recycled materials was analyzed.The high-value resource utilization of LFP cathode materials provides a feasible solution.The specific research work is as follows:In the early exploration of spent LFPcathode material recycling pretreatment and its performance attenuation,the effects of self-discharge,pure water and salt solutions with different concentrations on the deep discharge efficiency before disassembly of wasteLFPbatteries were studied.Under the conditions,the discharge conditions optimized for time/cost were explored;by studying various solvent dissolution methods and heat treatment methods,the separation effect between theLFPcathode material and the current collector aluminum foil and its effect on the chemical structure of theLFPwaste were investigated;by means of XRD and ICP analysis and determination,preliminary verification of the main cause of the performance degradation of waste LFP.In the process design and research of microwave hydrothermal reduction targeted repair ofLFPlithium loss defects,the advanced microwave technology was used to realize the feasibility of efficient application of "microwave-material" interaction and its characteristic thermal effect in direct regeneration strategy,avoiding particle agglomeration and performance degradation caused by high temperature and high pressure and long-term reaction in the conventional hydrothermal treatment.Compared with spent LFP,the microwave hydrothermal relithiation process regenerated rLFP at 150℃ for 1 h,which exhibited excellent morphology and electrochemical performance of 150.5 mAh g-1 at 0.1 C.Considering the potential value of anode waste graphite,realizing the simultaneous utilization of spent anode and cathode is beneficial to maximize the resource recovery rate,and the ingenious design of microwave hydrothermal cathode relithiation and anode regeneration graphene to reconstruct and regenerate LFP/MWrGO is further carried out.and research.Through the effective combination of the improved Hummer’s method and the microwave rapid reduction and exfoliation method,the appreciation of low-value anode waste graphite to high-performance MWrGO is realized;the surface charge of modified graphene is regulated by PDDA,and the repair process of microwave hydrothermal relithiation ofLFPis coordinated.Realize the "electrostatic self-assembly" of the two and reconstruct the new cathode material LFP/MWrGO.The regenerated rLFP/MWrGO-5%composite(containing 5%grapheneMWrGOdoping)exhibits significantly better electrochemical performance than the undoped rLFP with 161.4 mAh g-1 in a half-cell at 0.2C specific capacity and long cycle life(ie,94.9%capacity retention for 100 cycles at 0.2C).In addition,the mechanism analysis of microwave hydrothermal pre-lithiation and the economic and environmental analysis of the synergistic regeneration strategy are further conducted to demonstrate the applicability and economic and environmental benefits of the innovative strategy.In the ingenious design and research of "ice and fire duo" two-step repair synergistic in-situ nitrogen-doped carbon coating modification and reconstruction of regenerated LFP/3dC-N,especially for the optimization of lithium ion and electron transport channels,through the combination of low-temperature aqueous-solution relithiation method and the salt-template in-situ carbonization method can construct a three-dimensional interconnected porous nitrogen-doped conductive carbon network structure for its framework while directionally repairing the lithium loss defects of the spent LFP.The dual roles of glucose as reducing agent and carbon source in the twostep regeneration process were mainly explored.On the other hand,the effects of coating carbon content,pore structure,and nitrogen doping on the electrochemical properties of recycled materials were investigated by comparing with conventional solid-phase reduction roasting repair.This strategy effectively solves the problems of uneven lithium replenishment,particle agglomeration,and poor carbon coating after solid-phase annealing.Electrochemical test results show that the regenerated LFP@3dC-N cathode exhibits excellent electrochemical performance with proper supplementation of 20%glucose and urea.The new batteries with LFP@3dC-N as the cathode achieve high average reversible lithium storage capacities of 169.74 and 141.79 mAh g-1 at currents of 0.1C and 1C,respectively.After 200 cycles at 1C,a high reversible capacity of 143.78 mAh g-1 is still maintained,with a retention rate of over 95.7%.A fine-level reversible capacity of 107.18 mAh g-1 was obtained when the rate was increased to 5C.In summary,the two direct regeneration strategies designed in this paper show unique advantages in many aspects,such as lithium replenishment repair effect,performance efficiency and revenue optimization,comprehensive utilization of positive and negative electrodes,carbon coating modification,and regulation of ion transport channels.It has broad development prospects in the sustainable development strategy of lithium-ion battery recycling.