Research On The Attenuation Mechanism And Repair Technology Of Retired Lithium Iron Phosphate Battery Positive Electrode Material

As the increasing of electric vehicles,the number of scrapped Li Fe PO₄（LFP）batteries in electric vehicles is also increasing sharply year by year.Recycling the rapidly growing waste LFP batteries to ease people’s concerns about resource shortages（especially lithium）has become an urgent issue.In this work,the degradation mechanism of LFP cathode materials is investigated,and the direct regeneration of retired LFP cathode materials via the high-temperature solid-phase method is proposed.Furthermore,activated carbon nanotubes（CNTs）are introduced in the regeneration to enhance the electrochemical performance of the regenerated LFP cathode material.The degradation mechanism of waste LFP batteries is studied systematically,and the irreversible transformation of LFP cathode materials is considered to be the main reason.After long-term cycling,the Fe²⁺species in LFP cathode materials would be oxidized to Fe³⁺because of the formed Li loss,which leads to phase transition and structural collapse of part Li Fe PO₄.The transformation of chemical valence states and the destroyed structure of LFP greatly degrade the electrochemical performance of LFP.Therefore,the lithium supplement is crucial for regenerating waste LFP cathode materials.Meanwhile,the grain sizes,valence states and other microstructure of LFP are repaired via high temperature calcination with the addition of reductant.The regeneration conditions are obtained by adjusting temperature,Li⁺concentration and reductant dosage.It has been affirmed that compared with the waste LFP,not only the content of elements of regenerated LFP（R-LFP）has been restored to the raw one,but the crystal structure and Li⁺transport path are repaired.The discharge specific capacity of R-LFP at 0.05C is 148.55 m Ah·g^-1,which is about 96.36%of the new LFP that has not been used（N-LFP）.It indicates an excellent capacity recovery of R-LFP.In addition,the recovery rate from spent LFP cathode materials can be calculated to be 85.52%.Furthermore,the activated carbon nanotubes（CNTs）are doped in the regeneration process to improve the electrochemical performance of regenerated LFP.The three-dimensional conductive network composed of CNTs and LFP significantly reduces the electrode polarization and charge transfer impedance of LFP materials,and further increases the diffusion coefficient of Li⁺.The regenerated LFP（CNTs-LFP）shows excellent specific capacity recovery and cycle stability.The discharge specific capacity of CNTs-LFP at 0.05 C is 155.47 m Ah·g^-1,which is about 99.56%of N-LFP.At a rate of 1 C,the initial specific capacity of CNTs-LFP is 131.83m Ah·g^-1,and the capacity retention after 800 cycles is 70.84%,which is higher than that of the new LFP（about 68.50%）.This method is low in cost,simple in process,mild in conditions and easy to scale up,providing technical support for the sustainable development of the lithium-ion battery industry.