Preparation And Performance Of LiFePO₄ Composite And Conductive Binder Of Electrode

LiFePO₄ has been regarded as the most promising cathode material for lithium ion batteries because of low cost,environmental friendliness,good safety,high theoretical capacity and excellent cycling lifespan.However,LiFePO₄ has also suffered from low electronic and ionic conduction,which limits rate performance and energy density.In this work,we focus on LiFePO₄ material from synthesis of composite to fabrication of electrode.The goal is to enhance rate performance of LiFePO₄ without sacrificing tap density.A rheological phase-carbothermal method combined with high energy ball milling technology has been implemented to synthesize mesoporous carbon/LiFePO₄ nanocomposite?S-LFP@C?with size distribution of 95 nm.S-LFP@C nanoparticles are coated by 3⁵ nm porous carbon layer.Specific surface area of S-LFP@C nanocomposite is up to 48.0 m2·g^-1.S-LFP@C electrode exhibits excellent rate performance and cycling ability.At 10 C,discharge capacity is ca.100 m Ah·g^-1 and capacity retention is nearly 90 % after 1000 cycles.In addition,reduced particle size of intermediate helps to decrease calcination temperature and time.Iron powder is designed as a dual-functional source.One is the green reducing agent for graphene oxide.The other is the cheapest source for LiFePO₄ material.In addition,the adhesion of LiFe PO4 grains on the graphene sheets is enhanced by the electrostatic attraction between positive Fe2+ ions and negative reduced graphene oxide.The S-LFP@C/r GO electrode has delivered a discharge capacity of 110 m Ah·g^-1 at 10 C,and capacity retention is about 95% after 1000 cycles.In addition,Na3V2?PO4?3 is coated on the LiFePO₄ grains to improve low temperature performance.The LFP@NVP@C electrode exhibits a reversible discharge capacity of 80 m Ah·g^-1 at-20 oC with the rate of 10 C,which is 10% higher than that of LFP@C.As for LiFePO₄ electrode fabrication,the inert components in the electrode are designed and optimized for high-rate performance.Expandable graphite as the raw source is treated by thermal expansion,ultrasonic exfoliation in isopropanol and purification in HNO3 in order to synthesize multi-layer graphene.Showing excellent electronic conduction,this multi-layer graphene is used as the conductive agent in electrode fabrication.In as-prepared S-LFP@C/MG electrode,the electronic transfer pathway has been optimized.Therefore,the S-LFP@C/MG can exhibit a specific capacity of 120 m Ah·g^-1 and average discharge voltage of 3.3 V at 10 C.Additionally,furfuryl alcohol is adopted as the precursor of a novel binder.S-LFP@C has been adhesive onto current collector via an in-situ polymerization process of furfuryl alcohol,and the resultant poly?furfuryl alcohol??PFA?is the real binder.With PFA binder of 3.8 wt.% and no conductive additives in the electrode,S-LFP@C exhibited a specific capacity of 105 m Ah·g^-1 and so as S-LFP@C/PFA electrode with 101 m Ah·g^-1.At 1 C,the capacity retention is nearly 75% after 500 cycles.This result is attributed to that PFA is a conductive binder with a lower electric and ionic migration barrier,compared with traditional PVd F binder.according to density functional theoretical?DFT?calculation and synchronous X-ray absorption spectroscopy?s XAS?.Overall,in order to improve electrochemical performance of LiFePO₄ cathode material,various methods have been adopted,including particle-size control,graphene-modification and NASICON coating.As for enhancing energy and power density of LiFePO₄ electrode,the strategies are carried out,such as graphene conductive additives and conductive PFA binder.