Study On Graphene/phosphate-base Lithium-ion Battery Cathode Materials: Preparation, Characterization And Properties

The olivine based phosphate family LiMPO₄?Mn Fe,Mn,Co,Ni?has been recognized as potential cathode materials for rechargeable LIBs due to its low price,thermal stability,excellent cycling performance,and environmental friendliness.It is becoming the most promising material in electric vehicles.However,its wide application has always been limited by the bottlenecks of the poor intrinsic conductivity and Li⁺ion diffusion coefficient.Carbon-coated,metal-doped and morphology-controlled are the most effective ways for improving the electrochemical performance of LiMPO₄-based cathodes in LIBs.Graphene,which have high conductivity,has been proved to be a promising carbon material for improving the electrochemical of LiMPO₄ materials.Li FePO₄-rGO?LFPG?,LiMnPO₄-rGO?LMPG?and LiFe_xMn_1-xPO₄-rGO(LF_xM_1-x PG)composites were prepared via hydrothermal method by using graphene oxide?GO?as a precursor.A series of characterizations were used to analyze the composition and morphology of the materials,and then we investigated the electrochemical properties of the materials by electrochemical tests.Firstly,LFPG was prepared by using water and ethylene glycol as solvent.The wire-like LFP loaded on graphene nanosheets uniformly.Due to this novel structure and graphene,electronic and ionic conductivities of LFP were improved.FT-IR,XRD,Raman,XPS,TEM and SEM were used to characterize the composition and morphology of the materials.As cathode material for lithium ion batteries?LIBs?,the LFPG composits have excellent electrochemical performance,the electrochemical test results showed the specific discharge capacity of LFPG is 137.7 mA·h·g^-1 at current rate of 0.2 C and after 250 cycles,the Capacity retention rate nearly 100%.Secondly,LMPG was prepared by controlling the ratio of Li:Mn:PO₄=3:1:1.The length of the rod-like LMP in LMPG composites were 200 nm and uniformly loaded on graphene sheets.Compared with pure LMP,its size is reduced.As cathode material for lithium ion batteries?LIBs?,the LMPG composites have excellent electrochemical performance.A specific discharge capacity of LMPG up to 88.6mA·h·g^-1 obtained after 100 cycles at a current rate of 0.5 C,the Capacity retention rate nearly 90%.Finally,we synthesized LF_xM_1-x PG?X=0.9,0.8,0.5 and 0.1?composites by controlling the ratio of Fe²⁺and Mn²⁺.Compared with the other three composites,t LF_0.5M_0.5PG has better electrochemical performance.The LF_0.5M_0.5PG composite has the advantages of the other materials.As a cathode material for lithium ion batteries,the LF_0.5M_0.5G composit has excellent electrochemical performance.