| Lithium-ion rechargeable batteries are the most promising power system that can offer a higher operative voltage and energy density being used electric vehicles and hybrid electric vehicles.Among the positive components in lithium-ion batteries,LiFePO4 is of particular interest for use in large size batteries,due to its much more sources of rich sources material and low cost,environmentally friend,intrinsic structural and chemical stability that leads to safe and long cycle life batteries.The main obstacles for practical applications of LiFePO4 is its poor rate capability,which can be attributed to slow kinetics of lithium-ion diffusion coefficient and the poor electronic conductivity.In this paper,the FePO4×H2O obtained from waste water was use as ion source,oxalic acid and malic acid was use as composite carbon source,used mechanical activation-high temperature solid-state method synthesized LiFe1-xMxPO4/C(M=La3+,Nd3+,Er3+,Pr3+),combined with XRD,SEM,FTIR,CV,EIS and galvanostatic charge-discharge test,reaction comprehensive study how to focused on controlling the morphology,improving the electrochemical performance of LiFePO4 as well as its doped and coated derivatives;Using optimal technical conditions to prepare Li0.99Na0.01Fe0.995La0.05PO4/C,develop high-capacity,high-rate and good cycle performance LiFePO4/C material;LiFePO4/RGO composite was prepared with mechanically mixing using graphene as carbon source,one step,in-site graphene-coating method with graphene oxide as carbon source was developed to prepare the LiFePO4/RGO composite.The main results and new findings in this work are summarized as follows:(1)Using oxalic acid and malic acid as composite carbon source.Fe-site doped LiFe1-xMxPO4/C(M=La3+,Nd3+,Er3+,Pr3+;x=0.0025,0.005,0.0075,0.01)was synthesized by high temperature solid state method.The influences of different doping concentration of La3+,Nd3+,Er3+,Pr3+ on the structure,morphology and electrochemical properties of the material were investigated.The LiFe0.995La0.005PO4/C material synthesized under appropriate conditions has regular and uniform size distribution of particles.The initial discharge specific capacity of LiFe0.995La0.005PO4/C was 169.6 mAh·g-1 at 0.1 C rate and 140.1 mAh· g-1 at 1.0C rate,the capacity retention rate was 98.88%and 98.64%respectively after 20 cycles.Appropriate amount of Fe-site doped La3+ can effectively improve the electrochemical properties of materials.(2)Li-site doped Li1-xNaxFePO4/C(x=0.005,0.01,0.03,0.05)was synthesized by high temperature solid state method.The influences of different doping concentration of Na+ on the structure,morphology and electrochemical properties of the material were investigated.The Li0.99Na0.01FePO4/C material synthesized under appropriate conditions has regular and uniform size distribution of particles.The initial discharge specific capacity of LiFe0.995La0.005PO4/C was 168.6 mAh·g-1 at 0.1C rate and 142.3 mAh·g-1 at 1.0C rate.Li0.99Na0.01Fe0.995La0.005PO4/C sample prepared by high temperature solid phase method exhibits an enhanced electrochemical performances,the initial discharge specific capacities of Li0.99Na0.01Fe0.995La0.005PO4/C at the rates of 0.1C and 1C were 169.3 and 155.1 mAh g-1.Appropriate amount of Fe-site and Li-site co-doped can effectively improve the electrochemical properties of materials.(3)RGO was used as conductive additives to prepare LiFePO4/RGO composite.The RGO suspension was prepared by redox process,after SEM and FTIR characterization,The results of characterization showed the number of epoxide,hydroxyl,carbonyl and carboxyl groups which were located on the edge or surface of GO were significantly decreased after being reduced by Hydrazine monohydrate.LiFePO4/RGO composite was obtained by mechanically mixing the LiFePO4 and RGO powers,the results of measurement showed the introduced RGO did not destroy the structure of LiFePO4,and the RGO conductive additives disperse well among LiFePO4 particles which improved the reversible capacity and good rate performance of the composite. |
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