Study On Sulfur And Phosphorus Doping,morphology Control And Electrochemical Properties Of Nanometer Li₂FeSiO₄/C

The polyanionic cathode material Li₂Fe Si O₄ has received widespread attention because of its high theoretical specific capacity,low cost,and environmental friendliness.However,the lower electronic conductivity and slower ion diffusion rate limit its application to power batteries.Therefore,th is thesis focuses on the preparation and modification of Li₂Fe Si O₄ to enhance its comprehensive performance.The main research contents are as follows:（1）Li₂Fe Si O₄/C（LFS/C）,Li₂Fe Si O_3.99S_0.01/C（S1-LFS/C）,Li₂Fe Si O_3.98S_0.02/C（S2-LFS/C）,Li₂Fe Si O_3.97S_0.03/C（S3-LFS/C）nanoanode materials were prepared by solid-phase method,and their crystalline structures,microscopic morphologies and electrochemical properties were characterized by XRD,SEM,XPS,Raman,FTIR and constant current charge/discharge characterized the crystal structure,microscopic morphology and electrochemical properties of Li₂Fe Si O_3.98S_0.02/C.XRD showed that Li₂Fe Si O_3.98S_0.02/C had a P2₁monoclinic crystal structure,and the cell volume and layer spacing increased after sulfur doping.SEM shows that Li₂Fe Si O_3.98S_0.02/C has a nanosphere morphology with an average particle size of 45 nm,which is significantly smaller than the particle size of Li₂Fe Si O₄/C（67 nm）.The larger cell volume,layer spacing and smaller particle size enable Li₂Fe Si O_3.98S_0.02/C to exhibit better electrochemical properties.At 10 C,the discharge specific capacity of S2-LFS/C is 85 m Ah/g,which is much higher than that of LFS/C（34.3 m Ah/g）and has a better multiplier performance.1 At C,the capacity retention rate of S2-LFS/C after 100 cycles is 91.3%,which is higher than that of LFS/C（90.6%）and has a better cycling performance.（2）Two nanomaterials,Li_1.95Fe Si_0.95P_0.05O4/C（LFSPC）and Li₂Fe Si O₄/C（LFSC）,were prepared by solid-phase method.The crystal structure,microscopic morphology and electrochemical properties were characterized by XRD,SEM,TEM,XPS and constant current charge/discharge.XRD shows that both LFSPC and LFSC have Pmn2₁ orthorhombic crystal structures,and P doping does not change the crystal structure of lithium iron silicate,and the cell volume of the material is reduced after P doping.SEM shows that the average particle size of LFSPC is 25 nm smaller than that of LFSC（28 nm）.The smaller particle size is beneficial to shorten the Li⁺diffusion pathway and accelerate the Li⁺diffusion rate.EDS shows that P is uniformly distributed in the material as other elements,and TEM shows that LFSPC has clearer lattice stripes than LFSC.The constant current charge/discharge test showed that LFSPC has better charge/discharge performance,multiplicity performance and cycling performance.213.7 m Ah/g was the first discharge specific capacity of LFSPC at 0.1 C,which was higher than that of LFSC（144 m Ah/g）.79.3 m Ah/g was the discharge specific capacity of LFSPC at 10 C,which was much higher than that of LFSC（32.1 m Ah/g）.At 1C,the capacity retention rate of LFSPC is 85%after 200 cycles,which is higher than that of LFSC（78%）and has a better cycling performance.（3）Two nanoanode materials,tubular Li₂Fe Si O₄/C（TLFS）and Li₂Fe Si O₄/C（LFS）,were prepared by the solid phase method combined with the template method,and the structure,microscopic morphology and electrochemical properties were characterized by SEM,Raman,FTIR and constant current charge/discharge.SEM analysis shows that TLFS has a typical one-dimensional tubular morphology,and the particle size distribution of LFS ranges from 35 to50 nm,while that of TLFS ranges from 15 to 25 nm.the particle size of TLFS is significantly smaller than that of LFS,and the smaller particle size is beneficial to Li⁺diffusion.FTIR analysis shows that TLFS and LFS exhibit the same structural characteristic peaks.The constant current charge/discharge test showed that TLFS has better electrochemical performance.189.7 m Ah/g was the first discharge specific capacity of TLFS at 0.1 C,which was higher than that of LFS（147.4 m Ah/g）.71.9 m Ah/g was the first discharge specific capacity of TLFS at10 C,which was higher than that of LFS（63.4 m Ah/g）and had better multiplicative performance.At 1 C,the capacity retention rate of TLFS is 75%after 100 cycles,which is higher than that of LFS（65%）and has a better cycling performance.