Preparation And Modification Of Li₂FeSiO₄ Cathode Material For Lithium-ion Battery

In recent years,rechargeable lithium-ion batteries have been widely used in daily life such as portable electronic devices,electric vehicles and energy storage systems.With the development of economy and progress of science and technology,the demand for rechargeable lithium-ion batteries is gradually rising.Li₂FeSiO₄（LFS）has the advantages of high theoretical capacity(332 m Ah·g^-1),low cost,environment benignity and good thermal stability,and is considered as one of the most promising cathodes for the next generation of high energy density rechargeable batteries.However,the low conductivity(6×10^-14 S·cm^-1)and slow diffusion rate of lithium ions(1×10^-14 cm²·S^-1)severely limit the actual electrochemical performance of LFS.Therefore,we introduce carbon coating and ion doping to improve the shortcomings of LFS.The main contents are as follows:（1）Li₂FeSiO₄/C cathode material was synthesized by high-temperature solid phase method at 700°C using lithium carbonate,ferrous oxalate,silicon dioxide nanoparticles and pitch as lithium,iron,silicon and carbon sources.The carbon content in the material was controlled by adjusting the amount of pitch,and the optimal carbon content was selected by XRD,TG and constant current charge/discharge tests.The results showed that the electrochemical performance of the LFS/C-2 sample with 1 g of pitch addition and 13.5 wt%carbon content was better than the other samples,and the discharge specific capacity was143 m Ah·g^-1 at 0.2 C.（2）To solve the drawbacks of low electronic conductivity and small ion diffusion coefficient of LFS,non-metallic B³⁺doping was used to modify the LFS/C-2 samples.Li₂Fe Si_1-xB_xO_4-δ/C（x=0,0.01,0.03,0.05,0.07）composites were synthesized by high-temperature solid phase method using H₃BO₃ as the boron source.The structure and morphology of the composites were characterized by physical characterization such as XRD,Rietveld refinement,XPS and TEM,and the electrochemical perpormance of the materials were analyzed by constant current charge/discharge tests,CV and EIS tests.The discharge specific capacities of the Li₂Fe Si_0.95B_0.05O_4-δ/C samples were 160.7,142.1,120.6,103.1 and 80.6 m Ah·g^-1 at 0.2、0.5、1、2、5 C,respectively,and the capacity retention of 100 cycles at 1 C was 96.1%,and the Coulomb efficiency was more stable,mainly because the appropriate amount of B doping（5 wt%in this paper）could enhance the reaction kinetics,reduce the reaction polarization and promote the diffusion of lithium ions,thus improving the electrochemical performance of the material.（3）LFS/C-2 samples were modified by metal Zr⁴⁺doping.Li₂Fe_1-yZr_ySi O_4-λ/C（y=0,0.01,0.03,0.05）composites were synthesized by high-temperature solid phase method using Zr O（NO₃）₂ as zirconium source.Li₂Fe_0.97Zr_0.03Si O_4-λ/C samples exhibited the best electrochemical performance with discharge specific capacities of 161.8,138.8,120,102.7 and 79.8 m Ah·g^-1 at 0.2、0.5、1、2、5 C,respectively,and the capacity retention of 100 cycles at 1 C was 95%.The results show that the appropriate amount of Zr doping（3 wt%in this paper）can improve the multiplicity performance and cycling stability of Li₂FeSiO₄,because the appropriate amount of Zr can reduce the reaction polarization and increase the diffusion coefficient of lithium ions.