Preparation And Modification Of Li₂FeSiO₄ Cathode Materials For Lithium Ion Batteries

Polyanionic type Li₂FeSiO₄ had become one of the most popular cathode materials for lithium ion batteries,because of its high theoretical specific capacity,high energy density,excellent cycling performances and no environmental pollution.However,Li₂FeSiO₄ also had the problems of low electronic conductivity(10^-14 S·cm^-1)and poor ion diffusion coefficient(10^-17 S·cm^-2),which increased the polarization of electrochemical reaction and hindered the wide application of Li₂FeSiO₄.In this work,the preparation process of Li₂FeSiO₄ cathode material were optimized and the electrochemical characteristics was improved by doping and coating.The main contents were summarized as following:?1?Li₂FeSiO₄ cathode material was synthesized by citric acid assisted sol-gel method with LiNO₃,Fe?NO₃?₃,TEOS and C₆H₈O₇ as raw materials.According XRD,SEM and electrochemical test,the effects of the ratio of iron source and citric acid,calcination temperature and time on the phase structure,surface morphology and electrochemical properties of the samples were studied.The results showed that appropriate ratio of Fe/C,calcination temperature and time were helpful to prepare pure phase Li₂FeSiO₄.The impurity in the prepared material might result in larger internal impedance,hindering the transmission of lithium ions.The optimum synthesis process in this section was as following:the ratio of Fe/C was 1/2.0,and the calcination time at 650?C was 10 h.?2?Li₂FeSiO₄ cathode material was modified by Mn²⁺and V⁵⁺co-doping.Based on the XRD,SEM and electrochemical analysis,the enhancement mechanisms of Mn²⁺/V⁵⁺co-doping were explored.Mn²⁺doping can help to improve the first discharge capacity and stabilize the structure.The morphology of Li₂Fe_0.8Mn_0.2SiO₄ was controlled between 80 and 150 nm.The first discharge capacity was as high as128 mAh·g^-1 and was about 36%higher than the undoped.Further,Mn²⁺and V⁵⁺co-doping might play a synergistic role.When the doping concentration was x=0.01,the particle size was controlled between 50-150 nm,and the discharge specific capacity of the sample was as high as 144 mAh·g^-1 and increased about13%compared with Mn²⁺doped sample.?3?Li₂FeSiO₄ was then modified with sucrose as carbon source by hydrothermal method.Based on the XRD,SEM and electrochemical analysis,the effects of carbon coating on the surface morphology,chemical state and electrochemical properties were investigated.The results showed that the surface carbon coating could effectively improve the conduction between particles in the material,which was conducive to the transport of lithium ions.When the carbon coating was 2 wt%,the first discharge capacity of the material was 161 mAh·g^-1.The discharge capacity of the material was 20%higher than that of the uncoated and 71%higher than the original material.Meanwhile,the sample showed the best rate performance,because the conductive network formed in the carbonization process was helpful to improve the conductivity.