Synthesis And Performance Study Of Si Site Doped Li₂FeSiO₄ As Cathode Material For Lithium Ion Battery

Poly anionic silicate cathode material has the advantages such as high theoretical capacity,excellent safety,therefore Li₂FeSiO₄ is a very promising power battery polyanion cathode material.But the low electronic conductivity,poor reversible cycle performance and high rate performance of Li₂FeSiO₄,are the main shortcomings,to be solved.To obtain more Li+embedded,effective measures,such as carbon coating and metal cation doping would be taken to improve the cycle performance of the material and high rate performance.In this paper,covering different content of carbon and Si site doping metal elements have been employed to modify Li₂FeSiO₄.The thermogravimetric-differential thermal analysis（TG-DTA）,X-ray powder diffraction（XRD）,scanning electron microscope（SEM）,charge-discharge test,AC impedance（EIS）and cyclic voltammograms（CV）have been taken to research the structure,morphology and electrochemical performance.Li₂FeSiO₄/C composites were prepared by a sol-gel method.By controlling the amount of citric acid to control the carbon content in the composite material,according to stoichiometric amounts of citric acid:Fe=1,2,1.5,2.5 to synthesize Li₂FeSiO₄/C composite materials,the carton content were 4.63%,12.06%,19.94%,24.66%,respectively.Using the means of XRD and SEM to characterized material,we found that reunion phenomenon of the material particles is relatively serious when carbon content is small.With the increase of carbon content,reunion phenomenon is reduced.When carbon content is 19.94%,discharge specific capacity of the synthetic material up to 120.1 mA.h.g^-1,and cycle performance is best.Using the sol-gel method to synthesize composite material of Si doped with different amount of Aluminum,Li₂FeSi_1-xAl_xO₄/C,doping amount were 0.03,0.06,0.1 mol%.By means of material characterization of XRD and SEM,we found that doping Al element has no effect on Li₂FeSiO₄ crystal structure.However,with the increase of Al doping amount,the particle size of sample was gradually decreasing,the reunion phenomenon is slow down.Li₂FeSi_0.97Al_0.03O₄/C shows the best charge and discharge performance,the initial charge capacity is 143.18 mA.h.g^-1,the discharge capacity is 125.52 mA.h.g^-1.In the rate test,Li₂FeSi_0.94Al_0.06O₄/C shows the best high rate performance,charge and discharge current increased from 0.1 C to 0.5 C,its capacity to keep at a rate of 87.6%,and pure sample retention rate is only 62.87%.Cr-doped Li₂FeSi_0.9Cr_0.1O₄/C was prepared by the sol-gel method.By the characterization methods,such as XRD,SEM,we found a small amount of chromium has no effect on structure of Li₂FeSiO₄.The loose surface of Li₂FeSi_0.9Cr_0.1/C composite greatly increase the material specific surface area with more interspace,which is advantageous to the Li⁺migration between solid phase and liquid phase,to improve the electrochemical performance.The highest specific capacity of Li₂FeSi_0.9Cr_0.1O₄/C is 171.93 mA.h.g^-1,which significantly higher than the theoretical specific capacity 166 mA.h.g^-1,the first discharge specific capacity is 149.35 mA.h.g^-1,and its capacity to keep at a rate of 98.4%after 20 times.Relative to not doped sample,the sample of the doping Cr high rate performance also have obvious improve,rate from 0.1 C to 0.5 C,the capacity to keep at a rate of 86.3%.