Synthesis And Modification Of Li₂FeSiO₄ And Study On Its Electrochemical Properties As Cathode Materials For Lithium-ion Batteries

Li₂FeSiO₄ has been drawn considerable attention due to its lower cost and high safety,environment-friendly and nontoxicity, abundant reserves in the earth’s crust and its high theoretical capacity（potential capacity of 332 m Ah·g-1） with reversibly inserting/de-inserting. However, these orthosilicate materials suffer from the problems of poor electronic conductivity, slow diffusion of lithium ion, overlarge and uniform particle size and the overlong preparation time. These disadvantages limit its application.Aiming at the problems existing in the current research in this paper, Li2 Fe Si O4/C composite materials were synthetized by hydrothermal method and reflux-assisted sol–gel synthesis techniques, respectively. The different modification of coating carbon and substituting Fe2+ with rare earth ion of Nd3+ were conducted. Microstructure and electrochemical properties of the composite materials have been investigated systematically by using X-ray diffractometer, scanning electron microscope, infrared spectrometer, and battery detector and electrochemical workstation, and the significant results have been achieved.Jujube-stone-like Li2 Fe Si O4/C composites have been synthesized by hydrothermal method using the reactants of nano-silica（Si O2）, ferrous sulfate（Fe SO4?7H2O）, lithium hydroxide（Li OH?2H2O）, and the carbon source of glucose. The effect of coating carbon with different amounts of glucose and substituting Fe2+ with rare earth ion of Nd3+ for this material were systemically investigated. The results show that the structure of Li2Fe1-x Ndx Si O4/C（x=0, 0.01, 0.03, 0.05） materials synthesized by hydrothermal method are characterized as Pmn21 orthorhombic structure. The first discharge capacitiy of Li2Fe0.97Nd0.03 Si O4/C material at 0.2 C are 177 m Ah·g-1,and the stable discharge capacity of the material after five charge-discharge cycles is 161 m Ah·g-1, which is significantly higher than that of Li2 Fe Si O4/C.To deal with the problems in the hydrothermal synthesis method, the reflux-assisted sol-gel synthesis techniques was also used in this study. Ferrous oxalate（Fe C2O4?2H2O）and TEOS were acted as the reactants, and citric acid was used as the carbon source,reductant and chelating agent. The modified lithium iron silicate of Li2Fe1-x Ndx Si O4/C（x=0, 0.01, 0.03, 0.05） as cathode material of lithium ion battery were prepared by reflux-assisted sol-gel technique. The results show that the structure of Li2Fe1-x Ndx Si O4/C（x=0, 0.01, 0.03, 0.05） synthesized by reflux-assisted sol–gel technique were characterized as Pmnb orthorhombic structure. The maximum discharge capacity of Li2Fe0.97Nd0.03 Si O4/C at 0.2 C are 265 m Ah·g-1,and the discharge capacity of the material at seventh cycle is 186 m Ah·g-1, which is significantly higher than that of Li2 Fe Si O4/C material.The crystal structure of lithium iron silicate has not been changed by doping Fe2+with Nd3+, while the discharge capacity, cycle stability and the rate dischargeability of the material used as cathode of lithium ion battery have been significantly improved. At the same time, the charge transfer resistance on the electrode/electrolyte interface is noticeably decreased, and the exchange current density is significantly increased, and the dynamic properties of the materials have been distinctly improved. When the substitution quantity is 0.03, the charge transfer resistance of Li2Fe0.97Nd0.03 Si O4/C material is minimal,which is significantly lower than that of Li2 Fe Si O4/C material.