Preparation And Improvement Of Electrochemical Performance Of Li₂FeSiO₄ As Cathode Material For Lithium Ion Batteries

Lithium-ion batteries have been widely used in the 3C electronic products and are the most promising energy supply for hybrid electric vehicles and electric vehicles as the electrochemical energy storage and energy conversion device due to their many advantages,including high voltage,long cycle life,high energy/power densities,fast charge and discharge,wide operating temperature range,small size and environmental friendliness,etc.As an important part of lithium ion batteries,the cathode materials play decisive role in the output voltage,power density,energy density,cost and safety of the battery.It is necessary to develop novel and potential cathode materials.Li₂ FeSiO₄ is a potential polyanionic cathode material for lithium ion batteries and has the characteristics of rich raw material,low cost,good safety and environmental friendliness as well as high theoretical specific capacity of 330 m Ah g-1（the reversible insertion/extraction of two Li+ ions per formula unit,the highest among the polyanionic materials）.However,low electron conductivity and slow lithium ion diffusion rate seriously restrict its development.In this thesis,we have done a series of work to improve its electrochemical performance:（1）Li₂FeSiO₄/C/CMK-3 composites are successfully prepared by sol-gel method using citric acid and CMK-3 as carbon sources.The physical properties of samples are characterized by XRD,SEM,TEM and BET.Li₂ FeSiO₄ not only disperses on the CMK-3 surface,but also partly embeds in the channels of CMK-3 conductive matrix.In terms of electrochemical performance,the performance difference between the two materials with or without CMK-3 is compared.Compared to Li₂ FeSiO₄/C,Li₂ FeSiO₄/C/CMK-3 exhibits enhanced rate performance.The discharge capacities are 160,148,129,110,90,66 and 50 m Ah g-1 at 0.1,0.2,0.5,1,2,5,10 C,respectively.The greatly enhanced electrochemical performance of the Li₂ FeSiO₄/C/CMK-3 composites may be attributed to the fact that the pyrolysis carbon of citric acid is coated on the surface of Li₂ FeSiO₄ and forms a three-dimensional interpenetrating conductive network with CMK-3,which effectively limits the growth and agglomeration of Li₂ FeSiO₄ particles,and increases the electronic and ionic conduction throughout the electrode.（2）As we know,carbon is an ion insulator and hinders the transport of lithium ions.Therefore,co-modified Li₂ FeSiO₄/C/Ce PO₄ composites are prepared with the ion conductor Ce PO₄ and carbon.The physical properties of samples are characterized by XRD,SEM and TEM.It shows that Li₂ FeSiO₄ particles are deposited on Ce PO₄ nanorods,and the pyrolysis carbon of P123 is coated on the surface of Li₂ FeSiO₄ particles.In this framework,the ion conductor Ce PO₄ can accelerate the diffusion of lithium ion in the electrode material and the carbon layer can improve the electrical conductivity of the electrode material.The electrochemical performance of the Li₂ FeSiO₄/C/Ce PO₄ composites is optimized by adjusting the relative content of Ce PO₄ and carbon in the composites.As a result,the composites containing 6 wt.% Ce PO₄ exhibits the best electrochemical performance among all the Li₂ FeSiO₄/C/x Ce PO₄ samples.The average specific capacity of Li₂ FeSiO₄/C/x Ce PO₄（6 wt.%）at 1,2,5,10 and 20 C is 144,132,118,104 and 74 m Ah g-1,respectively.（3）Considering that excessive carbon coating will affect the tap density and lithium ion transport of the electrode material,we have successfully designed and prepared Li₂ FeSiO₄/C/Cu/Li₃PO₄ composites.The composite materials are prepared in one step by inducing Cu₃（PO₄）2 into the synthesis of Li₂ FeSiO₄.The as-prepared samples are characterized by XRD,EDS and XPS.These results prove that Cu2+ in the Cu₃（PO₄）2 is reduced by carbon to produce Cu0 and Li₃PO₄ is formed by the reaction of PO₄3-and Li+.In the Li₂ FeSiO₄/C/Cu/Li₃PO₄ composites,the Cu0 is encapsulated in the interior of Li₂ FeSiO₄ particles,improving the conductivity and the utilization of the active material.Li₃PO₄ not only exists in the interior of Li₂ FeSiO₄ particles,but also distributes in the particle surface by mixed with carbon,which can simultaneously enhance the lithium ion diffusion on the surface and in the bulk of Li₂ FeSiO₄.The electrochemical properties of composites containing different amounts of Cu0 and Li₃PO₄ are tested.The Li₂ FeSiO₄/C/Cu/Li₃PO₄ composites containing 2 wt.% Cu0 and 2.76 wt.% Li₃PO₄ exhibits the best electrochemical performance among all samples,and the average discharge specific capacities at 1,2,5,10,20 and 40 C are 165.8,142.9,119.2,102.1,83.3 and 60.4 m Ah g-1,respectively.（4）As we know,the interface modification mentioned above,such as the synthesis of Li₂ FeSiO₄/C/CMK-3,Li₂ FeSiO₄/C/Ce PO₄ and Li₂ FeSiO₄/C/Cu/Li₃PO₄,will not change the structure to improve the intrinsic properties of Li₂ FeSiO₄.Based on this,in the latter two parts of thesis,Li₂Fe1-x Mx SiO₄ is synthesized by a sol-gel method by substituting Fe2+ with Y3+ and Ti4+,respectively.（1）As to Li₂Fe1-x Yx SiO₄,XRD and XPS test results confirm that Y is successfully doped into the Li₂ FeSiO₄ crystal structure,causing lithium ion vacancy defects.By analyzing SEM,BET and Raman results,we can find that Y doping negligibly affect the morphology and dispersibility of the Li₂ FeSiO₄ particles as well as the structural properties of the coated carbon.UV-Vis is carried out to characterize Li₂ FeSiO₄ without carbon modified.For the first time,UV-Vis is used to prove that Y doping can reduce the band gap of Li₂ FeSiO₄ and improve its intrinsic conductivity.The effect of different Y doping concentration on the electrochemical properties of Li₂ FeSiO₄/C electrode is studied.Among all samples,Li₂Fe0.98Y0.02 SiO₄/C shows the best cycle and rate performance.The enhanced electrochemical performance is ascribed to its improved electronic conductivity,higher lithium ion diffusion coefficient and better structure stability due to the proper amount of Y doping.（2）The Li₂Fe1-x Tix SiO₄/C（x = 0,0.02 and 0.04）cathode materials are successfully synthesized.The physical and electrochemical properties of Li₂Fe1-x Tix SiO₄/C samples with different Ti doping content are systematically studied.The XRD and XPS confirm that Ti does enter the crystal structure of Li₂ FeSiO₄.Galvonostatic charge-discharge test shows that Li₂Fe0.98Ti0.02 SiO₄/C exhibits the best electrochemical performance,which can deliver the highest discharge capacity of 102.8 and 91.1 m Ah g-1 at 5 and 10 C high rate over 1000 cycles.Additionally,the electrochemical kinetic properties are further studied by electrochemical impedance spectroscopy,cyclic voltammetry and galvanostatic intermittent titration technique.They indicate that the enhanced electrochemical performance can be attributed to the effect of Ti doping,which not only enhances the structural stability and electronic conductivity,but also improves the lithium ion diffusion coefficient of Li₂ FeSiO₄/C.