Synthesis And Characterizations Of Cation Doped LiFeSO₄F Cathode Materials For Lithium-ion Batteries

Lithium-ion batteries become the ideal choice for the new generation of power sources due to its high specific capacity, high energy density, environmental friendly and no memory effect. However, besides the low-cost and long cycle life, safety property is also an essential factor for cathode material. As a new sulfate polyanionic material, Li Fe SO₄ F have some advantages, such as low-cos and environmental friendly. Especially, owning to the highly structural stability, it demonstrates good thermal stability which is even better than other commercial cathode materials of spinel Li Mn2O₄ and layered Li Co O2. However, the low electronic conductivity and the poor ion diffusion conductivity hinder the electrochemical performance of Li Fe SO₄ F at high current density. In this paper, to overcome the disadvantages mentioned above, we systematically investigate the effects of cation doped on the electrochemical properties of Li Fe SO₄F:Firstly, we discussed the crystal structures and electronic structures of Li Fe SO₄ F and Ti-doped Li Fe7/8Ti1/8SO₄ F by the first-principle calculation based on the density functional theory（DFT）. Ti ions were suggested to be situated in Fe（1） site of Li Fe7/8Ti1/8SO₄ F by evaluating the free energy and lattice parameters. The electronic structures indicated the reducing energy gap and increasing hole carriers which is benefitial to the enhancement of the electronic conductivity of Li Fe SO₄ F by ions doped.Secondly, we synthesised Ti-doped Li Fe1-2xTixSO₄F（x=0.01、0.02 and 0.03）by solvothermal method and studied their cathodic properties materials for lithium ion battery. And we examined the effect of Ti doping on the structure and electrochemical performance of Li Fe SO₄ F material by X-ray Diffraction（XRD）, Scanning Electron Microscope（SEM）, Transmission Electron Microscopy（TEM）, X-ray Photoelectron Spectrom（XPS）, Mossbauer spectroscopy（MS） and Uv-vis spectrometer（US）and so on. The results suggested:（1） Ti4+ ions were doped into the the Fe（1） site.（2） Cell volume was gradually decreasing with the increasing of titanium ion doping amount, because the ionic radius of Ti（0.605 ?） is smaller than that of Fe（0.78 ?）.（3） Morphology analysis showed the particles of Li Fe0.96Ti0.02SO₄ F distributed uniformly and the surfaces of the particles were clean and smooth, implying that there was no carbon coating on the surface. And Titanium element was uniformly distributed in the material.（4） The band gap of Li Fe0.96Ti0.02SO₄ F was reduced due to intrinsic conductivity of the material improved.（5）Li Fe0.96Ti0.02SO₄ F exhibited the best electrochemical performance. At 0.1C rate（15 m Ah/g）, discharge special capacity was achieved as 124 m Ah / g and remained as 109 m Ah/g after 50 cycles. And the capacity retention was 87.9%. At 2C（300m Ah/g） rate, discharge special capacity remained 24 m Ah/g. In contrast, discharge capacity of the pristine Li Fe SO₄ F was 96 m Ah/g and remained 69 m Ah/g after 50 cycles. And the capacity retention was 71.9%. At 2C rate, discharge capacity remained 9 m Ah/g.（6） Electrochemical impedance spectroscopy（EIS） analysis and cyclic voltammetry test（CV） results showed that Ti-doping improves the conductivity of Li Fe SO₄ F, which helped to reduce the charge transfer resistance, lithium ion diffusion resistance and the kinetic limitations during discharge process and improved the reversibility of the electrode and conductive properties of Li Fe SO₄ F.Finally, to further improve the rate performance of Li Fe SO₄ F, we synthesised Mg-doped Li Fe1-xMgxSO₄F（x=0.02 and 0.04） as cathode material for lithium ion battery by solvothermal method. By employing a series of structural and electrochemical characterizations, including XRD, SEM, TEM. The experimental results showed:（1） Cell volume was gradually decreasing with the increasing of magnesium ion doping amount. Because of the smaller ionic radius of Mg2+ ions（0.72?） in comparison with Fe2+（0.78 ?）, cell volume shrinked when replacing Mg2+ ions using Fe2+.（2） Morphological studies showed particle of Li Fe0.98Mg0.02SO₄ F were distributed uniformly and no carbon coating on the surface.（3）Li Fe0.98Mg0.02SO₄ F exhibited an excellent electrochemical performance. At 0.1C rate, discharge capacity was 101 m Ah/g and remained as 84.7 m Ah/g after 50 cycles with the capacity retention of 83.8%. At 2C rate, discharge capacity was 45 m Ah/g.（4） EIS and CV showed that ion doping improved the conductivity and ion diffusion rate of Li Fe SO₄ F, which is essential for reducing the charge transfer resistance and lithium ion diffusion resistance during chargr/discharge cycles. In conclusion, although the electrochemical performance of Mg-doped Li Fe SO₄ F was not superior to that of Ti-doped Li Fe SO₄ F, the cystal structure of Mg-doped Li Fe SO₄ F was stable and in favor of high rate discharge because a divalent element has less impact in crystal distortion than a trivalent element.In this paper, in order to improve intrinsic electronic conductivity and structural stability of Li Fe SO₄ F, we doped Li Fe SO₄ F with different cation ions. As a result, the electrochemical properties of the Li Fe SO₄ F were improved. This work provided a new attempt on modification of Li Fe SO₄ F.