Synthesis And Electrochemical Properties Of Li FeSO₄F Cathode Materials For Lithium Ion Batteries

Lithium-ion batteries have been widely applied in notebook computers, cell phones, laptop computers and other electronic devices due to their high energy/power densities, long cycle life and environmentally friendly. However, with the application of electric vehicles and large energy storage devices, searching for electrode materials, especially cathode materials, with higher energy density seems to be more essential. In 1997, Professor J.B.Goodenough reported the poly-anion material, Li Fe PO4, as cathode material for lithium ion batteries, and it attracted widespread attention because of its good structural stability and high safety performance. In 2010, J.M.Tarascon team using ionic liquids as solvents, prepared the polyanion-type compound, lithium iron fluorosulphate（LiFeSO₄F）, as cathode material for lithium ion batteries, and it has been found that the Li Fe SO4 F has higher working potential than that of Li Fe PO4, which is caused by the higher electronegativity of SO42- as compared to PO43-. Additionally, Li Fe SO4 F shows a three-dimensional framework which is very favorable for Li-ion transport. However, the disadvantages of Li Fe SO4 F, such as high synthetic cost, low electron conductivity and poor electrochemical kinetics, restrict its practical application. In this paper, in order to conquer the shortcomings mentioned above, we systematically investigate the effects of surface modification on the electrochemical performance of Li Fe SO4 F.Firstly, we select tetraethylene glycol as the solvents and successfully obtain Li Fe SO4 F at 260 oC. In addition, the thermal stability and safety property of the material are also studied. According to the DSC curves, the material decomposes at 385 oC with a minimal heat release of 79.4 J g-1 after the first charge, and further thermogravimetric and temperature dependent XRD results confirm that the minimal heat release at 358 oC is caused by the Li0.35 Fe SO4F decomposed to Fe2（SO4）3 and Li2（SO4）, accompanied with releasing the F2 gas. It indicates that Li Fe SO4 F can be used as a potential high safe cathode material for lithium ion batteries.Then, in order to improve the electrochemical performance of Li Fe SO4 F, we carry out surface modification for Li Fe SO4 F with multi-wall carbon nano-tube（MWCN） and graphene oxide（GO）, respectively. By means of a convenient in-situ synthesized process, we obtained the precursor Fe SO4 ? H2 O wrapped with one-dimensional MWCNT, then MWCNT wrapped LiFeSO₄F（LiFeSO₄F/MWCNT） is prepared by the reaction between the precursor and Li F. Further experiments prove that the LiFeSO₄F/MWCNT material shows improved electrochemical performance compared with the pristine Li Fe SO4 F. The charge-discharge test indicates a rising of the initial charge capacity to 123.9 m Ah g-1. Cyclic Voltammetry（CV） result shows that the ohmic polarization between the cathodic and anodic currents peak decreases. Additionally, the electrochemical impedance spectroscopy（EIS） shows not only the resistance of SEI film reduces, but the charge transfer resistance abruptly reduces from 366.8 Ω to 64.34 Ω. Incorporation of MWCNT improves the electrochemical kinetics of the materials, which results in enhanced electrochemical performance.As a kind of two-dimensional conductive carbonaceous material, graphene oxide can intimately wrap on the surface of Li Fe SO4 F, and improve the electronic conductivity of the material significantly. The DC electronic conductivity measurement shows that the electronic conductivity of the material improves from 8.16 × 10-11 S cm-1 to 1.65 × 10-4 S cm-1, the LiFeSO₄F/GO material exhibits much better electrochemical performance than that of the pristine Li Fe SO4 F, and it shows a discharge capacity of 113.2 m Ah g-1 at the 0.1 C rate with 99% capacity retention after 100 cycles. This is because the GO not only improves the conductivity of Li Fe SO4 F, but depresses the side reactions of the electrode and electrolyte, and promotes the charge transfer reactions at the electrode/electrolyte interface. Even at 2 C rate, a high specific capacity of 73.4 m Ah g-1 is still obtained for LiFeSO₄F/GO material, which is much higher than that of pristine Li Fe SO4 F. The CV and EIS results show the improvement of electrochemical kinetics. The lithium diffusion coefficient of the LiFeSO₄F/GO is about 20 times larger than that of the pristine Li Fe SO4 F, and the charge transfer resistance abruptly reduces from 366.8Ω to 35.4 Ω. The modification not only improves the electronic conductivity and electrochemical kinetics of the material, but isolates the direct contact of the active material with electrolyte, which depresses the formation of SEI film. Consequently, the LiFeSO₄F/GO material exhibits improved electrochemical performance.Finally, we prepared a poly-dopamine（PDA） coated Li Fe SO4 F cathode material via the oxidability of Fe3+. PDA contains several functional groups such as catechol, amine and imine, leading to the high hydrophilicity and robust wet-resistant adhesion. The PDA layer improves the hydrophilicity of the electrode, facilitates the lithium insertion/extraction, depresses the side reactions of the electrode with the electrolyte. In addition, the strong and robust wet-resistant adhesion of PDA could improves the connection of active material particles, conductive additive and collector, and releases the partial pressure caused by the large volume change during Li insertion/extraction. At 0.1 C rate, after 100 cycles, the discharge capacity of 97 m Ah g-1 still obtained, which is much higher than that of pristine Li Fe SO4 F. CV curves indicate the PDA layer coats on the surface of Li Fe SO4 F during the electrochemical cycle with high stability. EIS shows the charge transfer resistance reduces. The PDA coating layer improves the hydrophilicity of the electrode, depresses the side reaction of the electrode with the electrolyte, thus enhances the electrochemical performance of the electrode material.In this paper, in order to improve the electrochemical performance of Li Fe SO4 F, we modify Li Fe SO4 F by coating with different materials. These works give us a theoretical and technical guidance for basic research and practical applications of Li Fe SO4 F cathode materials in lithium-ion batteries.