Preparation Of Lithium-ion Batteries Cathode Material LiFePO₄/C Using Ionic Liquid As Carbon Source And Its Electrochemical Performance

In this thesis,we analyze the electrochemical performance of Li FePO₄/C?LFP/C?influenced by different methods.LFP/C composites were produced by using ionic liquid?IL?[VEIm]NTf₂ as carbon source for the first time ever.The LFP/C is first characterized by a variety of techniques including XRD,SEM,TEM and Raman spectroscopy to determine the quality of the electrode materials.The LFP/C is prepared by hydrothermal method followed by carbonization of[VEIm]NTf₂ and is characterized in a half-cell to measure the electrochemical performance.Because the excellent wettability of[VEIm]NTf₂,the derived carbon films tightly band to LiFePO₄?LFP?particles,and create electron paths between LFP particles,thus increase the electrical conductivity of LFP.The strongly banded carbon films limit the growth of LFP particles,therefore,reduce the path of Li⁺insertion and deinsertion.It is found that the LFP/C has significantly improved values of reversibility,cycle stability,rate performance,and the charge and discharge capacity.The improved performance of the LFP/C electrode can be attributed to the decreased resistance from the incorporated carbon films and the reduced Li⁺path created by the coating of carbon films on LFP particles.These results indicate that[VEIm]NTf₂ can be applied as carbon source for electrode materials in LIBs,which would suitable for widespread applications.In this study,the in-situ hydrothermal method is used to synthesize carbon film coated LFP particles with different carbon sources?IL and glucose?.The synthesized LFP/IAC and LFP/GAC samples?carbonization samples were named as LFP/IAC and LFP/GAC made with IL and glucose?are first characterized by a variety of techniques including XRD spectroscopy,FTIR spectroscopy,SEM microscopy,TEM microscopy and Raman spectroscopy.The structure characterization results indicate that the carbon sources have no effect on the LFP particles,but have remarkable influence on the coated carbon films.It is found that the IL carbon source can produce ultrathin carbon film of 1-2nm,which was rarely reported before.To compare the electrochemical performance of LFP/IAC and LFP/GAC,the obtained materials are assembled into coin type half-cells to measure the electrochemical properties.Compared with LFP/GAC sample,the LFP/IAC electrode exhibits a superior electrode reaction reversibility,a capacity retention of 160.6mAh·g^-1?1.47%decay rate?after 50 cycles and specific discharge capacity of 143.6mAh·g^-1 at 1C,which is much better than the performance of the electrode materials synthesized with glucose.The outstanding electrochemical performance of the LFP/IAC electrode can be attributed to the special carbon film structure which shortens the electron path between LFP particles.These results indicate that ILs are suitable carbon scours for the in-situ hydrothermal synthesis of LFP/C materials to produce high performance cathode materials for lithium ion batteries.LFP/C eletrodes were produced by using ionic liquid[BMIm][BF₄]as carbon source.The LFP and LFP/C eletrodes are characterized by a variety of techniques including XRD spectroscopy,TEM microscopy and Raman spectroscopy.The structure characterization results indicate that LFP and LFP/C electrode materials have high crystallinity and purity.The fluorine and boron codoped carbon films come from the carbonization of ionic liquid[BMIm][BF₄],which is only 3-5nm thick.Compared with LFP sample,the LFP/C electrode exhibits excellent rate performance.The first discharge capacity of the LFP/C electrode is still as high as123.9mAh·g^-1 at 5C rate.The discharge capacity of the LFP/C electrode is 160mAh·g^-1 after40 charge discharge cycle at 0.1C,which is much better than the performance of of LFP electrode material.The outstanding electrochemical performance of LFP/C electrode is due to the carbon layer of fluorine doped materials can effectively improve the rate performance and cycle performance,in addition to fluorine doping can also inhibit the growth of LFP crystals,shorten the lithium ion migration path to improve the conductivity of electrode materials.On the other hand,boron doping increases the number of charge carrier cavitation type,increases the conductivity of carbon.These results indicate that[BMIm][BF₄]can provide the fluorine source and the boron source to improve the electrochemical performance of the LFP electrode effectively.