Study On The Preparation And Electrochemical Performance Of Lithium Iron Phosphate/carbon Composite Materials

LiFePO₄/carbon（LiFePO₄/C）composites were prepared with LiH₂PO₄ and FeC₆H₅O₇·5H₂O as the precursors and the expanded graphite（EG）and polyacrylonitrile（PAN）-based carbon fiber as the carrier by in-situ sol-gel method and vacuum-infiltration process,followed by calcination process.And the Li-ion capacitors were assembled with LiFePO₄/C composites as the positive electrodes.The prepared electrode materials were characterized by TG-DSC,XRD,SEM,TEM,and electrochemical properties of them and their Li-ion capacitors were tested by cyclic voltammetry,impedance spectroscopy and galvanostatic charge-discharge methods.The influences of calcination temperature,carrier content,surfactant kind and content on the performance of the composites and their Li-ion capacitors were studied.And the best composite was compared with commercial LiFePO₄.The result shows that well-crystallized LiFePO₄ with the particle size of 30-80nm has been composited with EG uniformly by vacuum-infiltration process.S2prepared under the calcination temperature of 600℃and EG content of 10%shows the best electrochemical properties.The specific capacitance(C_pe)of S2 is 591.86 F/g measured by CV at the scanning rate of 5 mV/s,and the specific capacitance(C_ps)of its Li-ion capacitor is 55.33 F/g at the current density of 2 mA/cm².After 500 cycles,the Li-ion capacitor retains 100%of its initial capacitance at the current density of 5mA/cm².S2 and its Li-ion capacitor both exhibit better cycling stability than commercial LiFePO₄ and its Li-ion capacitor,which is due to the introduction of EG with high conductivity confirmed by impedance spectroscopy.LFP/EG-F127-2 which modified by the most hydrophilic F127 shows the highest C_pe of 752.87 F/g at the scanning rate of 5 mV/s.The C_ps of its Li-ion capacitor is 67.64 F/g at the current density of 2 mA/cm² and the Li-ion capacitor exhibits good cycling stability at the same time.When the PAN-based carbon fiber is the carrier,the well-crystallized LiFePO₄with the particle size of 30-80 nm has been composited with PAN-based carbon fiber successfully.With the increase of PAN-based carbon fiber content,the resistance of the composites decreases which is due to the introduction of PAN-based carbon fiber with good conductivity confirmed by impedance spectroscopy.The LFP/PAN CF-2 with the PAN-based carbon fiber content of 40%（and 0.25 g CTAB）shows the best electrochemical properties.The C_pe of LFP/PAN CF-2 is 632.37 F/g at the scanning rate of 5 mV/s,and C_ps of its Li-ion capacitor is 58.92 F/g at the current density of 2mA/cm².LFP/PAN CF-F127-2 which modified by the most hydrophilic F127 shows the highest C_pe of 658.56 F/g at the scanning rate of 5 mV/s.And its Li-ion capacitor exhibits better rate performance than the Li-ion capacitor with LFP/PAN CF,and good cycling stability at the same time.