Preparation And Electrochemical Performance Of TiN Modified LiFePO₄/C Cathode Materials

In order to improve the electrochemical properties of LiFePO₄/C, the surface ofLiFePO₄/C particles were coated with highly conductive non-oxide TiN. The effectsof TiN coating were studied by X-ray diffraction （XRD）, scanning electronmicroscopy （SEM）, high-resolution transmission electron microscopy （HRTEM）,galvanostatic charge/discharge test, electrochemical impedance spectroscopy, cyclicvoltammetry and potential step chronoamperometry.LiFePO₄/C/TiN composite materials with different amounts of TiN were madevia a suspension mixing method. The results of HRTEM showed that the incompletecarbon network could be repaired by nano-sized TiN. The amounts of additive TiNwere between4wt%and6wt%. The TiN coating could greatly enhance theelectrochemical performances of LiFePO₄/C, especially when the content of TiN was5wt%. The5wt%TiN-coated LiFePO₄/C sample exhibited the best electrochemicalperformance with discharge capacities of157.4mAh/g,152.7mAh/g,142.4mAh/g,134.3mAh/g,116.7mAh/g and88.1mAh/g at0.1C,0.2C,0.5C,1C,2C and5C,respectively. The material without TiN merely showed153.3mAh/g,144.6mAh/g,136.0mAh/g,120.2mAh/g,102.6mAh/g and65.0mAh/g. Through suspension mixingmethod, appropriate amount of TiN coating could improve the electrochemicalperformances, especially at high rates. The results of impedance spectroscopyindicated that TiN coating could decrease the Rctof LiFePO₄. The Rctof samplecoated with5wt%TiN was dropped from114.7to30.4compared with LiFePO₄/Csample. The lithium ion diffusion coefficient was calculated through potential stepchronoamperometry measurement. The results revealed that the5wt%TiN coatingsample had the highest lithium ion diffusion coefficient of2.20×10^-10cm²/s.LiFePO₄/C/TiN composite materials were also made by carbothermic methodand in-situ nitridation method under nitrogen-hydrogen and ammonia atmospheres,respectively. The possibilities of the reactions were studied by X-ray diffraction（XRD）. Suggested solutions for further research were also put forward on the basis ofin-situ nitridation method.