Study Of The Preparation And Performance Of Lithium Iron Phosphate As Cathode Materials For Power Lithium-ion Battery

Olivine-structured LiFePO₄,as the most promising cathode material for lithium ion batteries,has been received intensive research and development owing to its merits of high theoretical capacity,long cycle life,low cost and toxicity,superior safety,as well as environmental benignity.However,the inherent shortcomings of LiFePO₄ comparing with other cathode materials,such as the material’s low electrical conductivity and slow Li-ion diffusion,result in noteworthy capacity loss during high-rate discharge and poor electrochemical performance,making it unsuitable for commercial applications.Tremendous efforts have been devoted in our study to overcome these issues and improve the overall electrochemical performance of LiFePO₄,including decreasing Li FePO₄ particles to nanometer size,adjusting the crystal orientation of LiFePO₄ or coating with electronic conductive carbon.The research results and conclusions are listed as follow:Firstly,a high shear mixer has been adopted during the precursor preparation process,and size controlling of Li FePO₄ particles has been successfully achieved via this high shear mixer assisted hydrothermal method.Effect of rotor speed of the high shear mixer on the phase structure,morphology and size distribution of the precursor precipitate and Li FePO₄ particles were studied,in order to understand the reason that high shear mixer can successfully control the particle size of LiFePO₄.It is indicated that the Li FePO₄/C particles synthesized by the high shear mixer at the stirring rate of 1.3×104 rpm exhibit a reduced particle size of 220 nm,performing high discharge capacity of 160.1 mAh·g^-1 at 0.1 C and even high rate capacity of 90.8 m Ah·g^-1 at 20 C.Secondly,non-ionic surfactant Tween-80 is added during the precursor preparation process on the basis of high shear mixer assisted hydrothermal method.It is indicated that the use of Tween-80 can successfully reduce the grain size and modulate the crystal growth along the（010）facet of LiFePO₄ crystal.The LiFePO₄ particles synthesized via the Tween-80 modified hydrothermal method exhibit small mean diameter of ¹00 nm and a large I（020）/I（111）ratio of 1.19,which performing excellent discharge capabilities of 166.5 mAh·g^-1 and 119.6 mAh·g^-1 at the current rates of 0.1 C and 20 C,respectively.Lastly,a three-dimensional porous LiFePO₄/C composite with nano-sized LiFePO₄ particles embedded in an interconnected carbon network is successfully synthesized using unique Fe-based metal organic framework MIL^-100（Fe）as a porous template and the starting material of Fe and C by a carbothermal reduction reaction under reducing atmosphere.Then,the prepared LFP/CNWs composites are mixed with melamine to synthesize nitrogen-modified LFP/N-CNWs composites,expecting to further increase the electrical conductivity and consequently enhance the electrochemical performance of Li FePO₄ cathodes materials.This N-CNWs acted as a continuous conductive framework for the embedded Li FePO₄ nanosparticles,which increases the contact area between electrode materials and electrolyte,and then fasts the diffusion coefficient for electron and Li-ion,resulting in efficient utilization of lithium iron phosphate and high rate electrochemical performance.This synthesized LFP/N-CNWs composites deliver excellent discharge capacities of 161.5 mAh·g^-1 and 93.6 mAh·g^-1 at the current rate of 0.1 C and 20 C,respectively.