Research On Impact Flash Technology For LiFePO₄ Powder

LiFePO₄ is currently a mainstream cathode material for lithium batteries,however,the widespread use of Li Fe PO₄ is accompanied by many safety accidents.Recent studies have shown that the safety and electrochemical performance of Li Fe PO₄ degradation is closely related to the structural damage of its particles.Therefore,improving the strength of Li Fe PO₄ particles to reduce or avoid structural damage to the particles has become the key to improve the safety and electrochemical performance of the material.In this thesis,inspired by Flash Joule heating technology,a new powder particle modification technique is developed to change the property of Li Fe PO₄ by increasing its particle strength.The technology is named impact flash technology,its principle is:the high temperature and high pressure generated by the dynamic high pressure impact on the powder sample,while the strong interaction between the particles makes the local surface of the particles into a molten state,at this time through the pressure and temperature release mechanism--the slotted diaphragm at the front of the sample chamber,the diaphragm compression rupture of the partially molten particles instantaneously released into the external environment of normal temperature and pressure,to complete its rapid cooling and pressure relief.Due to this rapid temperature and pressure releasement,the molten state of the particle surface will changes into a glassy state,just like a dense shell that covers the surface of the particles.This thesis begins with the design of each device in the technology.The structure and dimensions of the new diaphragm-breakable target were designed to avoid the influence of sparse waves while meeting the functional requirements.The geometric structure and material of slotted diaphragm were designed by static pressure fracturing experiment and fractal theory.The purpose was to evaluate the stability of diaphragm fracturing and pressure release speed,and finally to provide reference for diaphragm selection in impact flash experiment.Based on the light gas gun,the high-speed impact flash experimental platform was built to conduct impact flash experiments on Li Fe PO₄ powder samples,and the recovered sample was analyzed by XRD to check whether the material had phase transition.In order to improve the problems and analysis results in the high-speed impact flash technology,we developed the impact flash technology with controllable sample pressure and temperature,conducted experiments again,and analyzed the experimental results by XRD,SEM and TGA-DSC to test whether the technology can improve the strength of Li Fe PO₄particles.The main conclusions obtained during the research process are as follows.（1）The impact speed is too high will cause damage to the device structure as well as the crystal structure of Li Fe PO₄.In the experiment,the diaphragm will break in advance,and the energy cannot fully applied to the sample,resulting in the high-speed impact flash technology can not achieve the ideal effect.（2）The impact flash technology can control the initial temperature of the sample and the pressure before breaking the diaphragm;the diaphragm made of 6061-T6 aluminum alloy with an effective thickness of 0.5 mm and a groove length of 32 mm can break sufficiently and quickly under high pressure;the rotation of the split pellet with the special structure of the circular sample chamber can enhance the interaction between the particles.（3）The characterization analysis of the recovered samples after the impact flash experiment shows that the technique has excellent ability to modify the morphology of Li Fe PO₄particles,which can effectively reduce the particle size and increase the sphericity,thus improving the particle strength and enhancing the safety and electrochemical performance of Li Fe PO₄.