Synthsis,Characteristics And Electrochemical Performance Of Three-dimensional Porous LiFePO₄/C Composites

In today's era of energy saving and environmental protection,the hybrid electric vehicles?HEVs?and electric vehicles?EVs?have emerged and become more and more popular.Lithium ion batteries?LIBs?are widely used as an excellent energy storage device.The performance of lithium-ion battery mainly depends on the cathode material.The olivine lithium iron phosphate?LiFePO₄?is considered as one of the promising cathode materials due to its high specific capacity(170 mAh g^-1),long cycle life,high safety performance and environmental friendliness.However,the low lithium-ion diffusion rate and the poor electron transfer ability greatly restrict the wide application of LiFePO₄ in the power battery field.Therefore,how to effectively improve the electronic conductivity and lithium ion diffusion rate of LiFePO₄ has become a hot research topic.The porous structure can provide rapid diffusion and transfer channels for lithium ions and electrons,increase the interface area of the active material/electrolyte and reduce the inert zone in the material.The carbon coating can effectively improve the electronic conductivity and restrain the material agglomeration.In this thesis,the porous Li FePO₄/C composites were prepared via the directional freeze-drying and hydrothermal methods with graphene modification.The morphology,phase composition and chemical structure of the composite materials were characterized and analyzed with the scanning electron microscopy,transmission electron microscopy,X-ray diffraction and Raman spectroscopy.The electrochemical performances of the composite materials were investigated by the charge-discharge test,cyclic voltammetry and electrochemical impedance spectroscopy.The main conclusions have been summarized as following:?1?The carbon-coated LiFePO₄ composites prepared by the directional freeze-drying showed a regular ordered three-dimensional porous structure.The unique ordered porous structure provides directional high-speed transport channels for lithium-ion and electrons,improving the kinetic properties of the material.Besides,the large number of nanopores in the porous wall could promote the impregnation of the active material with the electrolyte and effectively relieve the disruption of the material structure caused by volume expansion during the charging/discharging process.Therefore,the carbon-coated three-dimensional oriented porous Li FePO₄ composite?DLFP/C?exhibited high specific capacity,rate characteristics and cycle stability.?2?The three-dimensionally ordered porous graphene and LiFePO₄ composite materials?LFP/GA?were successfully synthesized by the directional freeze-drying method.The graphene sheets are uniformly embedded in the three-dimensional porous framework formed by the LiFePO₄ particles and are closly contacted with the LiFePO₄particles to speed up the electron transfer of the material.The ordered three-dimensional porous structure enriches the diffusion pathways of lithium ions and electrons and improves the lithium ion insertion/extraction kinetics.The introduction of graphene increases the specific surface area of the material,provides more active sites for Li⁺and increases the electrode/electrolyte interface area,thus,the electrochemical performance of Li FePO₄ is further improved.?3?The graphene-coated LiFePO₄ composite?LFP/GA?prepared by a hydrothermal method shows a three-dimensional porous spherical structure.The graphene coating and the construction of the three-dimensional conductive network effectively connect the isolated LiFePO₄ microspheres,coordinate the electron and lithium ion transfer process,and enhance the specific capacity,rate performance and cycle stability of the material.The suitable graphene coating state is the key to the successful modification.The LFP/GA-10%composites showed the best electrochemical performance,with a specific capacity of 168 mAh g^-1 at 0.1 C,a specific capacity of 155 mAh g^-1 at 1 C rate and a capacity retention of 96.3%after 800cycles.