Synthesis,Modification And Electrochemical Performance Of LiFePO₄ And LiNi_0.8Co_0.15Al_0.05O₂ Cathode Materials For Lithium Ion Batteries

At present,the energy crisis and environmental problems that the human society confronted with are becoming increasingly serious.Replacing the traditional fuel vehicles with the new energy vehicles is one of the most important ways to alleviate the energy crisis and solve the environmental problems.As a crucial component of the new energy vehicles,the power batteries still have problems in service life,safety performance,energy density and so on,which need to be focused on and intensively studied.In recent years,lithium-ion batteries?LIBs?have been widely applied in the consumer electronics,and rapidly extended to the electric vehicles?EVs?and hybrid electric vehicles?HEVs?.Since the performances of LIBs are greatly depended on the cathode materials,numerous efforts have been made to develop the high-performance cathode materials.In this thesis,we have intensively studied the synthesis,modification and electrochemical performances of the LiFePO₄ and LiNi_0.8Co_0.15Al_0.05O₂ cathode materials.The extremely low intrinsic electronic conductivity and lithium ion diffusivity of LiFePO₄ materials lead to the poor rate performance,and the high sensitivity to environmental and process parameters results in the difficult control of the stability of LiFePO₄ during the process of production.Controlling nanoparticles granularity and surface carbon coating are needed to improve the kinetic properties of the materials,and precise control of the process parameters is needed to ensure the stability of LiFePO₄ materials.Herein,the optimized key process parameters were determined by studying the response mechanism between different process parameters and the performance of LiFePO₄.The dynamic properties of LiFePO₄ were improved by optimizing the synthesis method and developing efficient carbon coating technology.Due to the high surface activity of the LiNi_0.8Co_0.15Al_0.05O₂ materials,the side reactions on the surface of materials easily led to the surface structure degeneration during the electrochemical process,and which finally resulted in many problems such as fast capacity fading and poor safety performance,and hindered the large-scale application of the LiNi_0.8Co_0.15Al_0.05O₂ materials.In this thesis,the compounds with excellent stability are employed to coat the surface or combine with the LiNi_0.8Co_0.15Al_0.05O₂ particles,the cycling and safety performances of LiNi_0.8Co_0.15Al_0.05O₂ were significantly improved.The main research work and conclusions are as follows:?1?The LiFePO₄/C composites have been successfully synthesized by adopting Li₂CO₃ and LiOH·H₂O as the mixed lithium sources.Since the melting point of LiOH is lower than that of Li₂CO₃,better melting state at the same temperature can be achieved when adopting the mixed lithium sources,and the transfer of lithium ions can be promoted during the high temperature synthesis process to obtain the pure phase LiFePO₄.The carbon content of the as-prepared LiFePO₄/C materials has been optimized to be ².0%.The discharge capacity of the as-obtained LiFePO₄/C composites was as high as 158.2 mAh/g,and after 100-day storage the capacity retention of full cells was still higher than 94.0%.With the increase of sintering time,the particle size of LiFePO₄/C increased.When the sintering temperature was higher than 750? and the sintering time was longer than 10 h,the morphology of the LiFePO₄/C particles will be out of control.The optimized sintering temperature and time were determined to be 750? and 10 h.?2?An efficient carbon coating process that innovatively using the mix carbon sources has been applied in different synthesis routes to obtain the LiFePO₄/C composite materials,including the solid state Fe₂O₃ route,FeC₂O₄ route,FePO₄ route and the hydrothermal FeSO₄ route.All of the prepared LiFePO₄/C samples are coated by a thin layer of continuous carbon with high conductivity.A small quantity of calcium compounds formed on the surface of LiFePO₄ particles which favor the surface stability and depress the side reaction between the active material and electrolyte.Therefore,the efficient carbon coating technology introduced in this work enables the excellent overall electrochemical performances of all the as-synthesized LiFePO₄/C samples.Depending on the procedures,the synthesized materials exhibit different features and can be promisingly applied as the potential cathode for lithium-ion batteries for different terminal markets.?3?The co-precipitation synthesized LiNi_0.8Co_0.15Al_0.05O₂ cathode material was modified by a coating layer of TiP₂O₇ through an ethanol-based process.The TiP₂O₇ coating layer can effectively suppress the structural degradation and ameliorate the surface status of the LiNi_0.8Co_0.15Al_0.05O₂ particles,and the intrinsic rhombohedral layered structure of the TiP₂O₇-coated LiNi_0.8Co_0.15Al_0.05O₂ was well maintained during the long-term cycling process,while the surface structure of the pristine LiNi_0.8Co_0.15Al_0.05O₂ was degraded from a rhombohedral R3 m layered structure to a cubic rock-salt NiO type structure.The thermal stability and cyclic performances of the LiNi_0.8Co_0.15Al_0.05O₂ electrode were remarkably improved by TiP₂O₇ coating.The total amount of heat release corresponding to the intensity of thermal runaway was 1075.5 and 964.6 J/g for the pristine LiNi_0.8Co_0.15Al_0.05O₂ and TiP₂O₇-coated LiNi_0.8Co_0.15Al_0.05O₂,respectively.The pouch shaped full cells that employing the TiP₂O₇-coated LiNi_0.8Co_0.15Al_0.05O₂ as cathode were able to perform more than 2200 cycles at 25? and more than 1000 cycles at 45? before the capacity retention fading to 80%.Besides,the LiMn_0.8Fe_0.2PO₄ material with high thermal stability was employed to modify the LiNi_0.8Co_0.15Al_0.05O₂ material,and considerable safety performance,rate capability and cycling stability were achieved.?4?The surface modification of the LiNi_0.8Co_0.15Al_0.05O₂ material was also conducted by FePO₄ coating through a liquid-phase process.The excellent structural and thermal stability of FePO₄ was utilized to improve the long-term reliability and safety of the LiNi_0.8Co_0.15Al_0.05O₂ material.The electrochemical properties of the LiNi_0.8Co_0.15Al_0.05O₂ material with different coating amount were intensively studied.The FePO₄ coating layer was able to prevent the side reaction between the LiNi_0.8Co_0.15Al_0.05O₂ particles and electrolyte,suppress the dissolution of the transition metal from the layered structure,and maintain the long-term stability.The optimized coating content of FePO₄ was 1.0%,and the best overall performance can be achieved in this situation.After 800 charge-discharge cycles,the capacity retention was as high as 95%,and after the 100-day storage at 25?,the capacity retention was still higher than 95%.The energy density,cycle life and safety of the LiNi_0.8Co_0.15Al_0.05O₂ materials were well balanced after the modification.