Study On Preparation And Performance Of Dual Ionic Thin Film Electrode Materials In Lithium-Ion Battery

Many efforts have been carried out for finding new high-performance storage-lithium materials in lithium-ion batteries.In the lithiation/delithiation processes,the traditional electrode materials often only offer single redox center site.Each cell unit can provide a kind of electrochemical redox reaction because of alone reaction site,which greatly restricts the electrochemical performance.Dual ionic materials,especially bianry cation structures,can provide two redox centers.Potential coupiling effect could exhibit advantages that traditional materials do not possess,and would be successfully applied to next-generation lithium ion batteries.However,the candates of novel materials are restricted by traditional synthesis method.Pulsed laser deposition?PLD?technology compared with the general electrode material preparation technology,it is regarded as a mature thin film deposition technology and easily realize the tailor of material components,especially some electrode materials are difficult to synthesize,such as fluoride,oxy-fluoride,and nanocomposite system.In addation,because the thin film electrode does not need to add conductive agents,it has been considered as an ideal system for electrochemical mechanism research.In this paper,series of dual ionic typical thin film materials are deposited by PLD,which are used as electrodes of Li-ion batteries.The charge-discharge performance and electrochemical reaction characteristics of these films were measured by galvanostatic charge-discharge and cyclic voltammetry.X-ray diffraction?XRD?,high-resolution electron microscopy?HRTEM?,selected area electron diffraction?SAED?,atomic force microscopy?AFM?,and X-ray photoelectron spectroscopy?XPS?are used to determine the composition of the materials in their electrochemical processes.The structure and structure have been tested and characterized to explore the electrochemical reaction mechanism of these nanometer thin films.It mainly includes the following systems:?1?Fabrication of dual ionic typical Ni Fe₂O₄ Thin Films and Electrochemical Characteristics.NiFe₂O₄ nanocomposite thin films are successfully fabricated by pulsed laser deposition by adjusting composition of targets and deposition parameters.Through the characterization of its electrochemical processes and the changes in the composition and structure of the films during charge and discharge,their electrochemical properties characterization and detecting composition and structure changes in charge and discharge,The large irreversible capacity loss could be ascribed to the irreversible reaction of the electrolyte in the first cycle,generated metallic Ni and Fe nanoparticles and Li₂O,followed by the reversible reaction of NiO and Fe₂O₃.After the first cycle,the lithiation/delithiation of the electrode material went through a similar reaction course and showed a certain cycle performance.Due to the irreversible reaction in the first cycle,the capacity loss of the electrode material is relatively large,and the subsequent cycle performance is poor due to the conversion reaction cause the volume expansion?2?Electrochemical Performance of LiPON coated NiFe₂O₄ thin Film Electrode.LiPON as an"artificial SEI layer"is generally used to inprove the cyclic stability of volume expansion materials.A 50nm thick LiPON protective layer is coated on the surface of the Ni Fe₂O₄ film electrode by magnetron sputtering to fabricate LiPON coated NiFe₂O₄ nanocomposite thin film electrode,and its electrochemical performance is studied.At the current density of 5?A/cm² for 50 cycles,LiPON coated NiFe₂O₄ is obviously superior to the electrochemical performance of Ni Fe₂O₄.Through SEM and TEM characterization,due to the presence of the LiPON coated layer,the volume of the thin film electrode is significantly suppressed in charging and discharging.At the same time,XRD spectra shows that the LiPON coated did not affect the structure of NiFe₂O₄.?3?Preparation and Electrochemical Characteristics of LiF-NiFe₂O₄ nano-composite cathode thin Films.LiF-NiFe₂O₄ nanocomposite cathode electrode is prepared by PLD.Ultilizing the formed M-O-F structure during the charging process,the electrochemical redox potentials of the nanocomposite material are increased,and the NiFe₂O₄ negative electrode is converted into a positive electrode.Electrochemical characterization results show that the average potential is above 2.5 V because of the exsiting of dual redox reaction.The initial discharge can obtain a specific capacity of235 mAh g^-1,followed by a reversible capacity of about 220 mAh g^-1,and an average capacity attenuation of less than 0.2%for every cycle.The electrochemical reaction and reaction mechanism in the reversible process have been studied.The charge-discharge reaction mechanism is different from the traditional lithiation/delithiation processes.It is a reversible conversion reaction and mainly includes the reversibility Conversion reaction of M-O-F framework and LiF-MO nanocomposites.The research in this dissertation has a certain reference value and guiding significance for exploring the synthesis of novle dual inonic electrode materials for secondary lithium ion batteries,surface modification and modification of negative electrode materials,the surface capacitance effect of high potential positive electrode nanomaterials and its influence on electrochemical properties.These new electrode materials also have good application value in solid-state thin-film batteries.