Solvothermal Synthesis And Lithium-ion Storage Properties Of Iron Based Perovskite Fluorides

In order to alleviate environmental pollution and energy crisis,efficient and environmentally friendly energy storage devices have made great progress in the past few decades.Lithium ion batteries（LIBs）have been successfully commercialized.However,the shortcomings of commercial anode materials such as slow dynamics,low theoretical specific capacity and poor safety seriously hinder the further development of lithium-based storage devices,so it is urgent to design and develop new high-performance anodes for lithium ion storage.In recent years,perovskite fluorides（KMF₃）have attracted extensive attention in energy storage for their"soft"crystal structure,wide electrochemical working potential and abundant electrochemical active sites.From the perspective of lithium ion storage,its octahedral framework filled with K atoms provides a three-dimensional transport path for lithium ions,thus it has high ionic conductivities,and the high reversible capacity of two electrons can be provided by the conversion reaction of transition metals(M^II)theoretically.Therefore,these compounds are promising anode materials for lithium ion storage.Unfortunately,the K-M-F bond is a strong ionic bond,resulting in a wide band gap in KMF₃which is not conducive to electron transport,so the electrochemical performance of unmodified KMF₃is poor.In this thesis,firstly,KFeF₃was modified by carbon engineering to construct electron transport path.Then,the effect of entropy on electrochemical performance of KMF₃is further explored by using the high entropy strategy,which provides a new idea for the design of the next generation of electrode materials.The relevant research results are as follows:（1）High purity cubic perovskite fluoride KFeF₃nanocrystals were prepared by one-step solvothermal method and KFeF₃-CNTs was prepared in the same way by adding one-dimensional carbon nanotubes with good electronic conductivity.The electrochemical impedance spectroscopy（EIS）of the KFeF₃electrode was decoupling by relaxation time distribution（DRT）during the first discharging/charging process and attained four characteristic time constants（τ₁,τ₂,τ₃,andτ₄）.The addition of CNTs reduced the charge transfer resistance of KFeF₃,and the specific capacity of KFeF₃-CNTts was 381.4 m Ah g^-1after 180 cycles at the current density of 100 m A g-1.（2）Ti₃AlC₂was etched by in-situ HF formation of LiF/HCl,and a clay-like MXene was further exfoliated by ultrasound.Then,KFeF₃was anchored on MXene nanosheets by solvothermal method.The synergistic effect between KFeF₃and MXene was conducive to improving the mixed conductivity of ions and electrons,speeding up ion transport and charge transfer.CV results show that the dynamic process of lithium storage in KFeF₃-MXene is controlled by the mixture of pseudocapacitance and diffusion,and the discharge capacity of the half cell at 0.1-3.2A g^-1is 133-509 m Ah g^-1.When KFeF₃-MXene negative electrode is matched with LiFePO₄positive electrode,the discharge specific capacity of the full cell is 103 m Ah g^-1（calculated by the mass of the positive electrode）after 100 cycles under the current density of 50 m A g^-1,and the capacity retention rate reaches 86%,showing high practicability.（3）Medium entropy/high entropy perovskite fluoride（TM4/TM5）was prepared by mild ball milling combined with hydrothermal method.The results showed that the five transition metal cations were disordered distributed in the M position of KMF₃,and the increase of configuration entropy was realized by the increase of components.The charge-discharge curves of TM5 and TM4 have no obvious platform after the first discharging process,showing the lithium storage is controlled by pseudocapacitance,and the reaction is highly reversible.At the current density of 100 m A g^-1,TM5electrode has a specific capacity of 389 m Ah g^-1after 50 cycles,and has a higher capacity retention and better cycling stability than TM4 electrodes.The improvement of electrochemical performance may be related to the entropy stabilization effect of high entropy materials and the synergistic effect of various transition metal cations.The results of ex-situ XRD and TEM indicate that the slow diffusion effect of the high entropy material protects the lattice structure well during the first diacharging process,and could reduces the volume strain during the charge/discharge cycling,which is conducive to the cyclic stability of the electrode.In this thesis,there are 60 figures,7 tables and 137 reference articles...