Research On Transition Metal Fluoride Cathode For Lithium/sodium Ion Batterie

The development and utilization of new energy is an effective means to solve the current environmental pollution and energy shortage.However,many clean energy sources cannot be directly used by human beings.It is feasible to convert them into controllable,clean and economical electric energy.Due to its high output voltage and high energy density,lithium-ion batteries（LIBs）have been a popular option among these as mature energy storage and conversion devices in electric vehicles,wearable technology,and smart grids.However,conventional intercalation-type cathode materials for LIBs,such as LCO and NMC,are very close to their theoretical limits,and further increase in their energy density may compromise battery safety.As a result,performance breakthroughs in materials research are required.By transferring two or three electrons per metal atom/ion,it’s indeed possible to achieve the release of all potential electrochemical reactions in the lithiation and de-lithiation of perovskite metal oxides,which appears to provide an important reference option for obtaining maximum specific capacities.Additionally,compared to the intercalation material,the mechanism of the conversion reaction between the electrode and the alkali metal does not depend on the size of the alkali metal ion,which theoretically results in improved cycling stability.There are now still many unanswered questions regarding the conversion and storage mechanisms of the TMFs-based cathode in lithium/sodium ion batteries.Moreover,due to variety and complexity of the electrochemical environment,it is still very difficult to determine how the catalytic activity of excess metals impacts the organic components in the cell.The special electronic structure properties of 3d transition metal element allow the analysis and study of relevant scientific problems from a magnetic point of view.Transition metal fluoride cathode is usually accompanied by corresponding magnetic changes during the charge/discharge（electron transfer）process,and the magnetic changes are closely related to the changes in valence,particle size,and structure of the alkali metal ion battery during electron transfer,so we have combined magnetic testing with electrochemical testing and constructed a highly sensitive in-situ real-time magnetic testing device independently,which provides strong support for tracking transition metal magnetic changes The device provides strong support for tracking the magnetic changes and charge transfer of transition metals.In this study,the transition metal fluoride conversion mechanism and catalytic mechanism are investigated based on in situ magnetic testing technique,as well as the storage mechanism of alkali metal ions through kinetics of the material is also systematically investigated.The two main components of the contents are as continues to follow:（1）To reveal the influence of the evolution of CEI on the electrochemical properties of CoF₂ surface.In this study,we first prepared CoF₂@C cathode materials with carbon nanotubes on the surface by a two-step synthesis method of high-temperature carbonization and gas fluorination,and then electrochemical tests revealed that CoF₂@C cathodes exhibited superior electrochemical performance in fluorinated electrolytes（FEC）than in conventional carbonate-based electrolytes（EC）without fluorine.By altering the chemical composition and physical characteristics of the CEI,changing the electrolytes has an influence on the electrochemical performance,according to interfacial characterization methods.It was discovered that the original FEC-based CEI was thinner,more uniform,and richer in inorganic Li F than the EC-based CEI,and that it could protect the electrode and reduce electrolyte breakdown over continuous cycling.In addition,in situ magnetic testing of the FEC-based CEI showed that it effectively inhibited the catalytic activity of metal Co,ensuring its stability during lengthy cycling.Among them,we also discovered the transitional mechanism of CoF₂ partial reduction at 1.0 V.By tracking the evolution of FEC-based CEI,this work not only explains the cause of the improved performance of transition metal fluorides but also offers recommendations for future studies on anode CEI containing transition metals.（2）Kinetic analysis of lithium/sodium ion storage mechanism of Fe F₃.In this work,first we prepared anhydrous Fe F₃@C transformed cathode materials with honeycomb shape.In Li-ion batteries,Fe F₃@C displays astounding long cycle stability and excellent multiplicative performance.We investigated this material’s Li⁺storage mechanism using kinetic methods and observed that pseudocapacitance and reversible redox work together synergistically to produce capacitive storage that is dominant.Additionally,we have performed electrochemical studies and investigated how this material is stored in sodium ion batteries.We found that the cathode stores sodium ions comparable to how lithium ions are stored.Finally,we also successfully assembled the lithiated Fe F₃@C with graphite into a full cell,revealing its practical value.This work not only prepared the transformed cathode material with lithium/sodium ion co-storage but also systematically investigated its lithium/sodium ion storage mechanism,which is a key step for the practicalization of transition metal fluoride.