Study On Synthesis Of Carbon-based Transition Metal Fluoride Composites And Properties Of Lithium-ion Batteries

As an important electronic component,lithium-ion batteries play an important supporting role in the performance of electronic products.However,the energy density of lithium-ion batteries composed of Li Fe O₄,Li Co O₂ and Li Ni_1/3Co_1/3Mn_1/3O₂ traditional cathode materials is close to its theoretical limit.Therefore,it is imperative to develop cathode materials for lithium batteries with higher capacity.Transition metal fluorides,due to the high electronegativity of fluorine,make them have a relatively high voltage platform,so they can be used as positive electrode materials;Phytocathode materials have higher theoretical specific capacities,making them promising candidates for next-generation high-energy-density batteries.However,due to the strong ionic bond characteristics,transition metal fluorides have low ionic and electronic conductivity,resulting in high voltage hysteresis and poor electrochemical reaction kinetics;there is also severe volume expansion during electrochemical conversion,which deteriorates the cycle Stability,reduced Coulombic efficiency.In order to solve these problems,this paper mainly constructs carbon-based transition metal fluorides with different structures,studies the relationship between electrode material structure and electrochemical performance,and clarifies its electrochemical stability mechanism.The main research contents are as follows:（1）One-step fluorination-etching technique to prepare porous Fe F₃ composite cathode material.Using glucose as the carbon source,ferric nitrate hexahydrate as the iron source,and silica aerogel as the pore-forming agent,an iron-carbon mixture intermediate was generated by high-temperature carbonization;then,polytetrafluoroethylene（PTFE）was used as the The fluorine source,the monomer produced by pyrolysis can not only fluorinate the carbonized iron-carbon composite,but also etch the silicon dioxide in it,leaving abundant pores in the matrix,thereby constructing Fe F₃ with a porous structure（p-Fe F₃@C）composites.The rich pore structure of the composite material can provide more ion transmission paths and improve the rate performance of the electrode;at the same time,the carbon fluoride generated in situ during the high-temperature fluorination process can overlap with the electron cloud in Fe F₃,improving the conductivity of the electrode material sex.The electrochemical performance test proves that the p-Fe F₃@C composite has excellent electrochemical performance.The p-Fe F₃@C electrode has a high specific capacity of 230 m Ah g^-1 at a current density of 0.1 C(1 C=237 m A g^-1);the capacity retention rate is92.5%after 200 cycles at a current density of 1 C.Analysis of the surface components of the electrode after cycling shows that the structure can induce the formation of a uniform,thin and stable Li₂CO₃/Li F-rich cathode-electrolyte interface（CEI）,which is conducive to structural stability and cycle stability.In addition,Li-ion full cells assembled with pre-lithiated graphite anodes also exhibit excellent electrochemical performance.（2）Construction of cubic structured carbon cage limited Ni F₂-Fe F₂ materials based on Prussian blue for lithium metal full batteries.A lithium-free cathode material（C@Ni F₂-Fe F₂）with a cubic structured carbon cage limited domain Ni F₂-Fe F₂ bimetallic fluoride was prepared by using nickel ion-exchanged Prussian blue as the precursor,silica as the structural limiting agent with polydopamine coating,and PTFE as the fluorine source for fluorination and etching.The hollow structure of the carbon cage not only improves the electrical conductivity of the material but also effectively alleviates the volume expansion of the active material inside,thus further enhancing the electrochemical performance of the metal fluoride.The half-cell test results show that C@Ni F₂-Fe F₂ lithium free cathode material with the current density is 0.1 C(1 C=560.8 m A g^-1)has a high specific capacity of490 m Ah g^-1;at a current density of 1 C,it still has a specific capacity of 320 m Ah g^-1 after1000 cycles.On the negative side,based on the restricted domain structure of the carbon cage,the transition metal fluoride can be used as a pro-lithium material to induce the preferential deposition and encapsulation of lithium metal inside the carbon cage,and the Li F generated by the reaction between the transition metal and lithium can enhance the stability of the solid electrolyte interface film（SEI）,so it is conducive to suppressing the volume expansion and dendrite growth of lithium metal and achieving the cycle stability of the lithium metal negative electrode.Electrochemical performance tests show that:Li||Cu/C@Ni F₂-Fe F₂ half cell with the current density is 0.2 m A cm^-2 and surface capacity0.2 m Ah cm^-2 conditions can be stabilized for 500 cycles;the assembled C@Ni F₂-Fe F₂@Li||C@Ni F₂-Fe F₂@Li symmetrical cell can be cycled stably for 1200 h at a current density of 0.5 m A cm^-2 and a surface capacity of 0.5 m Ah cm^-2 with an overpotential of 38m V.The lithium-metal full cell assembled with C@Ni F₂-Fe F₂ after depositing 3 m Ah has an initial specific capacity of 598.5 m Ah g^-1 at 1 C current density and remains at 486.2 m Ah g^-1 after cycling to 50 turns with a capacity retention rate of 81.2%.（3）Preparation of high entropy metal fluorides in the carbon cage-limited domain.Spherical high-entropy Prussian blue analogs（HE-PBA）containing five metal elements were prepared by co-precipitation method using salt solutions of five metals Fe\Co\Ni\Mg\Mn;subsequently,a carbon-cage-limited high-entropy metal fluoride（C/HE-PBA-F）was constructed by Si O₂ surface modification and PTFE etching.The spherical shell of the carbon material not only confines the high-entropy fluoride inside the carbon cage,but also enhances the conductivity of the high-entropy fluoride;at the same time,the synergistic effect between different metal elements of the high-entropy material can both enhance the specific capacity of the electrode and stabilize the structure of the high-entropy material.The half-cell test results show that the specific capacity is 463.3 m Ah g^-1 at a current size of 0.05A g^-1;the discharge capacity is still maintained at 96.8%after 500 cycles at a current size of0.2 A g^-1 and 252.3 m Ah g^-1 after 1300 cycles.the kinetic performance test results show that the C/HE-PBA-F electrode has higher lithium-ion diffusion coefficient and reactivity.The SEM analysis of the electrode morphology after cycling showed that the active material maintained a more regular spherical structure after 200 cycles.