Design And Performance Study Of CoF₂ And FeF₃ Cathode Materials For Lithium Battery

Metal fluorides are highlighted as new cathode materials for next-generation lithium and lithium-ion batteries due to their significant advantages such as high theoretical specific capacity,moderate operating voltage,and low price.However,the practicality of metal fluoride cathodes typically suffers from the rapid capacity decay for their poor electronic/ionic conductivity and unstable electrode-electrolyte interphase.In this thesis,two honeycombed metal fluoride（CoF₂ and FeF₃）composite cathodes are firstly developed,and the feasibility of using Prussian blue precursor to synthesize FeF₃ cathode is proved then the synthesis method is further optimized.Besides,the FeF₃ cathodes are successfully coupled with Li metal-free anodes to construct lithium-ion full cells,which provides a platform for the future research on metal fluorides.The main research achievements are as follows:（1）The Co（NO₃）₂/PVP gel prepared by sol-gel method is carbonized at high temperature then fluorinated at low temperature to synthesize the honeycombed CoF₂@C nanocomposite with high specific surface area up to 180.4 m2 g^-1,in which the CoF₂ nanoparticles with size of 5-25 nm are evenly embedded in the honeycombed carbon framework.The as-produced CoF₂@C nanocomposite can deliver a high-capacity utilization of～365 m Ah g^-1（far higher than commercial sample）and an average capacity retention of 81.9%over 300 cycles at a current density of 110 m A g^-1,as well as a reasonable capacity of～205 m Ah g^-1 at 1100 m A g^-1.Such excellent electrochemical performance is due to the unique configuration that achieves the nanoconfinement of conversion reaction in the metal fluoride cathode.（2）A honeycombed FeF₃@C nanocomposite cathode is successfully synthesized by the same method,in which the FeF₃ nanoparticles（less than40 nm）uniformly embedded in the honeycombed carbon matrix.The high-loaded honeycombed FeF₃@C(～3.5 mg cm^-2)cathode offers a reversible specific capacity of 369.9 m Ah g^-1 after 500 cycles in a Li coin half-cell,corresponding to noticeable areal capacity of～1.25 m Ah cm^-2.Furthermore,it also can be run in Li pouch half-cell with cathode area of84 cm² delivering average nominal capacity of～50 m Ah each cycle,and coupled with prelithiated Si anode in a full-cell.Research indicates that the honeycombed FeF₃@C cathodes show partially reversible phase transition in a complete discharge/charge,which offers abundant fresh phase interface,contributing to growing pseudocapacitance and Li⁺migration ability as the cycle proceeds.The pseudocapacitance compensates for lost capacity caused by incompletely reversible phase transition,and enables superior electrochemical properties.（3）Microcubic FeF₃@C composite,where the nanosized FeF₃particles（<40 nm）are encapsulated by graphitized carbon and linked through surrounding amorphous carbon matrix,is synthesized through the Prussian blue microcubes.When using as the cathode of coin-type lithium batteries,it can achieve stable and ultralong lifespan（over 1000 cycles）at FeF₃ mass loading of～2 mg cm^-2,ascribing to the compact and thick wrapping of carbon shell and stable cathode solid electrolyte interphase（CEI）during cycling.Besides,the FeF₃-Li pouch cell,FeF₃ full batteries with pre-lithiated Li₄Ti₅O₁₂ and pre-lithiated mesocarbon microbeads anodes are successfully constructed.To interpret the capacity rising of as-constructed FeF₃ cathodes within initial cycles,the detailed electrochemical behaviors and electrode kinetics are investigated.The results show that the decay of the high-potential intercalation process cannot catch up with the activation of the low-potential conversion reaction during FeF₃ discharge is the reason for their rising capacity within initial cycles,accompanied by the increased Li⁺diffusion coefficient（resulted from the amorphization of FeF₃ particle）.（4）A brief manufacturing process to fabricate the FeF₃/C nanocomposite is proposed,where the commercial Prussian blue powder was sequentially carbonized and fluorinated in one pot.Research shows that the optimal carbonization temperature is 600°C.The FeF₃/C nanocomposite synthesized under carbonization temperature of 600°C combined with 3D porous free-standing cathode architecture can demonstrate long-term stability over 760 cycles along with the record areal capacity of～4.3 m Ah cm^-2 at ultrahigh mass loading of～11.3 mg cm^-2,suggesting the remarkable practicality.As-reported synthesis strategy endows the merits of cheap raw materials,high yield,facile synthesis,and scalability,which can reconcile the current contradiction between cost-ineffectiveness synthesis in lab and poor electrochemical performance of industrialized FeF₃ powder,and shed the light on future investigation even industrialization.