Study On Synthesis And Performance Of Iron-based Phosphate As Cathode Materials For Lithium/Sodium-ion Storage

Because of the advantages of good safety and low cost,iron-based phosphate materials are useful as cathode materials for batteries.However,the problem of low intrinsic conductivity and slow diffusion of ions,limits their industrial applications.In this paper,LiFePO4F and FePO4 two kinds of iron-based phosphate as the main research object.In order to improve the conductivity of the material,by means of the particle nano-sphericalization,Ag nanoparticles surface modification and the composite structure of the bowl-shaped structure with carbon,to improve their the electrochemical performance.LiFePO4F nanospheres with high purity were synthesized by a solid-state route associated with chemically induced precipitation method.The materials prepared under different conditions were tested.Cyclic voltammetry results show that the redox peak(3.09 V/2.61 V)corresponds to the Fe2+/Fe3+ redox couple in the two-phase transition of Li2FePO4F/LiFePO4F.Its electrochemical behaviors results show that the initial discharge capacity is 110.2 mAh g-1 at 0.5 C,after 200 cycles is still retained 104.0 mAh g-1 with the retention rate of 94.4%.The excellent cycle performance is mainly attributed to the uniform nanospheres-like morphology which is not only beneficial to shorten the transport distance of ions and electrons,but also improve the interface area between electrode and electrolyte,and thus improve the kinetics of Li ions.In order to further improve the electrochemical performance of LiFePO4F,Ag decorated LiFePO4F nanospheres have been synthesized via a precipitation method with in-situ reduction of Ag+.The Ag nanoparticles with size of-10 nm are in-situ grown on the surface of LiFePO4F nanospheres with impressive electrochemical performance.It delivers a high discharge capacity of 148.7 mAh g-1 at 0.1 C.It is worth mentioning that the Ag-decorated LiFeP04F nanospheres reveal superior cycling stability.The initial discharge capacities of Ag-decorated LiFePO4F reaches up to 120.3 mAh g-1 at 0.5 C,and the capacity retention is as high as 96.1%after 300 cycles,which is remarkable higher than that of pure LiFePO4F nanospheres.Therefore,Ag modification can significantly increase the electronic conductivity and lithium ion diffusion coefficient of the material,thereby improving the electrochemical performance of the material.we design and construct bowl-like FePO4@MCHB/rGO,in which FePO4 nanoparticles grown on microporous carbon hollow spheres and then are assembled with continuous cross-linked networks of GO,further thermal reduction and get the material.This material is superior to FePO4@MCHB and pure FePO4 materials in lithium ion battery/sodium ion battery.The discharge specific capacity of the material in SIBs at 1C has 122.4 mAh g-1,and there are still 93.5 mAh g-1 after 1000 cycles.The discharge capacity of the material in LIBs at 1C has 119.6 mAh g-1,and after 1000 cycles still has 107.8 mAh g-1,demonstrating excellent cycle stability.Such unique bowl-like structural design not only preserves all the advantages of hollow structures but also increases the packing density.The GO component plays the role of"cushion" as it could quickly realize capacity response,buffering the impact to FePO4 under the conditions of high-rate charging or discharging.Therefore,this unique structural design can greatly improve the electronic conductivity of FePO4,resulting in excellent electrochemical properties in lithium/sodium storage.