Synthesis And Performance Of Iron-based Phosphate Electrode Materials For Na-ion Batteries

In the past decades,great progress has been made in the application of lithium-ion batteries（LIBs）.Due to the less lithium resources and the increasing demand,the cost of LIBs has increased a lot.Sodium-ion batteries（SIBs）have the similar chemical properties to that of the LIBs and the cost of SIBs is much lower.So SIBs have the potential to be a sustainable choice for LIBs.Among the investigation for electrode materials of SIBs,phosphate materials have gained much attention for the stable structure and safety performance.And the iron-based polyanionic compounds are also the important part for the electrode materials of SIBs,due to the low cost and environmental friendliness.In this thesis,the research content concern on the synthesis and performance of iron-based phosphate electrode materials for Na-ion batteries.Firstly,the subsolid us phase relation of Na₂O-FeO-P₂O₅ and electrochemical propriety of Na_2-2x-2x Fe_1+x+x P₂O₇ have been determined.Secondly,we synthesis a new sodium ferrous orthophosphate Na_x Fe₄（PO₄）₃（1.1<x<1.2）and study the electrochemical propriety for the first time.（1）The subsolidus phase relations of Na₂O-FeO-P₂O₅ ternary system under the 95%Ar+5%H₂ atmosphere has been systematically investigated by X-ray diffraction（XRD）.There are 8binary compounds,5 ternary compounds and 19 three-phase regions within this ternary system.In this system,the Na_2-2x-2x Fe_1+x+x P₂O₇ solid solution phase with the homogeneous range of-0.05≤x≤0.22 is determined.The corresponding lattice parameters are obtained by the refinement results and decrease with the x.And the electrochemical proprieties of Na_2-2x-2x Fe_1+x+x P₂O₇ with x content as cathode material for sodium-ion battery are investigated by galvanostatic charge/discharge mode.The results show that the excess Fe-ion（x>0）has an unfavorable effect on the electrochemical proprieties by decreasing the ionic conduction of the compounds.（2）A new sodium ferrous orthophosphate Na_x Fe₄（PO₄）₃（1.1<x<1.2）with P21/n symmetry has been successfully synthesized via a solid-state reaction for the first time.The structure consists of FeO₅ pyramids,FeO₆ octahedron and PO₄ tetrahedra which form large six-sided tunnels along^b direction and three-or four-sided tunnels running along^a direction.When evaluated as the anode materials for sodium-ion batteries,the first discharge and charge capacities of 553 and 264 mAh g^-1 for Na_1.1Fe₄（PO₄）₃/C and 583 and 315 mAh g^-1for Na_1.2Fe₄（PO₄）₃/C,respectively,can be obtained at 10 mA g^-1 current density.Compared to the Na_1.1Fe₄（PO₄）₃/C,Na_1.2Fe₄（PO₄）₃/C exhibits a better cycling performance,where the charge capacity of about 100 mAh g^-1 can still be maintained after 15 cycles.