Syntheses,Crystal Structures,and Properties Of Several Novel Polyanionic-based Cathode Materials For Sodium-ion Batteries

Energy shortage and environment pollution have threatened the survival and development of human society.In order to maintain the sustainable development of human society,it is very crucial to develop renewable resources.In this situation,the development of efficient and convenient energy storage technology has become a research hotspot of researchers to meet the needs of energy.The electrochemical energy storage is the most suitable for energy development.In the past three decades,the Li-ion batteries(LIBs)have been developed deeply and used extensively for electronic products,electric and power grid storage.Due to the scarcity of material and high-cost,further development of Li element is limited.Therefore,it has become a hot topic to research new sodium ion batteries(NIBs)in the energy storage field.The traditional lithium and sodium ion batteries are mostly transition metal oxides,in recent years,the polyanionic cathode materials have become the research focus of lithium and sodium ion battery electrode materials.Compared with other cathode materials,the polyanionic compounds have higher voltage and thermal stability.According to the anion species,it can be divided into phosphates,borates,sulfates and molybdate positive materials.In this paper,four novel crystal have been prepared by high temperature solution reaction,including Nax My(Mo O4)3(M= Mn,Fe,Co;x=2.67,3;y=1.67,1.5)and Na0.48Mn1.22PO4.Polycrystalline samples were synthesized by high temperature solid state reaction and sol-gel method.Herein,we mainly report the synthesis,structure and other properties of four novel compounds.We synthesized the Nax My(Mo O4)3 by high temperature solution reaction,it belongs to the alluaudite-type structure compounds.Na2.67Mn1.67(Mo O4)3 crystallizes in the space group C2/c(No.15)of the monoclinic system with unit cell parameters a=12.7445(16)?,b=13.6694(17)?,c=7.1892(9)?,β=112.459°,V=1157.4(3)?3 and Z=4.Na3Co1.5(Mo O4)3 crystallizes in the space group C2/c(No.15)of the monoclinic system with unit cell parameters a=12.6202(18)?,b=13.5668(19)?,c=7.1687(10)?,β=112.24°,V = 1136.1(3)?3 and Z=4.Na3Fe1.5(Mo O4)3 crystallizes in the space group C2/c(No.15)of the monoclinic system with unit cell parameters a=12.6905(17)?,b=13.6561(19)?,c=7.1792(10)?,β=112.62°,V=1148.48(100)?3 and Z=4.M atom is coordinated with six O atoms to form a distorted octahedron,which are coupled by edge sharing to form Mn2O10 dimer.A two-dimensional infinite sheet is consist of M2O10 dimers and Mo(2)O4 tetrahedra through sharing corner.The sheets are linked together by Mo(1)O4 tetrahedra along a direction,to form a three-dimensional framework.The morphology and element content were analyzed by SEM and EDX.The ball-milled samples of Nax My(Mo O4)3 were electrochemically tested at room temperature in coin cells with sodium metal as the reference and counter electrodes,we also studied the charge/discharge curve of Nax My(Mo O4)3 at different current rates.The first discharge capacity of Na2.67Mn1.67(Mo O4)3 can reach 92 m Ah/g with an average voltage of 3.5V.Na0.48Mn1.22PO4 crystallizes in the space group R-3(No.148)of the trigonal system with unit cell parameters a = 15.3672(17)?,c = 43.503(5)? and V = 8896.91(2)?3.The polyhedrons of Mn(1)to Mn(9)are interconnected with each other through sharing corners,edges and faces to construct a 3D Mn-O-Mn network.The PO4 tetrahedrons reinforce the 3D Mn-O-Mn network.The Na atoms reside in the cavities of network.The ball-milled samples of Na0.48Mn1.22PO4 were electrochemically tested at room temperature in coin cells with sodium metal as the reference and counter electrodes.Na0.48Mn1.22PO4,with an average voltage of 3.2 V,can be very effectively cycled against Na metal electrode.In addition,there exists small amounts of Mn3+ by XANES analysis,and magnetic analysis indicate that the predominant spin exchange interactions are antiferromagnetic in the material.