Studies On The Controlling Synthesis And Electrochemical Performance Of Hydroxyl Ferric Phosphate

As a natural mineral.the crystalline compound of hydroxyl ferric phosphate(Fex(PO4)y(OH)z·nH2O)has been widely used in many fields,such as biological applications,catalysts,lithium ion batteries and so on.Hydroxyl ferric phosphate can be treated as one kind of promising cathode materials in lithium-ion batteries because of its high structural stability,high theoretical capacity and simple synthetic method.Although the theoretical capacity of Fe5(PO4)4(OH)3·2H2O(180 mAh g-1)is higher than that of commercial LiFePO4(170 mAh g-1),the former's high-capacity mechanism for reversible lithiation-delithiation and the actual functions of crystal water(i.e.,nH2O)are still ambiguous so far.In this thesis,the controlling synthesis(i.e.,hollow single-crystalline octahedra and spherical aggregates)and electrochemical performances of hydrated hydroxyl ferric phosphate(Fe5(PO4)4(OH)3·2H2O)are chosen as main topics.Simultaneously,by comparison with the structural and electrochemical properties of hydroxyl ferric phosphate samples with different contents of crystal water,the influence of crystal water in lattice structure on their electrochemical performances is also focused.This dissertation is mainly divided into two parts,which is shown as below.(1)In this part,hollow single-crystalline octahedra of hydrated hydroxyl ferric phosphate(Fe5(PO4)4(OH)3·2H2O,edge length?1.0 ?m,a known orthorhombic phase)and its dehydrated counterpart(Fe5(PO4)4(OH)3,edge length?0.9 ?m,a fitting orthorhombic phase)are uniformly prepared for the first time,facilitating the comparative studies on their structural and electrochemical properties.As a lithium-ion battery cathode within 1.5-4.5 V vs.Lit+/Li,the hydrated sample can deliver an initial discharge capacity of 176.6 mAh g-1at 5 mA g-1,which is close to the theoretical value of 180.0 mAh g-1,By comparison,no matter how many is charge-discharge current rate,reversible capacity of the hydrated microcrystallites in each cycle is much higher than that of the dehydrated counterparts.In a word,it is the solvent of 1,2-propanediol that mainly determines the hydro-/solve-thermal formation of Fe5(PO4)4(OH)3·2H2O hollow single-crystalline octahedra,and it is the presence of crystal water that modifies the cell parameters of orthorhombic lattice and promotes the reversible lithiation-delithiation capability of microcrystallites.(2)In this chapter,spherical aggregates of Fe?(PO4)4(OH)3·2H2O(diameter?1.57?m)are prepared through a solvothermal route at first,then a simple heat treatment(350? or450?)in air atmosphere is used to obtain Fe5(PO4)4(OH)3·13H2O(diameter?1.49 ?m)or Fey(PO4)4(OH);(diameter?1.47 ?m)spherical aggregates,and then,by an ultrasonically complexing method,the corresponding three composites of graphene oxide(GO)-containing hydroxyl ferric phosphate samples are finally fabricated.To investigate the high-capacity mechanism of Fe5(PO4)4(OH)3·2H2O as lithium-ion battery cathodes and to explore the actual functions of crystal water,additive GO and their synergistic effect on the electrochemical properties of hydroxyl ferric phosphate samples,systematic structural and electrochemical measurements have been adopted for the above-mentioned.The results show that,at 50 mA g-1and in the 120th cycle,reversible capacities of Fes(POa)a(OH)3·2H2O,Fe5(PO4)4(OH)3·1.3H2O and Fey(PO4)4(OH)3 cathodes are 128.7.63.5 and 46.5 mAh g-1,respectively.This indicates that crystal water has a positive effect on electrochemical performance.Also.under the exactly same operation conditions,reversible capacities of the three composite electrodes are correspondingly higher than those of the non-GO-containing counterparts:Fes(PO4)4(OH)3.2H2O/GO,155.1 mAh g-1:Fe5(PO4)4(OH)3.1.3H2O/GO,117.5 mAh g-1;Fes(PO4)4(OH)3/GO,98.0 mAh g-1.At a current density of 180 mA g-1(1C),a long-term cycling performance of a Fe5(PO4)4(OH)3.2H2O/GO composite cathode presents a still high reversible capacity of 140.6 mAh g-1in the 250th cycle.This indicates a synergistic effect of crystal water and additive GO on the significantly improved electrochemical properties of hydroxyl ferric phosphate dihydrate.