Synthesis,Modification And Electrochemical Performances Of Transition Metal Oxides(Sulfides)

In this thesis,KMn₈O₁6 and Na₂Mn₈O₁6 as cathode materials,and transition metal sulfide?nickel sulphide,copper sulphide?,and a-Fe₂O₃ as anode material for lithium-ion batteries?LIBs?,were synthesized and further modified.The crystal structure and morphology of the obtained-samples were investigated.It was found that the modification by ions doping for KMn₈O₁6 and ?-Fe₂O₃ could efficiently improve electrochemical performances of these designed materials,which could be applied in LIB industry in the future.The main results are as follows:?1?Single-crystal Na-OMS-2?OMS:octahedral molecular sieve?nanorods were prepared through a hydrothermal method by using manganese sulphate and sodium dichromate as reactants.All samples were characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,and transmission electron microscope?TEM?.Reaction time plays an important role in controlling the crystal phases,shapes of the nanorods.The mechanism of electrochemical reaction was also discussed.The electrochemical performance of the samples was studied by cyclic voltammetry?CV?,eletrochemical impedance spectroscopy?EIS?and constant current charge-discharge tests.As cathode material for LIBs cycled between 1.5 and 4.2 V at a current density of 50 mA/g,the initial discharge capacity of NMO-4?prepared at 120? for 20 h?is 123.4 mAh/g and the capacity retention was 123.9 mAh/g after 100 cycles.The result demonstrates that Na₂Mn₈O₁6 cathode materials have a high reversible discharge capacity and a good cycling stability.?2?KMn₈O₁6 nanorods were synthesized via a reflux method with MnSO4·H₂O and KMnO₄ as reactants,SnCl₄·5H₂O(molar ratio of Sn⁴⁺ and KMn₈O₁6 is 0.03,0.05 and 0.15,respectively)as dopant.The microstructures and morphologies of the KMn₈O₁6 nanorods and Sn⁴⁺-doping KMn₈O₁6 nanorods were characterized using XRD,BET,SEM,and TEM.The electrochemical measurements demonstrated that ions-doping KMn₈O₁6 nanorods(molar ratio of Sn⁴⁺ and KMn₈O₁6 is 0.03)presented a much higher reversible discharge capacity?151.4 mAh/g?and went up to a discharge capacity of 159.1 mAh/g after 100 cycles,compared with non-doping KMn₈O₁6 nanorods?129.8 mAh/g?.These results demonstrate that the Sn⁴⁺-doping on the crystal structure of KMn₈O₁6 can enhance electrochemical property of material during the charging and discharging process.?3?Nickel sulfide??,?-NiS?rods were synthesized via a simple solvothermal process at 180? in the absence of template or surfactant.The sample was charaterized by XRD,SEM and energy dispersive X-ray spectroscopy?EDS?.The results show that the first discharge capacity of NiS electrode was 589.7 mAh/g at 1.0-3.0 V with the current density of 50 mA/g and the initial discharge capacity was 1050.3 mAh/g of the NiS electrode cycled between 0,01-3.00 V at the same current density.It is preliminarily concluded that the rapid loss of its specific capacity may due to the agglomerate phenomena of active material and the irreversibility of Li₂S formation during the charging and discharging process.?4?Copper sulfide was prepared by a hydrothermal process,using CUCl2·2H₂O and thiourea as reactants,FeCl₃·6H₂O and SnCl₄·5H₂O were used as dopants during the reaction.XRD,SEM,TEM,and EDS were introduced to analyze the crystal structures,morphologies,and compositions of the products.It indicated that the products were heterogeneous phases of both CuS?hexagonal?and Cu₁.8S?cubic?phases,which existing in undoped and doped samples.Larger change in lattice parameter is found in the cubic phase when the ions-doping is applied.This may indicate that the dopant actually enter into the cubic phase.The electrochemical performance of the samples was studied by CV and constant current charge-discharge tests.The results demonstrated that Sn-doping sample synthesized at 180? for 24 h,presented an intial discharge capacity of 210.2 mAh/g,and reduced to 118.7 mAh/g after 40 cycles.The capacity retention was 56.5%.The electrochemical property of Sn-doped sample was significantly improved,compared with Fe-doped sample?the intial discharge capacity was 167.6 mAh/g?on the same synthesis condition.?5?Pure and Cu-doped a-Fe₂O₃ cubes were obtained via a mixed solvothermal route by using FeCl₃·6H₂O and hexamethylenetetramine?HMT?as reactants.The crystal structure and morphology of the as-synthesized products were characterized by XRD,field-emission SEM,and TEM.The results of constant current charge-discharge tests show that monodisperse and uniform ?-Fe₂O₃ microcubes were composed of nanoparticles.The electrochemical measurement data indicate that the reversible specific capacity of Cu-doped a-Fe₂O₃ was higher than that of pure a-Fe₂O₃,as anode materials of LIBs.It was found that Cu²⁺ ions could obviously reduce the electrochemical impedance of a-Fe₂O₃ electrode,which resulted in the increase of the space of Li⁺ access.It was beneficial to the lithium ion intercalation and deintercalation process,and leaded to the improved cycle stability.