Synthesis,Modification And Properties Of Nickel-Based Compounds (Oxides,Carbonates) In Lithium-Ion Batteries

Lithium-ion batteries（LIBs）have dominated the field of power for portable electronic devices in recent years because of their high energy density and ability to provide high power output.However,current LIBs technology cannot meet the growing energy and electricity needs of electric vehicles.Therefore,it is necessary to develop anode materials with higher energy density to replace the most used graphite anode.In addition,at present,polyvinylidene fluoride（PVDF）binder mainly used for LIBs also greatly affects its performance,so the development of new binders is also the future direction of LIBs.In this thesis,transition metal oxide and transition metal carbonate are taken as the research objects,their synthesis,modification,and then their electrochemical performance are explored.A new type of binder is synthesized,and its influence on the performance of LIBs is also explored.The specific research content is as follows:（1）Fe₃O₄/Ni O composite were synthesized by one step hydrothermal method.X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）were used to characterize the crystal structure and morphology of the samples.The electrochemical performance of Fe₃O₄/Ni O composite was tested using a constant current charge and discharge system and electrochemical workstation.The morphology is flower-like spheres,which are composed of a large number of nanosheets with the diameter of 150 nm.These loose layered nanostructures can effectively alleviate the volume expansion of materials in the process of charge and discharge.Meanwhile,large specific surface area can provide more reaction sites.Ultra-thin nanosheets can also reduce the diffusion distance of lithium ions,so that the material exhibit high specific capacity,excellent cycling stability and long cycle performance in LIBs.At the current density of 100m A g^-1,the initial discharge capacity of Fe₃O₄/Ni O composite can reach 1670 m Ah g^-1,and can still maintain at 1021 m Ah g^-1after 100 cycles.At 1 A g^-1,the discharge capacity of Fe₃O₄/Ni O composite can still maintain at 500 m Ah g^-1and the coulomb efficiency（CE）is always stable at 99%after 1000 cycles.Lower impedance and better rate capability are also shown in the subsequent electrochemical impedance test and pseudocapacitance behavior test.（2）Using sodium carboxymethylcellulose（CMC-Na）as raw material,CMC-H was prepared by acidizing excess hydrochloric acid,and then the final product CMC-Li was obtained through CMC-H react with excess lithium hydroxide（Li OH）solution.Subsequently,the Fe₃O₄/Ni O composite electrodes were prepared using PVDF,CMC-Na and CMC-Li as binders,respectively.The electrochemical performance tests find that the CMC-Li binder can increase the electrochemical reactivity of Fe₃O₄/Ni O electrode while not affecting the electrochemical reaction mechanism.In addition,CMC-Li will dissociate a part of Li⁺ions,thus increasing the content of freely moving Li⁺ions in LIB,so that the initial coulomb efficiency（ICE）and charge-discharge specific capacity of electrode are improved.Compared with PVDF and CMC-Na as binders,at the current density of 100 m A g^-1,the cycling performance of Fe₃O₄/Ni O electrode is also best using CMC-Li as binder.It can still maintain at 1544 m Ah g^-1after 100 cycles.At different current densities,its rate performance of Fe₃O₄/Ni O electrode is also the best,using CMC-Li as binder.Finally,the results of AC impedance test show that the charge transfer impedance is the lowest when CMC-Li binder is used.The Fe₃O₄/Ni O electrode behaves faster intercalation dynamics of lithium ions and higher conductivity when CMC-Li is used as binder.（3）Ni_xMn_1-xCO₃（x=0.2,0.25,0.5）composites with different components were prepared using a mixture of ethanol and water as solvent,Mn（CH₃COO）₂·4H₂O,Ni（CH₃COO）₂·4H₂O and urea as reactants.With the increase of Ni（CH₃COO）₂·4H₂O,the morphology of the material gradually changes from hexahedron to hazelnut.Among them,the Ni_0.25Mn_0.75CO₃composite material has the smallest size,reaching the nanometer level.The bimetal in Ni_xMn_1-xCO₃has the synergistic effect,which makes it have higher electronic conductivity,electrochemical reactivity and lithium storage performance than the single metal carbonate.At 100 m A g^-1,the initial specific discharge capacity of Ni_0.25Mn_0.75CO₃can reach 1506 m Ah g^-1,and can still maintain 954.5 m Ah g^-1after 100cycles.At 1 A g^-1,the specific capacity of Ni_0.25Mn_0.75CO₃remains at 595 m Ah g^-1and the CE remains stable at 99%after 500 cycles.In the subsequent electrochemical impedance test and pseudocapacitance behavior test,it is found that the Ni_0.25Mn_0.75CO₃electrode also behaves the lowest impedance and the best rate capability.