Study On Ion Exchange Technology For The Construction Of Transition Metal Compound Electrode Materials And Their Lithium Storage Performance

Lithium-ion batteries(LIBs)have made a huge contribution to the development of modern society as an energy storage device and will continue to have an influence for a long time in the future.Energy storage materials are one of the important components of LIBs.Traditional intercalation-based energy storage materials are no longer able to meet the demand for higher energy density batteries due to the limitations of the energy storage mechanism.A class of transition metal compounds(TMCs)nanomaterials based on the conversion-based energy storage mechanism has attracted the attention of many researchers due to their high theoretical capacity,wide variety and abundant sources.However,the disadvantages of TMCs,such as huge volume changes during charging and discharging leading to structural collapse,low electronic conductivity and poor ion diffusion rate,have limited their further development.Ion exchange(IE)technology,as a mild method for the synthesis of nanomaterials,not only enables precise morphological design of products at the nanoscale,but also allows precise in situ regulation of elemental composition and content,and has attracted the attention of scholars in recent years.In summary,in this thesis,transition metal oxides(TMOs)and transition metal sulphides(TMSs)will be studied for their morphological modification and modulation of elemental components to enhance electrochemical performance using IE technology.The major studies are as follows:(1)IE-assisted carbon coating strategy for the preparation of freestanding flexible electrodes.Firstly,Mg(OH)₂ nanosheets were electrochemically deposited on carbon cloth(CC)as precursors for ion exchange,followed by hydrothermal coating with glucose as the carbon source and high temperature annealing treatment to performance carbon coating,and then IE reaction with Ni²⁺.Finally,the carbon coating NiO-carbon cloth(C@IENiO-CC)freestanding electrode was successfully prepared by low temperature dehydration treatment.The three-dimensional porous nanosheet array morphology enhances the wettability with the electrolyte and can provide more ion transport pathways;in addition,unlike conventional carbon coating methods,this IE-assisted carbon coating strategy successfully avoids the reduction of NiO to Ni by carbon at high temperatures,ensuring the purity of the active material NiO,while effectively enhancing the structural stability and electronic conductivity of NiO nanosheets;and the residual Mg²⁺can achieve in situ doping of NiO,which enhances the disorder of the lattice structure,thus serving to boost conductivity of NiO grains and the diffusivity of Li⁺.Benefiting from these merits,C@IENiO-CC exhibits excellent lithium storage performance.At a high current density of 0.25 m A cm^-2 and a high loading of 4 mg cm^-2,C@IENiO-CC has a high areal capacity of 3.08 m Ah cm^-2;the areal capacity is maintained at 2.5 m Ah cm^-2 after300 stable cycles at a current density of 0.5 m A cm^-2;The areal capacity remains at 1.78 m Ah cm^-2 even at high current densities of 8 m A cm^-2.In addition,the flexible electrode maintains excellent electrochemical stability under different bending deformation conditions.(2)Preparation of Ni-doped FeS₂ lithium-free cathode by IE technology.A series of FeS₂ with different contents of Ni atoms doped(Ni-FeS₂)was successfully prepared under room temperature and atmospheric pressure by controlling the time of IE reaction using Ni(OH)₂ as precursor with Fe²⁺occurring IE and then via high-temperature sulphidation treatment abtained Ni-FeS₂.The in situ Ni atom doping serves to optimise the electronic and crystal structures,increase the electronic conductivity and active reaction sites,enhance the catalytic activity of FeS₂ and accelerate the conversion of the intermediate polysulphide.Thanks to the facile and efficient in situ doping technology of the IE strategy,the specific surface areas of the prepared Ni-FeS₂ with different Ni doping contents were all larger than those of the conventional high-temperature synthesized FeS₂,while the morphology of FeS₂ was not significantly affected by different Ni doping contents.The electrochemical test results showed that the Ni-FeS₂-2h sample with 2 h of IE reaction had superior electrochemical performance,and at a current density of 0.2 A g^-1 cycling for 100 cycles,still had a specific capacity of 320m Ah g^-1;at high current densities of 5 A g^-1,it remains with a capacity of 95 m Ah g^-1.(3)Preparation of carbon composite Ni-doped FeS₂ lithium-free cathode materials(C/Ni-FeS₂)by IE technology.C@Mg O nanosheets were used as precursors to prepare C/Ni-FeS₂ by two consecutive Mg²⁺/Ni²⁺and Ni²⁺/Fe²⁺IE reactions and sulfidation treatments.The mass of carbon material used for carbon composite by this IE strategy is much smaller than the proportion of carbon in the composite/coating means by the conventional method,which contributes to the actual energy density;in addition,the continuous IE strategy achieves the purpose of improving the conductivity of FeS₂ by carbon composite and fulfilling the doping of Ni atoms,and both of which synergistically also effectively enhance the reaction kinetics of FeS₂.The electrochemical test results show that the Li⁺diffusion coefficient of C/Ni-FeS₂is 2.92×10-13 cm² s^-1;the specific capacity is 450 m Ah g^-1 and the energy density is 850 Wh kg^-1 at a current density of 0.2 A g^-1 for 450 cycles with a voltage window of 1-3 V;with a capacity of 288 m Ah g^-1 despite a high current density of 5 A g^-1,it exhibits excellent electrochemical properties.