Preparation And Electrochemical Performance Of Fe,Co-MOFs Derivatives And Their Composites

Lithium-ion batteries have been widely applied in electronic products and energy vehicles because of their high energy density,high working voltage and long cycle lifespan.However,traditional commercial graphite can not meet the demand for new anode materias with high energy density.Therefore,it's urgent to develop high-performance anode materials.Due to the high porosity,large specific surface area,and structural and functional diversity,metal-organic frameworks(MOFs)and their derivatives have been considered as promising candidate electrode materials.However,the development of MOFs are restricted by poor conductivity and inevitable volume change,which affected their rate performance and cycle stability.Based on above considerations,the present thesis focuses on the preparations of four types of electrode materials(CoFeP,Cu_7.2S₄/Co₃S₄,CoSe₂@rGO,CoSe₂@GA)through the strategies of phosphating,sulfidation,selenization,and the integration with graphene,using Fe-MIL88B and CoCo-PBA as self-sacrificial templates.In addition,the electrochemical properties of these materials are studied in detail.The research contents are as follows.1)Fe-MIL88B as self-templates was synthesized by a hydrothermal method.Then,CoFe-MIL88B was prepared by a doping with cobat metal through a hydrothermal method.Afterwards,the CoFeP and FeP materials were prepared by the oxidation and the following phosphation.The morphology of as-obtained CoFeP was similar to the precursor with semi-hollow nanorod.In the subsequent electrochemical performance test,CoFeP displayed an excellent lithium-storage performance.At a current density of 0.1 A g^-1,CoFeP had a specific capacity of 897.2 m Ah g^-1 after 87cycles,and the specific capacity of CoFeP remained at 478.5 m Ah g^-1 after 800 cycles at a highe current density of 1 A g^-1.The specific capacity was significantly improved,compared to pure FeP.The excellent lithium-storage performance was attributed to the doping of cobalt atom which improved the overall conductivity,whereas the hollow structure accelerated the electrode conduction.2)Cu_7.2S₄/Co₃S₄ was successfully prepared by a hydrothermal treatment and the doping of hetero-metals,using CoCo-PBA nanocubes as templates.A series of characterizations were carried out on the final product,in which the morphology of CoCo-PBA was inherited.The coexistence of Cu_7.2S₄ and Co₃S₄ was determined by X-ray diffraction.The electrochemical performance of the composite Cu_7.2S₄/Co₃S₄was significantly improved,comparing with pure Co₃S₄ and intermediate product Cu-Co₃S₄.At a current density of 0.1 A g^-1,the specific capacity of Cu_7.2S₄/Co₃S₄remained at 850 m Ah g^-1 after 100 cycles.The specific capacity of Cu_7.2S₄/Co₃S₄remained 200 m Ah g^-1 under a large current density of 1 A g^-1 after 800 cycles.The high specific capacity at small current densities was mainly due to the doping of Cu²⁺and Cu_7.2S₄ which contributes to the specific capacity.The high stability at high current densities was caused by the doped cobalt atoms which increased the electrode conductivity.3)The CoCo-PBA precursor was synthesized by a room-temperature solution method.CoSe₂@rGO was successfully prepared by a simple self-assembly,freeze-drying treatment,and high-temperature selenization.SEM results show that the cubic structure of CoCo-PBA was maintained,and CoSe₂ was evenly distributed in three-dimensional network of rGO.This composite structure can effectively suppress the volume change during the cycles.Electrochemical tests show that the capacity remained at 1000 m Ah g^-1 when the current density is 0.1 A g^-1 after 90 cycles.Compared with pure CoSe₂(280 m Ah g^-1),the improved electrochemical activity of CoSe₂@rGO can be ascribed to the pseudo-capacitance behavior,and highly reversible formation/decomposition of SEI film during the charge/discharge process.4)The CoCo-PBA nanoparticles were wrapped into the network of GA by a self-assembly method,using graphene aerogel(GA)as the substrate.The resultant composite CoSe₂@GA was obtained by a high temperature selenization.CoSe₂@GA exhibited a good cycle stability when applied as anode material of lithium-ion batteries,due to the synergistic effect of selenide and GA.The specific capacity of the composite was 1405 m Ah g^-1 after 90 cycles at 0.1 A g^-1.When the current density increased to 1 A g^-1,the composite still had a specific capacity of 600 m Ah g^-1,after500 cycles.The combination of CoSe₂with GA improved the overall conductivity and alleviated the agglomeration and volume expansion of selenide particles during the cycles.