Study On The Preparation Of Nickel/cobalt-based Bimetallic Selenide And Its Sodium Storage Performance

Currently,lithium-ion batteries(LIBs)have been widely used in portable electronic devices,electric vehicles,and grid energy storage.This is mainly due to its advantages of high energy density,long cycle life,high safety and environmental friendliness.However,LIB s are difficult to meet requirements of large energy and high power for large equipments,with the limited reserves of lithium resources.Therefore,designing and developing high-capacity electrode materials with high natural abundance is the key to solve the above problems.The energy storage mechanism of sodium-ion batteries(SIBs)is similar to LIBs,But the storage of sodium resources are more abundant than lithium and are environmentally friendly.Therefore,SIBs are regarded as the most promising alternatives to LIBs.However,the ionic radius of Na is larger than that of Li(1.02 A vs 0.76 A),which results in many anode materials that are widely used in LIBs are not suitable for SIBs.Therefore,it is very necessary to develop high energy anode materials for SIBs.Compared with single metal selenides,polymetallic selenides have higher conductivity,richer redox active sites and better electrochemical performance,so it is becoming popular and is now a hot of subject in energy storage and conversion system.However,the high rate and cycle stability of the transition metal selenide(TMSs)are poor,and the volume expansion of the material during charging and discharging leads to easy powdering.In view of the above problems of TMSs,this thesis constructs binary transition metal selenide and carbon composite materials by hydrothermal and high temperature selenization strategies to buffer the selenide cyclic volume expansion effect,so that the performance of selenide materials has been improved to a certain extent.The main contents are summarized as follows:(1)The NiCo-MOFs precursor were synthesized by hydrothermal method,and then by a facile one-step high-temperature selenization and carbonization treatment,NiCoSe4 multiple hollow microspheres(NiCoSe4@NC)with nitrogen-doped carbon skeleton were successfully prepared.Further,NiCoSe4@NC@rGO is obtained by using a hydrothermal strategy to wrap graphene oxide to enhance the conductivity of the material and protect the structural stability of the material.The electrochemical performance of NiCoSe4@NC@rGO in different voltage ranges and different electrolytes was tested.The results shown that the material has stable long-cycle performance in the voltage range of 0.3?3 V and in ether electrolytes.The NiCoSe4@NC@rGO composite maintained a reversible capacity of 293 mAh g-1 after 1500 cycles at a current density of 1 A g-1,and the capacity decay rate per cycle was 0.025%.In the ester electrolyte,the performance of the material decreases rapidly,and at the same time,the performance is unstable in the voltage range of 0.01?3 V.In addition,the material showed a capacity of 277.8 mAh g-1 at a rate of 10 A g-1.Further,pseudo-capacitance analysis was conducted through cyclic voltammetry curves with different sweep speeds.The results showed that pseudocapacitance contribution is the main reason for the excellent rate performance of the material.(2)Controlled synthesis of nano-scale porous Fe2CoSe4 and Fe2NiSe4 by scalable,simple hydrothermal method and high temperature calcination.Full contact between the nano-scale porous composite material and the electrolyte also shortens the Na+diffusion path and can also relieve volume expansion during the material circulation process.Therefore,the binary selenide of prepared exhibits excellent sodium storage performance.Fe2CoSe4 and Fe2NiSe4 materials have a specific capacities of 352 and 282.2 mAh g-1 after 2100 cycles at a current density of 1.0 A g-1.In addition,at a high rate of 30 A g-1,Fe2CoSe4 and Fe2NiSe4 have specific capacities of 166 and 224 mAh g-1,respectively.Further,the electrochemical impedance of the two materials was tested,and the results showed that the RCt value of the composite material was kept at a very low level after the charge and discharge cycle,which ensured rapid charge transfer and ions diffusion,thus was beneficial to improving the electrochemical performance of SIB s.