Preparation Of High-performance Transition Metal-based Compound Electrode Materials And Their Application In Hybrid Ion Capacitors

In recent years,global warming and environmental pollution may become problems that people need to solve urgently due to the rapid consumption of fossil fuels.In particular,lithium-ion batteries and supercapacitors regarded as dominated electronic devices have high energy density as well as high power density,respectively.Neither batteries nor supercapacitors by themselves can so far satisfy the demand for reliable electrochemical energy storage systems due to their own shortcomings and limitations.The most important task for us is to find an energy storage device with both high energy-power density and long cycle life.Recently,ion-capacitor have gradually attracted widespread attention because they combine the two different reaction types and provide a bridge between them.Considering these merits,ion-capacitors are expected to be widely employed in new energy-saving vehicles and the storage of solar and wind energy.Although great efforts have been made to design promising electrode material and applied them to ion capacitors,the development of ion capacitors is still hindered by many problems,such as the slow dynamics of electrode materials,resulting in great limitations on the rate performance of the device,etc.Therefore,further design of suitable electrode materials to provide fast kinetics is the key to large-scale applications of ion capacitors.In this work,the microstructure,phase information and electrochemical properties of the synthesized dendrite-structured FeF₂ composed of nanoparticles,P-CVO-N@GO and a layered Fe₂(Mo O₄)₃(L-FMO)microsheet electrode materials were studied in depth by a variety of ex-situ testing methods and first-principles calculations to study its electrochemical reaction mechanism.The main contents are as follows:(1)The dendritic FeF₂ multilevel structures composed of nanoparticles were synthesized by a mild solvothermal method coupled with the further calcination strategy.Though electrochemical test,we can see that the as-prepared the FeF₂ exhibits excellent rate performances(109 m A h g^-1at 1 A g^-1)and stable cycling stabilities for 250 cycles.The electrochemical results show the feasibility of employing nano-structured FeF₂ as cathode material for lithium-ion batteries.It is hoped that study will provide useful inspiration for the development of emerging cathode.(2)We developed a facile solvothermal and high temperature calcination strategy to synthesize a L-FMO microsheet assembly composed of ultra-thin nanosheets(?140nm)and applied then into sodium-ion capacitors.Considering that L-FMO is a new anode material and it's sodium storage mechanism is not very clear,this paper employed various ex-situ characterizations to systematically study the reaction mechanism.Such as ex-situ HRTEM and XPS characterization was analyzed,indicating the occurrence of the conversion reaction.Furthermore,the assembly of sodium ion capacitors shows that the device not only possess high energy-power density,but also achieves superior long cycle stability.(3)A uniform porous P-CVO-N@GO composite was prepared via a simple solvothermal reaction and high-temperature solid-phase treatment.The composite show promising electrochemical performance in potassium-ion battery.The excellent electrochemical performances can be ascribed to the following characteristics:the smaller CVO nanocages are evenly distributed in the graphene matrix,which achieves shorter diffusion paths of ions and electrons;graphene recombination further improves the conductivity,which greatly improves the transport efficiency of carriers in it;as a porous structure,the micropores inside the composite material can effectively adapt to the volume expansion caused by the insertion of charge carriers between CVO layers.In addition,ex-situ XPS?HRTEM characterizations and DFT calculations confirmed that the electrochemical energy storage mechanism of the CVO//K battery for the first time,demonstrating a conversion reaction in the discharge process of low potential range.And CVO was observed during charging,verifying the completely reversible reaction mechanism of CVO in potassium ion batteries.Benefiting from the advantages,potassium ion capacitors are proposed,which show high energy-power density as well as ultra-long cycle stability.