Preparation Of Transition Chalcogenides And The Mechanism Of Sodium Storage

Lithium-ion batteries(LIBs)are widely used in energy storage by virtue of high energy density and light weight.At the same time,the problem of low lithium resource reserves has become increasingly prominent.There is an urgent need to trigger a search for other earth abundant elements to substitute lithium for an energy storage system in the long term.Among next-generation energy storage systems,Sodium-ion batteries(SIBs)have attracted great attention as potential alternatives due to the overwhelming superiority in low cost and abundant natural resources.However,the larger ionic radius of alkali ions usually results in sluggish reaction kinetics,unstable solid electrolyte interphase(SEI)layer,and collapsing of the host materials crystal structure,which gives rise to inferior electrochemical performance.In this case,it is highly desirable to explore appropriate anode materials with high capacity and cycling stability.Among the well-developed electrode materials,transition metal chalcogenides(TMCs)have been widely considered as the anode candidate,originated from their rich redox reactions and the weaker metal–sulfur/selenium bond favoring conversion reaction in comparison with their oxide counterparts.Nevertheless,most of them are still suffer from low intrinsic electric conductivity and inevitable volume changes during prolonged cycling,which cause poor rate performance and fast capacity fading.To solve these issues,in this thesis,we designed and prepared some TMCs through nanostructure design,molecular engineering and other methods to obtain anode materials with outstanding cycling stability and excellent rate performance for SIBs.The main research contents and conclusions are as follows:Firstly,the hierarchical Co Se₂ nanospheres assembled from nanosized building blocks were successfully prepared through a solvothermal method and subsequent selenization process.The characterizations of morphology and structure show that the diameter of the hierarchical Co Se₂ solid nanospheres is about 550 nm.This hierarchical nanostructure can decrease the ionic transport distance,as well as alleviate volume expansion.Therefore,the Co Se₂ anode material maintains a reversible capacity of 388 m A h g^-1 after 100 cycles at 0.2 A g^-1,and can provide a capacity of 425 m A h g^-1 at 20 A g^-1 for SIBs.Then,the hierarchical hollow spheres,Ni_0.33Co_0.67SSe(HPS-NCSSe),are successfully synthesized through a facile sulfurization and selenization reaction for high-performance sodium storage.It can be found that anion exchange reaction during sulfurization process could induce hollow structure,while,Ni doping process results in the decreased particle size.Besides,anion and cation substitution will increase the conductivity of the metal selenide.Owing to the nanostructure engineering and the coordinated strategy of cation and anion substitution,NCSSe hierarchical hollow spheres have excellent Na storage performance.Specifically,between 0.01 and 3 V,the electrode delivered excellent rate capacity of 608 m A h g^-1at 20 A g^-1 and superior capacity of 533 m A h g^-1 at 5 A g^-1 after 2000 cycles.Finally,the sodium ion storage mechanism and kinetics of NCSSe are studied.A combination of in situ X-ray diffraction and ex situ transmission electron microscopy measurements fundamentally reveal that the Na storage process in NCSSe is a reversible conversion reaction,which results in excellent long-period stability.The ultrafast Na⁺storage kinetics of the NCSSe anode is attributed to the surface-dominant reaction process and shortened Na⁺diffusion paths.In sum,the structure and molecular design of TMCs have been carried out in this thesis,and an anode material with excellent electrochemical sodium storage properties has been effectively obtained.It provides insight into the design and optimization of transition metal sulfur selenide based on hierarchical nanostructures.