Preparation And Performance Study Of New Conversion-based Sulfide Compounds For Potassium Ion Batterie

The structure of the electrode material with micro-and nano-scale is one of the factors determining the electrochemical performance of the battery.Therefore,the investigation of relationship between the structure of electrode material and the performance of battery during the electrochemical reaction is of critical significance.Even through the conversion-based sulfide and selenide electrode materials have been actively studied and regarded as a potential electrode material of potassium ion batteries,many problems occurred during the the charging and discharging processes,such as low ion diffusivity,poor reaction kinetics of potassium ions,insufficient mechanical strength,and serious side reactions.In this paper,to clarify the failure mechanism of electrode materials,we have developed efficient and non-destructive characterization methods,combined with advanced characterization techniques and detailed theoretical analysis,realizing multi-dimensional online diagnostic evaluation of electrode materials under cross-scale and multi-modal,unraveling the structure evolution mechanism of electrode materials,and clarifing the structure-performance relationship.This work offers some insights to design an electrode material with high capacity,high rate,and high stability.In conclusion,the preparation and properties of sulfide and selenide electrode materials are studied via structural design,bulk doping,and interface engineering.The main research contents and results are as follows.（1）Construction of multi-dimensional NiS2 and investigation of its potassium storage mechanismConversion-based NiS2 from one to three dimensions were synthesized to establish the structure-performance relationship of the different dimensions and the mechanical degradation caused by cycling and evoke some insights to construct the high-performance conversion-based anode materials by dimensional engineering method for potassium ion batteries.Compared to the low-dimensional conversion-based anode（such as sphere-NiS2 and tube-NiS2）,the multi-dimensional flower like-NiS2 composed of interleaved sheets provides a 3D network of ion transport and reduces stress concentration,exhibiting a super-stable structure and robust electrochemical performance.Synchrotron X-ray tomography confirm that the flower-like NiS2 has low morphology complexity and the existence of interconnected visualized ion diffusion paths within its multi-dimensional structure.Furthermore,the induced structural analysis of the finite element simulation shows that the multi-dimensional electrode material can alleviate the uneven stress distribution during potassium ion storage.This design idea is suitable for the fabrication of conversion-based anodes with high capacity and superior stability.（2）Structural modulation of Mn-doped ZnSe to improve potassium storage stabilityTo address the problem of poor cycle stability of potassium ion batteries in the work of chapter 1,we synthesized flower-like zinc selenide（ZnSe）via hydrothermal method,and then the Mn-doped ZnSe with modulated electronic structure was obtained through atomic-level structure engineering,reducing the mechanical degradation of the battery anode and improving the ion transport kinetics.The crystal structure changes,ion/electron migration paths,and micromechanical stress evolution mechanisms were probed in detail by using density functional theory calculations,nanoindentation techniques,and synchrotron radiation X-ray imaging.Results show that the heterogeneous tuning of the electronic structure can alleviate the internal strain caused by potassification and improves the structural stability of the battery anode,revealing the close correlation between doping chemistry and mechanical stability,opening up a new pathway towards the preparation of highly stable potassium ion batteries.（3）Multilevel interface design of CoSe2/Fe Se2/hollow carbon fibers with high potassium storage capacityTo address the issue of low capacity of potassium ion batteries in the work of chapter 2,we synthesized dual carbon-modified interfacial region with high lattice matching degree including a heterostructure composed of cobalt diselenide/iron diselenide（CoSe2/Fe Se2）and an electronically conductive interface composed of 2D sheet carbon and 1D hollow carbon fibers（HCF）.Advanced characterization techniques,including ex-situ soft X-ray absorption spectroscopy,synchrotron X-ray tomography,ultrasonic transmission mapping,and density functional theory,were used to reveal electrode reaction kinetics,electrode evolution and failure mechanism.Results suggest that the CoSe2/Fe Se2/HCF heterostructure composed of the same crystal system and space group can tailor the redox kinetics of conversion-based metal selenides,and the 2D sheet carbon and 3D HCF network can alleviate the physicochemical degradation of the electrode and the occurrence of side reactions.In addition,the CoSe2/Fe Se2/HCF anode still maintains a reversible capacity of 423.8m Ah g^-1 after 100 cycles at 0.2 A g^-1,with a capacity retention rate of 95.4%and a high capacity of 118 m Ah g^-1 after 1500 cycles at a high current density of 5 A g^-1.