Synthesis And Electron Microscopy Studies On The Cathodes For Multivalent Ions Batteries

With the rapid development of society,the growing population and the worsening environmental problems,we have to accelerate the transition from an energy structure depending on non-renewable fossil fuels to that based on renewable clean energy.Renewable clean energy includes solar energy,wind energy,geothermal energy,tidal energy,nuclear energy,biological energy,etc.Although these energy sources are inexhaustible,they are characterized by volatility,intermittentness,unevenness and randomness,which also puts forward higher requirements on energy storage systems and technologies.Nowadays,lithium-ion battery technology has been rather mature and widely used worldwide,but the reserves of lithium in the earth’s crust are very low,resulting in high cost of lithium-ion batteries（LIB）.In addition,the frequent occurrence of LIB explosion indicates that safety performance urgently needs to be improved.Meanwhile,the dendritic problem of metal lithium anode has not been addressed,and carbon-based anode materials severely limit the capacity of LIB.In recent years,beyond-LIB,such as sodium,potassium,calcium,magnesium,and aluminum ion batteries,have attracted researchers’a great deal of attention.Among them,calcium,magnesium,and aluminum metal ion batteries are all multivalent metal ion batteries.The characteristics of carrying multiple charges make it possible to provides higher capacity,and there is no dendrite problem,which rule out of metal anodes.In addition,the content of these elements in the earth’s crust is relatively high,leading to low cost,which can promote the development of large-scale energy storage systems.Although significant strengths,the feature of multivalent metal ion batteries,carrying multiple charges,induces strong Coulomb interaction between metal ions and the host cathode material,which make it difficult to explore cathode materials compatible with multivalent ions batteries.Research on the electrochemical redox mechanism of electrodes can facilitate the study of excellent electrode materials.Recently,anion redox has been a research hotspot,especially for lithium and sodium ion batteries,which is expected to achieve higher energy density.The anion redox also occurs and couples with cation redox of cations in different ways for multivalent ion batteries,thereby either increasing a higher capacity or promoting a stable and reversible reaction.In this doctoral dissertation,with the aid of imaging and spectrum technology of electron microscopy,combined with X-ray diffraction,X-ray photoelectron spectroscopy,X-ray absorption spectroscopy and other methods,the redox mechanisms of the cathodes for multivalent ion batteries have been studied.First,a series of Fe_xCo_1-xS₂（0<x<1）samples were synthesized by hydrothermal method.Compared with others,Fe_0.5Co_0.5S₂,as a cathode material for magnesium ion batteries,exhibits the highest specific capacity and better cycle stability.By GITT,EIS and structural characterization,the typical insertion reaction during discharge is confirmed.The analyses of EELS and XPS draw conclusion that all of Fe,Co,and S elements act as redox centers to compensate charge in the electrochemical process.Secondly,the redox mechanism of Mo₆S₈ cathode for aluminum ion batteries was figured out.The results of EELS,XAS and XPS demonstrate that the first aluminum ion intercalation causes Mo oxidation and S reduction,and with more aluminum ion intercalation Mo is reduced,the oxidation state of S is kept.Combined with i DPC-STEM and XRD refinement of atomic scale imaging,the structure evolution of Mo₆S₈ during the intercalation of aluminum ions was studied.According to the result of DFT calculation,the cation-synergy stabilizing anion redox is revealed.It is concluded that when the first aluminum ion is inserted,the Mo-Mo bond length in the Mo₆ metal cluster shrinks sharply,the upward metal antibonding orbitals and the downward Mo-S antibonding orbitals render electrons transfer from Mo₆ cluster to S₈ cube,manifesting oxidation of Mo and full reduction of S.with more aluminum ions insertion,the Mo-Mo bond length in the Mo₆ metal clusters is slightly extended,and Mo is reduced,S does not change.The studies about anion redox mechanism of metal sulfides is conducive to the design and exploration for stable and high-capacity cathode materials of multivalent ion batteries,diminishing the cost of rechargeable ion batteries with excellent electrochemical performance.