Phase Regulation And Synchrotron Radiation X-Ray Spectroscopic Investigations Of Transition Metal-Based Anode In Lithium Ion Battery

The research and development of battery materials promote the progress of electrochemical energy storage technology,especially for lithium-ion battery systems.Although the innovation of electrode materials is a complicated and time-consuming process,it is an important factor to seize the commanding heights of the lithium-ion battery industry.Various transition metal-based anode materials have been favored by researchers due to their high theoretical capacity and unique physical and chemical properties.However,there are structural stability issues such as volume expansion and irreversible phase transition during cycling.To accelerate the commercialization process,reasonable and effective phase structure design will endow the next generation of ion batteries with higher energy density and longer cycle life.In addition,a comprehensive understanding of the structural evolution resulting from phase regulation is the key to unraveling the mechanism of enhanced electrochemical performance,of which synchrotron radiation spectroscopy is the touchstone.The highly selective and locally sensitive characterization techniques can track the structural changes of materials at the atomic scale,thus revealing the electrochemical reaction processes.The intrinsic link between phase regulation and electrochemical performance has been elucidated.This dissertation focuses on the structural stability of conversion-type transition metal sulfides and intercalation-type transition metal carbide(MXene).The controllable crystal phase strategy and in-situ derived heterojunction strategy have been used to optimize the charge transfer and ion transport,as well as electrochemical reaction sites of electrode materials for high capacity and cycling stability.With the help of synchrotron radiation X-ray absorption spectroscopic technology,we have investigated the changes in the electronic structure,chemical environment and geometric structure,as well as the lithium storage mechanism.The enhancement mechanism induced by the structural modulation is well understood.The structureactivity relationship between electrochemical performance and structure has been studied.Related works provide the experimental basis and design ideas for the structure regulation and mechanism characterization of new energy storage materials.The main research contents of this dissertation are as follows:1.The crystal phase engineering is used to modulate the internal phase structure to change the original physicochemical properties,thus enhancing structural stability and electrochemical properties.We have developed a heat-treated crystal phase regulation strategy to controllably synthesize hollow tubular cobalt sulfide composites(cubic phase:C-CoSx and hexagonal phase:H-CoSx)with different crystal structures,which can be used as lithium-ion battery anodes.Combining various characterization methods,the different phase structures and compositions of cobalt sulfide were studied.Afterward,the differences in electrochemical properties were compared.Synchrotron radiation X-ray absorption fine structure(XAFS)result showed that H-CoSx has lower oxidation state,enhancing the structural stability of the active component.Therefore,the H-CoSx anode exhibited more excellent rate performance(1203 mAh g-1 at 0.1 A g1 and 190 mAh g-1 at 10 A g-1)and higher cycling stability after 200 cycles at 0.5 A g-1.Further,the enhancement mechanism of H-CoSx in electrochemical performance was systematically investigated by ex-situ XAFS spectroscopies.It was confirmed that HCoSx has multiple redox modes.The electrochemical reaction process involving both anions and cations enhances the lithium storage capacity.This work provides a favorable observation platform for the controllable crystal phase modulation by synchrotron radiation characterization and a certain experimental basis for the preparation of energy storage materials with new crystal phase.2.The design and synthesis of heterostructures is another effective strategy to improve the lithium storage capacity and stability.The heterostructure(hetero-Mo2C)coupled with amorphous MoS2 and fluorine-free Mo2C was constructed by a one-step in-situ hydrothermal method.The green and safe fluorine-free etchant and streamlined preparation process expand the research scope of MXene heterojunction materials.Theoretical calculation results showed that the formation of the heterostructure changes the charge arrangement,enhances the charge transport and lithium-ion adsorption capacity,as well as oxidation resistance,thus improving the stability of the material.In addition,synchrotron radiation XAFS studies revealed that pure Mo2C MXene exhibits a distinct phase transition due to irreversible structure changes during lithium storage.However,hetero-Mo2C represents a unique lithium storage mechanism that can inhibit the further oxidation of MXene,exhibiting a reversible structural transformation.Benefiting from the reversible structural evolution during the reaction,the heterojunction MXene material obtained excellent lithium storage capacity of 1242 mAh g-1(0.1 A g-1)and 162 mAh g-1(8 A g-1)and long cycle life of 1200 cycles.This work provides new insights into the rational design of novel heterostructures and the study of reaction mechanisms through synchrotron radiation spectroscopy.3.Based on the previous research aimed at the heterostructure construction,we further developed in-situ surface-derived endogenous heterostructures without additional additives.This strategy enhances the cycling stability of MXene heterojunction materials.The precursor MAX particles were slightly modified by ball milling technology,and then etched with concentrated hydrochloric acid to obtain small-sized endogenous heterostructure(S-Mo2C).The phase of MAX was changed during ball milling,and the heterostructure of Mo2C MXene coupled with Mo3C2 was obtained after etching.XAFS results showed the introduction of Mo3C2 brings a large number of defects,which increases the electrochemical lithium storage sites.Meanwhile,the strong interfacial coupling and the nanostructure promoted ion diffusion and electron transfer,thereby improving the electrochemical reaction kinetics.Thanks to the synergistic effect of the heterostructure and a large number of defect sites,the S-Mo2C heterojunction solves the problems of slow ion migration rate and poor stability of MXene materials in the lithium storage process.As expected,the designed S-Mo2C electrode exhibited excellent lithium storage capacity:1041 mAh g-1 at 0.1 A g-1 and 433 mAh g-1 at 5 A g-1,and cycling stability of 637 mAh g-1 after 600 cycles at 1 A g-1.In addition,the ex-situ XRD and XAFS analyses confirmed that S-Mo2C has a reversible intercalation reaction mechanism,revealing the source of lithium storage performance.Combined with synchrotron radiation spectroscopy,this work opens up new ideas for the preparation of heterostructures and the structural optimization of highperformance energy storage materials.