Fabrication And Mechanism Investigations Of 2D Transitional Metal Sulfides And Selenides For Alkali Metal Ion Batteries

Alkali metal ion batteries,such as lithium-ion batteries(LIBs),sodium-ion batteries(SIBs),and potassium ion batteries(PIBs)have been used extensively as power sources for a variety of portable electronic devices,including laptops,computers,digital cameras,and cell phones.Over the last two decades,tremendous effort has been devoted to developing SIBs and PIBs as the next generation alternative candidates of LIBs due to their low cost,high abundance in the crust of earth,and similar energy storage mechanism to LIBs.Additionally,sodium and potassium are not the same as lithium that could react with aluminum current collectors,which allows aluminum foils to replace copper foils used in lithium batteries as current collectors,thereby further reducing cost and weight.However,the large ionic radii of Na+ and K+ lead to slow kinetics,large volume expansion,and short cycle life,hindering their practical applications.Therefore,the research and development of new anode materials with a high specific capacity,high rate performance,and long-cycle stability need to be strengthened.In this doctoral thesis,we have fabricated a series of bimetallic sulfides and selenides involving conversion and alloying/dealloying mechanisms,and investigated the correlation between their electrochemical mechanism and electrochemical performances as anode for alkali metal ion batteries.2D transitional metal selenides heterostructures are well-known electrode materials in alkali ion batteries owing to the huge surface area,high mechanical strength,and shorter diffusion path.However,the cycling performance is a paramount challenge in terms of the electrochemical conversion reaction.Herein,we have orderly fabricated 2D Se-rich ZnSe/CoSe2@C heterostructure which significantly enhances the intrinsic conductivity during charging/discharging and greatly improve the cyclability.The interaction between potassium ion and Se-rich nanocomposite plays a very crucial role,which is probed for its strong reversibility during intercalation/deintercalation.Thus,the capacity delivered by 2D ZnSe/CoSe2@C-2 heterostructure is mainly contributing to the conversion and alloy/de-alloy processes,where K+ may highly insert or de-insert into Se-rich ZnSe/CoSe2@C.The electrode delivered an outstanding reversible charge capacity of 214 mA h g-1 at 1 A g-1 after 4000 cycles for PIBs,and also obtained 262 mAh g-1 when the electrode was coupled with PTCDA cathode in the full cell.The electrochemical conversion mechanism of the optimized electrode during cycling was investigated through in-situ XRD,ex-situ HRTEM,and Raman.First principle calculations further verify that the ZnSe/CoSe2 anode possesses high potassium ions adsorption ability with high diffusion kinetics.This work opens up a new window for investigating a novel electrode material with wonderful capacity and long durability.Bimetallic sulfides are the leading promising anode candidates for sodium and potassium ion batteries owing to high theoretical capacities and the synergistic effect of dual metal reaction kinetics.However,the poor conductivity and large volume expansion impeded their electrochemical conversion reactions.Herein,we successfully decorated Co1-xS/ZnS hollow nanocubes heterostructure on the surface of reduced graphene oxide through hydrothermal and post-heat vulcanization techniques.This unique Col-xS/ZnS@rGO nanocomposite can provide a more stable conductive network which may significantly seize the relationship between Na+/K+ intercalation and conversion during cycling process which resultantly obtained ultra stable cycling performance.The optimized nanocomposite presents remarkable electrochemical performance in PIBs,which presents 274 mA h g-1 and sustained the charge capacity up to 245 mA h g-1 at high current density of 1 A g-1 after 2000 cycles.More importantly,the electrode also demonstrated an outstanding cycling performance for K+ full cell configuration when PTCDA was used as a cathode.A series of electrochemical tests,in-situ XRD,ex-situ HRTEM and GITT were revealed to investigate phase transition and ion diffusion kinetics of Co1-xS/ZnS@rGO electrode.The hollow nanocube structure of bimetallic sulfide prevents from structural collapse and perpetuates the durability of the electrode for long cycling performance.This work will accelerate the fundamental construction of bimetallic sulfide hollow nanocubes electrodes for energy storage applications.Developing metal sulfide anodes for potassium ion batteries is crucial for large-scale applications,due to the high theoretical capacity and abundant active sites.However,the intrinsic low conductivity and poor cycling stability are hampered their practical applications.Given this,the rational design of hybrid structures with high stability and fast charge transformations is the critical approach.Herein,we demonstrated CoS2ZnS@rGO hybrid nanocomposites with stable cubic phases,the synergistic effect of bimetallic sulfide nanoparticles,and highly conductive 2D rGO nanosheets,which hold excellent long term cyclability for potassium ion storage.Such hybrid nanocomposites delivered a remarkable ultra-stable cycling performance in PIBs of 159,106,and 80 mA h g-1 at 1,1.5,and 2 A g-1 after 1800,2100,and 3000 cycles,respectively.Moreover,the full cell configuration with PTCDA organic cathode(CoS2/ZnS@rGO‖PTCDA)exhibited better electrochemical performance.Besides,when the CoS2/ZnS@rGO nanocomposites were used in SIBs,the electrode demonstrated a reversible charge capacity of 259 mA h g-1 after 600 cycles at 2A g-1.In-situ XRD and ex-situ HRTEM characterizations further confirm the conversion reactions of CoS2/ZnS during insertion/desertion processes.Our synthesis would be general to other bimetallic sulfide hybrid nanocomposites.This strategy opens a new road map for exploring the hybrid nanocomposites with feasible phase engineering for excellent electrochemical performances in energy storage applications.