Controllable Synthesis And Electrochemical Mechanism Of Transition Metal Chalcogenides

Developing efficient and stable electrode materials is becoming one of the key technology for exploring new energy storage and conversion systems.Transition metal chalcogenides（TMCs）pose diverse phase structure and play a vital role in the field of electrode materials.The electronic structure in both the 3d and 4s orbitals of TMs（especially in 3d family）can be potentially influenced by the local environment.The chemical reaction states of TMs in energy storage systems can be devided into two categories:（i）the chemical reaction of TMs occurred in the bulk of electrodes in Li-ion batteries,Zn-in batteries,and Na-ion batteries,and their oxidation and reduction properties have potential influence towards the energy storage properties;（ii）the chemical adsorption and desorption of intermediates with TMs occurred on the surface of electrodes in Zn-air batteries,CO₂ series batteries,and water-splitting devices.However,the electrochemical properties of the TMs can be potentially influenced by many factors,such as the crystal structure,crystallinity,interface structure,and the local doping state,thus affecting the electron transfer,energy density,and cycling stability.Therefore,it is an urgent tusk to systematically investigate the influential mechanism of local environment towards the electrochemical properties of TM.In this work,we synthesized series of single phase,integrated electrodes,heterointerface and high disordered TMCs,and precisely controlled their crystal structure,micromorphology,and interfaces states.Based on the advanced characterization techniques and theoretical calculations,we realized the influence mechanism of the local environment of TMs（both on the surface and in the bulk）towards the energy strorage mechanism.This work is significantly important for designing efficient electrode materials.We have done the following detail work in controllable synthesizing TMCs and exploring their electrochemical mechanism.（1）We developed a facile and green polyols-based solution chemical process for controllable tuning the composition,phase structure and morphorlogy of TMCs,such as Ni-based sulfides and selenides,by injecting cationic precursors into hot base anionic solution.The influence mechamism of the synthesized TMCs towards oxygen evolution and hydrogen evolution（OER and HER）were systematically studied.（2）The above synthesized single phase of catalysts are easily integrated,and thus influencing the electron transfer rate and lowering the surface absorption and desorption properties of surface TMs.Therefore,we synthesized integrated electrodes by in situ sulfurizing or seleniding the Nickle Form（NF）,and further in situ electrochemically oxidized the surface of precatalysts to form core-shell structure.The elemental ratio and oxidation degree can be well tuned to optimizing the synergestic effect in the well conductive electrodes,and accomplishing efficient overall water splitting application.（3）Alhough the TMCs have high electronic conductivity,their little active sites exposure,low intermediates adsorption/desorption rate confined the catalytic performance.Designing heterointerface structure can introduce excellent properties of single phase into the composites,and enhancing the electronic coupling effect,thus improving the active sites exposure.Therefore,we developed successive ion injection method to prepared transition metal selenides hybrids.The high dense heterointerface structure of the nanohybrides were well controlled by tunning the injection interval and other factors.The experimental and theoretical results show that the transition metal Ni and Co in high dense interfaces can be oxidized in a relatively lower overpotential,and that the Ni can be more easily oxidized than that of Co at low overpotenail,indicating the Ni mainly responsible for OER/ORR at the starting stage.The NiSe₂/CoSe₂-N with atomic level interfaces display an overpotential of 285 m V at 10 m A cm^-2.（4）The above results are studying the influence of microstructure towards surface TMs.However,the TMs in the bulk can be potentially influenced by crystal structure,crystallinity,and defects,et.al.Their oxidation or reduction behavior have can triger the cell perameter change or even phase change,thus influencing the ionic or electronic transfer.Therefore,it is important to investigate the influence mechanism of local environment towards the bulk TMs.We select Li₂Mn_3/4Cr_1/4O₂F as target,which has high disordered structure and weak crystallinity,and using Li₂Mn_3/4Mo_1/4O₂F,and Li₂Mn_2/3Nb_1/3O₂F as references.The results show that the higher disordered structure and amorphous area in the Li₂Mn_3/4Cr_1/4O₂F can relief the inner stress that caused by the lattice parameter change during charging/discharging,which is benefit for stabilizing the crystal structure.Using in-situ hard XAS analysis,we also observed that the Mn and Cr can be dual redoxed simultaneously during charging/discharging,which means that the redox potential of Mn and Cr is relatively high（～3.6-3.8 V）and thus can restrain the oxygen redox at relatively high voltage,thus decreasing the transition metal dissolution and lattice constrain and ensuring the stability of the lattice structure.