Interface Modulation Of MoS₂-based Catalysts And Study On Li-O₂ Battery Performance

As the market share of electric vehicles continues to increase,consumers are placing greater demands on their endurance.However,the energy density of traditional Li-ion batteries has hit a plateau,prompting researchers to explore energy storage devices with higher energy densities that are better suited for electric transportation systems.Despite the significant advantage in energy density over other batteries,Li-O2 batteries are still constrained by such critical factors in practical applications,including limited cycling life,safety concerns regarding metallic lithium anodes,electrode material stability and cost consideration.In order to promote the industrial application and development of Li-O2 batteries,finding a suitable cathode catalyst material is crucial to effectively enhance the electrochemical performance.A stable and efficient cathode catalyst could enable significantly reduced battery overpotentials,increased Coulombic efficiency,enhanced cycling stability and promoted kinetics of the sluggish oxygen reduction/evolution reactions.Transition metal sulfides typically exhibit high catalytic activities and diverse chemical and physical property due to their structures,which can be easily tuned by adjusting their synthesis conditions.These characteristics make transition metal sulfides widely studied and applied in areas such as carbon dioxide reduction,electrochemical water splitting and electrochemical energy storage.In the case of LiO2 batteries,the generation and decomposition of the main products during charging and discharging occur on the cathode side,demonstrating that the microstructure,compositional makeup and electronic structure of the cathode catalysts significantly affect the formation and decomposition of Li2O2 to ultimately determine battery performance.In this thesis,the modification method of carbon materials by transition metal sulfides and the heterojunction structure constructed from bimetallic sulfides were performed to regulate the structure of molybdenum sulfides,and their reaction mechanisms with various discharge products were intensively investigated.The main thesis investigations are listed as follows:（1）Novel 2H-MoS₂ modified nitrogen-doped hollow mesoporous carbon spheres were constructed as efficient cathode catalysts for Li-O2 batteries.The relationship between battery performance and structural properties was further explored.Using a one-step hydrothermal synthesis method,the organic precursor was carbonized by heat treatment,and the nitrogen-doped hollow carbon spheres modified with molybdenum disulfides were synthesized.We found that the synergistic effect of nitrogen-doped mesoporous carbon and MoS₂ endowed the composite catalyst with excellent electrocatalytic activities,and the growth and decomposition mechanisms of Li2O2 during the charge and discharge process are investigated.The modified material achieved discharge/charge specific capacities of 15563/15292 mAh g-1 at a current density of 100 mA g-1,and excellent stability with 145 cycles under a cutoff capacity of 1000 mAh g-1.（2）A cathode catalyst material with a heterojunction structure of NiS₂-MoS₂ was synthesized,and its electrocatalytic performance and charge/discharge mechanisms were further studied.Bimetallic transition metal oxide precursors are obtained using a one-step hydrothermal method,and NiS₂-MoS₂ heterojunctions with rich heterojunction interfaces were prepared by tube furnace sulfidation.Combining experimental results with theoretical calculations,the promoting effect of the heterojunction interfaces on the formation and decomposition of products during the charge and discharge processes was explored at the atomic level.The Li-O2 cells with this catalyst delivered discharge/charge specific capacities of 16528/16471 mAh g-1 at a current density of 100 mA g-1,and stable cycling for 450 cycles under the conditions of a current density of 1000 mA g-1 and a cutoff capacity of 1000 mAh g-1.This work provides new insights into the structure modulations and improvements of transition metal sulfides,as well as their catalytic mechanisms for the cathode in Li-O2 batteries.