Structural Design And Energy Storage Mechanism Of High-Performance MoS₂-Based Cathode Materials For Aqueous Zinc-Ion Batteries

Aqueous zinc ion batteries(AZIBs)have gained extensive attention owing to high energy density(5855 mAh cm-3 and 820 mAh g-1),good water compatibility,and low redox potential(-0.763 V vs.standard hydrogen electrode)of Zn anode.Nevertheless,the electrode structure degradation and sluggish diffusion dynamics caused by the strong electrostatic interactions between bivalent Zn2+ and host materials severely hinder the development of AZIBs.Therefore,developing efficient,stable,and rapid Zn-storage cathode materials is one of the key points to promote the development of aqueous Zn-ion batteries.To address the above problems,the present thesis innovatively designs three novel MoS2-based composites as cathodes for AZIBs.The aqueous Zn-storage activities of the asprepared MoS2-based materials are gradually optimized through structural modification and energy-storage-mechanism promotion strategies,meanwhile,the electrochemical properties,electrode process kinetics,and charge storage mechanisms of the as-prepared materials are systematically evaluated and explored.The main investigations are outlined as follows:(1)A novel MoS2/graphene hybrid was synthesized based on the interlayer expansion engineering,in which the intercalated graphene significantly expands the MoS2 interlayer spacing from 0.62 nm to 1.16 nm.The MoS2/graphene contains abundant metallic 1T-MoS2 and some oxygen-containing groups remain in the intercalated graphene,endowing the MoS2/graphene material with high ionic/electronic conductivity and good hydrophilicity.During the hydrothermal reaction,MoS2/graphene nanosheets self-assemble into a flower-like structure,which significantly suppresses the stacking of graphene and MoS2 layers,facilitates electrolyte penetration,promotes Zn2+transport,and maintains structural stability.Benefiting from these structural merits,MoS2/graphene cathodes show high-rate tolerance(283.9 mAh g-1 and 141.6 mAh g-1 at 0.1 A g-1 and 5 A g-1,respectively)and excellent cycling performance(capacity retention of 87.5%after 1000 cycles).The high Zn2+ diffusion coefficient and low Zn2+ migration barriers of MoS2/graphene electrode are systematically demonstrated by electrochemical measurements and theoretical computations.Multiples ex-situ measurements elaborate the charge storage mechanism of the reversible phase transition between 1T-and 2H-MoS2 induced by Zn2+storage/release behaviors during cycling.As a proof of concept,the assembled quasi-solid-state Zn-ion batteries using MoS2/graphene cathode exhibit good application prospects.(2)In-situ molecular engineering is implemented to implant the lattice oxygen and structural defects into the MoS2 structure(D-MoS2-O),successfully activating the MoS2 inert basal plane and expanding the interlayer distance(0.96 nm),achieving more efficient three-dimensional Zn2+ transport along the c-axis and ab plane.The rich metallic 1T-MoS2 endows the D-MoS2-O with good hydrophilicity and high electronic conductivity.The D-MoS2-O nanosheets are vertically grown on the carbon cloth substrates,effectively restraining the stacking MoS2 layers and promoting electrolyte infiltration and Zn2+diffusion.Benefiting from the above merits,D-MoS2-O cathode delivers excellent high-rate capability(289.1 mAh g-1 and 102.4 mAh g-1 at 0.1 A g-1 and 10 A g-1,respectively)and long cycling durability(capacity retention of 90.5%after 1000 cycles).Electrochemical measurements demonstrate the high Zn2+diffusivity of D-MoS2-O electrode,and theoretical computations verify the feasible three-dimensional Zn2+ transport and low Zn2+diffusion barriers in D-MoS2-O structure.In addition,the quasi-solid-state Zn-batteries were assembled using D-MoS2-O cathode and can work stably even under severe bending conditions,demonstrating good practicality.(3)The proton storage chemistry in Zn-MoS2 batteries has been successfully deciphered by rationally designing the cathode and electrolyte.The poly(3,4ethylenedioxythiophene)was intercalated into MoS2 gallery(MoS2/PEDOT)based on the exfoliation and reflux methods,in which the intercalated PEDOT significantly enlarges the MoS2 interlayer distance from 0.62 nm to 1.29 nm but also enhances the electronic conductivity and reinforces the layered structure.More importantly,the MoS2/PEDOT cathode experiences a highly reversible Zn2+/H+co(de)intercalation mechanism.The proton storage chemistry induced by PEDOT can screen the electrostatic interactions between the MoS2/PEDOT and divalent Zn2+,unveiling a new concept of "proton lubricant" accelerating Zn2+diffusion.Benefiting from the synergism of structural modification and working mechanism optimization,MoS2/PEDOT electrodes afford an admirable high-rate performance(312.5 mAh g-1 and 83.6 mAh g-1 at 0.1 A g-1 and 15 A g-1,respectively)and longterm cycling stability(capacity retention of 90.1%after 4000 cycles).In addition,the fiber-shaped quasi-solid-state Zn-batteries are fabricated employing MoS2/PEDOT electrode and present good cycling stability at low temperatures and deformation states,illustrating great application prospects.