Simulation Studies Of Metal-Ion Coordination And Diffusion Mechanisms In Bulk And Interfacial Water-in-Salt Electrolytes

In recent years,aqueous electrolytes have received great research interests due to their high safety,environmental friendliness,and low manufacturing costs.Compared to organic liquid,polymer,inorganic solid,and ionic liquid electrolytes,aqueous electrolytes have significant advantages in ion conductivity,interfacial wetting,and safety.However,the electrochemical stability window of aqueous electrolytes is relatively narrow,which largely hinders the output voltage and energy density of aqueous batteries.Recently,it has been found that high-concentration water-in-salt（WIS）electrolytes can effectively inhibit the decomposition of water molecules,thereby broadening the electrochemical stability window of aqueous electrolytes and increasing the energy density of aqueous batteries.Currently,experiments have successfully synthesized a series of WIS electrolytes based on monovalent and divalent metal ions,but there is still a lack of fundamental understanding about the structure and properties of metal ions with different ionic sizes and valences in WIS electrolytes.To this end,this thesis employs molecular dynamics（MD）simulations to systematically investigate the coordination structure and diffusion properties of typical monovalent and divalent metal cations in high-concentration WIS electrolytes and explore the behavior of Li⁺ion-based WIS electrolytes in two-dimensional Ti₃C₂O₂ nanosheet slits.In Chapter 2,we conducted MD simulations to study the coordination and transport mechanisms of monovalent metal ions（Li⁺,Na⁺,and K⁺）with different ionic sizes in WIS electrolytes.Our simulation results showed that in high concentration WIS electrolytes,the coordination stability of metal cation with water molecules and anions in the first solvation layer gradually decreases with increasing ion size.For Na⁺and K⁺ions,the coordination stability between metal cation and anions is higher than that between metal cation and water molecules.This is probably due to the Na⁺and K⁺ions have a larger size,which would weaken their binding ability to water molecules,while enlarge their interactions with anions.Upon increasing the ionic size,the diffusion rates of anions,cations,and water molecules are also found to be increased,but the diffusion mechanisms of all three components tend to adopt a diffusion motion,which is independent of ion size.In addition,we found that the average number of hydrogen bonds（HBs）formed among water molecules around the metal cation increases as the ionic size increases,but the HBs strength gradually decreases.In Chapter 3,we further studied the coordination structure,coordination stability,and diffusion dynamics of divalent metal ions(Zn²⁺and Mg²⁺)in WIS electrolytes.The simulation results show that in low-concentration salt-in-water（SIW）electrolytes,the metal cations are mainly surrounded by water molecules,while in high-concentration WIS electrolytes,there exist both water molecules and anions around the metal cations.Compared to low-concentration SIW electrolytes,the coordination configurations of divalent metal cations increase in high-concentration WIS electrolytes.In addition,as the concentration of metal salts increases,the diffusion rate of anions,cations,and water molecules decreases,as well as for the average HBs number between water molecules.Furthermore,the coordination stability of cation-water molecules increases gradually when increasing metal salt concentration,but it is still weaker than the coordination stability between metal ions and anions.This is mainly because the electrostatic interaction between metal cations and anions is stronger than that between metal cations and water molecules,and furthermore the water molecules around the metal cations at high concentrations can seldomly form HBs,which weakens the interaction between water molecules and metal cations.In Chapter 4,we investigated the structural and dynamic properties of a lithium-ion-based WIS electrolyte in Ti₃C₂O₂ nanosheets,with a special interest in the effect of slit pore size on ion behavior.Our simulation studies show that at small pore size,lithium ions can form a trilayer structure within the slit.As the pore size gradually increases,the lithium ions in the slit would transform into a bilayer structure.When the slit pore size is further increased,the lithium ions would form a trilayer structure.Upon increasing the pore size,the orientation of TFSI anions in the slit changes from parallel to the Ti₃C₂O₂ surface to perpendicular to the Ti₃C₂O₂ surface.In addition,it is noted that water molecules tend to be parallel to the Ti₃C₂O₂ surface as the slit pore size increases.Further,the interaction between lithium ions and water molecules in the first solvation layer becomes weakened with respect to the pore size,while the interaction with TFSI anions gradually strengthens.Finally,we found that the diffusion coefficients of each component in the slit increases as the interlayer spacing increases.