| Commercial secondary batteries have been widely used in various industries,including alternative fuel vehicles,base stations,aerospace and the military industry.The most common used liquid electrolytes have encountered some safety problems,e.g.leakage,combustion,explosion and short circuit in batteries.Therefore,an urgent demand in daily life boosts the development of high energy lithium-ion batteries with higher safety performance.Compared with organic liquid electrolytes,polymer electrolytes present potentially the best solution for lithium-ion batteries due to their tunable mechanical properties,low density,good flexibility,less leakage,less flammable nature,strong designability,and scalable production in industries.However,the widespread development of polymer electrolytes is hindered by some important issues,including the failure of the Nernst-Einstein(NE)relationship,the unclear ionic solvation sheath structure,the distribution of ion species,the chemical nature of the experimentally measured transport number(transference number),and the unperfected mechanism of ion transport,etc.Therefore,a deep understanding of the microstructures and ion transport in polymer electrolytes is beneficial to developing the next-generation high-performance polymer electrolytes.This work aims to study the microstructures and ion transport of different kinds of polymer electrolytes using theoretical simulation,guiding the design of polymer electrolytes and promoting the development of ion transport mechanisms.The main research objectives of this work include two types of polymer electrolytes:all-solid-state and gel-based electrolytes.The first part is to study the ion association and transport properties in PEO-Li TFSI electrolytes at different salt concentrations.This work has revealed two regions of salt concentration related to the properties of ion association;these two characteristic regions present significantly different microscopic structural features,including ion solvation sheath structures,distribution of ion species,and the structures of ion clusters.Through the study of the effect of ion association on the Li~+diffusion at low salt concentrations,the dynamics equilibrium between free ions and ion pairs is an important feature of ion association.At high salt concentration,the motion of ion clusters contributes to the total Li~+diffusion,but the content of free ions is the highest and their motion is the slowest;the mechanisms of the dynamics of ions,ion clusters,and polymers are also revealed.In the second part,the microstructures and dynamic behavior of Poly-DOL-based gel polymer electrolytes in the bulk phase and on the surface of lithium metal electrode were investigated.In bulk electrolytes,the lithium solvation sheath structures consist of one layer of solvent/anion molecules,and the interaction between these molecules and lithium ions is composed of Van der Waals interaction and weak attraction;the influence of polymer molecular weight and solvent content on the solvation sheath structures,ion transport,ionic conductivity and ion transference etc.was researched in order to establish the empirical law regulating the transport properties of polymer electrolytes.We have constructed three kinds of electrolyte-lithium metal interface models:polymer-in-liquid type,liquid-in-polymer type and concentrated-solution type;the increase of polymer and salt concentration is beneficial to the improvement of the interfacial stability;Analysis of the Li~+solvation sheath structure shows that its composition is different between parallel and perpendicular to the metal surface,which may have a certain impact on the deposition of lithium ions.Lithium metal has a certain promotion effect on ion diffusion,while the use of polymer and high salt concentrations inhibits this effect.The analysis of the relevant properties of the gel polymer electrolyte in this work can provide an in-depth understanding of the impact of lithium metal anodes on battery performance. |
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