Molecular Dynamics Simulation Of A New Solid Polymer Electrolyte

Compared with traditional liquid electrolytes,solid polymer electrolytes?SPEs?have excellent properties such as high safety,good mechanical properties,and high energy density,which have become important trends and directions for future energy development.However,SPEs face the problems such as easy crystallization of polymers and low ionic conductivity at room temperature,in practical applications and commercialization.To this end,the development of new SPEs system or modification of polymers has become the main measure,and the modification methods include the addition of inorganic fillers,blending,grafting,and addition of ionic liquids?IL?.It is significance to clarify the transport sites,diffusion properties,and existing microscopic interactions of Li⁺in the SPEs system,but there are still few research reports on the molecular level of SPEs systems.Therefore,it is necessary to analyze the dynamic behavior of Li⁺from a microscopic perspective.In this paper,molecular dynamics?MD?simulation method was used to introduce organic-inorganic hybrid compounds?polyhedral oligomeric silsesquioxane-ionic liquid,POSS-IL?to a low molecular weight polyethylene oxide?PEO?-based SPE system,and a series of corresponding SPEs models were constructed.Through the calculation of microscopic interactions in the SPEs simulation systems,the effect of inhibiting polymer crystallization,kinetics,conductivity and other properties of Li⁺,the addition mehods of POSS-IL?blending and grafting?,the carbon chain of the imidazolium cation,the influence of factors such as length?C_nMIM,n=3,4,5,6?,POSS concentration,and number of PEO grafts on system performance were studied and analyzed.The main research contents are as follows:?1?Combining POSS and low molecular weight PEO in a direct blending manner to construct a simulation system of PEO/PC/PVDF-HFP/POSS-IL/Li TFSI,which was referred to POSS-C_nMIM.The spatial distribution functions?SDFs?,radial distribution functions?RDFs?,mean square displacement distribution functions?MSDs?,self-diffusion coefficients?D?,glass transition temperature?T_g?and electrical conductivity???were investigated and analyzed.The results showed that the introduction of POSS-IL can not only provide additional transmission sites for Li⁺,but also effectively inhibit polymer crystallization.Especially when n=3,POSS-C₃MIM had the best crystallization inhibition effect on the system.At the same time,the electrical conductivity of POSS-C₃MIM SPE at 318.15K was an order of magnitude higher than that of the blank system,reaching 1.20×10^-3 S/cm.In the study of the effect of POSS concentration on the comprehensive performance of the system,it was found that when the mass fraction of POSS-C₃MIM was 1.55 wt%,the system had the highest self-diffusion coefficient and the best effect of inhibiting polymer crystallization,which showing good comprehensive performance.?2?Combining POSS and low molecular weight PEO in a grafting way to construct a simulation system of PEO/PC/PVDF-HFP/POSS-IL/Li TFSI,which was recorded as POSS-PEOm?m=1-7,take an integer?.By calculating RDFs,MSDs,D,T_g,?and other related properties,the effects of POSS-IL graft addition and the number of grafted low molecular weight PEO segments on the system performance were analyzed.The results showed that,similar to blending,Li⁺also had a strong interaction with the oxygen atoms at the end of the long chain of POSS in the grafting system.When m=3-6,it had a better effect on inhibiting polymer crystallization,and the polymer segments can move faster in these systems,which was beneficial to the transmission of Li⁺.The calculation of electrical conductivity showed that when m=reaches the order of 10^-2 S/cm at 358.15K,which had been greatly improved,compared with conventional PEO-based SPEs.