Molecular Dynamics Simulations Of Mass Transportation Controlled By Porous Materials

In the past decades,with the miniaturization of electronic devices,there has been an increasing demand for new generation of energy materials,which has raised intensive interest in the research of lithium batteries,among which lithium-sulfur(Li-S)batteries are expected to be applied in a new generation of energy storage devices due to their excellent theoretical capacity and environmentally friendly properties.However,the industrial production and commercial applications of lithium-sulfur batteries are still challenging,due to the low electrical conductivity of sulfur,the expansion of Li₂S during its formation,and the penetration of lithium dendrites into the diaphragm.In addition,the dissolution and migration of polysulfides(Li PS)lead to performance degradation in batteries(i.e.,the"shuttle effect")during charging and discharging processes,making the development of high-performance lithium-sulfur batteries extremely challenging.Porous materials can be used to tune the transport of polysulfides and electrolyte due to their adjustable structures and high specific surface area.Many studies have shown that porous materials have the potential applications in enhancing the comprehensive performance of lithium-sulfur batteries.In this work,molecular dynamics simulations(MD)have been used to investigate the diffusion mechanism of lithium ions controlled by metal-organic framework-based polymer electrolytes and to reveal the transportation mechanism of electrolyte and polysulfide tuned by carbon nanotubes as a diaphragm.The contents and results of this research are listed as follows.1.MD Simulations of Metal-Organic Framework-Regulated Polyionic LiquidsThe polymer matrix provides channels for ion transportation through the movement of polymer chains.MOFs filler not only effectively improve the dissociation rate of lithium salts,the conductivity of lithium ions,and the mobility of polymers,but also prevent the dissolution of polysulfides in the electrolyte of the cathode via adsorption.First,MD simulations were carried out to study the shift of orientations of polyionic liquids(PILs)with MOFs in comparison before the involvement of MOFs for reveling the interaction mechanism between HKUST-1 and PILs.Subsequently,the adsorption mechanism of MOFs,PILs and polysulfides were analyzed at the atomic level using density functional theory(DFT)calculations,and the microscopic process mechanism of MOFs on the enhancement of properties in lithium-sulfur batteries was inferred based on our theoretical studies.2.MD Simulations of Domain-Controlled by a Double-Shelled Hollow Carbon Sphere(DSHCS)on PolysulfidesThe double-shelled hollow carbon nanotubes have excellent material transport properties in the lithium-sulfur battery diaphragm due to their unique structures,which provide channels for electron transportation and adjust the diffusion rate of soluble polysulfides through the domain-controlling effect in the confined space.Firstly,MD simulations were used to simulate eight carbon nanotube models with pore diameters ranging from 1.2 to 11 nm to explore the transportation law of polysulfides and electrolyte substances,and it was found that the diffusion rate of each substance increased with the increasing of the pore diameter from 1.2 to 3.5 nm.When the pore diameter was larger than 3.5 nm,the diffusion rate of each substance decreased with the increasing of the pore diameter.In addition,the effect of temperature and pore length on the transportation was investigated,showing that increasing in the temperature could increase the diffusion rate of substances,while the increase of pore length did not effectively improve the diffusion.Compared with the model of single shelled structure,this double-shelled structure can effectively restrict the polysulfide while remain the transportation of electrolyte and ions.Theoretically our model proved that porous materials had excellent performance in controlling the electrolyte transportation and polysulfide.In summary,we have applied MD simulations with the combination of DFT calculations to explore the microscopical transportation process of lithium-sulfur batteries regulated by porous materials.The relationship between the structure and performance was investigated by our research,which provides theoretical guidelines for adjusting the order of lithium-sulfur batteries when the transportation was manipulated by the porous materials.