Theoretical Study On Modifying Materials For Separator To Suppress The Shuttle Effect Of Polysulfide In Li-S Batteries

Renewable and reliable sources of clean energy,such as wind and solar energies,are not only critical for the global economy but will also aid in the mitigation of environmental and health hazards caused by fossil fuels.However,the intermittent nature and variable of the output of these renewable energy may cause stability problems to grid network.And to smooth out the variations,electricity storage systems are needed in the grid.Among the next-generation rechargeable batteries,Li-S batteries have been proved to be one of the most attractive and promising candidates due to their high energy density(2600 Wh·kg^-1)and theoretical capacity(1672 mAh·g^-1).Despite great progress has been achieved,some challenges including obtain high loading content of sulfur and avoid the loss of active materials caused by the dissolution of the intermediate polysulfides in the electrolyte still exist in the practical application of Li-S batteries.In view of this,effectively anchoring the LiPSs from being dissolved to electrolytes is critical to enhance the electrochemical performances of Li-S batteries.Separator is one of the important parts of the batteries,whose performance has huge effect on the overall performance of batteries considering its key functions in separating the anode and cathode,as well as permiting the free flow of lithium ions.Especially,some separators modified with certain materials,can demonstrate strong chemical interaction effects on immobilizing polysulfides,thus bringing benefits to the improved performances of Li-S batteries.In this thesis,the modification of the separator by using polar anchoring materials may be convenient and efficient approach to trap polysulfides preventing them from shuttling between the cathode and the anode.The major contents and research results are listed below:?1?Four kinds of non-metallic monolayer materials?N-MMLMs?,including C₃N₄,C₂N,BN,and Graphene,have been investigated as the anchoring materials to immobilize lithium polysulfides?LiPSs?of entire lithiation species by the density functional theory?DFT?calculations.The results show that the anchoring effect benefits from both the chemical binding and the physical confinement of S-related species by the N-MMLMs.C₃N₄ and C₂N can provide proper binding energies and enhance the redox kinetics.The dispersion interaction region is mainly concentrated in the vicinity of sulfur atoms,and the strong anchoring effect comes from the bonding of Li-N/C-S and charge transfer.The adsorption/desorption of multiple Li₂S on anchoring materials can be controlled by modifying the charge state.Considering the role of the electrolyte,the schematic diagram demonstrates that strong-coupled interaction formed by C₂N and C₃N₄ can alleviate the interactions between LiPSs and solvent as well as the consequent dissolution.?2?Combining experimental with theoretical approach,a separator is designed by coating hexachlorocyclotriphosphazene?HCCP?decorated rGO,which provides effective anchor sites for immobilizing LiPSs.The results illustrate that our designed separator can moderately bind with LiPS species,keep the intact LiPSs configuration and avoid their dissolution into the electrolyte.The conductivity of HCCP/rGO before and after the adsorption of LiPSs was also explored.The results show that HCCP/rGO can be used as electronic diffusion channels for the subsequent electrochemical reaction.Synergetic effect of the effective barrier and good electrical conductivity within the HCCPs/rGO sheets efficiently anchors the LiPSs and achieves excellent electrochemical performance.Moreover,the interaction between HCCP derivatives and LiPSs was also studied by DFT calculations in order to select suitable anchoring material.The HCCP derivatives are predicted to be promising anchoring materials for high performance Li–S batteries,and different substituent on P atoms have their own advantages for the immobilization of LiPSs.?3?The Pt single-atom was"fixed"on the polar surface of the WS₂ materials,sufficient adsorption energy is crucial to ensure the immobilization of Pt atoms,the position of W atoms was the most favorable site for all adatoms.The DFT was carried out to confirm that PtWS₂ could be used in Li-S batteries to suppress the shuttle effect of LiPSs and increase the conductivity of the WS₂ materials.Due to the interaction of Pt atom on S atom,the structure of long-chain LiPSs were seriously distorted.At the same time,we studied the potential of PtWS₂ monolayer as electrode materials for Li,K,Mg and Al ion batteries.Almost all of the cations in different sites were effectively adsorbed on the PtWS₂monolayer.Thus we suggest that PtWS₂ is a very promising separator or an electrode material for Li-S batteries or maybe even Li,K,Mg and Al ion batteries.