Investigations On The Mechanical And Diffusion Properties Of SEI Film On The Lithium Metal Anode

In recent years,lithium metal has become one of the most promising electrode materials for high-energy-density batteries.The solid electrolyte interface(SEI for short)on the surface of lithium metal electrode plays an important role in the electrochemical process,but it is also subject to large deformation.The structural failure of SEI will promote the growth of lithium dendrites,leading to the degradation of battery performance and safety problems.In this thesis,the SEI on the surface of lithium metal anode is considered as the main object of study.The density functional theory is used to systematically study the deformation damage and size effects of mechanical properties of SEI at nanometer scale.The mechanism of lithium diffusion behavior under mechanical regulation is discussed.And the energy barrier and path under strain condition are given.This thesis aims to obtain original results in the fundamental mechanical properties and mechanochemical coupling mechanism of SEI,which could solve the failure problem of SEI and provide theoretical support for promoting the development of new generation of efficient and pollution-free lithium batteries.For SEI nanofilms,this thesis considers ionic conductor materials LiF and Li2O as research objects,studing the mechanical properties of LiF and Li2O crystal films and exploring the surface diffusion behavior of lithium ions on the crystal films under strain conditions.The results show that the mechanical properties of the two nanofilms have obvious size effects,and the Young’s modulus and the ideal strength decrease with the increase of the thickness of the films.During the battery operation,the initial formation of the ultra-thin SEI has extremely high strength,which could withstand large structural deformation.In terms of diffusion,the relatively low energy barrier on the surface of LiF film is more conducive to the transport of lithium ions.Due to the volume expansion of lithium metal anode,surface diffusion of lithium ions on SEI film is affected:tensile strain reduces the energy barrier of lithium ion on LiF film and increases the energy barrier on Li2O film.For the interfaces in SEI,LiF and Li2O are selected to construct heterogeneous interface system,exploring the mechanical properties of LiF/Li2O interface system and the mechanisms of the interface diffusion of lithium ions under strain regulation.Compared with the LiF and Li2O bulks,the Young’s modulus and tensile strength of the heterogeneous interface system are largely reduced,indicating that the difference of mechanical properties of LiF and Li2O leads to the performance degradation of the LiF/Li2O system.In addition,the presence of vacancy defects and interface inclusions inside the battery further decreases the mechanical properties of the interface system.In terms of diffusion,tensile strain increases the energy barrier of lithium ions through the LiF/Li2O interface,which will reduce the electrochemical reaction rate.For the interactions between SEI and lithium metal anode,this thesis considers LiF,Li2O and lithium metal as examples to study the mechanical properties of SEI/Li heterogeneous interface systems and analyze the influence of irregular lithium metal surface on the mechanical properties of the interface systems.The results show that Li2O/Li interface system has higher strength and critical tensile strain,showing that the mechanical stability between Li2O and lithium metal is better than that between LiF.In the process of electrochemical reaction,the nonuniform diffusion performance of lithium ions leads to the irregular deposition surface of lithium,resulting in the reduction of mechanical strength of the SEI/Li interface systems,which increases the possibility of disbonding between the SEI and lithium metal,and thus reduces the cycle performance of the battery.