Theoretical Simulation Of Lithium Anode Inhibiting Dendrite Growth And Lithium Ion Diffusion

Chemical power supply realizes the reversible conversion of electric energy and chemical energy and is an important carrier to improve energy efficiency.Lithium metal has the lowest electrode potential and mass density,which is the best choice for current high energy density energy storage system,such as lithium battery anode material.However,it is easy to generate dendrites in the charging and discharging process,forming "dead lithium",which reduces the efficiency of the battery,has serious safety risks,and limits its practical application.Self-assembled molecular membranes and li-containing alloys show unique advantages in the inhibition of lithium dendrite growth,but the thermodynamic mechanism of the inhibition of lithium dendrite growth is not clear.In this paper,the mechanism of dendrite formation and growth inhibited by alkyl phosphoric acid self-assembled molecular membrane and the mechanism of lithium ion diffusion in lithium alloy were studied by using Density Functional theory(DFT).Specific contents include:(1)Mechanism of dendrite growth inhibition by alkyl phosphoric acid self-assembled molecular membrane.The formation of self-assembled molecular film on lithium surface and its effect on the geometrical and electronic structure of metal surface were studied,which led to the change of lithium ion deposition and diffusion behavior.First,it was found that the alkyl phosphate molecule transferred electrons to the lithium surface through the phosphate group to form a bond with the lithium.With the increase of chain length,the van der Waals interaction between alkyl phosphoric acid molecules is stronger than the interaction with lithium surface.At high coverage,the interaction between alkyl phosphoric acid molecules dominates the stability of molecular films.Secondly,the selfassembled molecular film causes tensile strain to the atoms on the lithium surface.At the same time,the negative charge value on lithium surface decreases due to charge transfer.Thirdly,the oxygen of the alkyl phosphoric acid molecular membrane head group can anchor the lithium ions beyond the quota,forming the lithium oxygen bond,enhancing the adsorption energy of the surface for lithium.When the adsorption energy is greater than the cohesion energy of lithium,lithium atoms tend to uniformly adsorb and deposit on the surface of the material,which can effectively avoid the agglomeration of lithium atoms into dendrites.Finally,the diffusion barrier indicates that the diffusion barrier value of lithium atom at the most stable adsorption site is only 0.28 e V,and the self-assembled molecular membrane can effectively promote the diffusion of lithium atom without strong limiting effect in a certain region.In summary,it is found that self-assembly of alkyl phosphoric molecules with a certain chain length on lithium surface can uniformly improve the lithium philicity of electrode surface and reduce the diffusion energy barrier of lithium atoms,thus obtaining uniform dendrite-free lithium deposition and improving the coulombic efficiency and cycle life of the battery.(2)Study on diffusion mechanism of Lithium ion in lithium alloy anode.The effects of alloy type,vacancy and interfacial lithium concentration gradient on the diffusion behavior and mechanism of lithium ion were studied.Firstly,it is found that the formation of alloy can reduce the cohesion of the system and increase its stability.Secondly,compared with pure lithium,lithium alloy is easier to achieve the diffusion of lithium ion by direct diffusion mechanism.Diffusion energy barrier calculation shows that Li3 Ag alloy phase has better diffusion ability of lithium ion.Thirdly,the diffusion barrier of different vacancy concentration and distribution indicates that the increase of vacancy concentration can effectively reduce the energy barrier to be overcome.Finally,the calculation of the composite model with different lithium concentration gradients shows that the larger the lithium concentration difference at the interface,the more favorable lithium ion diffusion is.In conclusion,the diffusion property of lithium ion can be improved by selecting suitable alloy and adjusting the vacancy concentration or interfacial lithium concentration gradient of alloy.