High-throughput Molecular Dynamics Simulation Of Lithium-ion Transport In Garnet-type Solid Electrolyte

In recent years,all-solid-state lithium batteries without liquid organic electrolytes have attracted wide attention due to their high safety,high energy density and other factors.As a key material in all-solid-state batteries,solid electrolytes replace the electrolyte and diaphragm in traditional lithium batteries,play the role of conducting ions and isolating electrons,which requires solid electrolytes exhibit room temperature lithium ion conductivity(10^-2S·cm^-1)and a wider electrochemical window as compared with electrolyte.Solid electrolytes with garnet-like structure has gained extensive research interests due to its high ionic conductivity(10^-4S·cm^-1),mechanical strength,excellent chemical stability and electrochemical stability.The main obstacle of its development is the insufficient lithium ionic conductivity.In this work,we study the phase transition mechanism and cubic phase stabilization methods by molecular dynamics simulations and first-principle calculations.The influence of the size of lithium ion transport channel bottleneck on diffusion performance are also studied.The phase transition from tetragonal to cubic phase was simulated by molecular dynamics,and the redistribution of lithium ion was studied.The phase transition of lithium garnet was caused by the site energy difference and driven by entropy.With the flowing of lithium ions from 32g,8a and 16f sites to 16e site in sequence,the arrangement of lithium ions turned from order（tetragonal phase）to disorder（cubic phase）.First-principles calculations confirmed that the order of the flowing is caused by site energy difference.The migration pathway in both tetragonal and cubic phase were studied by the combination of spatial distribution,occupancy distribution and local structure of lithium ions network.Lithium ions exhibited two-dimensional diffusion in tetragonal phase and three-dimensional diffusion in cubic phase.The difference of diffusion mechanism was caused by the distribution difference of lithium ion.The effect of supervalent element doping on lithium ion occupancy was studied.Lithium ion concentration played a key role in the stabilization of cubic phase.Low valence element doping could also stabilize cubic phase.A series of doping elements have been selected through equivalent doping design.The bottleneck size was more suitable than the lattice constant and volume to measure the influence of structure effect on diffusion performance.Elements doping with large ionic radius could increase lattice parameters and introduce structural distortion.The lithium ion transport channel could not be scaled up equally.The relationship between lithium ion conduction and the bottleneck size were studied by performing high-throughput molecular dynamics simulations.Larger bottleneck size helped to increase diffusion coefficient and reduce diffusion barrier.The bottleneck size of equivalent doping with single element ranged from 1.83 to 1.91?,which was smaller than the sum of O^2-（1.40?）and Li⁺（0.64?）.