The Surface/Grain Boundary Properties And Lithium-Ion Transport Dynamics In Perovskite-Type Solid Electrolyte LLTO

Due to its excellent safety and high energy density,all-solid lithium-ion battery is expected to improve the safety of batteries,and has a broad application in the field of power batteries and large-capacity new energy storage.Solid electrolyte,as the key material of all solid state lithium ion battery,determines the performance of battery system fundamentally.Among the solid electrolytes,the perovskite-type（ABO₃）solid electrolyte has attracted much attention due to its rich compositions and high ionic conductivity.Among them,the lithium titanate lanthanum Li_3xLa_（2/3）-x□_（1/3）-2xTi O₃（abbreviated LLTO,□represents A hole）is considered to be a potential solid electrolyte materials because of its highest ionic conductivity(at room temperature as high as 10^-3S/cm).However,the growth of lithium dendrites remains a major challenge for LLTO solid electrolytes.Therefore,using molecular dynamics simulation and first-principles calculations,we studied the surface/grain boundary and Li transport properties of LLTO solid electrolyte as well as the influenceof lithium concentration in this paper,with the aim to understand how Li content in LLTO surface/grain boundary tunes the surface/grain boundary performance.Our results can provide theoretical guidance for inhibiting the growth of lithium dendrites in LLTO solid electrolyte.The effects of terminal Li content on the stability,electronic structure and Li⁺diffusion properties of LLTO surface were studied by molecular dynamic simulation combined with first-principles method.The results show that the（001）plane with La/O/Li-atomic terminal is the most stable crystal plane in both Li-poor and Li-rich phases,and its surface energy firstly decreases and then increases with the increase of Li content.The electronic structure analysis shows that with the increase of Li content,the transition from metal to semiconductor is found on LLTO（001）surface of Li-poor phase and Li-rich phase.The results of Li⁺transport properties show that LLTO（001）surfaces all have two-dimensional diffusion channels along the ab plane,and the maximum Li⁺diffusion coefficient and the lowest Li⁺diffusion energy barrier are 0.42e V and 0.30 e V when the terminal Li content reaches 0.38 and 0.40,respectively.Therefore,changing the Li content is helpful to improve the surface stability of LLTO（001）,open the surface band gap,and improve the Li⁺migration performance.The improvement of these properties is conducive to the growth of surface lithium dendrites.The stability,mechanical properties,Li⁺diffusion properties and the influence of Li content on the Li⁺diffusion properties at the LLTO grain boundary were studied by molecular dynamics simulation.The results show that,compared with other perovskite and polycrystalline materials,the formation energy of six grain boundary（GB）structures is lower than 1.30 J/m²,indicating that these GB structures may exist in high concentration in real materials.In terms of mechanical properties,Young’s modulus（E）,shear modulus（G）and volume modulus（B）of grain boundary structure are all smaller than those of corresponding bulk phase,which means that grain boundary structure is more unstable than bulk phase.In addition,we found that the activation energy of Li⁺in both Li-poor and Li-rich phases is lower than that of grain boundary structures,indicating that the existence of grain boundary hinders the movement of Li⁺.Finally,we study the effect of lithium content on Li⁺diffusion at grain boundaries.The results show that the higher the lithium content at grain boundaries,the wider the Li⁺diffusion range.