(Invited) Strain Effects on Li Ion Conductivity – Migration, Defect Correlations, and Structural Changes

Abstract

Elastic or plastic strain at cathode/electrolyte interfaces can affect the charge transfer kinetics in all-solid-state Li-ion batteries. Quantifying this effect both for Li ion conduction within the solid electrolyte material and for the Li transfer ability at the interface is important for understanding the kinetics of solid state batteries. Depending on the strain tensor, the impact could be beneficial or, alternatively, detrimental. While it is generally known that isotropic tensile strain increases the ionic conductivity and compressive strain decreases it [1], how different strain fields act upon Li ion conduction is not yet assessed. Existing studies show a non-trivial decrease/increase of Li-ion migration energies depending on relative alignment of migration direction and strain field [2], making quantitative estimates of conductivity changes difficult to predict. Here, we study the effect of strain on Li ion conductivity of β-Li3PS4 by means of Ab Initio Molecular Dynamics (AIMD) simulations. Ionic conductivity is directly calculated from AIMD trajectories using Nernst–Einstein equation, as it is linearly proportional to the diffusivity of the center-of-mass of all Li-ions [3]. Our calculations indicate that biaxial strain could substantially promote correlated motions of Li-ions, which in turn could increase ionic conductivity of β-Li3PS4. The effect depends on direction of applied strain, and the underlying structural changes increasing ionic conductivity are discussed.[1] Y. Wang, W.D. Richards, S.P. Ong, L.J. Miara, J.C. Kim, Y. Mo and G. Ceder, Design principles for solid-state lithium superionic conductors, Nat. Mater. 14 , 1026–1031 (2015)[2] C. O’Rourke and B. Morgan, Interfacial strain effects on lithium diffusion pathways in the spinel solid electrolyte Li-doped MgAl2O4, Phys. Rev. M 2, 045403 (2018)[3] A. Marcolongo and N. Marzari, Ionic correlations and failure of Nernst-Einstein relation in solid-state electrolytes, Phys. Rev. M 1, 025402 (2017)