3d Chemo-Mechanical Modeling of Microstructure Evolution and Anisotropic Deformation in Naxv2(Po4)3 Cathode Particles for Sodium-Ion Batteries

Abstract

The cathode material NaxV2(PO4)3 of sodium-ion batteries displays complicate phase segregation thermodynamics with anisotropic deformation during (de)intercalation. A virtual multiscale modeling chain is established to develop a 3D anisotropic chemo-mechanical phase-field model based on first-principles calculations for NaxV2(PO4)3. This model accounts for diffusion, phase changes, anisotropic misfit strain, and anisotropic elasticity. The multiwell potential of NaxV2(PO4)3 is constructed, which captures phase segregation into a sodium-poor phase NaV2(PO4)3 and a sodium-rich phase Na3V2(PO4)3. The elastic properties of NaV2(PO4)3 are determined by first-principles for the first time. Furthermore, we address how elastic effects and crystal orientation influence the full 3D microstructure evolution, including phase evolution, interface morphology, and stress evolution in NaxV2(PO4)3 particles. We find that the quasi-equilibrium single wave propagation along [010] is determined by the anisotropic elasticity tensor. The anti-orthotropic symmetry property of the elasticity tensor leads to the striking behavior of warping of the interface. Furthermore, the phase boundary motion is thermodynamically affected by the crystal orientation, which is controlled by minimization of the interface area. It is found that the [010] crystal orientation is mechanically more reliable and recommended for NaxV2(PO4)3 electrode design. Apart from yielding information about the properties of NaxV2(PO4)3, the findings of this work may offer an opportunity to achieve improved mechanical stability of the phase separating electrode materials by engineering the crystal orientation.

Keywords: Sodium-ion batteries, Phase segregation, Anisotropic elasticity, Phase-field approach, First-principles