Mechanically Coupled Phase-Field Modeling of Microstructure Evolution in Sodium Ion Batteries Particles of NaxFePO4

Abstract

The microstructure evolution of the cathode material NaxFePO4 of sodium-ion batteries is investigated during insertion, using a mechanically coupled phase-field model. A direct comparison between NaxFePO4 and LixFePO4 is made in terms of the microstructure evolution and the stress evolution. The dynamics of single wave propagation in spherical particles of NaxFePO4 is obtained, and the interface morphology between phases that goes across the particle dynamically changes to minimize its proportion. When mechanics is accounted for, the interface gets more widened for NaxFePO4, and its miscibility gap is significantly reduced. In contrast to the constant stresses in each phase occurring in shrinking-core dynamics, both, tensile and compressive stresses coexist in each phase, and the related gradient of hydrostatic stress induces NaxFePO4 a steeper concentration inhomogeneity in each phase. It is expected that the particle surface of the species-rich phase is more prone to cracking. Compared with LixFePO4, the stress magnitudes at the interface are smaller in NaxFePO4. Although the miscibility gap of NaxFePO4 is smaller, the stress magnitudes at the particle surface are larger in this material, which makes it less mechanically reliable.