Microstructure evolution and intermediate phase-induced varying solubility limits and stress reduction behavior in sodium ion batteries particles of Na[formula omitted]FePO[formula omitted] ([formula omitted

Abstract

The cathode material NaxFePO4 (0<x<1) of sodium-ion batteries displays complex phase segregation processes with the existence of an intermediate phase, and large volume change during charging/discharging. A chemo-mechanical phase-field model is developed to capture the thermodynamics of phase segregation along with the structural change that occurs in NaxFePO4. The multiwell potential of NaxFePO4 for the full range of concentration is constructed for the first time. This new model not only captures phase segregation into a sodium-poor phase FePO4 and a sodium-rich phase Na2∕3FePO4 but also the solid-solution phase NaxFePO4 (2∕3<x<1). The microstructure evolution in the whole processes of sodiation and desodiation is investigated. The stress assisted diffusion induces the striking behavior of the maximum solubility limit going beyond 2/3 even within two-phase coexistence. Further, the formation of an intermediate phase leads to varying solubility limits which agrees with recent experimental observation, as well as a stress reduction behavior. Finally, our work suggests that prolate NaxFePO4 particles are mechanically more reliable due to nearly stress-free phases. We expect that the intermediate phase-induced stress reduction behavior provides a new concept for improving mechanical stability and thus better battery performance.