Electrode and Electrolyte Materials From Atomistic Simulations: Properties of LixFEPO4 Electrode and Zircon-Based Ionic Conductors

Abstract

LixFePO4 orthophosphates and fluorite- and pyrochlore-type zirconate materials are widely considered as functional compounds in energy storage devices, either as electrode or solid state electrolyte. These ceramic materials show enhanced cation exchange and anion conductivity properties that makes them attractive for various energy applications. In this contribution we discuss thermodynamic properties of LixFePO4 and yttria-stabilized zirconia compounds, including formation enthalpies, stability, and solubility limits. We found that at ambient conditions LixFePO4 has a large miscibility gap, which is consistent with existing experimental evidence. We show that cubic zirconia becomes stabilized with Y content of ~8%, which is in line with experimental observations. The computed activation energy of 0.92eV and ionic conductivity for oxygen diffusion in yttria-stabilized zirconia are also in line with the measured data, which shows that atomistic modeling can be applied for accurate prediction of key materials properties. We discuss these results with the existing simulation-based data on these materials produced by our group over the last decade. Last, but not least, we discuss similarities of the considered compounds in considering them as materials for energy storage and radiation damage resistant matrices for immobilization of radionuclides.

Pyrochlore Compounds From Atomistic Simulations

“…However, different applied approximations (e.g., exchange-correlation functionals) result in different quality of the computed structural data ( Perdew et al, 2008 ; Blanca-Romero et al, 2014 ). In a series of papers on orthophosphates we demonstrated that a very good match to the structural parameters, especially the Ln -O bond lengths, can be obtained with the PBEsol exchange-correlation functional ( Perdew et al, 2008 ) and the DFT + U method with the Hubbard U parameter derived from first principles ( Blanca-Romero et al, 2014 ; Beridze et al, 2016 ; Kowalski et al, 2021 ). Finkeldei et al (2017) have shown that PBEsol functional can predict accurate value of the lattice parameter of the Nd 2 Zr 2 O 7 pyrochlore.…”

“…The core electrons of atoms have been represented by the ultrasoft pseudopotentials (Vanderbilt, 1990), with the relevant planewave energy cutoff of 50 Ryd. Following our broad experience on computation of lanthanide orthophosphates and zirconates, and uranium-oxides (e.g., Beridze and Kowalski, 2014;Blanca-Romero et al, 2014;Li et al, 2015;Beridze et al, 2016;Kowalski et al, 2021) here we applied a parameter free DFT + U approach. In this method, the Hubbard U parameter values are computed from first principles using the linear response method of Cococcioni and de Gironcoli (2005).…”

“…Zirconia-based materials, including pyrochlore, are considered as prospective ionic conductors for solid state electrolytes (Wuensch, 2000;Diazguillen et al, 2008;Kumar et al, 2008;Mandal et al, 2008;Shlyakhtina and Shcherbakova, 2012;Anithakumari et al, 2016). One of the striking examples is yttria-stabilized zirconia -one of the fastest solid state ionic conductors known (Kowalski et al, 2021). The oxygen conduction in a stoichiometric pyrochlore occurs due to jumps of oxygens, or oxygen vacancies, between the neighboring 48f sites.…”

“…In general, zirconia-based compounds are known to show high ionic conductivity. Among these, yttria-stabilized zirconia is considered as one of the fastest ionic conductors ( Ahamer et al, 2017 ; Kowalski et al, 2021 ). In pyrochlore-type compounds, the ionic conduction happens through hopping between 48 f sites.…”

“… Bukaemskiy et al (2021) constructed a model for oxygen diffusion in zirconia doped with trivalent elements, in which the oxygen conductivity was assumed proportional to the probability of a vacancy jump between clusters in a certain typical configuration occurring more frequently due to SRO. Such a model has been successfully applied together with atomistic simulations to derive the ionic conductivity in yttria-stabilized zirconia materials ( Kowalski et al, 2021 ). Here, with simulations of energies of defects formation on anion sublattice and activation barriers for oxygen diffusion we will discuss the mechanisms of ionic conductivity in a series of zirconate pyrochlores.…”