Point-defect chemistry for ionic conduction in solid electrolytes with isovalent cation mixing

Abstract

Recent studies on solid electrolytes for electrochemical devices such as batteries and fuel cells have focused on investigating migration of mobile ions in ion-conducting crystalline solids, whereas the temperature dependence of the concentrations of charge carriers has received little attention. In this study, the role of point-defect concentrations on the conductivity of fluoride ions in BaF₂–CaF₂ solid solution was investigated using first-principles calculations. The BaF₂–CaF₂ solid solution exhibits ionic conductivity that is several orders of magnitude higher than that of the unmixed compounds, BaF₂ and CaF₂. The calculated equilibrium point-defect concentration of a mixed structure with a composition of Ba_0.5Ca_0.5F₂ and unmixed structures shows that the cation mixing enhances anion Frenkel-pair concentrations by several orders of magnitudes. Although doped ions have the same valence state as that of an ion of a host material, a size mismatch between the doped and host ions could increase mobile defects. Activation, migration, and formation energies of mobile fluorine defects are compared among the examined structures by employing molecular dynamics simulations combined with machine-learning potentials and nudged elastic band calculations. The defect formation energy in the mixed phase is at least 0.75 eV lower than that in the unmixed cases, whereas the difference in migration energy is less than 0.15 eV. These results indicate that the increase in defect concentrations caused by cation mixing contributes to the enhancement of ionic conductivity. The formation energies of fluoride-ion vacancies and interstitial fluoride ions were also compared among fluorite-structured AF₂ (A = Ca, Sr, Ba, Cd and Pb), which exhibits opposite trends between the two defects except for PbF₂ when plotting as a function of ionic radius. The observed trends of the vacancy and interstitial defects correlate with the bonding strength parameter between neighboring F and A atoms and the space size of the interstitial site, respectively. The exceptional behavior of PbF₂ is attributed to 6s² lone-pair electrons of Pb²⁺. In Ba_0.5Ca_0.5F₂, the formation energy of each defect is close to the lower energy between BaF₂ and CaF₂, resulting in enhanced defect concentrations. The results indicate that the complementary exploitation of point-defect chemistry can provide a comprehensive understanding of the ionic conduction mechanism, leading to the rational design of materials with higher ionic conductivity.