Oxygen Vacancies in Fast Lithium-Ion Conducting Garnets

Abstract

Fast Li-ion conducting garnets have shown excellent performance as chemically stable solid state Li electrolytes even at room temperature. However, because of phase formation and Li loss during preparation, reliably obtaining high Li-ion conductivities remains challenging. In this work, we show that an additional defect chemical species needs to be considered, namely, oxygen vacancies. We prove the existence of oxygen vacancies in all six investigated sample types: Ta-, Al-, and Ga-stabilized cubic Li 7 La 3 Zr 2 O 12 (LLZO) polycrystals and Ta-stabilized LLZO single crystals. Isotope exchange threedimensional analysis was used to characterize surface oxygen exchange (k*) and bulk oxygen diffusion (D*) enabled by the oxygen vacancies present in the LLZO variants. Remarkably high k* values of 10 −11 −10 −8 cm s −1 and D* values of 10 −15 −10 −11 cm 2 s −1 were found at 350 °C in air. In a further data analysis, the differences between the compositions are investigated, the concentration of oxygen vacancies is estimated, and the possible effects on the cation defect chemistry and phase formation of LLZO are discussed.

Nonstoichiometry and Li‐ion transport in lithium zirconate: The role of oxygen vacancies

“…Generally, more lithium is lost for Fe, Al, and Ga doping that induces more oxygen vacancies compared to Ti and Nb doping. From a viewpoint of defect chemistry, oxygen vacancies, as a donor, can change the Li stoichiometry of lithium oxides …”

“…From a viewpoint of defect chemistry, oxygen vacancies, as a donor, can change the Li stoichiometry of lithium oxides. 16 As shown in Figure 8B, the bulk Li-ion conductivity increases from Nb 5+ to Fe 2+ , and the conductivity increase is accompanied by the increase in oxygen and lithium vacancy concentrations for Nb 5+ , Ti 4+ , and Ga 3+ , except for Fe 2+ where lithium vacancy concentration is lower. From Ga 3+ to Fe 2+ , the activation barrier continues to drop (0.71-0.58 eV) and the bulk Li-ion conductivity increases, too.…”

“…Oxygen vacancies induced by cation doping or lithium loss will have an impact on the Li + diffusion in LZO. Recently, Kubicek et al 16 confirmed the existence of oxygen vacancies in multiple doped cubic Li 7 La 3 Zr 2 O 12 structures, and suggested these oxygen vacancies can influence Li + conductivity by reducing Li concentration, and by elastically deforming the lattice to affect lithium migration pathways. The former scenario associates directly with the concentration and the later links to the mobility of Li + , and these 2 factors jointly governs Li + conductivity.…”

Progress of the Interface Design in All‐Solid‐State Li–S Batteries

“…Later the gallium‐substituted LLZO was prepared under the atmosphere of dry oxygen and exhibits the conductivity as high as 1.3 × 10 −3 S cm −1 . This high conductivity was ascribed to the exclusion of the contact with moisture in air and the exclusion or reduction of oxygen defects in crystalline structure …”

Zirconium‐Based Materials for Electrochemical Energy Storage

“…Besides, Fleig et al . prove existence of the oxygen vacancies in six sample types (Al‐, Ta‐, and Ga‐stabilized LLZO polycrystals and the Ta‐stabilized LLZO single crystals) (Figure ).…”