Oxygen Vacancies in Fast Lithium-Ion Conducting Garnets Markus Kubicek,* ,† Andreas Wachter-Welzl, † Daniel Rettenwander, § Reinhard Wagner, ‡ Stefan Berendts, ∥ Reinhard Uecker, ⊥ Georg Amthauer, ‡ Herbert Hutter, † and Jü rgen Fleig † † Institute of Chemical Technologies and Analytics, Technische Universitä t Wien, Getreidemarkt 9/164EC, 1060 Vienna, Austria ‡ Department of Chemistry and Physics of Materials, University of Salzburg, Jakob Haringer Straße 2a, 5020 Salzburg, Austria § Center for Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, United States ∥ Department of Chemistry, Technische Universitä t Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany ⊥ Leibniz Institute for Crystal Growth (IKZ), Max-Born-Straße 2, 12489 Berlin, Germany 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 three- dimensional 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. ■ INTRODUCTION Replacing today’s liquid-electrolyte Li-ion batteries with all-solid state batteries is highly desirable to avoid safety and durability issues such as dendrite-driven short circuit or thermal runaway. Since its introduction by Murugan et al. in 2007, 1 the garnet Li 7 La 3 Zr 2 O 12 (LLZO) has received a great deal of attention as a solid electrolyte with significant Li-ion conductivity at room temperature in air. 2−5 In numerous doping studies since then it was attempted to optimize the cation composition of LLZO with respect to Li-ion conductivity and stability. 6,7 Essential is the stabilization of cubic phase(s) and avoiding the tetragonal phase because cubic LLZO shows Li-ion conductivity that is ∼2 orders of magnitude higher. 2,6,8 Also the influence of moisture and CO 2 on the stability of different LLZO compositions requires attention, because LLZO can degrade or decompose in ambient air. 9,10 A common strategy is to introduce substituents acting as donors such as Al 3+ , Fe 3+ , or Ga 3+ on the Li + sites or Nb 5+ , Ta 5+ , Bi 5+ , or Mo 6+ on the Zr 4+ site and thereby reduce the Li stoichiometry per formula unit from 7 to an optimum usually between 6 and 7, depending on the cation substituent. 11−25 A great challenge in the preparation of highly conductive LLZO is optimizing the synthesis route. On one hand, high temperatures are necessary to form the garnet phase; on the other, Li loss via volatile Li compounds is commonly observed at high temper- atures. 26 Therefore, an excess of Li and/or a protective covering to slow Li loss is regularly used during sintering to finally acquire the desired LLZO composition. 27,28 In most of today’s research, optimizing the cation compositions of Li garnets is attempted for improving the properties of LLZO, while oxygen anion stoichiometry is considered to be fixed at 12 oxygen atoms per formula unit and therefore largely ignored. In this work, we show that oxide anion defects indeed exist in LLZO and that their contribution to the total defect chemistry of LLZO cannot be neglected. Only a few studies are known to the authors that speculate about the existence of oxygen vacancies in LLZO or consider them to potentially play a role in the phase formation and defect equilibria of LLZO. 13,29−31 The main argument for formation of oxygen vacancies given there is that Li + loss is connected to simultaneous O 2− loss due to charge neutrality. Here, we give direct proof that indeed oxygen stoichiometry can vary in LLZO. Via isotope exchange depth profiling using 18 O 2 as a stable isotope tracer and by subsequent time of flight secondary-ion mass spectrometry (ToF-SIMS) analysis, we verify that oxygen vacancies are present or even abundant in all investigated LLZO materials, including single crystals, polycrystals, and different cation substituents (Ta 5+ , Ga 3+ , and different Al 3+ concentrations). The oxide tracer diffusion coefficient at 350 °C is surprisingly high (up to D* = 8.2 × 10 −12 cm s −1 ) and is even close to that of yttria-stabilized zirconia, a fast oxygen-ion conductor. Consequently, we show that oxygen vacancies need to be considered to understand the Li-ion conductivity of LLZO. Acting as donors, they directly Received: March 29, 2017 Revised: August 8, 2017 Published: August 11, 2017 Article pubs.acs.org/cm © 2017 American Chemical Society 7189 DOI: 10.1021/acs.chemmater.7b01281 Chem. Mater. 2017, 29, 7189−7196