Transition Metal Migration Can Facilitate Ionic Diffusion in Defect Garnet-Based Intercalation Electrodes

The importance of metal migration during multielectron redox activity has been characterized, revealing a competing demand to satisfy bonding requirements and local strains in structures upon alkali intercalation. The local structural evolution required to accommodate intercalation in Y-2(MoO4)(3) and Al-2(MoO4)(3) has been contrasted by operando characterization methods, including X-ray absorption spectroscopy and diffraction, along with nuclear magnetic resonance measurements. Computational modeling further rationalized behavioral differences. The local structure of Y-2(MoO4)(3) was maintained upon lithiation, while the structure of Al-2(MoO4)(3) underwent substantial local atomic rearrangements as the more ionic character of the bonds in Al-2(MoO4)(3) allowed Al to mix off its starting octahedral position to accommodate strain during cycling. However, this mixing was prevented in the more covalent Y-2(MoO4)(3), which accommodated strain through rotational motion of polyhedral subunits. Knowing that an increased ionic character can facilitate the diffusion of redox-inactive metals when cycling multielectron electrodes offers a powerful design principle when identifying next-generation intercalation hosts.