Reversible ion exchange and structural stability of garnet-type Nb-doped Li₇La₃Zr₂O₁₂ in water for applications in lithium batteries

H⁺/Li⁺ ion exchange and structural stability of the high ionic conductivity Nb-doped Zr-garnet Li_6.75La₃Nb_0.25Zr_1.75O₁₂ (LLNZO) are investigated in this study. Relationships between ion exchange and Li-population per unit cell, which are necessary to establish the practical framework of garnet electrolytes, are deduced for garnet oxides within ion-exchange process. H⁺/Li⁺ ion exchange of cubic LLNZO powder is performed continuously in distilled water and products with various exchange levels are obtained via this simple method. FTIR spectra show the evolution of H-O bonding through the ion-exchange process. A maximum of 74.8% exchange of Li⁺ by H⁺ was found, consistent with a preferential replacement of octahedrally coordinated Li. The cubic garnet phase is maintained throughout all levels of proton exchange observed. The formation of garnet-type solid solution of Li_6.75-xH_xLa₃Nb_0.25Zr_1.75O₁₂ is indicated by well-resolved lattice fringes as well as the linear evolution of crystal lattice parameters with the ion exchange level. The reverse ion exchange of H⁺ by Li⁺ is successfully achieved in Li⁺ containing aqueous solutions, demonstrating its high structural stability and good compatibility for promising applications in lithium batteries.