Reversible H+/Li+ Ion Exchange and Structure Stability of Garnet-Type Nb-Doped Li7La3Zr2O12 in Water

The lithium garnet-type oxide is considered as a promising candidate of solid electrolytes to be applied in all solid lithium batteries for its high ion conductivity, chemical stability vs. metal lithium and wide electrochemical potential window. In order to further improve the ion conductivity, dopants like Nb and Ta have been adopted and much progress has been achieved. The total lithium-ion conductivity of Nb-doped Li₇La₃Zr₂O₁₂ was as high as 0.8mS/cm. In this work, its reversible H⁺/Li⁺ion exchange reaction and the structural stability in aqueous solutions are systematically investigated.

Cubic garnet phase Li_6.25La₃Zr_1.75Nb_0.25O₁₂ (LLNZO) was prepared by a conventional solid-state reaction process at 950^oC. The H⁺/Li⁺ion exchange reaction of the LLNZO powder was performed in distilled water and the pH value was recorded simultaneously. The process was carried out continuously for 4 times. The lithium content and crystal structure of the garnet powder were measured by ICP and XRD.

The pH value of the distilled water increased quickly after the cubic garnet powder was added in it and finally reached a constant value. The garnet powder was then filtered and the dispersion was repeated with fresh water for deeper ion exchange.  The products with different H⁺/Li⁺ ion exchange rate were obtained via this way for further dynamics analysis of ion exchange process. The remaining lithium content of the garnet powder was gradually declined after the ion exchange happened. FTIR spectra was tested to identify the ion exchange. It was found that the cubic garnet phase was maintained even the ion exchange rate of lithium ion was up to 74.8% and was confirmed by the high resolution lattice imaging and XRD analysis. The stoichiometry of garnet-type Li_6.25-xH_xLa₃Zr_1.75Nb_0.25O₁₂, as well as the crystal lattice parameters, was evolved with the H⁺/Li⁺ ion exchange reaction in aqueous solutions. It is noticeable that the reverse ion exchange of H⁺ by Li⁺was achieved by controlling the reaction temperature, pressure, the lithium concentration and the pH of the solutions. The successful re-exchange indicated the high stability of the Nb doped cubic garnet phase.

Fig.1 XRD results of Li_6.25La₃Zr_1.75Nb_0.25O₁₂powder and products from distilled water treatment

Fig.2 The evolution of the constant pH value and remaining lithium content versus water treating stages

Fig.3 TEM and electron diffraction images of Li_6.25La₃Zr_1.75Nb_0.25O₁₂with 74.8% lithium ion exchange

Reference

(1)   Truong, L. and V. Thangadurai (2011). Chemistry of Materials 23(17): 3970-3977.

(2)   Shimonishi, Y., A. Toda, et al. (2011). Solid State Ionics 183(1): 48-53

(3)   Ishiguro, K., Y. Nakata, et al. (2013). Journal of the Electrochemical Society 160(10): A1690-A1693.