NbO2-derived Nb substitution in garnet-type Li7La3Zr2O12: Structural stabilization and local electronic structure revealed by experimental XPS and theoretical XAFS

Garnet-type Li7La3Zr2O12, (LLZO) solid electrolytes are among the most promising candidates for solid-state electrolytes due to their high electrochemical potential window and good chemical stability against Li anodes. However, to achieve high-room temperature conductivity, high-temperature synthesis for longer sintering durations and Li+ ion vacancies induced with dopants within the structure are required. In this study, we report Nbx-doped Li7La3Zr2O12 (x = 0.05, 0.1, 0.2, 0.6, 1, 2) solid electrolytes synthesized with NbO2 source for Zr4+ sites, which leads to a cost-effective procedure that reduces sintering temperature and time. In literature, the Nb2O5 source is widely studied. Here, for the first time, the effect of the NbO2 source on the structure was studied with experimental methods, and the FEFF 8.20 code for theoretical X-ray Absorption Fine Structure (XAFS) calculations was employed to probe the effect of Nb on the local electronic structure. Experimental results showed that a doping amount less than 0.6 mol is not enough to stabilize the cubic phase. 0.6 mol Nb doped Li6.4La3Zr1.4Nb0.6O12 solid electrolyte sintered at 1030 °C for 20 min stabilizes in the cubic phase and sintered at 1100 °C for 6 h shows the maximum ionic conductivity of 4.36 × 10−4 S cm−1 at room temperature. Temperature-dependent theoretical XAFS analysis reveals a distinct functional advantage for Nb-substituted compositions for thermal resilience. By correlating high-resolution X-ray photoelectron spectroscopy (XPS) with extended XAFS (EXAFS) analysis, this work establishes a direct link between niobium substitution and the resulting electronic modifications in Nb-LLZO electrolytes, revealing how the oxidation state of niobium and the concomitant evolution of the lithium and oxygen bonding environments dictate the material’s structural integrity.