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

Garnet-type Li 7 La 3 Zr 2 O 12 , (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 Nb x -doped Li 7 La 3 Zr 2 O 12 (x= 0.05, 0.1, 0.2, 0.6, 1, 2) solid electrolytes synthesized with NbO 2 source for Zr 4+ sites, which leads to a cost-effective procedure that reduces sintering temperature and time. In literature, the Nb 2 O 5 source is widely studied. Here, for the first time, the effect of the NbO 2 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 Li 6.4 La 3 Zr 1.4 Nb 0.6 O 12 solid electrolyte sintered at 1030 o C for 20 min stabilizes in the cubic phase and sintered at 1100 o 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.