The perovskite-type oxide Li3xLa2/3-xTiO₃ (LLTO) is a promising solid electrolyte but suffers from high grain-boundary resistance. This study compares B-site substitutions (M = Ta⁵⁺, Zr⁴⁺, V5 +) in Li0.3La0.57Ti0.99M0.01O3 prepared via the sol–gel method to improve dopant incorporation and assess their impact on grain-boundary resistance. X-ray diffraction confirms a cubic perovskite (Pm-3m) with dopant-dependent lattice changes: Ta5+ and Zr4+ expand the unit cell, whereas V5+ causes slight contraction and conversion to tetragonal perovskite (P4/mmm). Raman spectroscopies reveal systematic shifts of the A1g/Eg modes, indicative of modified TiO6 octahedral distortion and bonding. Densification for Ta- and Zr-doped LLTO ceramics is achieved at 1300 °C, while V-doped LLTO tends to form a more porous morphology. Electrochemical impedance spectroscopy demonstrates that Ta⁵⁺ and Zr⁴⁺ incorporation markedly lowers grain-boundary (GB) resistances, enhancing overall ionic transport, while V⁵⁺ deteriorates both bulk and intergranular pathways. The corresponding activation energies confirm facilitated bulk transport for Ta (Ea,G = 0.212 eV) and Zr (0.237 eV). Collectively, the structural–spectroscopic–electrochemical evidence establishes a clear structure–transport correlation and identifies Ta5+ as effective dopants to simultaneously enhance bulk conductivity and intergranular connectivity in LLTO electrolytes. © 2026 The Authors, The authors gratefully acknowledge the financial support provided
by the Colombian Ministry of Science, Technology and Innovation (Minciencias) through project No. (Contract No. 2023–0685). Additional support was received from the project “Development of scientific
capabilities for the investigation of infrastructure deterioration processes
in renewable ene rgy systems in the department of Choc´o (Colombia)”
(BPIN 2020000100330), conducted by the Technological University of
Choc´o “Diego Luis CÓrdoba” and the Center for Research in Renewable
Energies and Climatology (CIER), and funded by the Science, Technology
and Innovation Fund (STIF) of the General System of Grants (GSG)
of Choc´o, Colombia. Financial assistance from the Elisa project
(CNS2023–145494) funded by the Spanish Ministry of Science and
Innovation (MCIN/AEI/10.13039/501100011033) and the European
Union through the “NextGenerationEU”/PRTR initiative, is also gratefully
acknowledged, as well as additional support from the 2024ICT207
and COOPB23103 projects of CSIC. Maycol F. Mena acknowledges
Minciencias for the Doctoral Scholarship 2020 (BPIN 2020000100391),
and Robin Picca acknowledges the Erasmus+ Programme for supporting
the educational training internship (Agreement No. 177941–2023). The
authors also wish to thank Sara Serena, Pamela Vargas, and Felipe
Cuasquer from ICV-CSIC, for their invaluable assistance with experimental
measurements and technical support, which were essential to the
success of this research. The authors thank the Research Support Services
of the University of Extremadura (SACSS-SAIUEx). Special thanks
to Dr. Liliana T. L´opez of CIDEMAT-AdeA and Dr. H´ector D. Agudelo of
GIERMET-UTCH for their support in analyzing the XRD and XPS results, Peer reviewed