Investigating the structural, mechanical and electronic stability of Li_7-xLa₃Zr_2-xNb_xO₁₂ garnet-type solid electrolyte

Materials Modelling Centre, University of Limpopo, Private Bag X1106, Sovenga 0727, South Africa

^* Corresponding author: tibanekhumbulani01@gmail.com

Abstract

The garnet-type oxide Li₇La₃Zr₂O₁₂ (LLZO) is considered a leading solid electrolyte for solid-state batteries due to its high lithium-ion conductivity and thermodynamic stability against lithium metal. However, at room temperature, LLZO forms a low-conductivity tetragonal phase. Supervalent doping has been demonstrated to induce lithium vacancies, thereby stabilizing the highly conductive cubic phase and enhancing structural stability. Despite extensive studies, the mechanistic understanding of supervalent substitution remains elusive. Therefore, in this study, density functional theory was employed to investigate the structural, mechanical, and electronic properties of pristine LLZO and niobium-doped Li_7-xLa₃Zr_2-xNb_xO₁₂ system with Nb concentration (x = 0.25). All systems were found to be thermodynamically and mechanically stable, satisfying Born stability criteria and exhibiting ductile behaviour. Phase transition was observed with Nb-doped content: a monoclinic phase at 12.5% (Li_6.75La₃Zr_1.75Nb_0.25O₁₂). Electronic structure analysis via density of states reveals that the doped system is an insulator, with a decreased band gap from 4.345 eV (pristine LLZO) to 3.734 eV upon doping. These findings substantiate the electronic stability and structural reconfigurability of Nb-doped LLZO systems.