Migration properties of Li⁺in Li_1+<i>x</i>Al_<i>x</i>Ti_2–<i>x</i>(PO₄)₃

Abstract

NASICON-type materials are specific skeleton structures in which ions move in three dimensions. Li_1+<i>x</i>Al_<i>x</i>Ti_2–<i>x</i>(PO₄)₃ (LATP) is a promising NASICON-type solid-state electrolyte for Li-ion batteries, due to its relatively high Li⁺ conductivity, chemical stability to air and moisture, and mechanical strength. Motivated by this, we study the doping and electronic properties of Li_1+<i>x</i>Al_<i>x</i>Ti_2–<i>x</i>(PO₄)₃ (<i>x</i> = 0.00, 0.16, 0.33, 0.50) and the transport properties of Li⁺ in them by using first-principles calculations based on density functional theory as implemented in Vienna <i>ab initio</i> Simulation Package (VASP). The results indicate that Al can substitute Ti to form a stable structure. When the Al doping concentration is <i>x</i> = 0.16, the average bond length of Li—O bond is longest and the bonding strength is weakest, this may lead to the expansion of channels for Li⁺ migration, which facilitates the diffusion of Li⁺. With the increase of Al doping concentration, the strength of Ti—O bond remains almost unchanged. The electronic structure calculations exhibit that with the increase of Al doping concentration, the bandgap of LATP does not change much, and LATP shows semiconductor characteristic. The differential charge results indicate that more electrons are localized on O-atoms surrounding the Al-dopant, causing the AlO₆ groups to form polarization centers. The study on the migration properties of Li⁺ indicates that Li⁺ exhibits different migration characteristics in three different migration modes (vacancy migration, interstitial migration, and cooperative migration). With the increase of Al doping concentration, the migration barrier of Li⁺ increases via vacancies involving only lattice site migration, and the migration barrier for LATP-0.16 is lowest (0.369 eV). While in interstitial migration involving only interstitial sites, the migration barrier of Li⁺ decreases accordingly. When the Al doping concentration is <i>x</i> = 0.50, the migration barrier is lowest (0.342 eV). In terms of cooperative migration, this migration mode involves both vacancy and interstitial sites, so the migration barrier first decreases and then increases with the increase of Al doping concentration. Thus, our study suggests that by varying the concentration of Al doping, the interstitial Li⁺ content, migration channel structure, and the migration performance of Li⁺ can be changed favorably. Our results provide a theoretical basis for improving the ion conductivity of Li in LATP by varying the Al doping concentration in experiment.