Lithium Lanthanum Titanates:  A Review

Abstract

To date, the highest bulk lithium ion-conducting solid electrolyte is the perovskite (ABO3)-type lithium lanthanum titanate (LLT) Li3 x La(2/3)-x □(1/3) - 2 x TiO3 (0 < x < 0.16) and its related structure materials. The x ≈ 0.1 member exhibits conductivity of 1 × 10-3 S/cm at room temperature with an activation energy of 0.40 eV. The conductivity is comparable to that of commonly used polymer/liquid electrolytes. The ionic conductivity of LLT mainly depends on the size of the A-site ion cation (e.g., La or rare earth, alkali or alkaline earth), lithium and vacancy concentration, and the nature of the B−O bond. For example, replacement of La by other rare earth elements with smaller ionic radii than that of La decreases the lithium ion conductivity, while partial substitution of La by Sr (larger ionic radii than that of La) slightly increases the lithium ion conductivity. The high lithium ion conductivity of LLT is considered to be due to the large concentration of A-site vacancies, and the motion of lithium by a vacancy mechanism through the wide square planar bottleneck between the A sites. It is considered that BO6/TiO6 octahedra tilting facilitate the lithium ion mobility in the perovskite structure. The actual mechanism of lithium ion conduction is not yet clearly understood. In this paper, we review the structural properties, electrical conductivity, and electrochemical characterization of LLT and its related materials.

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“…Half-cells based on LiCoO 2 fi lms covered with LLTO fi lms deposited by pulsed laser deposition could be cycled for hundreds of cycles. [ 111 ] The disadvantage of bulk LLTO is that it requires high annealing temperatures (often > 1000 ° C [ 107 ] ) to obtain this high ionic conductivity. Thin fi lms might not withstand these annealing steps due to the formation of interfacial layers, internal stress and even the formation of cracks.…”

Recent progress in solid oxide and lithium ion conducting electrolytes research

“…This finding is the most attractive feature of the investigated garnet-type oxides compared to other ceramic lithium ion conductors. For comparison, the best lithium ion conductor based on perovskite LLT exhibits a total conductivity that is about two orders of magnitude lower than the bulk conductivity [68][69][70]. Furthermore, impedance and DC electrical investigations confirm the absence of electrolyteelectrode interface resistances in the case of Li 6 ALa 2 Ta 2 O 12 (A=Sr, Ba) when lithium metal is employed as reversible electrode (Fig.…”

“…The ionic conductivity is highly sensitive to the lithium content, and a dome-shaped dependence of the conductivity on the Li content was found (Fig. 9) [70]. LLT substitution of 5 mol % Sr for La exhibits a slightly higher conductivity (1.5×10 −3 S/cm at 27°C) than the pure LLT, while complete substitution of other transition metal ions for Ti decreases the ionic conductivity ( Fig.…”

“…Among them, the x≈0.1 member of LLT exhibits the highest bulk lithium ion conductivity of 10 −3 S/cm at room temperature with an activation energy of 0.40 eV [68,69]. Stramare et al recently reviewed the lithium ion transport properties of LLT and structurally related materials [70].…”

“…NMR studies reveal that at low temperature ( > room temperature), the lithium ion hops between cages through the bottleneck in the ab plane, and at high temperature the lithium ions hop in three directions 6. The application of LLT as lithium ion electrolyte is not favorable because the compound is not stable in direct contact with elemental lithium and rapidly undergoes [70] Li-insertion with consequent reduction of Ti 4+ to Ti 3+ , leading to high electronic conductivity. Furthermore, LLT has been investigated with regard to its stability against lithium and intercalation of lithium using several electrochemical methods that include galvanostatic insertion, coulometric titration, and discharge/ charge characteristics.…”