Influence of Ga Content on the Ionic Conductivity of Li1+XGaXTi2-X(PO4)3 Solid-State Electrolyte Synthesized by the Sol-Gel Method

Abstract

In this study, NASICON-type Li 1+X Ga X Ti 2-X (PO 4 ) 3 (x = 0.1, 0.3 and 0.4) solid-state electrolytes for all-solid-state batteries were synthesized through the sol-gel method. In addition, the influence on the ion conductivity of solid-state electrolytes when partially substituted for Ti 4+ (0.61Å) site to Ga 3+ (0.62Å) of trivalent cations was investigated. The obtained precursor was heat treated at 450 °C, and a single crystalline phase of Li 1+X Ga X Ti 2-X (PO 4 ) 3 systems was obtained at a calcination temperature above 650 °C. Additionally, the calcinated powders were pelletized and sintered at temperatures from 800 °C to 1,000 °C at 100 °C intervals. The synthesized powder and sintered bodies of Li 1+X Ga X Ti 2-X (PO 4 ) 3 were characterized using TG-DTA, XRD, XPS and FE-SEM. The ionic conduction properties as solid-state electrolytes were investigated by AC impedance. As a result, Li 1+X Ga X Ti 2-X (PO 4 ) 3 was successfully produced in all cases. However, a GaPO 4 impurity was formed due to the high sintering temperatures and high Ga content. The crystallinity of Li 1+X Ga X Ti 2-X (PO 4 ) 3 increased with the sintering temperature as evidenced by FE-SEM observations, which demonstrated that the edges of the larger cube-shaped grains become sharper with increases in the sintering temperature. In samples with high sintering temperatures at 1,000 °C and high Ga content above 0.3, coarsening of grains occurred. This resulted in the formation of many grain boundaries, leading to low sinterability. These two factors, the impurity and grain boundary, have an enormous impact on the properties of Li 1+X Ga X Ti 2-X (PO 4 ) 3 . The Li 1.3 Ga 0.3 Ti 1.7 (PO 4 ) 3 pellet sintered at 900 °C was denser than those sintered at other conditions, showing the highest total ion conductivity of 7.66 × 10 -5 S/cm at room temperature. The total activation energy of Li-ion transport for the Li 1.3 Ga 0.3 Ti 1.7 (PO 4 ) 3 solidstate electrolyte was estimated to be as low as 0.36 eV. Although the Li 1+X Ga X Ti 2-X (PO 4 ) 3 sintered at 1,000 °C had a relatively high apparent density, it had less total ionic conductivity due to an increase in the grain-boundary resistance with coarse grains.Key words Li 1+X Ga X Ti 2-X (PO 4 ) 3 , sol-gel method, trivalent cation, ionic conductivity, solid-state electrolyte.