Application of Al-Doped LLTO Solid Electrolyte Prepared By Sol-Gel Method in Lithium-Oxygen Batteries

To date, lithium lanthanum titanate (LLTO) of the nominal formula Li_3xLa_{2/3-x 1/3-2x}TiO₃ with perovskite structure have been considered to be promising solid electrolyte materials for lithium-oxygen battery due to a numerous outstanding advantages such as: (i) a high lithium conductivity at room temperature, (ii) a high lithium diffusion coefficient, (iii) a low electronic conductivity, and (iv) an electrochemical window larger than 4 V. However, the LLTO materials have also suffered from a few disadvantages such as insufficient total conductivity due to the large grain boundary resistance and difficulty in controlling Li⁺ content and Li⁺ conductivity of the materials especially after exposure to high annealing temperature. In addition, when the LLTO contacts directly with Li metal, Ti⁴⁺ in LLTO can be reduced into Ti³⁺ by metallic Li leading to increase in the electron conductivity of LLTO.

In this study, we investigated the feasibility of application of Al-doped LLTO ceramics with nominal formula of (Li_1/3La_5/9)_1.005Ti_0.99Al_0.01O₃ (denoted as A-LLTO) in Li–O₂ batteries. A-LLTO was synthesized using sol-gel modified Pechini method in the aim of (i) reducing the synthesis temperature (up to 300-400 ^oC compared to the conventional solid state reaction method); (ii) lessening loss of lithium and gaining homogeneous mixture easily. As referred above, since LLTO cannot contacts directly with Li metal, prior to being assembled in organic- type Li-O₂ battery as an electrolyte separator On the A-LLTO ceramics with 16 mm diameter and 200 m thickness, a thin protective layer of LiPON of 200 nm in thickness was sputtered for separation of A-LLTO from the Li metal anode. The perovskite material was used as catalyst for cathode material of the batteries.

The XRD patterns of A-LLTO ceramics obtained after annealing were compared and matched with Li_0.33La_0.56TiO₃ (Code: 01-087-0935; Tetragonal) confirming purely the tetragonal crystalline structure P4/mmm space group of perovskite phase without secondary phases. After polished and heat etched at, the surface of the ceramics was observed using SEM. T ceramics was composed of grains of typical rectangle shape connected together closely. The largest grain size reached 100 m.

Ionic conductivity of A-LLTO measured using electrochemical impedance spectroscopy (EIS) exhibited the relatively high conductivity of 3.17 10^-4 S/cm. This is attributed to the low total activation energy of 0.358 eV and the somewhat large crystal grain size compared with ceramics prepared at the same temperature.

The Li-O₂ cells employing the A-LLTO ceramic electrolytes were tested in galvanostatic and constant capacity mode. Effect of current density and operating temperature on the charge-discharge behavior of Li-O₂ cell was investigated. The Li-O₂ cell employing A-LLTO electrolyte layer exhibited the satisfactory performance.