Electrochemical Characteristics of (Li0.33La0.56)1.005Ti0.99Al0.01O3 Ceramic As a Solid Electrolyte for 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 batteries 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, (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 LLTO contacts directly with Li metal, Ti⁴⁺ in LLTO can be reduced into Ti³⁺ by metallic Li leading to increase in the electronic conductivity of LLTO.

 In this study, we focused on enhancement of the total conductivity of LLTO ceramics by improving the bulk conductivity using Al³⁺ dopant and the grain boundary conductivity using excess Li₂O. Al-doped LLTO powders of (Li_0.33La_0.56)_1.005Ti_0.99Al_0.01O₃ with a various amount of 10-35% excess Li₂O in stoichiometry were synthesized using sol-gel modified Pechini method, and then pelletized and sintered at 1350 ^oC.

 The XRD patterns on the synthesized powders were compared and matched with Li_0.33La_0.56TiO₃ (PDF#01-087-0935; Tetragonal) confirming pure tetragonal crystalline structure (P4/mmm space group) of perovskite phase without secondary phases. With the amount of excess Li₂O 15%, crystal grains grew significantly. The largest grain size reached 100 m. The grains with typical rectangle shape were connected together closely.

 The ionic conductivity of the obtained materials was measured using electrochemical impedance spectroscopy (EIS). Obviously, the bulk and grain boundary conductivities increased with the addition of excess Li₂O. A maximum total conductivity of 3.17 10^-4 S/cm was achieved for the sample with 20% excess Li₂O, which is 4 times higher than that of the sample without excess Li₂O. The high conductivity of the sample with 20 % excess Li₂O is attributed to the minimum total activation energy of 0.358 eV and the somewhat large crystal grain size, 50 m.

 To realize the application of LLTO materials as a solid electrolyte separator in Li-air batteries, a lithium phosphorous oxynitride (LIPON) interlayer fabricated using sputtering method was employed to prevent direct contact between Li metal and LLTO. The Li-O₂ cells employing the LLTO ceramic electrolytes were tested in galvanostatic mode. Initial results showed that the Li-O₂ cell employing LLTO electrolyte layer exhibited the satisfactory performance.