Ionic Conductivity of the Epitaxial Li0.33La0.55TiO3 Solid Electrolyte Thin Films

Solid-state lithium batteries composed of solid electrolytes with high ionic conductivity and high stability has several advantages such as improved safety, increased energy and power density and absence of leakage problems compared to the conventional Li ion batteries containing organic liquid electrolytes. Among the most investigated ionic conductions, Li_0.33La_0.55TiO₃ (LLTO) is reported to have the highest bulk ionic conductivity at room temperature (RT) (order of 10^-3 S/cm).^1,2 Ionic conductivity of amorphous LLTO and epitaxial Li_0.17­La_0.61TiO₃ films grown on Al₂O₃ and Nb:SrTiO₃ substrates are reported to be in the order of 10^-6 and 10^-4 S/cm respectively at RT. ^1,2 Ionic conductivity of epitaxial LLTO films and impacts of the Ti valence change on the electrical and structural properties have not yet been clearly studied. In the present case, we report on the ionic conductivity and the effect of oxygen partial pressure during the growth of LLTO epitaxial film grown on SrTiO₃ (STO) (100) and (111) substrates and explore some distinct conductivity behavior. Epitaxial LLTO films (150 nm thick) on STO (100) and (111) substrates were prepared via pulsed laser deposition (PLD) employing KrF laser at 800 ^oC in 10, 50, 100 and 200 mTorr oxygen partial pressures. Structural and orientation of the films were characterized employing Bruker X-ray diffractometer (XRD) and scanning transmission electron microscopy (STEM). Thin lamellas of the samples for surface and cross sectional transmission electron microscope (TEM) studies were prepared by cutting with focused ion beam. The lamellas were studied with TEM operated at 300 kV. Impedance measurements were carried out on all the films at the temperature range of RT – 250^o C employing an impedance analyzer. XRD and TEM studies (Fig.1 (a)) shows that the films grown on STO (100) and (111) substrates exhibits epitaxial (100) and (111) orientations with perovskite tetragonal structure. RT ionic conductivity of LLTO films grown on STO (100) and STO (111) substrates decrease from 3.12 x 10^-4 to 3.10 x 10^-5 S/cm and 1.8 x 10^-4 to 2.6 x 10^-5 S/cm with increasing oxygen partial pressure (during the film growth) from 10 to 200 mT respectively (Fig. 1(b)). Conductivity of all the LLTO films increases exponentially with increasing temperature. Activation energy (E_g) for all the films were calculated from the temperature dependent conductivity measurements employing the Arrhenius relationship σ = σ₀ exp (-E_g/K_BT), where K_B is Boltzmann constant. E_g value for all the films are seen to be around 0.34 eV, agreeing well with the reported bulk grain activation energy values. LLTO films grown at 10 mT are seen to have conductivity close to the bulk conductivity (in the order of 10^-4S/cm) and indicates that these films can be used as a solid electrolyte in high power all solid-state batteries.

 

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[1] Z. Zheng, H. Fang, F. Yang, Z. K. Liu, and Y. Wang, Journal of The Electrochemical Society, 161(2014) A473-A479.

[2] S. Kim, M. Hirayama, W. Cho, K. Kim, T. Kobayashi, R. Kaneko, K. Suzuki and R. Kanno, CrystEngComm, 16 (2014) 1044