Are Oxide Electrolytes Moisture Insensitive? Effect of Corrosion at the Grain Surface of Li3xLa2/3-XTiO3 Inhibiting Li-Ion Transport

Introduction

Solid state electrolytes presenting high lithium ions mobility are of great challenge to improve safety and durability of secondary Li-ions batteries. Among the solid electrolytes, lithium lanthanum titanates and related compounds have attracted much attention due to their remarkably fast lithium conduction in bulk (up to 1 mS/cm at RT [1]) and have successfully been used as separator in an aqueous Li-air battery cell [2]. The processing and stability of these materials in air present a great advantage simplifying the production process and hence the production costs. Unfortunately Li_3xLa_2/3-xTiO₃presents a high grain boundary resistivity and low densification properties decreasing the total conductivity to values lower than 0.01 mS/cm. To date no complete study has clearly explained the origin of the high grain boundary resistivity. In this work, we identify and solve the high grain boundary issue by changing the processing conditions for ceramic samples.

 Experiments

Pure compounds corresponding to the stoichiometry Li_0.34La_0.55TiO₃ (LLTO) were synthesized using the citrate route. Four samples were prepared in different atmospheres: air, synthetic air, oxygen and argon to evaluate the influence of the presence of moisture and gas composition. The crystal structure was analysed by Rietveld refinement of X-ray diffraction data and the microstructure observed by transmission electron microscopy (TEM). The conductivity of the samples was measured by impedance spectroscopy and ⁷Li and ¹H-NMR analyses enable to follow the corrosion process and its influence on the ion mobility.

 Results and Discussion

We observed that LLTO is air-sensitive and its transport properties are highly influenced by moisture during the processing. A corrosion process occurs at the surface of the grains forming an insulating lithium carbonate phase after consecutive reactions with H₂O and CO₂[3]. This secondary phase prohibits the lithium diffusion from grain to grain and is responsible forthe increase of the grain boundary resistivity. A clear understanding of lithium diffusion was obtained by impedance and NMR experiments as it reveals the presence of lithium ion blocking secondary phase and its dramatic effect on the grain boundary conductivity.  The room temperature conductivity at the grain boundary can be increased by almost a decade when using dry sintering atmospheres as it prevents the protonation of the surface of the particles by Li-H exchange as observed by TEM. The sintering atmosphere plays also a crucial role for the densification of the ceramics.

  Conclusions

The grain boundary resistivity issue in LLTO ceramics has been identified and then solved by using moisture-free atmospheres during the material processing. These results offer new prospect of cost effective processing of high conductivity ceramic materials for applications in Li battery technologies.

  References

[1]          Inaguma, Y., Chen, L., Itoh, M., Nakamura, T., Uchida, T., Ikuta, H., and Wakihara, M. Solid State Communications 86(10), 689–693 June (1993).

[2]          Inaguma, Y. and Nakashima, M. Journal of Power Sources 228(0), 250–255 April (2013).

[3]          Boulant, A., Bardeau, J. F., Jouanneaux, A., Emery, J., Buzare, J.-Y., and Bohnke, O. Dalton Transactions 39(16), 3968–3975 (2010).