Amorphous Lithium Aluminate As Solid Electrolyte Produced By Pulsed Laser Deposition
Authors: Taormina Riccardo , Di Fonzo Fabio 
Published: July 2022

Solid state batteries are deemed to become the cornerstone of the future electric mobility. Nevertheless, research on solid electrolytes is still ongoing due the many limitations of current polycrystalline materials. The isotropic and non-periodic structure of amorphous ceramics have shown to contribute to increase the overall ionic conductivity of the material by decreasing the grain-boundary resistive contribution. At the current state, the most promising amorphous material happened to be lithium phosphate oxynitride LiPON (σLi = 10-6 S/cm, ), which demonstrates that the absence of grain boundaries, allows the formation of lithium small dendrites which can grow inside the material without cracking it, avoiding short life cycle of the battery over high current densities [1]. Gao et al. [2] addressed the limited ionic conductivity of LiPON to the strong bond between the PO 4- group with Li+: for this reason, elements with weaker electronegativity than P, such as Al, can generate an ionic bond with O with weaker electrostatic force, regulating the kinetics of Li+ transport and speeds up the diffusion process [3]. In this scenario, Lithium Aluminate (LiAlO2) and Nitrogen-doped Lithium Aluminate (LiAlON) result in a competitive position for the development of an innovative amorphous-glassy electrolyte: very few studies have been conducted on the development of lithium aluminate based solid electrolytes at the present time, mainly due to its low processability at the amorphous phase and the low ionic conductivity of the crystalline phase, more common in the traditional sintering processes.
In this study, we demonstrate for the first time the possibility to obtain with Pulsed Laser Deposition (PLD), a completely amorphous LiAlO2 solid electrolyte with a room temperature ionic conductivity of 10-10 S/cm. Thanks to the PLD processing, the grade of polymorphism can be easily controlled as well as film thickness range (10nm up to 10um) and film porosity. By controlling the deposition atmosphere, different content of nitrogen doping has been achieved, promoting the formation of the highly ionic conductive LiAlON (almost two order of higher conductivity). Electrochemical analysis such as DC polarization and Impedance Spectroscopy, revealed the wide electrochemical voltage stability against lithium metal and the high ionic conductivity of the solid electrolyte. A multi-layer approach for the direct deposition of the solid electrolyte over lithium metal surface is proposed, allowing the realization of symmetric cell test and plating/stripping test. Good protection of Li metal substrate has been observed from the LiAlO2 SSE over 24h, hindering oxidation and degradation of the sample.
[1] - Nowak, Berkemeier, and Schmitz, “Ultra-Thin LiPON Films – Fundamental Properties and Application in Solid State Thin Film Model Batteries.”
[2] - Gao et al., “Screening Possible Solid Electrolytes by Calculating the Conduction Pathways Using Bond Valence Method.”
[3] - Guan et al., “Superior Ionic Conduction in LiAlO 2 Thin-Film Enabled by Triply Coordinated Nitrogen.”