Improvement of the conductivity of sol-gel derived Li-La-Zr-O thin films by the addition of surfactant

Abstract

Li–La–Zr–O thin films were successfully fabricated using the sol-gel spin coating method and the effects of the addition of surfactant in the solution were studied. X-ray diffraction, scanning electron microscopy and Atomic Force Microscope were used to investigate the characteristics of these films and an amorphous structure and smooth surface on the films were revealed. The ionic conductivity of the film was investigated by impedance analysis. The results showed that the room-temperature ionic conductivity of the film increases with the content of surfactant, from 1.8 × 10⁻⁶ S/cm for 0.5 wt% to 3.9 × 10⁻⁶ S/cm for 1.5 wt%. However, further increasing the content of surfactant can deteriorate the property of the thin film on the contrary. The improvement of the property can be attributed to the improvement of the morphology of the film. We propose that this Li–La–Zr–O thin film is a promising solid electrolyte for thin-film lithium-ion batteries.

Introduction

Since the commercialization of lithium batteries, we have witnessed their great growth in production and sales worldwide. Now rechargeable lithium ion batteries are key components of consumer electronic devices such as mobile phones, laptops, and digital cameras for their unique advantages over other competing battery technologies [1]. However, they still face many future challenges such as safety problems arising from flammable organic solvent and growth of lithium dendrites [2], [3]. Replacing liquid electrolytes in conventional lithium-ion batteries with solid electrolytes has been proposed as an appealing way to deal with the safety problems [4], [5], [6]. Among all kinds of all-solid-state lithium batteries, thin-film batteries with the maximum voltage and energy storage densities have received great attention [7], [8]. However, preparation of thin-film solid electrolytes with high ionic conductivity under low cost condition remains a major obstacle. Currently, the most widely used thin-film solid electrolyte is lithium phosphorus oxynitride amorphous glass (LiPON) with an ionic conductivity in the order of 10⁻⁶ S/cm at room temperature [9], [10], [11]. However, LiPON is not stable in air and sensitive to water and oxygen. As a result, it is necessary to find a more stable thin-film solid state electrolyte.

A plenty of inorganic solid electrolytes have been explored to date, such as Li_1.5Al_0.5Ge_1.5(PO₄)₃ with an ionic conductivity up to 5 × 10⁻³ S/cm, Li_4−xGe_1−xP_xS₄ with conductivity of 2.2 × 10⁻³ S/cm at room temperature, and Li_0.34La_0.5TiO_2.98, whose bulk lithium conductivity was reported to be as high as 10⁻³ S/cm at room temperature [12], [13], [14]. Among these electrolyte materials, the garnet type Li₇La₃Zr₂O₁₂ (LLZO), first reported by Murugan and coworkers in 2007, has attracted the most interest due to its high ionic conductivity and great chemical/electrochemical stability [15], [16]. Great efforts have been made to prepare thin film LLZO and a variety of thin film growth methods such as pulsed laser deposition (PLD), radio frequency (RF) magnetron sputtering and sol-gel process have been tried [17], [18], [19], [20], [21], [22], [23], [24]. For example, Kim and Hirayama reported the preparation of epitaxial LLZO thin films by PLD with a conductivity as high as 1.0 × 10⁻⁵ S/cm in 2013 [23]. However, the PLD deposition area usually makes large scale manufacturing limited. Nong et al. have prepared amorphous Li-La-Ti-Zr-O thin film electrolyte by RF magnetron sputtering with a conductivity of 2.83 × 10⁻⁶ S/cm in 2015, but the RF sputtering method is still a lit bit of expensive [17]. Among all kinds of thin film growth methods, sol-gel process has many advantages over other vacuum-based processes: flexible synthesis conditions, easy control of chemical components, simplicity of the process, low cost, and high yield. We reported the preparation of Li-La-Zr-O thin films by sol-gel process with a conductivity of 1.67 × 10⁻⁶ S/cm in our previous work, and found that the morphology has an important impact on the performance of the thin film [18]. It has been reported that the addition of surfactant can greatly improve the morphology of the thin-film [20]. In addition, Kiyoharu Tadanaga et al. have proven that the addition of an ionic surfactant can improve the quality and conductivity of the LLZO thin films [25].

In this work, the influence of surfactant additives to the conductivity of the LLZO thin film was investigated and it is revealed that moderate content of surfactant can improve the quality of the film significantly while too much surfactant may deteriorate the property of the thin film on the contrary. By optimizing the concentration of the surfactant to 1.5 wt%, an optimum ionic conductivity of 3.92 × 10⁻⁶ S/cm was obtained.