(Invited) Application of Liquid Metals in Battery Technology
Authors: Ota Hiroki 
Published: December 2024

Stretchable devices have many potential applications, including wearable electronics, robotics, and health monitoring. These mechanically adaptable devices and sensors can seamlessly integrate with electronics on curved or soft surfaces. Given that liquids are more deformable than solids, sensors and actuators utilizing liquids encased in soft templates as sensing elements are particularly suited for these applications. Such devices, leveraging ultra-flexible conductive materials, are referred to as stretchable electronics.

Liquid metals (LMs) have emerged as one of a leading material in this field. In recent years, interest in liquid metals has surged, notably in flexible and soft electronics. When considering liquid metals, mercury often comes to mind due to its fluid state at room temperature. However, its high toxicity precludes its use in wearable technology. Instead, gallium-based liquid metals are preferred due to their safety in such applications.

Gallium alone melts at about 30°C, but an alloy of 75% gallium and 25% indium lowers the melting point to 15°C. Adding 10% tin further reduces it to -19°C. These gallium-based liquid metals, which form low-viscosity eutectic alloys, have extremely low melting points and high biocompatibility. In addition, they rapidly form a thin oxide layer on their surface, which complicates patterning on substrates. To address this, metal nanoparticles like nickel can be blended using ultrasonic probing to create a malleable paste.

These materials are still under research to explore additional functionalities. Liquid metals are particularly promising for self-healing materials and advanced wiring technologies for sensors and smart devices in stretchable electronics. More recently, their application in battery technologies in addition to sensors and wiring has been proposed. With ongoing advancements in flexible and stretchable electronics, the flexibility of lithium-ion batteries, essential for powering these devices, is also under investigation. This presentation discusses research on flexible battery electrodes using liquid metal and on materials for stretchable battery packages.

In our first study, liquid metal served as a battery electrode, integrating the reaction and current collecting layers into a single process, thus simplifying manufacturing. However, this integration results in lower conductivity compared to traditional two-layer electrodes. By employing materials such as Li4Ti5O12 (LTO) or Li2TiS3 (LTS) with liquid metal, we developed a high-conductivity, printable liquid metal electrode ink that combines both functions.

In a second application, liquid metal was used as an package for stretchable batteries. Recent studies on batteries have primarily focused on enhancing their stability and lifespan, with less attention to packaging. Conventionally, aluminum laminate film is used to prevent moisture and gas permeation in highly deformable batteries. Our study introduced a novel approach using a layer-by-layer technique to apply a thin liquid metal coating on a gold-coated thermoplastic polyurethane film, resulting in a stretchable packaging film with excellent gas barrier properties. This innovation not only enhances the battery's operational stability but also allows it to function reliably in atmospheric condition.

The applications for liquid metals are extensive and hold promise for further exploration in various fields.