Printable Liquid Metal Electrodes for Lithium Ion Battery
Authors: Ozawa Yuta , Nishitai Yuuki , Ochirkhuyag Nyamjargal , Usami Natsuka , Kanto Moeka , Ueno Kazuhide , Ota Hiroki 
Published: December 2024

Recent developments in the Internet of Things (IoT) technology led to research on flexible devices for next-generation wearable devices.[1,2] As devices advance, the Lithium ion batteries (LIBs) used to operate them are also being actively researched to make them flexible. The battery electrode contains a reaction layer and a current collector layer. Flexible battery electrodes, in which these layers are integrated, are fabricated in a single process, simplifying the electrode fabrication process. However, to date, integrated electrodes have lower conductivity than conventional two-layer electrodes, and the process that maintains conductivity has not been simplified. This study proposes a liquid metal electrode ink that simultaneously plays the role of the reaction and current collector layers with high conductivity and can be formed in a single process via printing(Figure 1 a, b, c). It employs a Gallium (Ga) alloy liquid metal with high conductivity and high deformability containing Ni powder added for viscosity adjustment,[3] embedded with Li4Ti5O12 (LTO) or Li2TiS3 (LTS) as active materials. These liquid metal electrodes have high electronic conductivity and can function as cathode and anode electrodes for lithium flexible batteries on various substrates such as polyethylene terephthalate (PET) and polypropylene (PP), which have insulating properties, by simply applying the material to the substrate without the need for solvent vaporization.
In this study, we evaluated the printing characteristics of the developed liquid metal electrode and its electrochemical properties in a half-cell using a gel electrolyte as a separator with a Li foil as a reference electrode. As printing characteristics, the desired thickness can be controlled by screen printing at a film thickness of 50 μm or more (Fig. 2a), and patterns can be successfully printed by applying mask printing (Fig. 2b). This is effective for the future practical use of flexible batteries with multi-pixel electrodes. Fig. 2c indicates the results of laser microscope measurements of the surface of the printed liquid metal electrode (LTO), and Fig. 2d shows the results of elemental analysis using SEM-EDX. From these results, it was observed that Ga and Sn, which are liquid metal components, Ni from the added Ni powder, and Ti, which is a component of the active material LTO, are dispersed on the surface.
As electrochemical characteristics, it was confirmed that the printed liquid metal electrode has conductivity of about 104 S cm-1, and can exhibit charge-discharge performance regardless of the conductivity of the substrate (Fig. 3a). In a demonstration using a PP cup, it was demonstrated that LEDs can be turned on as a power source by constructing liquid metal electrodes on a curved surface (Fig. 3b). In charge-discharge tests using laminated cells with a Li reference electrode, both LTO and liquid metal electrodes with LTS exhibited Coulomb efficiencies close to 100 % (Figure 3c). In a dynamic bending test using the fabricated laminated cell, the LEDs stably emitted light even when the cell was bent up and down from a flat state (Figure 3d). This demonstrates the robustness of the LEDs against deformation.
The liquid metal electrode we have developed can be used directly for simple printing techniques, and is expected to be applied to complex construction using the dispensing method [4] and mass production technology by Roll-to-Roll printing [5] in the future. Our concept of fabricating new printable liquid metal electrodes by mixing active materials with liquid metal expands the possibilities of lithium-ion battery research, simplifies the LIB manufacturing process, and contributes to efficient electrode production methods in future LIBs.
[1] T. Yokota, P. Zalar, M. Kaltenbrunner, H. Jinno, N. Matsuhisa, H. Kitanosako, Y. Tachibana, W. Yukita, M. Koizumi, T. Someya, Sci Adv 2016, 2, DOI 10.1126/sciadv.1501856
[2] K. Takei, W. Honda, S. Harada, T. Arie, S. Akita, Adv Healthc Mater 2015, 4, 487.
[3] M. D. Dickey, Advanced Materials 2017, 29, DOI 10.1002/adma.201606425.
[4] A. Cook, D. P. Parekh, C. Ladd, G. Kotwal, L. Panich, M. Durstock, M. D. Dickey, C. E. Tabor, Adv Eng Mater 2019, 21, DOI 10.1002/adem.201900400.
[5] R. Abbel, Y. Galagan, P. Groen, Adv Eng Mater 2018, 20, DOI 10.1002/adem.201701190.

Figure 1