Materials for new energy applications have recently attracted a number of research interests due to concerns about the depletion of fossil fuels and the construction of sustainable societies. In particular, lithium and sodium ion batteries (LIBs and NIBs) with higher energy density are essential for next generation energy storage devices such as electric vehicles, hybrid electric vehicles and smart grids. A large proportion of the components of these devices are generally inorganic materials in crystalline form. Since the crystal shape, outer plane, size and crystallinity drastically affect the above properties, it is necessary to control these properties simultaneously. Nowadays, all solid-state LIBs and NIBs have attracted much interest due to a number of potential advantages, including energy densities, cost, size, safety and operating temperature. However, there are still serious problems to be solved before practical use. For example, the diffusion of lithium and sodium ions at the interface between different solid materials, including active materials and electrolytes, is still poor for charge/discharge operation in batteries.

In this context, our group has been investigating "nature-mimetic flux growth" of inorganic crystals for these applications. Flux growth is a type of liquid phase crystal growth technique in which molten metals and molten metal salts are used. Fluxes act as solvents at temperatures above their melting and/or eutectic points. As the growth conditions of inorganic crystals are similar to those found in the Earth's crust, we call it "nature-mimetic". Flux grown crystals are characterised by high quality without thermal stress, idiomorphic shape with specific crystal planes and controlled shape and size.

Recently, with the aim of achieving all-crystalline (solid state) LIBs and NIBs, we have applied the flux method to battery materials. We expected that flux crystal growth would allow (I) crystal shape control of active and solid electrolyte materials, (II) construction of good interfaces in electrodes between cathodes, solid electrolytes and anodes. The second theme would be possible if electrode materials could be dissolved and densely recrystallized on their surfaces. As a result, smooth ion transport through bulk materials and their interfaces would be realized in all-crystalline (solid) state LIBs and NIBs. Our concept using the flux method would provide a new aspect to lead an innovation in all solid state LIBs and NIBs. Details of the research progress will be presented at SIPS2024.

Acknowledgement     This research was partially supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), the 3rd period of SIP, JST-GteX, Aichi Grant and JSPS Grant-in-Aid for Scientific Research (KAKENHI).