Monday, July 14, 2014
2:00 PM (Refreshments 15 minutes beforehand)
RLE Haus Room, 36–428
CRYSTAL ENGINEERING BASED ON FLUX COATING APPROACHES FOR ADVANCED LITHIUM ION RECHARGEABLE BATTERIES
Prof. Nobuyuki Zettsu
Center for Environmental Science & Technology, Shinshu University,
Department of Environmental Science & Technology, Shinshu University,
CREST, Japan Society and Technological Agency
Lithium-ion rechargeable batteries (LIBs) are expected to be increasingly used as power sources for smart grid systems, hybrid electric vehicles, pure electric vehicles, and many other applications. In general, besides the current collector, an LIB consist of three layers, namely, cathode and anode electrode materials, electrolyte, and many of additive materials that facilitate lithium-ion and electron transportation. Currently, however, the energy performance of LIBs is not at par with combustible sources of energy, and improvements, including increases in their energy density, lifetimes, and safety, are necessary if LIBs are to be adapted more widely. Both improvement of the energy density and the lifetime of LIBs are generally limited by the properties of the cathodes.
LiCoO2 has been the most widely used cathode materials in commercial LIBs owing to its good capacity retention, rate capability, and high structural reversibility below 4.2 V vs. Li+/Li. Charging up of the layered LixCoO2 material from LiCoO2 to Li0.5CoO1.5 resulted in large anisotropic volume change of the host lattice in c‑axis direction as large as 7%. Such large volume changes, however, offered peeling of the additive materials from the active material particles. This fact leads the problem of capacity fade and poor rate capability associated with the material breaking away into the electrolyte.
In this regard, great efforts have been made to improve the capacity retention. Very recently, nanostructured approaches have been demonstrated to improve the electrochemical performance of electrode materials as well as their structural durability. For instance, those nanostructures decrease the diffusion length of lithium ion in the insertion/extraction reaction and increase surface area, which results in the higher capacities at high charge/discharge rates due to shorter diffusion pathways for ionic transportation and electronic conduction. Additionally, nanostructured materials enable to buffer the stresses caused by the volume variation occurring during the charge/discharge process. However, nanostructured materials are not an ultimate solution to achieve the all requirements of future advanced LIBs. One of the primary reasons is that some of the intrinsic material properties, such as low conductivities, cannot be simply improved by just transforming them into nanostructured materials. Moreover, to nano-sized materials leads to aggregation of them without some surface modification, resulting in capacity fade due to both inhomogeneous distribution and low tap density.
On the basis of these backgrounds, in order to enhance energy density and power density of nano-sized LiCoO2 single crystals-based cathodes, we newly propose additive-free electrodes which constructed of densely packed idiomorphic nanosized-LiCoO2 crystal layer directly growth from a current collector substrate surface with high crystal face orientation and homogeneous distribution, fabricated by flux coating using a LiNO3-LiOH mixed flux. We here discuss structural characterization of the LiCoO2 crystal layer, its formation mechanism, and its additive-free electrode characteristics.
FLUX INNOVATION – ENVIRONMENTAL AND ENERGY MATERIALS
Prof. Katsuya Teshima
Center for Energy and Environmental Science, Shinshu University, Nagano, Japan.
Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, Nagano, Japan.
Energy conversion and storage, producing alternative fuels, and environmental purification are of growing importance to realize sustainable societies. Although some of promising devices used for the above applications are proposed and constructed, there are still difficulties to fabricate those next-generation devices satisfying required performances. The difficulties lie in decreasing defects in materials and constructing well-connected interfaces between different materials because these are general causes of scattering of conductive carriers. For example, all-solid-state lithium-ion rechargeable batteries (LIBs) have advantages in packaging density and intrinsic safety compared to conventional liquid-electrolyte-based LIB, whereas it is challenging to prepare high-quality solid electrolytes achieving smooth Li-ion conductivity. In addition, constructing a well-connected interface at solid electrolytes / active materials, and active materials / current collectors is important to prevent electrons and Li ions from scattering.
We believe flux growth and flux coating approaches are consider to be promising route to overcome the above difficulties. Flux method is high temperature solution growth method using molten metals or metal salts as solvents (fluxes), and is an environmentally-friendly, simple and low-cost process. You can obtain high quality crystals at temperatures well below the melting points. In addition, crystals can grow in an unconstrained fashion; that is, they can grow free from mechanical or thermal constraints into solution and therefore develop facets. Furthermore, flux coating method is a coating technique based on flux concept. Various high-quality crystal layers (films) have been fabricated directly on various substrates.
This lecture presents recent research activities of my laboratory on flux growth of functional crystals for energy and environmental applications such as LiCoO2, LiNi0.5Mn1.5O4 Li4Ti5O12 as active materials for LIB, Li7La3Zr2O12, and Li5La3Nb2O12 as solid electrolytes, NaTaO3, Ta3N5 SrNbO2N, SrTaO2N as photocatalysts for water-splitting, and CuInS2 as light absorbing layer of solar cells. The layered products consisting of the above crystals have been also prepared by flux coating on various substrates. The detail fabrication techniques and their physical properties will be presented.