Strain Imaging of Electrode Materials in Li-Ion Batteries

Abstract

Li-ion batteries operate through the migration of Li ions between the electrodes. Therefore, non-destructive observation of the migration with high spatial resolution is important.When charging and discharging, Li-ions are extracted or inserted into electrode materials, which generally causes changes in volume. An atomic force microscope (AFM) can detect and image these volume changes through current collectors with high spatial resolution, which enables us to investigate Li-ion migration without destruction. AFM detects surface displacements synchronizing the charging/discharging cycle, and provides strain images along with topography of the current collector.1) Here, we present our results of lithium titanate oxide (LTO: Li x Ti5O12). LTO is an anode material with high safety having a theoretical capacity of 160 mA h g−1 (from LiTi2O4 to Li2Ti2O4). But the phase change from spinel to rock-salt lead to very small volume changes. Therefore, it might be difficult to observe the Li migration by this method.However, Li4Ti5O12 with a spinel structure has high resistivity, whereas charged Li7Ti5O12 with a rock-salt structure is a conductor. The former is a transparent semiconductor and the latter is a dark metal. The state of charge of LTO can be observed by an optical microscope. The spatial resolution is not enough though the method is easy-to-use.2) High-resolution method has been proposed.3) Photoinduced strain imaging discriminates semi-conductors having bandgap from metallic materials with no bandgap. The method images bandgap variations based on the detection of electric strains caused by electron-hole pair generation. When electron-hole pairs are generated by photon injection, strains are induced, because the lattice constants are determined as the energy of the electron band structure becomes minimum, and the generation changes the energy configuration. The generation of electron-hole pairs is determined from the magnitude relationship between photon and bandgap energies. Bandgap energies can be measured quantitatively by scanning the injected photon energy. When the photon energies are larger than bandgap energies, strains are generated by electron-hole pairs generation. Therefore, mapping of the strains reveals changes in bandgap energies, that is, state-of-charge of LTO.The lithium metal anode has fruitful advantages that are the lowest electrode potential of Li+/Li and low density of Li, which are −3.04 V and 0.534 g/cm3, respectively. However, the lithium dendrite issue is a serious barrier standing in way of the development of high-energy-density batteries with a lithium metal anode.We have been successful in imaging electrochemical reactions in the interface between lithium metal and electrolyte using strain imaging. Precipitation and dissolution of lithium ions in the surface of lithium metal, and electrolyte flow induced by the gradient of lithium-ion concentration should cause the surface displacements of the current collector. We will be able to get a lot of knowledge and information from this observation.