Reconstruction of the LiCoO2 Near-Surface Region upon Overgrowth of Lipon
Authors: Sugimoto Yuta , Ohuchi Satoru , Asano Tetsuya , Kume Toshiro 
Published: February 2020

Recently, all-solid-state batteries have been actively studied for a next-generation secondary battery. All-solid-state batteries have an advantage in safety compared with the current flammable liquid electrolyte-based lithium ion batteries because of its excellent chemical stability. In addition, since this improved stability will eliminate the need for safety apparatus, the energy density of the all-solid-state battery module is also expected to be improved. One of the major issues of all-solid-state batteries towards their practical application is high interfacial resistance between the solid electrolyte and electrode active materials. While it has been speculated that space charge layer formation at the interface is a major factor for the high interfacial resistance, the interfacial phenomena still remains unclear due mainly to the difficulty in analysis of the interface in bulk battery form. In this study, we prepared thin-film battery with a well-defined pristine interface using thin film technique, and studied the changes in the crystal structure and electronic state in the vicinity of the interface between the cathode active material and solid electrolyte upon charging and discharging.

　In this study, we used LiCoO2 as an positive-electrode active material, and LiPON as a solid electrolyte. LiCoO2 was epitaxially grown in (104) orientation after forming the SrRuO3 as a current collector on SrTiO3(100) substrate by Pulsed laser deposition (PLD). LiPON was deposited on LiCoO2 by RF sputtering method using Li3PO4 as a target under N2 atmosphere. Li metal was deposited by vapor deposition as a negative electrode. In all the film formation processes, the samples were kept in vacuum to prevent the surfaces from being exposed to air in order to maintain clean and pristine interfaces.

　Cross sectional transmission electron microscope (TEM) images of the as-deposited film revealed the presence of the interfacial layer between LiCoO2 and LiPON which corresponds to neither LiCoO2 nor LiPON. The thickness of the interfacial layer was about 15 nm. From the results of TEM-EDS and electron diffraction, the interfacial layer is composed of elements of Li, Co and O, and its structure is similar to LiCoO2 but with larger lattice constant than LiCoO2. In addition, from TEM-EELS measurement results, Li-K edge of the interface layer was found to be shifted to the low-energy side compared with that of LiCoO2. This indicates that the Li in the interfacial layer is bounded more weakly than that of LiCoO2.

　By charging and discharging the above thin film battery at a current density of 4.33 μA/cm2(1/20C), the discharge capacity of 140 mAh/g was obtained. The characterisitic plateau associated with the oxidation-reduction of LiCoO2 was observed, which confirms excellent charge and discharge characteristics of the thin film battery. Taking the cross sectional TEM images after charging and discharging, the interface layer disappeared after charging while the interface layer was found to appear again after discharge. From this result, it can be inferred that the interface layer was formed when the Li concentration of LiCoO2 in the vicinity of the interface was high. The effect of interfacial layer on the electrochemical properties are to be discussed at the presentation.