Interface Structure and Transport Properties of Thin-Film Solid-State Batteries with an Epitaxial Cathode

Abstract

A problem of solid-state Li ion batteries is a relatively low ionic conductivity at the interface between electrodes and electrolytes that limits the charging/discharging rate. For designing the solid-state batteries, understanding the mechanism behind the low interface conductivity is crucial. Investigation of the atomic structure of the interfaces and its relation to the ionic conductivity would be helpful for this purpose. However, atomic-scale observation of the buried interfaces is often difficult in practical batteries which consist of grain aggregates. Recent progress in thin-film deposition techniques of electrodes and electrolyteshas opened a path to study the interface quantitatively. By using an all-in-vacuum battery fabrication system [1], we succeeded in fabricating a very low solid-electrolyte/electrode interface resistance in thin film batteries with an epitaxial cathode. The well-defined crystalline interfaces allow the analysis of atomic-scale structure and its transport properties. For the structure analysis, we used X-ray crystal truncation rod (CTR) scattering which enables the atomic-scale analysis in a non-destructive manner.The analysis of two interfaces will be presented: a solid-electrolyte/electrode interface and an electrode/current collector interface. In the former case, we studied Li(negative electrode)/Li3PO4(solid electrode)/LiCoO2(positive electrode) thin-film Li batteries with low and high (more than 10 times) interface resistances. It turned out that the low-resistance interface has an atomically well-ordered LiCoO2 surface and a disordered surface CoO2 layer can results in the high-resistance interface, demonstrating the crucial importance of the structural ordering in the interfacial layer for realizing a low-resistance interface [2]. In the second topic, we focused on electrode/current collector interface that can also affect the battery performance. We found a drastic improvement of the charging/discharging property by inserting 1.2 nm-thick band insulator LaAlO3 into the LiCoO2 (electrode)/Nb-doped SrTiO3 (current collector) interface. The CTR analysis revealed that a formation of electric dipole at the LaAlO3/Nb-doped SrTiO3 interface, which causes a energy band alignment preferable for the electronic conductance between the electrode and the current collector.