Local Structure and Composition Variation at Surface of Lithium-Ion Conducting Solid Electrolyte

Abstract

Recently, lithium ion conducting solid electrolytes attract many interests because of their potential application to all-solid-state batteries (ASSBs), which are supposed to be one of candidates of next-generation energy storage devices with high safety and reliability [1-3]. With great efforts of development of solid electrolytes, several materials with high lithium ionic conductivity have been reported, which facilitated hope of realization of ASSBs [2,4-8]. Now, researchers face several problems on application of solid electrolytes to ASSBs. Most of those problems are related to interfaces of between solid electrolytes and active materials, for example, contact areas, interdiffusion of elements on assembly and/or electrochemical processes of ASSBs. To solve these issues, much attention has been paid to design and fabrication techniques. In addition to the problems on the fabrication, several reports point out intrinsic phenomena related change in ion concentration at interfaces of solid electrolytes [1, 9, 10]. We have so far studied the local structure at the interfacial region by fabricating nano-composites of active materials and solid electrolytes, and confirmed that the interfacial region with distorted lattice ranges for at least 20-50 nm in thickness [9, 10]. Although more researches with variety of experimental analysis are required for further comprehension of the interfacial phenomena, difficulty in the analysis of solid/solid interface limits experimental tools. It is known that surface of solid electrolytes is also related to interfacial resistance. Kliewer proposed cation depletion at surface of ionic crystals when free energy of defect formation is lower for cation than for anion [11]. In addition, we observed that ball-milling of a poor ionic conductor (Li2SiO3, LSO) resulted in increase in ionic conductivity without change in activation energy [9, 12]. In this study, three different surface-sensitive techniques, grazing incidence X-ray diffraction (GIXD), attenuated total reflectance Fourier transform infrared (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS), were employed to analyze the local structure and composition around the surface region of a lithium ion conducting solid electrolyte sheet (NASICON-type in the system of Li-Al-Ti-Ge-Si-P-O, Ohara Inc. abbreviated as LICGC). It was revealed that the local structure and composition changed depending on depth from the top surface of the sheet. At the very surface, there was a layer with expanded lattice (Figure 1) and high Li composition (Figure 2). And with increasing the depth from the surface, lattice shrank sharply, and then expanded again gradually. The change in the lattice seemed to be accompanied by Li and Al composition. It was supposed that the change in the Li composition and structure is induced by combination of chemical reaction, segregation, and defects distribution [13].