Atomic Resolution STEM and Spectroscopic Characterization of Battery Related Materials

Abstract

The properties of lithium battery strongly depend on the diffusion of lithium ions during charge/discharge process. Since this behavior determines the stability, lifetime and reliability, direct visualization of Li site is required to understand the mechanism of the diffusion of lithium ions. Annular bright field (ABF) scanning transmission electron microscopy (STEM) is useful imaging technique to directly observe the both light and heavy element columns [1]. In this technique, an annular detector is located within the bright‐field (direct‐scattered) region, and the columns display absorptive‐type contrast. Figure 1 shows ABF STEM images observed from [001] of (a) olivine Li x FePO 4 and (b)delithiated olivine (FePO 4 ). It can be seen that Li column contrast appears in Li x FePO 4 , but disappears in FePO 4 . Fig.1 (c) (d) shows HAADF STEM images observed from the same region of (a) and (b), indicating that the cation frame work columns are almost the same before and after delithiation [2]. In this study, light elements in several lithium battery related materials are directly observed by ABF STEM, and the mechanism of lithiation/delithiation is discussed based on the observation results. The properties of thin‐film batteries is influenced by the atomic structures of the embedded interfaces, such as electrode/electrolyte and electrode/current‐collector interfaces, as well as the grain and domain boundaries. Detailed analyses of these interface structures, which provides insights into formation mechanisms of the interfaces and the effects of microstructure on electrochemical properties, is essential for understanding the mechanism of lithiation /delithiation and for obtaining the guideline to design the thin film devices. In this study, the epitaxial growth mechanism of a typical cathodic LiMn 2 O 4 thin film is investigated by exploring the detailed structural and compositional variations in the vicinity of the film/substrate interfaces. STEM observation shows the LiMn 2 O 4 film forms an atomically flat and coherent heterointerface with the Au(111) substrate, but that the crystal lattice is tetragonally distorted with a measurable compositional gradient from the interface to the crystal bulk [3]. The growth mechanism is interpreted from the chemical and physicomechanical effects, which is related to the complex interaction between the internal Jahn‐Teller distortions induced by oxygen non‐stoichiometry and the lattice misfit strain. In addition, the microstructures for La 2/3‐x Li 3x TiO 3 (LLTO) and La (1‐x)/3 Li x NbO 3 (LLNO) electrolytes are characterized by ABF STEM. It was found that the unique structures of the domain boundaries (DBs) in LLTO affect the Li‐ion mobility and ionic conductivity. DBs in LLTO are consisted of two types: frequently occurring 90° rotation DBs and a much less common antiphase‐type boundary [4]. The 90° DBs are found to have coherent interfaces consisting of interconnected steps that share La sites, with occupancies of La sites higher than in the domain interiors. The DBs show different degrees of lattice mismatch strain depending on Li content. The lattice strain and resultant Li and O vacancies and the high La occupancy at DBs are considered to be the reason for lower interdomain Li‐ion mobility, which has a deleterious effect on the overall Li‐ion conductivity. LLNO is found to have complex modulated crystal structures with partially ordered distributions of A cations and vacancies. This involves a long‐range layer‐ordering of A cations into alternating La/Li‐rich and La/Li‐free layers parallel to (001)p, and a short‐range sinusoidal columnar ordering of A cations within the La‐rich layers.