Atomic-Scale Tracking of a Phase Transition from Spinel to Rocksalt in Lithium Manganese Oxide

For the intercalation type cathode in lithium-ion batteries, the structural framework of electrode is expected to remain unchanged during lithium insertion and extraction. Unfavorable phase transition in electrode materials, which has been frequently observed, modifies the structural framework, which leads to capacity loss and voltage decay. Here, we track atoms motion/shift in lithium manganese oxide during a phase transition from spinel to rocksalt by using atomically resolved aberration corrected scanning transmission electron microscopy and spectroscopy. We find that when given energy, the transition metal cation can readily hop between oxygen tetrahedral and octahedral sites in oxygen deficient lithium manganese oxide similar to lithium diffusion behavior, which leaves the anion structure framework almost unchanged. During this phase transition, the intermediate state, migration length, and atomic structure of phase boundaries are revealed, and the mechanism is discussed. Our observations help us to understand the past experimental phenomena and provide useful information to stabilize the structure of electrode materials and thus improve the cycling life of lithium-ion batteries

Direct Observation of Impurity Segregation at Dislocation Cores in an Ionic Crystal

Dislocations, one-dimensional lattice defects, are known to strongly interact with impurity atoms in a crystal. This interaction is generally explained on the basis of the long-range strain field of the dislocation. In ionic crystals, the impurity–dislocation interactions must be influenced by the electrostatic effect in addition to the strain effect. However, such interactions have not been verified yet. Here, we show a direct evidence of the electrostatic impurity–dislocation interaction in α-Al₂O₃ by visualizing the dopant atom distributions at dislocation cores using atomic-resolution scanning transmission electron microscopy (STEM). It was found that the dopant segregation behaviors strongly depend on the kind of elements, and their valence states are considered to be a critical factor. The observed segregation behaviors cannot be explained by the elastic interactions only, but can be successfully understood if the electrostatic interactions are taken into account. The present findings will lead to the precise and quantitative understanding of impurity induced dislocation properties in many materials and devices

A New Rechargeable Sodium Battery Utilizing Reversible Topotactic Oxygen Extraction/Insertion of CaFeO_<i>z</i> (2.5 ≤ <i>z</i> ≤ 3) in an Organic Electrolyte

At present, significant research efforts are being devoted both to identifying means of upgrading existing batteries, including lithium ion types, and also to developing alternate technologies, such as sodium ion, metal–air, and lithium–sulfur batteries. In addition, new battery systems incorporating novel electrode reactions are being identified. One such system utilizes the reaction of electrolyte ions with oxygen atoms reversibly extracted and reinserted topotactically from cathode materials. Batteries based on this system allow the use of various anode materials, such as lithium and sodium, without the requirement to develop new cathode intercalation materials. In the present study, this concept is employed and a new battery based on a CaFeO₃ cathode with a sodium anode is demonstrated

Assessment of Strain-Generated Oxygen Vacancies Using SrTiO₃ Bicrystals

Atomic-scale defects strongly influence the electrical and optical properties of materials, and their impact can be more pronounced in localized dimensions. Here, we directly demonstrate that strain triggers the formation of oxygen vacancies in complex oxides by examining the tilt boundary of SrTiO₃ bicrystals. Through transmission electron microscopy and electron energy loss spectroscopy, we identify strains along the tilt boundary and oxygen vacancies in the strain-imposed regions between dislocation cores. First-principles calculations support that strains, irrespective of their type or sign, lower the formation energy of oxygen vacancies, thereby enhancing vacancy formation. Finally, current–voltage measurements confirm that such oxygen vacancies at the strained boundary result in a decrease of the nonlinearity of the <i>I</i>–<i>V</i> curve as well as the resistivity. Our results strongly indicate that oxygen vacancies are preferentially formed and are segregated at the regions where strains accumulate, such as heterogeneous interfaces and grain boundaries