Exploring the Chemistry and Probing the Nature of Over-stoichiometric Earth Abundant Transition Metal-based Disordered Rock Salts

University Of New Mexico, Albuquerque NM

Investigators

Abstract

PART 1: NON-TECHNICAL SUMMARY Rock salts are one of the most commonly known structural types of materials, featuring the common salt (NaCl)'s atomic arrangement with cations and anions occupying the Na and Cl sites, respectively. Disordered rock salts, in which the Na and Cl sites are randomly occupied by different cation/anion elements, have been arising as a new research field for both fundamental materials science and battery materials development. A typical rock salt structure is subject to the site balance rule that constrains the ratio of cations to anions to 1:1, fundamentally limiting the synthesizable composition of disordered rock salts. The site balance rule also essentially poses a trade-off between theoretical capacity and charge compensation contribution, when the materials are used in Li-ion or Na-ion battery electrodes. The proposed study explores the chemistry of over-stoichiometric disordered rock salts (i.e., allows cation to anion ratios greater than 1:1) and aims to understand the relationship between over-stoichiometry, fundamental materials properties, and energy storage mechanisms. The materials development and the understanding of the structure-property relationship will contribute as a knowledge pool for building earth-abundant element-based Li-ion/Na-ion batteries without being strictly limited by compositional constraints, thus generating new material-level solutions for next-generation energy storage technologies. In parallel to the research activities, the project serves as an education platform that converts the research subjects to a series of student lab projects, course, and outreach materials. These educational activities aim to involve students from K-12, undergraduate, to graduate levels and local K-12 science educators, and to minimize the gap between fundamental physical science education and real-world materials science research. PART 2: TECHNICAL SUMMARY The objective of the proposal is to study the chemical space and understand the structure-property relationship of over-stoichiometric disordered rock salts with earth abundant transition metal-based compositions, as Li- and Na-ion battery cathode materials. In recent years, although a significant progress has been made in the development of disordered rock salt-based battery cathode materials, the composition, structure, and property of over-stoichiometric disordered rock salts remain as a significant knowledge gap. This project, supported by the Ceramics program in the Division of Materials Research, aims to develop a new series of over-stoichiometric Li(Na)-Ti-Mn(Fe)-O-F-based disordered rock salts as model materials, characterize their crystallographic properties, and understand the atomic coordination behavior by probing their electronic and vibrational structures. The project will further systematically evaluate the electrochemical properties of the developed materials as Li/Na-ion battery cathodes and correlate the parameters of the chemical space to with detailed electrochemical properties. Furthermore, the study will probe the energy storage mechanism of the over-stoichiometric disordered rock salts via a set of closely integrated approaches including X-ray crystallography, X-ray spectroscopy, resonance Raman spectroscopy, and near-field infrared spectroscopic imaging. These concerted efforts are expected to offer the materials research community a new chemical space for battery material exploration, enhance the knowledge of the relationship between electronic/vibration structures and cation over-stoichiometry, and ultimately lead to discovery of materials design strategies for building optimized over-stoichiometric battery cathode materials. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.