Fine tuning of structural and physical properties of transition metal halides by electrochemical intercalation

PART 1: NON-TECHNICAL SUMMARY Intercalation chemistry, the process of ions moving in and out of a materials structure, lies at the center of how commercial Li-ion batteries work. The concept also is at the core of emerging technologies, including electrochromics, desalination, thermal switching and resistance switching materials. Regarding battery technology, oxide materials have been the intercalation materials of choice for a long time. Nevertheless, they suffer from degradations associated with ion intercalation that slowly deteriorate battery performance upon prolonged charge and discharge of the battery which limits the battery’s lifetime. For this project, which is supported by the Solid State and Materials Chemistry Program in NSF’s Division of Materials Research, a new class of intercalation materials using chlorine atoms, in place of oxygen, in the host structure, is synthesized. The reversibility of ion intercalation in halide materials is studied and compared with that in oxide materials, drawing structure-property relationship for this novel class of intercalation materials. The project provides new avenues to design more robust intercalation materials for Li-ion batteries. Additionally, outreach activities are organized as part of this project to engage with communities, and educational opportunities are provided for undergraduate students which has the potential to develop further the US workforce. PART 2: TECHNICAL SUMMARY The objective of this research, which is supported by the Solid State and Materials Chemistry Program in NSF’s Division of Materials Research, is to reveal the factors governing the electrochemical intercalation of alkali cations into transition metal halides, such that a future generation of Li- or Na-ion battery technologies can be developed. To this end, the principal investigator and his research group at Boston College carry out an experimental study combining the synthesis of novel lithium- or sodium-containing transition metal halides with measurements of the physical properties that govern their electrochemical (de)intercalation. The central hypothesis guiding this work is that layered halides can offer a fast alkali cation (de)intercalation while avoiding damaging structural transitions that plague the extraction of lithium or sodium from oxides at high potential. By mapping the chemical landscape that governs the redox chemistry of layered halides, this work seeks to lay the fundamental understanding to how ligand polarizability, size and electronegativity modify the redox properties of layered materials. Novel metastable polymorphs are synthetized and, by comparing their structural features and electrochemical response with that of more thermodynamically stable ones, competition existing between intra- and inter-layer interactions for intercalated layered halides can be revealed. Combined with electrical and magnetic measurements, the results are integrated to find out how cations intercalation impart the competition existing between inter- and intra-layer interactions in transition metal layered halides. Methodologies and knowledge gathered in this work serve to identify promising intercalation materials with tunable electronic properties. Collectively, this work advances redox chemistry of transition metal halides for rechargeable batteries and paves the way towards the development of new halide compounds. The research efforts are complemented by participating in an outreach program serving students in grades 8-12, and by engaging undergraduate student researchers in the project. 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.