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

插层化学，即离子从材料结构中进出的过程，是商用锂离子电池工作的核心。这一概念也是新兴技术的核心，包括电致变色、海水淡化、热开关和电阻开关材料。就电池技术而言，氧化物材料长期以来一直是首选的插层材料。然而，随着电池的长时间充放电，它们会受到与离子嵌入相关的退化的影响，这种退化会慢慢降低电池的性能，从而限制电池的使用寿命。该项目由美国国家科学基金会材料研究部固态与材料化学项目支持，在宿主结构中合成了一类新的嵌入材料，使用氯原子代替氧。研究了离子在卤化物材料中嵌入的可逆性，并与氧化物材料中的可逆性进行了比较，得出了这类新型嵌入材料的结构-性能关系。该项目为设计更坚固的锂离子电池插层材料提供了新的途径。此外，作为该项目的一部分，还组织了外展活动，与服务不足的社区接触，并为有可能进一步发展美国劳动力的本科生提供教育机会。这项研究得到了美国国家科学基金会材料研究部固态和材料化学项目的支持，其目的是揭示控制碱阳离子电化学插入过渡金属卤化物的因素，从而开发出下一代锂离子或钠离子电池技术。为此，首席研究员和他在波士顿学院的研究小组进行了一项实验研究，将新型含锂或含钠过渡金属卤化物的合成与控制其电化学（脱）插层的物理性质的测量相结合。指导这项工作的中心假设是，层状卤化物可以提供快速的碱阳离子（脱）插层，同时避免了从高电位的氧化物中提取锂或钠的破坏性结构转变。通过绘制控制层状卤化物氧化还原化学的化学景观，这项工作旨在为配体的极化性、大小和电负性如何改变层状材料的氧化还原性质奠定基本的理解。合成了新的亚稳态多晶，并通过比较其结构特征和电化学响应，揭示了层间和层内相互作用之间存在的竞争。结合电和磁测量结果，研究了在过渡金属层状卤化物中，阳离子插入如何影响层间和层内相互作用之间的竞争。在这项工作中收集的方法和知识有助于确定具有可调谐电子特性的有前途的插层材料。总的来说，这项工作推进了可充电电池过渡金属卤化物的氧化还原化学，并为开发新的卤化物化合物铺平了道路。研究工作的补充是参与外展项目，为8-12年级代表性不足和服务不足的学生提供服务，并让本科生研究人员参与该项目。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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 chloride atoms, in place of oxygen, in the host structure, is synthetized. The reversibility of ions 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 underserved 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 serves 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 to outreach program serving underrepresented and underserved students in grade 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.