Mechanical Activation Enhanced Solid-State Reaction and Electrochemical Properties of NaCrO2

摘要：高能球磨广泛用于生产航空航天工业中应用的镍基和铁基高温合金。近年来，该技术也被用于制造电池电极材料。通过固态和材料化学项目的这项奖励，首席研究员试图了解在室温下由高能球磨引起的机械活化如何改变NaCrO2（高温下固态反应的产物）的结构缺陷，以及NaCrO2的结构缺陷如何影响NaCrO2的电化学性能。通过实验和理论建模与仿真的无缝结合，本项目在原子水平上发展了对机械的理解。新发现的知识将用于指导合理的设计和机械活化合成NaCrO2，以获得可控的结构缺陷和所需的掺杂剂，从而为na离子电池提供卓越的容量保持，高往返能量效率和长循环寿命。这种钠离子电池可以在电网规模的电力储存中发挥关键作用，以广泛整合可再生能源，使美国人负担得起清洁能源，并使技术更环保，更节能。通过这个项目，本科生将有机会参与伊利诺伊理工学院为期一学期的“跨专业项目”研究。“化学在锂离子电池中的作用”的演讲和实践演示在高中的科学课上进行，其中有很高比例的少数民族学生没有被充分代表，以激励他们追求科学，工程和技术方面的职业。技术摘要：本项目为固体与材料化学资助项目，首次研究了反应产物NaCrO2的机械活化程度、固体反应条件和结构缺陷之间的关系。本项目研究了电化学充放电循环中NaCrO2的容量保持与NaCrO2结构缺陷和掺杂的关系。利用原位高能x射线衍射（HEXRD）揭示了合成过程中NaCrO2的反应途径和动力学、机械活化的影响以及结构缺陷的演变。在电化学循环过程中，原位HEXRD和原位x射线吸收（XAS）也用于定义NaCrO2的晶体结构变化和结构缺陷演变以及Cr离子的局部结构和氧化态，而密度泛函理论（DFT）计算有助于在原子水平上解释实验结果，并提出提高充放电循环中容量保持的途径。此外，通过高通量第一性原理计算来指导掺杂实验，进一步提高具有所需结构缺陷的NaCrO2电极的稳定性。作为将该项目科学发现转化为可行技术的第一步，在项目接近尾声时，最好的NaCrO2将用于制造半电池，以证明其卓越的容量保持高达2000次循环，具有高往返能源效率（90%）。为了加速新创造的知识在科学界和工业界的传播，主要研究人员计划使用从2000个循环实验中获得的数据来预测长期特性（10,000个循环）。

Non-Technical Abstract:High-energy ball milling is widely used to produce nickel- and iron-based superalloys for applications in the aerospace industry. In recent years, the technique has been adopted for fabrication of battery electrode materials as well. Through this award by the Solid State and Materials Chemistry Program, the principle investigator seeks to understand how mechanical activation induced by high-energy ball milling at room temperature alters structural defects in NaCrO2 - the product of solid-state reactions at high temperature - and how the structural defects in NaCrO2 affect the electrochemical properties of NaCrO2. Through seamless integration of experiments and theoretical modeling and simulation, this project develops mechanistic understandings at the atomic level. The newly created knowledge is used to guide rational design and synthesis of NaCrO2 with mechanical activation to obtain controlled structural defects and desired dopants that yield superior capacity retention, high round trip energy efficiency and long cycle life for Na-ion batteries. Such Na-ion batteries can play a critical role in grid-scale electric energy storage for widespread integration of renewable energy, making clean energy affordable to Americans and the technology greener and more energy efficient. Through this project, undergraduate students are offered opportunities to participate in research through a semester long "Inter-professional Project" at the Illinois Institute of Technology. Presentations on "Roles of Chemistry in Lithium-ion Batteries" with hands-on demonstrations are given in the science classes of high schools with a high percentage of under-represented minority students to inspire them to pursue careers in science, engineering and technology.Technical Abstract:This Solid State and Materials Chemistry-funded project constitutes the first investigation of relationships among the degree of mechanical activation, solid-state reaction conditions and the structural defects in the reaction product NaCrO2. This project investigates the dependence of capacity retention of NaCrO2 over electrochemical charge/discharge cycles on the structural defects and dopants in NaCrO2. In-situ high-energy X-ray diffraction (HEXRD) are conducted to unravel the reaction pathway and kinetics, effects of mechanical activation, and structural defect evolution in NaCrO2 during synthesis. In-situ HEXRD and in-situ X-ray absorption (XAS) are also performed during electrochemical cycling to define the crystal structure change and structural defect evolution of NaCrO2 and the local structure and oxidation state of Cr ions, while density functional theory (DFT) calculations help interpret experimental results at the atomic level and suggest pathways to improve capacity retention over charge/discharge cycles. Additionally, high-throughput first-principles calculations are carried out to guide doping experiments to further enhance the stability of the NaCrO2 electrode with the desired structural defects. As the first step in translating the scientific discovery made in this project towards a viable technology, near the end of the project the best NaCrO2 will be used to fabricate half cells to demonstrate the superior capacity retention up to 2,000 cycles with high round trip energy efficiency ( 90%). To expedite the dissemination of the newly created knowledge in scientific community and industry, the principle investigators plan to predict the long-term properties (10,000 cycles) using the data derived from 2,000 cycle experiments.