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

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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.