Chemical Bonding in Redox Active Oxyfluoride Cathode for Li-ion Battery.

The ever-growing demands of modern technology necessitate the development of high-performance, sustainable battery electrodes. Their electrochemical behavior relies primarily on transition metal oxide cathodes. In a recent development, fluorine-substituted disordered rocksalt oxyfluoride cathodes have gained popularity as next generation cathode materials. This thesis delves into the exciting potential of oxyfluoride cathodes, where the element fluorine plays a previously underestimated but crucial role in unlocking their next-generation capabilities. This work focuses on systematically understanding the fundamental interactions between fluorine, transition metals, and oxygen, and translating these insights into the design and optimization of novel cathode materials for lithium-ion (Li-ion) batteries. The initial research established a robust mechanochemical synthesis method for oxyfluoride cathodes, enabling the investigation of their electrochemical potential. Utilizing a suite of characterization techniques, particularly X-ray absorption spectroscopy (XAS), revealed the active participation of fluorine in the redox reaction, contradicting the prevailing view of its passive spectator role. This discovery challenged existing paradigms and opened new avenues for cathode design by leveraging the unique properties of fluorine. To unlock the intricacies of the redox mechanism within oxyfluorides, in Chapter 3, Li2MnO2F and Li2Mn2/3Nb1/3O2F, two model oxyfluoride systems were studied. XAS served as a powerful tool to probe the electronic structure of the materials. Detailed analysis of the Mn and O K-edge XAS data confirmed their established roles in the redox process. However, the F K-edge data revealed a previously unknown phenomenon: fluorine not only participates in covalent bonding with the transition metal, but also actively contributes to the redox reaction. This remarkable finding highlighted the crucial role of fluorine in determining the overall electrochemical performance of oxyfluoride cathodes, emphasizing the need for further exploration and optimization. To further expand the quest of finding new solid compounds, in chapter 4, Li2NiO2F and Li2CoO2F were synthesized. Elucidating the exact role of F in modulating the formal redox chemistry of a late metal like Ni and Co was the motivation of this study. Using both XAS techniques, I have found that on highest oxidation state of Ni and Co, O does not participate in redox mechanism, whereas counter intuitively, F chimes in and shows greater redox participation at Ni(IV) and Co(IV) states. Driven by the potential to push the boundaries of fluorination, the research ventured into exploring the "practical limit" of fluorine incorporation in chapter 5. By systematically increasing the fluorine content, the synthesis of Li2MOF2 (M = Mn, Ni, Co) phases with a remarkable 67% fluorination level was achieved, exceeding previous records. These highly fluorinated materials exhibited improved electrochemical performance compared to the 33% fluorinated version, highlighting the potential of this strategy for further development. This thesis overall aims to significantly contribute to the battery community by posing a fundamental inquiry: the role of fluorine (F) in cathode materials. Through extensive investigation, it was demonstrated that fluorine, despite being the most electronegative element, cannot be simplistically regarded as purely 'ionic'. Instead, it forms covalent bonds with transition metals and actively participates in redox reactions. By delving into the fluorine bonding dynamics with manganese (Mn), cobalt (Co), and nickel (Ni), this study unveiled a periodic trend in fluorine covalency. Furthermore, the work has provided a compelling comparison between Li2MO2F, a system with 33% of fluorine, and Li2MOF2, the most heavily fluorinated compound known to date. This juxtaposition aims to underscore the impact of fluorination on the electrochemistry of oxyfluoride cathodes for lithium-ion batteries. Looking beyond the immediate application in batteries, the research also explored the broader implications of understanding and defining covalency in transition metal-containing materials. By developing novel methods to probe and quantify these interactions using XAS, this work lays the foundation for advancements in diverse scientific fields, potentially impacting areas like catalysis, magnetism, and materials science.