Investigation of the Redox Activity in Mn-Based Oxyfluorides

Li-ion batteries offer excellent electrochemical performance, are widely used in vehicle electrification and portable devices, and have huge potential for grid storage applications. Their electrochemical behavior relies primarily on transition metal oxide cathodes. In a recent development, Mn has been used in Li-rich cathode materials with a cationic disordered rock-salt (DRX) structure [1,2]. These Mn-based DRX materials are promising candidates for next-generation Li-ion battery cathodes because of their large energy densities and basic favorable features of Mn, such as sustainability. Along with the use of Mn in a DRX structure, substitution of some of the oxygen by fluorine imparts improved cyclability [1-6]. It has been proposed that Li-site distribution played an important role in the initial capacity of these materials and the metal-redox capacity and reversibility was improved by fluorination [5,6]. However, the contribution of the different ions to this improvement has not been fully elucidated. Fluorine and oxygen have different bonding interactions with transition metals, raising the question of how they may comparatively participate in redox compensation. To address this question and determine the role of a mixture of anions in improving the energy density in such cathode materials, we conducted a deep dive into Li2MnO2F as a model cathode material [6]. We interrogated the covalent interaction between the oxygen 2p states, fluorine 2p states, and the transition metal 3d orbitals, and their respective contribution to the charge compensation mechanism using X-ray absorption spectroscopy (XAS). XAS allowed us to resolve the role of both oxygen and fluorine in the electrochemical activity and how covalent interactions are affected by redox, both in extent and in their reversibility. References: Gerbrand Ceder et. al. "The Configurational Space of Rocksalt‐Type Oxides for High‐Capacity Lithium Battery Electrodes." Advanced Energy Materials 4, no. 13 (2014): 1400478. Lee, Jinhyuk et.al. "Unlocking the potential of cation-disordered oxides for rechargeable lithium batteries." science 343, no. 6170 (2014): 519-522. Freire, Melanie et. al. "A new active Li-Mn–O compound for high energy density Li-ion batteries." Nature materials 15, no. 2 (2016): 173-177. Reed, J., G. Ceder, and A. Van Der Ven. "Layered-to-spinel phase transition in Li x MnO2." Electrochemical and Solid-State Letters 4, no. 6 (2001): A78. Lee, Jinhyuk, et al. "Reversible Mn 2+/Mn 4+ double redox in lithium-excess cathode materials." Nature 556, no. 7700 (2018): 185-190. 7/8 Lun, Zhengyan, et al. "Design Principles for High-Capacity Mn-Based Cation-Disordered Rocksalt Cathodes." Chem 6, no. 1 (2020): 153-168.