Evaluate the Limits of F Covalent Bonding with Transition Metals at High Valent States in Li2MO2f ( M = Mn, Co, Ni ) Oxyfluoride
Authors: Roy Indrani , Cabana Jordi 
Published: February 2024

The capacity of transition metal oxides as Li-ion battery cathodes are limited by instabilities that arise when high states of charge are achieved1. Oxyfluorides with a disordered rock-salt structure have emerged as attractive alternatives2, but the role of F in their electrochemical function, particularly when metals reach high formal oxidation states, remains to be ascertained so far. In our recent study3, using X-ray Absorption Spectroscopy (XAS) measurements of Mn, O and F, we revealed the existence of Mn-F covalent interactions in Li2MnO2F. The results challenged the assumption of F as largely a spectator ion, providing instead a nuanced picture of redox compensation in oxyfluorides. They suggested the existence of unique knobs of design of battery cathodes in these chemical spaces, by manipulating the covalent interactions between transition metals and two different anions. To further expand the understanding of F participation in the covalent bonding and its electrochemical effect on oxyfluoride materials, we synthesized Li2CoO2F and Li2NiO2F4. Solid fluorides of ions like Co(IV) and Ni(IV) are known to be aggressive oxidizers5. Analysis of the transition metal fluoride literature reveals that oxidation states of IV and higher could lead to unstable phases for late transition metals, particularly Co or Ni5. Unlike oxides, rather than reductively releasing F2, they have a strong propensity to act as highly oxidizing F- donors6,7, being able to oxidize even other halogen cations to their VII state. Yet the oxidation of Ni(II) features prominently in oxyfluoride cathodes that were recently discovered. This puzzle and the exact role of F in modulating the formal redox chemistry of a late metal like Ni and Co in the presence of O remains to be elucidated. To address this question and determine a periodic trend on the role of a mixture of anions in improving the energy density in such cathode materials, we conducted a deep dive into Li2CoO2F and Li2NiO2F. 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 XAS. These two oxyfluorides data were compared with the previous Li2MnO2F data to understand the effect of varying the transition metal and how that affects the overall electrochemistry of these materials. We also have estimated the M-O and M-F hybridization and provided a periodic trend. This study allowed us to define a rule to manipulate each element in a oxyfluoride that determines the electrochemical properties of these cathode materials.

References:

Bak, S. M.; Hu, E.; Zhou, Y.; Yu, X.; Senanayake, S. D.; Cho, S. J.; Kim, K. B.; Chung, K. Y.; Yang, X. Q.; Nam, K. W. ACS Appl. Mater. Interfaces 2014, 6 (24), 22594–22601.

Lee, J.; Kitchaev, D. A.; Kwon, D. H.; Lee, C. W.; Papp, J. K.; Liu, Y. S.; Lun, Z.; Clément, R. J.; Shi, T.; McCloskey, B. D.; Guo, J.; Balasubramanian, M.; Ceder, G. Nature 2018, 5567700 2018, 556 (7700), 185–190.

Roy, I.; Kumar, K.; Li, H.; Sunariwal, N.; Alexander, G.C.B.; Freeland, J.W.; rodolakis, F.; Cabana, J. Chem. Mater. 2023, 35, 5, 2107–2113.

Xu, X.; Pi, L.; Marie, J.-J.; Rees, G. J.; Gong, C.; Pu, S.; House, R. A.; Robertson, A. W.; Bruce, P. G. J. Electrochem. Soc. 2021, 168 (8), 080521.

Riedel, S.; Kaupp, M. Coord. Chem. Rev. 2009, 253 (5–6), 606–624.

Bartlett, N.; Chambers, R. D.; Roche, A. J.; Spink, R. C. H.; Chacón, L.; Whalen, J. M. Chem. Comm. 1996, No. 9, 1049–1050.

Lucier, G.; Shen, C.; Casteel, W. J.; Chacón, L.; Barlett, N. J. Fluor. Chem. 1995, 72 (2), 157–163.