In recent years, cation-disordered Li-excess rocksalts (DRX) have emerged as a promising new class of high-energy cathode materials for lithium-ion batteries. [1] Aside from the desirable Co-free chemistry, these compounds offer exceptionally large charge storage capacities by utilizing the redox reactions of both cationic transition-metals and anionic oxygen in the lattice. While early research focused on DRX oxides, which met with significant challenges in voltage stability and capacity retention upon cycling [2-3], recent studies shifted towards oxyfluorides with a substantial level of F substitution. It was found that incorporating F into the anionic sublattice can reduce oxygen gas release, impedance rise and capacity fade, consequently improving cathode cycling stability. [4-5] To this end, developing synthesis methods to incorporate large F content in the lattice as well as designing and optimizing oxyfluoride chemistry for both high energy density and cycling stability are imperative.

While high F substitution levels (up to 30-40 at.%) in DRX have been achieved through mechanochemical synthesis, the method has limitations in industrial application due to poor scalability. Solid-state synthesis, on the other hand, are readily scalable and often offers drop-in replacement in materials processing. In this presentation, we show our recent effort in developing calcination-based fluorination approach to achieve high-level fluorination of Mn-redox-active DRX materials. [6] The unique behavior of capacity rise upon cycling of a new class of Mn-rich DRX oxyfluoride cathodes will be reported. Our understanding in how chemistry can impact local and long-range structures and their evolution during electrochemical cycling will also be presented, as well as perspectives on future directions in DRX development.

References

1. Lee, J.; Urban, A.; Li, X.; Su, D.; Hautier, G.; Ceder, G. Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries. Science 2014, 343, 519.
2. Yabuuchi, N.; Takeuchi, M.; Nakayama, M.; Shiiba, H.; Ogawa, M.; Nakayama, K.; Ohta, T.; Endo, D.; Ozaki, T.; Inamasu, T.; Sato, K.; Komaba, S., High-Capacity Electrode Materials for Rechargeable Lithium Batteries: Li3NbO4-based System with Cation-Disordered Rocksalt Structure. Natl. Acad. Sci. 2015, 112, 7650.
3. Chen, D.; Kan, W. H.; Chen, G. Understanding Performance Degradation in Cation-Disordered Rock-Salt Oxide Cathodes. Energy Mater.2019, 9, 1901255.
4. Lee, J.; Papp, J. K.; Clément, R. J.; Sallis, S.; Kwon, D.-H.; Shi, T.; Yang, W.; McCloskey, B. D.; Ceder, G. Mitigating oxygen loss to improve the cycling performance of high capacity cation-disordered cathode materials. Commun. 2017, 8, 981.
5. Lun, Z.; Ouyang, B.; Kitchaev, D. A.; Clément, R. J.; Papp, J. K.; Balasubramanian, M.; Tian, Y.; Lei, T.; Shi, T.; McCloskey, B. D.; Lee, J.; Ceder, G. Improved Cycling Performance of Li-Excess Cation-Disordered Cathode Materials upon Fluorine Substitution. Energy Mater.2018, 9,1802959.
6. Ahn, J.; Chen, D.; Chen, G.. A Fluorination Method for Improving Cation-Disordered Rocksalt Cathode Performance. Energy Mater.2020, 10, 2001671.