(Invited) High-Energy Mn-Rich Disordered Rocksalt Cathodes
Authors: Ahn Juhyeon , Chen Guoying 
Published: November 2022

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.

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