Accelerating the Electrochemical Formation of the δ Phase in Manganese-Rich Rocksalt Cathodes

Recently, advancements in Mn-based disordered rocksalt (DRX) cathodes have demonstrated not only high energy density but also an electrochemical transformation to a spinel-like phase, now referred to as the δ phase 1,2,3 . The δ phase possesses a unique domain structure, a flattened voltage profile and improved rate capability. However, two important challenges remain: the necessity to mill the material to reduce particle size after synthesis, and the prolonged cycling required to complete the transformation to spinel-like partial order. To address these challenges, we present the development of an electrochemical formation method that enables rapid and in-situ formation of this partially disordered spinel phase from a DRX cathode, including in micron-sized single crystals. To gain a deeper understanding of this transformation process, we carry out a systematic study to determine whether the transformation to the δ phase can be accelerated through a precycling formation process. By employing this novel electrochemical approach to purposefully stimulate the transformation, we successfully reduce the time required from several weeks to as little as one day for Mn-rich DRX. Extended cycling and synchrotron X-ray diffraction confirm that these materials exhibit both equivalent performance and resulting structure to those obtained through conventional cycling over a much longer duration. Further characterization of this material using HAADF and 4-D STEM reveals the complex local structure of the spinel-like partial ordering. It is hoped that such a practical electrochemical formation method allows for an acceleration of research on these earth-abundant and energy-dense materials. Li, L.; Lun, Z.; Chen, D.; Yue, Y.; Tong, W.; Chen, G.; Ceder, G.; Wang, C. Fluorination-Enhanced Surface Stability of Cation-Disordered Rocksalt Cathodes for Li-Ion Batteries. Adv Funct Mater 2021, 31 (25), 2101888. https://doi.org/10.1002/adfm.202101888. Ahn, J.; Giovine, R.; Wu, V. C.; Koirala, K. P.; Wang, C.; Clément, R. J.; Chen, G. Ultrahigh-Capacity Rocksalt Cathodes Enabled by Cycling-Activated Structural Changes. Adv. Energy Mater. 2023. https://doi.org/10.1002/aenm.202300221. Cai, Z.; Ouyang, B.; Hau, H.-M.; Chen, T.; Giovine, R.; Koirala, K. P.; Li, L.; Ji, H.; Ha, Y.; Sun, Y.; Huang, J.; Chen, Y.; Wu, V.; Yang, W.; Wang, C.; Clément, R. J.; Lun, Z.; Ceder, G. In Situ Formed Partially Disordered Phases as Earth-Abundant Mn-Rich Cathode Materials. Nat. Energy 2023, 1–10. https://doi.org/10.1038/s41560-023-01375-9.