Fluorine-Mediated Molten-Salt Crystallization Regulates Phase Evolution and Structural Stability in Sodium Layered Oxides
Chenguang Shi, Junyi Yao, Xiaowen Zhan, Jeongok Choe, Xixian Yang, He Zhou, Amir Ashraf Elnashar, Rupayan Ghosh, Callum Connor, Anika Tabassum Promi, Weibo Huang, Hongyu Wang, Sooyeon Hwang, Kim Kisslinger, Si Athena Chen, Dali Yang, Lu Ma, Hui Zhong, Jianming Bai, Xianghui Xiao, Dennis Nordlund, David Reed, Xiaolin Li, Kejie Zhao, Feng LinAbstract
Ni- and Co-free P2-type Fe–Mn layered oxides with high Na concentrations are promising cathodes for sodium-ion batteries, but their practical use is limited by phase instability during Na-excess synthesis and structural degradation during cycling and storage. Although large single-crystal-like architectures could improve stability, controlled phase-selective crystallization in these cathode systems remains challenging. Here, we show that fluorine-assisted molten-salt synthesis enables controlled formation of large-particle Fe–Mn–Ti P2 cathodes with enhanced phase selectivity and structural coherence. Compared with Na-excess processing alone, NaF-assisted flux growth expands the P2 phase stability window and promotes large particles with uniform morphologies while preserving local layered ordering. Multiscale characterization further reveals more-reversible structural evolution and more homogeneous Mn redox behavior in the NaF-assisted material, together with a gradient-like fluorine distribution involving surface-enriched NaF-like species and lower level interior fluorine environments. These coupled structural, morphological, and chemical features collectively mitigate crack formation and deliver good cycling stability. In summary, NaF-assisted molten-salt synthesis shows promise in addressing key limitations of Ni- and Co-free P2-type Fe–Mn layered cathodes.