PNNL

Abstract

Lithium- and manganese-rich (LMR) layered oxides are promising high-energy cathodes for next-generation lithium-ion batteries yet their commercialization has been hindered by a number of performance issues, including capacity decay, voltage fade and impedance rise. Fluorination has been explored as a mitigating approach, however, results from past polycrystalline-particle based studies are inconsistent and the mechanism for performance improvement in some reports remains unclear. In the present study, we use well-defined single-crystal LMR as a platform and developed an in situ fluorination method that leads to fluorinated Li1.2Ni0.2Mn0.6O2 (F-LNMO) single-crystal samples with no apparent impurities. High fluorination levels lead to decreased oxygen activities, reduced side reactions at high voltages, and broadly improved cathode performance including higher initial coulombic efficiency, discharge energy and energy retention. Detailed characterizations using a range of advanced microscopy and spectroscopy techniques reveal particle-level Mn3+ concentration gradient from the surface to the bulk of F-LNMO crystals, corresponding to the formation of a Ni-rich LiNixMn2-xO4-yFy (x > 0.5) spinel phase on the surface and a “spinel-layered” coherent structure in the bulk where domains of a LiNi0.5Mn1.5O4 high-voltage spinel phase are integrated into the native layered framework. This work provides fundamental understanding of LMR fluorination and key insights for future development of high-energy Mn-based cathodes with an intergrown/composite crystal structure.