Structural and Electrochemical Insights from the Fluorination of Disordered Mn-Based Rock Salt Cathode Materials

Recent studies have shown that disordered rock salt (DRS) oxyfluorides with Li excess are interesting candidates as cathode materials for Li-ion batteries. However, these materials have not been able to achieve the desired technological level yet owing to structure stability issues and the lack of direct evidence of the underlying Li+ (de)insertion mechanism. In this work, we demonstrate how fluorine can increase the stability of the DRS structure and improve the lithium diffusion in a percolation network concomitant with accommodated structural Mn oxidation. Therefore, we synthesized four representative Mn-based DRS materials in the Li1-xMn2/3Ti1/3O2Fx (0 <= x <= 1) chemical system to clarify the effect of fluorine on the structural and electrochemical properties. All of the compositions have achieved higher than 210 mAh g(-1) initial capacity and good cyclability, mainly for high F/Li ratios. The ex situ Raman spectroscopy analysis shed light on the lithium diffusion pathways inside the structure and on the effect of fluorine on the Li+ (de)insertion during cycling. Thanks to the F-enrichment, the amount of Li+ trapped as LiO4 tetrahedral sites can be reduced, allowing more efficient lithium transport. The structure stability is reinforced by its (MnO6)-O-II octahedral local ordering, resulting in better capacity retention. The study demonstrates the viability of the fluorination strategy toward developing cobalt-free cathode materials with enhanced performance.