Cation-disordered rocksalt (DRS) materials have shown good initial reversibility and facile Li+ insertion and extraction in the structure at high rates. However, all of the Li-rich oxyfluorides introduced so far suffer from short cycle lifetimes and severe capacity fading. In the current study, we combine the strategy of using high-valent cations with partial substitution of oxygen anions by fluorine ions to achieve the optimal Mn4+/Mn2+ double redox reaction in the composition system Li2Mn1–x Ti x O2F (0 ≤ x ≤ 2/3). While Ti-rich compositions correlate to an O-oxidation plateau and a partial Mn3+–Mn4+ redox process at high voltages, owing to the presence of Ti3+ in the structure, a new composition Li2Mn2/3Ti1/3O2F with a lower amount of Ti shows better electrochemical performance with an initial high discharge capacity of 227 mAh g–1 (1.5–4.3 V window) and a Coulombic efficiency of 82% after 200 cycles with a capacity of 136 mAh g–1 (>462 Wh kg–1). The structural characteristics, oxidation states, and charge-transfer mechanism have been examined as a function of composition and state of charge. The results indicate a double redox mechanism of Mn4+/Mn2+ in agreement with Mn–Ti structural charge compensation. The findings point to a way for designing high-capacity DRS materials with multi-electron redox reactions.

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+x Mn2/3Ti1/3O2F x (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 MnIIO6 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.

The existing classes of Na-based cathode materials and their chemistries are still limited, mainly with respect to the increasing demand for alternative post-Li technologies. In this letter, a newly synthesized Na-rich disordered rock salt (DRS) oxyfluoride with the nominal composition Na2MnO2F is reported as a cathode candidate for Na-ion batteries (SIBs). Rietveld refinement analysis confirmed that the synthesized compound has a DRS structure with larger lattice compared to Li-rich homologues. During the first cycle, up to 1.7 Na+/f.u. can be extracted at a slow rate, while a better capacity retention and cycling stability are obtained at high rate, reminiscent of electrode–electrolyte interaction. Further, X-ray absorption fine structure (operando and ex situ) confirmed the Mn oxidation state evolution upon cycling in agreement with the cyclic voltammetry redox profile, emphasizing the reversible Na+ (de)insertion and change of the Mn local ordering. This work is an additional input to the limited series of cathode candidates for SIBs.

The demand for high-performance lithium-ion batteries and thus efficient cathode materials is steadily increasing. In addition to a high energy density and long lifetime, these should also be cost-effective and environmentally benign. Manganesebased materials have particular potential because manganese is available in sufficient quantities and can be supplied at a comparatively low cost. Hence, in this study, manganese-based disordered rock salt oxyfluorides Li 2 Mn 1−x V x O 2 F (0 ≤ x ≤ 0.5) are synthesized as cathode materials for lithium-ion batteries using high-energy mechanochemical ball-milling. The effect of partial vanadium substitution on the sample properties is analyzed, focusing on the electrochemical properties. Furthermore, a heat treatment process for stabilization of the samples is followed, where the morphology and structure of the samples are studied by powder X-ray diffraction and electron microscopy (SEM/TEM). The oxidation states of the transition metals in the synthesized compositions are further investigated using X-ray absorption nearedge structure spectroscopy. The data analysis reveals that the heat treatment resulted in increased symmetry and reduced defects of ball-milled compounds, but it may also affect the local fluorination degree in the structure. However, the results show that this treatment process has a beneficial effect on capacity retention of the formulated electrodes (∼81% after 100 cycles), a faster response to the change of cycling rate, and less increase in charge-transfer resistance of the samples during cycling. Such a structural improvement attributed to mitigation of the surface/bulk defects is an additional input to the series of cathode candidates of low temperature stability.

Manganese based disordered rocksalt systems have attracted attention as Co-free and high capacity cathode materials for Li-ion batteries. However, for a practical application these materials are considered as metastable and exhibit too limited cyclability. In order to improve the structural stability of the disordered rocksalt Li1+xMn2/3Ti1/3O2Fx (0 ≤ x ≤ 1) system during cycling, we have introduced a mild temperature heat treatment process under reducing atmosphere, which is intended to overcome the structural anomalies formed during the mechanochemical synthesis. The heat-treated samples presented better electrochemical properties, which are ascribed to a structural defect mitigation process both at the surface and in the bulk, resulting in improved crystal structure stability. In addition, the optimized particle size and the smaller BET surface area induced by the recrystallization contributes to the observed enhanced performance. Among the studied compositions, the heat treated Li2Mn2/3Ti1/3O2F sample displayed better electrochemical performance with a discharge capacity of 165 mAh g−1 after 100 cycles at 0.1 C (∼80% of the initial capacity), when combined with further conditioning of the cells. The results point explicitly towards a guided stabilization approach, which could have a beneficial effect regarding the application of DRS oxyfluoride materials for sustainable LIBs.