Synthesis and Structure Stabilization of Disordered Rock Salt Mn/V-Based Oxyfluorides as Cathode Materials for Li-Ion Batteries

Abstract

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.

Realising higher capacity and stability for disordered rocksalt oxyfluoride cathode materials for Li ion batteries

“…The capacity of LMTOF@C at 0.5C was similar to ∼200 mA h g −1 achieved by a composition of Li 1.2 Mn 0.6 Ti 0.2 O 1.8 F 0.2 , but the latter used a significantly lower (dis)charge current of 30 mA g −1 (equivalent to ∼0.08C). 32 The capacity of LMTOF@C was also competitive with 210 mA h g −1 at 0.1C achieved by Li 2 Mn 0.5 V 0.5 O 2 F (LMVOF), 47 but again LMVOF was tested at 1/5th of the (dis)charge C rate compared with LMTOF@C. The comparisons show LMTOF@C exhibited faster Li + percolation kinetics due to the higher amount of Li + and electronegative F − . Electrochemical impedance spectroscopy (EIS) was used to further investigate the effects of C coating.…”

Over‐Stoichiometric Metastabilization of Cation‐Disordered Rock Salts

“…An important avenue for future research would be to conduct more exploration of over-stoichiometric chemical space of DRXs and explore the relationship between composition, structure, functionality, and mechanism in the over-stoichiometric chemical space. Fine-tuning the structure and properties of over-stoichiometric DRXs via various materials/particle engineering techniques [84,85] is also highly worth exploring. Findings of this work suggest a new controllable variable to tune the properties of DRXs and provide further promise for utilizing DRXs as a means to address outstanding challenges in LIB cathodes.…”

Unravelling the Chemical and Structural Evolution of Mn and Ti in Disordered Rocksalt Oxyfluoride Cathode Materials Using Operando X-ray Absorption Spectroscopy

“…Recently, vast research progress has been made in the field of Li-rich disordered rocksalt (DRS) materials aiming to exploit high power and energy density batteries with environment-friendly and nature-abundant metals, such as Mn, Ti, and others. − Manganese is an attractive redox-active transition metal for Li-rich DRS materials, as it allows multielectron transfer. In addition, manganese is an inexpensive and earth-abundant element which is economically desirable for manufacturing high-energy Li-ion batteries. , Compositions within the formula range Li 2 Mn 1– x Ti x O 2 F (0 ≤ x ≤ 2/3)LMTOF, which combine the high-valent cations substitution and the partial substitution of F – for O 2– anions, have been proposed as cobalt-free high-energy cathode materials for Li-ion batteries.…”