Recently, new types of cation disordered rocksalt (DRS) have been reported which show good reversibility. In our study we combined the strategy of using high-valent cations with partial substitution of fluorine for oxygen anions in disordered rocksalt-structure phase to achieve optimal Mn²⁺/Mn⁴⁺ double-redox reaction in the composition system Li₂Mn_xTi_1-xO₂F (1/3 ≤ x ≤ 1). we synthesized 4 different compositions (Li₂Mn^IIIO₂F, Li₂Mn^II_1/3Mn^III_1/3Ti^IV_1/3O₂F, Li₂Mn^II_1/2Ti^IV_1/2O₂F and Li₂Mn^II_1/3Ti^III_1/3Ti^IV_1/3O₂F). Two of them were synthesized for the first time, Li₂Mn^II_1/3Mn^III_1/3Ti^IV_1/3O₂F and Li₂Mn

^II_1/3Ti^III_1/3Ti^IV_1/3O₂F. By studying the electrochemical properties of different compounds we found that Ti⁺⁴ in the structure keeps Mn at the second state of charge, thus enabling a double redox reaction of Mn²⁺/Mn⁴⁺. By investigating the electrochemical properties of all samples we found that the sample with the composition Li₂Mn_2/3Ti_1/3O₂F showed the best electrochemical properties with initial high discharge capacity of 227 mAh g^-1 in the voltage window of 1.5-4.3 V and 82% of capacity retentionafter 100 cycles. However, fluorination might lead to several issues such as synthesis limitation, lithium diffusion issues due to preferable strong Li-F bonds, etc. thus, two more different samples based on the Li₂Mn_2/3Ti_1/3O₂F composition were synthesized and their properties were investigated (Li_1.5Mn^II_1/3Mn^III_1/3Ti^IV_1/3O₂F_0.5 and Li_1.25Mn^II_1/3Mn^III_1/3Ti^IV_1/3O₂F_0.25) in order to find the proper amount of fluorine in the structure which promises the electrochemical behavior. In the following the effect of fluorine on lithium diffusion was investigated by ex-situ Raman studies. These studies shed light on the diffusion pathways of lithium ions during charge and discharge process. The structural characteristics are examined using X-ray diffraction patterns, Rietveld refinement, energy-dispersive X-ray spectroscopy and scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The oxidation states and charge transfer mechanism are also studied further using extended X-ray absorption fine structure and X-ray photoelectron spectroscopy in which the results approve the double redox mechanism of Mn²⁺/Mn⁴⁺ in agreement with Mn-Ti structural charge compensation. The findings pave the way for designing high capacity electrode materials with multi-electron redox reactions.

References:
[1]: Chen, R.; Ren, S.; Knapp, M.; Wang, D.; Witter, R.; Fichtner, M.; Hahn, H., Disordered Lithium‐Rich Oxyfluoride as a Stable Host for Enhanced Li+ Intercalation Storage. Advanced Energy Materials 2015, 5, (9), 1401814.
[2]: Lee, J.; Kitchaev, D. A.; Kwon, D.-H.; Lee, C.-W.; Papp, J. K.; Liu, Y.-S.; Lun, Z.; Clément, R. J.; Shi, T.; McCloskey, B. D., Reversible Mn 2+/Mn 4+ double redox in lithium-excess cathode materials. Nature 2018, 556, (7700), 185-190.