Towards Better Stability and Reversibility of Mn²⁺/Mn⁴⁺ Double Redox Activity in Disordered Rocksalt Oxyfluoride Cathode Materials

Recently discovered cation disordered rocksalt (DRS) materials have shown high reversibility towards Li ions due to facile insertion and extraction in the structure. Nonetheless, issues with manganese containing compounds have been identified due to instabilities of the structure and/or solubility of manganese in the electrolyte. In our study, we combined the use of high-valent cations with partial substitution of fluorine for oxygen anions in DRS-structure phase to achieve optimal Mn²⁺/Mn⁴⁺ double-redox reaction in the composition system Li₂Mn_xTi_1-xO₂F (1/3 ≤ x ≤ 1). Four 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) have been synthesized and optimized according to the starting precursors and their oxidation states. By studying the electrochemical behavior of different compounds, we found that Ti⁺⁴ keeps Mn at the second state of charge in the structure which lead to double redox reaction of Mn²⁺/Mn⁴⁺. Galvanostatic cycling and rate capability tests of the sample with the composition Li₂Mn_2/3Ti_1/3O₂F showed notable electrochemical kinetics and cycling performance, with initial high discharge capacity of 227 mAh g^-1 in the voltage window of 1.5-4.3 V and 82% coulombic efficiency after 100 cycles.In a further study, synthesis related issues and limitations due to fluorination were examined. In fact, lithium diffusion can be impeded due to preferable strong Li-F bonds. Thus, two more 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 which compromises the structural/chemical stability and electrochemical storage against Li-ions.We will present and discuss the structural characteristics of these selected DRS materials using XRD-Rietveld refinement analysis, energy-dispersive X-ray spectroscopy and scanning electron microscopy. The oxidation states and charge transfer mechanism using a combination of CV and XPS will be also addressed, 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.

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