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

Abstract

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.

Borate‐Based Surface Coating of Li‐Rich Mn‐Based Disordered Rocksalt Cathode Materials

“…The data for the uncoated sample in this table were taken from our previous work. [5] The surface morphologies of LMTOF samples before and after coating with B 2 O 3 were first analyzed by SEM, while EDX mapping was used to elucidate the distribution of elements (Figure S3, Supporting Information). In the uncoated sample, the fine particles with spherical morphology were agglomerated and formed secondary particles.…”

“…[4] So far, Mn-redox-based disordered materials have shown promising electrochemical performance but with considerable capacity fading during long-term cycling. [5] The capacity degradation is linked to structural instability, surface reconstruction associated with irreversible oxygen loss, and manganese dissolution into the carbonate-based electrolyte. [6] Especially, nanocrystalline DRS materials synthesized by ball-milling show a higher reactivity with electrolyte.…”

Influence of Chloride Ion Substitution on Lithium-Ion Conductivity and Electrochemical Stability in a Dual-Halogen Solid-State Electrolyte

“…The ZrO 2 (99%, Aldrich; 5 μm particle size) sample was similarly prepared under ambient conditions. Details of the sample preparation and data acquisition can be found elsewhere. , …”

Research Progress in Lithium‐Excess Disordered Rock‐Salt Oxides Cathode

“…Both the synthesized Li 2 Mn 2/ 3 Nb 1/3 O 2 F and Li 2 Mn 1/2 Ti 1/2 O 2 F exhibited higher Mn-redox capacities (Figure 6b) and lower proportion of O-redox as compared with the previous Mn 3+ -based DRXs during cycling. Then, Moghadam et al [66] synthesized four different ratios of Li 2 Mn 1Àx Ti x O 2 F (x = 0, 1/3, 1/2, 2/3) to reveal the influence of Ti content on electrochemical performance. The material exhibited the highest initial capacity when x = 1/ 2, while Li 2 Mn 2/3 Ti 1/3 O 2 F with x = 1/3 showed an initial capacity of 227 mAh/g and a capacity ~136 mAh/g after 200 cycles with the most superior cycling stability.…”