Anionic Redox Reactions in Manganese-Based Binary Layered Oxides for Advanced Sodium-Ion Batteries

Oxygen 2p-electron, unhybridized with transition metals (TMs), is a critical species for the generation of an anion-based redox reaction of O2–/O– for high-energy-density cathodes in lithium-ion and sodium-ion batteries (LIBs and SIBs, respectively). More importantly, oxygen redox activity has been highlighted as a breakthrough to increase the intrinsic low redox potential for SIBs because its reaction theoretically and experimentally occurs at ≈4.2 V versus Na+/Na. Here, we present in detail the thermodynamic, structural, and chemical origins of stabilized Ni2+ (redoxable) and Mn4+ (nonredoxable), determined by the different electronegativity values in Mn-based binary layered oxides, without excess monovalent element in the TM layer, for the rational use of an anion-based redox reaction. The Ni solubility into Na[Mn1–xNix]O2 has the highest value at x = 0.5 owing to the decrease in Mn3+ instability with the maximally stabilized Ni2+, which is understood by the thermodynamic mixing enthalpy value, M–O (M: Mn and Ni) and Na–O bonding lengths, and qualitative and quantitative electronic structure investigations. Utilizing the cumulative redox reaction, combined with the cationic and anionic species, the thermodynamically stabilized Na[Mn0.5Ni0.5]O2 is predicted to show a double redox reaction of Ni2+/Ni4+ and a subsequent anion-based redox reaction of O2–/On– combined with a partial Ni redox contribution, indicating a much higher redox potential behavior as compared with that of a single redox reaction of Mn3+/Mn4+ in NaMnO2. Our more concrete understanding of the thermodynamic, structural, and chemical origins, coupled with the stabilized Ni2+ (variant) and Mn4+ (invariant) species for redox reactions in the energetics in crystal field theory, is a critical factor in boosting the use of the Mn-based layered oxides and overcoming the limitations of low energy density for SIBs.