Peroxide Formation for Voltage Hysteresis in O2-Type Lithium-Rich Layered Oxides

Abstract

Development of energy storage systems with high energy density is of vital importance for realizing a sustainable society. Although lithium-ion batteries (LIBs) are the state-of-the-art energy storage technology, their energy density is limited in part by the specific capacity of the positive electrode (cathode) materials. For example, conventional cathode materials, i.e., layered transition-metal oxides LiMO2 (M = transition metal), deliver a modest capacity of approximately 160 mAh/g, where the dominant mechanism of charge compensation for lithium-ion (de)intercalation is the valence change of the transition metal. Further increase in the cathode capacity requires an additional redox center.Integrating an anionic-redox (or oxygen-redox) capacity with the conventional cationic-redox capacity is a promising strategy for large-capacity battery cathodes exceeding present technical limits. However, most oxygen-redox cathodes exhibit a large charge/discharge voltage hysteresis (> 0.5 V), resulting in poor energy efficiency and impractical implementation. Considering immediate electron transfer (O2– → O– + e –) against subsequent structural deformation (O–O dimerization), the overall hypothetical mechanism of the oxygen-redox reactions is described as a square scheme (Figure 1) 1: if an oxidized oxide ion (O–) is stable, it directly contributes to a non-polarizing discharge capacity (nonpolarizing O redox).2 Meanwhile, unstable O– dimerize to form stable peroxo-like O2 2–, which may be accelerated by cation migration. The O2 2– dimers provide a polarizing discharge capacity (polarizing O redox) and an unstable reduced dimer (e.g., O2 4–) decomposes to O2–. The O–O dimerization is prone to result in release of O2 gas (O2 evolution) by their excessive oxidation. However, experimental verification of the square scheme is limited in part due to complicated structural changes during the oxygen-redox reactions. For example, O3-type Li1.2Ni0.2Mn0.6O2 (O3: lithium ions occupy octahedral sites between the MO2 layers, and the packing arrangement of the oxide ions is ABCABC) exhibits irreversible structural degradation such as layered-to-spinel transformation and surface cation densification upon cycling.In this work, we focus on O2-type lithium-rich layered transition-metal oxides that possess structural integrity against the oxygen-redox reactions (O2: lithium ions occupy octahedral sites between the MO2 layers and the packing arrangement of the oxide ions is ABCBA). O2-type Li1.12–y Ni0.17Mn0.71O2 delivers a large reversible capacity greater than 200 mAh/g with minimal voltage decay and capacity fading upon cycling. Combination of X-ray absorption/emission spectroscopy, magnetic susceptibility measurements, and density functional theory calculations indicates bond-forming 2O– → O2 2– and bond-cleaving O2 4– → 2O2– processes. These results emphasize the importance of suppressing the formation of O2 2– and maximizing the contribution of the nonpolarizing O2–/O– redox couple to develop energy-efficient oxygen-redox battery electrodes. Furthermore, based on the electrochemical kinetic analysis, particularly for the 2O2– → O2 2– transformation, technical strategies to quantify non-polarizing and energy-efficient oxygen redox will be discussed.References M. Okubo, et al., Acc. Mater. Res. 2021, 3, 33–41.A. Tsuchimoto, et al., Nat. Commun. 2020, 12, 63.K. Kawai, et al., Energy Environ. Sci. 2022, 15, 2591–2600. Figure 1