Unified Picture of (Non)Hysteretic Oxygen Capacity in O3‐Type Sodium 3d Layered Oxides

Abstract

A unified picture of the generalized mechanism that elucidates the (non)hysteretic oxygen capacities of O3‐type Na1−x[Li2/6Mn3/6M1/6]O2 (M = Mn4+, Ni4+, and Ti4+) layered oxides is suggested to provide a critical factor in inducing ideal reversibility for an oxygen redox (OR) reaction using the new concept, the “potential‐pillar” effect. Considering that there is no formation of interlayer oxygen–oxygen (OO) dimers at x = 1.0 in Na1−x[Li2/6Mn3/6Ti1/6]O2, the phase stabilities reveal that the biphasic reaction occurs in Na1−x[Li2/6Mn3/6Ni1/6]O2 (0.5 ≤ x ≤ 1.0), and the monophasic reaction takes place in Na1−x[Li2/6Mn3/6Ti1/6]O2 during desodiation. The electronic structures of cations and anions unambiguously show OR reactions over the full vacancy range, and the oxygen ions comprising TiO6 are relatively deactivated compared with those of NiO6 upon electrochemical OR reaction. This is deemed as an intriguing “potential‐pillar” effect, in which the chemically stiff Ti4+(3d)O(2p) bond locally retains a strong electrostatic repulsion between the mixed layers. This unified concept based on an in‐depth understanding of the three cathode models not only accounts for the origin of the (non)hysteretic oxygen capacities, but also provides an exciting local structure viewpoint for harnessing the full potential of OR reactions for high energy‐density sodium‐ion batteries.

Strong Anionic Repulsion for Fast Na Kinetics in P2‐Type Layered Oxides

“…This hypothesis is denoted as the “potential‐pillar” effect, which was proposed in our systematic computational work. [ 35 ] In detail, it conceptually combines the “cationic repulsion” and “anionic repulsion” effects, and this interesting concept can be an exciting design parameter for triggering fast Na kinetics for high‐power‐density in SIBs, especially OR‐related binary Mn—Ni layered oxide cathodes.…”

Correlating concerted cations with oxygen redox in rechargeable batteries

“…105,106 Therefore, the rigid structure of the nonbonding O-TM state and no electrons available in the valence band in the electron configuration can serve as a ''potentialpillar'' to stabilize structures. 107,108 Ti 4+ cation tends to effectively maintain the electrostatic repulsion of the lattice O 2À in Fig. 9 (a) Index of several common types of transition metals in cathodes through the periodic table of elements.…”

“…the M-layer due to the nonreactive nature during the voltage window, [108][109][110] leading to the stabilization of oxygen redox. With the rise of inactive Ti 4+ (not available for charge compensation), Li 1.2 Ti 0.6 Mn 0.2 O 2 instead of Li 1.2 Ti 0.4 Mn 0.4 O 2 exhibits higher capacity and better cycle performance due to the strengthened networks of strong electrostatic repulsion.…”

Data‐Driven Decoupling Structural Feature Correlation for Harnessing Anionic Capacity in Na Layered Oxide

“…Interestingly, interlayer M migration into the Na layer rarely occurs during desodiation because of their different environments, as compared with those for LIBs. [12,[24][25][26] In contrast to this mechanism, O3-type Li 2 IrO 3 is regarded as an ideal cathode model featuring a nonhysteretic and reversible (nHR) redox capacity without 5d cation migration upon cycling. [27,28] Therefore, the underlying reaction mechanism was studied intensively to unlock the origins in correlation with the electrochemically reversible features.…”