Revealing the Origin of Transition-Metal Migration in Layered Sodium-Ion Battery Cathodes: Random Na Extraction and Na-Free Layer Formation.

Cation migration often occurs in layered oxide cathodes of lithium-ion batteries due to the similar ion radius of Li and transition metals (TMs). Although Na and TM show a big difference of ion radius, TMs in layered cathodes of sodium-ion batteries (SIBs) can still migrate to Na layer, leading to serious electrochemical degeneration. To elucidate the origin of TM migration in layered SIB cathodes, we choose NaCrO2 , a typical layered cathode suffering from serious TM migration, as a model material and find that the TM migration is derived from the random desodiation and subsequent formation of Na-free layer at high charge potential. A Ru/Ti co-doping strategy is developed to address the issue, where the doped active Ru is first oxidized to create a selective desodiation and the doped inactive Ti can function as a pillar to avoid complete desodiation in Ru-contained TM layers, leading to the suppression of the Na-free layer formation and subsequent enhanced electrochemical performance.

Intrinsic Origin of Nonhysteretic Oxygen Capacity in Conventional Na-Excess Layered Oxides.

An intriguing redox chemistry via oxygen has emerged to achieve high-energy-density cathodes and has been intensively studied for practical use of anion-utilization oxides in A-ion batteries (A: Li or Na). However, in general, the oxygen redox disappears in the subsequent discharge with a large voltage hysteresis after the first charge process for A-excess layered oxides exhibiting anion redox. Unlike these hysteretic oxygen redox cathodes, the two Na-excess oxide models Na2IrO3 and Na2RuO3 unambiguously exhibit nonhysteretic oxygen capacities during the first cycle, with honeycomb-ordered superstructures. In this regard, the reaction mechanism in the two cathode models is elucidated to determine the origin of nonhysteretic oxygen capacities using first-principles calculations. First, the vacancy formation energies show that the thermodynamic instability in Na2IrO3 increases at a lower rate than that in Na2RuO3 upon charging. Second, considering that the strains of Ir-O and Ru-O bonding lengths are softened after the single-cation redox of Ru4+/Ru5+ and Ir4+/Ir5+, the contribution in the oxygen redox from x = 0.5 to 0.75 is larger in Na1-xRu0.5O1.5 than that in Na1-xIr0.5O1.5. Third, the charge variations indicate a dominant cation redox activity via Ir(5d)-O(2p) for x above 0.5 in Na1-xIr0.5O1.5. Its redox participation occurred with the oxygen redox, opposite to the behavior in Na1-xRu0.5O1.5. These three considerations imply that the chemical weakness of Ir(5d)-O(2p) leads to a more redox-active environment of Ir ions and reduces the oxygen redox activity, which triggers the nonhysteretic oxygen capacity during (de)intercalation. This provides a comprehensive guideline for design of reversible oxygen redox capacities in oxide cathodes for advanced A-ion batteries.

Uncovering the Structural Evolution in Na-Excess Layered Cathodes for Rational Use of an Anionic Redox Reaction.

A new paradigm based on an anionic O2-/On- redox reaction has been highlighted in high-energy density cathode materials for sodium-ion batteries, achieving a high voltage (∼4.2 V vs Na/Na+) with a large anionic capacity during the first charge process. The structural variations during (de)intercalation are closely correlated with stable cyclability. To determine the rational range of the anion-based redox reaction, the structural origins of Na1-xRu0.5O1.5 (0 ≤ x ≤ 1.0) were deduced from its vacancy (□)/Na atomic configurations, which trigger different interactions between the cations and anions. In the cation-based Ru4+/Ru5+ redox reaction, the □ solubility into fully sodiated Na2RuO3 predominantly depends on the crystallographic 4h site when 0.0 ≤ x ≤ 0.25, and the electrostatic repulsion of the linear O2--□-O2- configuration is accompanied by the increased volumetric strain. Further Na extraction (0.25 ≤ x ≤ 0.5) induces a compensation effect, leading to Na2/3[Na□Ru2/3]O2 with the □ formation of 2b and 2c sites, which drastically reduce the volumetric strain. In the O2-/On- anionic redox region (0.5 ≤ x ≤ 0.75), Na removal at the 4h site generates a repulsive force in O2--□-O2- that increases the interlayer distance. Finally, in the 0.75 ≤ x ≤ 1.0 region, the anionic O charges are unprotected by repulsive forces, and their consumption causes severe volumetric strain in Na1-xRu0.5O1.5. Coupling our mechanistic understanding of the structural origin with the □- and Na-site preferences and the electrostatic interaction between lattice O and vacancies in Na1-xRu0.5O1.5, we determined the rational range of the anionic redox reaction in layered cathode materials for rechargeable battery research.