Anion–Cation Synergetic Contribution to High Capacity, Structurally Stable Cathode Materials for Sodium‐Ion Batteries

Abstract

Layered transition metal oxides as promising cathode materials for sodiumion batteries have been extensively studied to obtain superior electrochemical properties. Since the cationic redox materials have almost reached the theoretical capacity limits accompanied by the migration and disproportionation of transition metals, anionic redox counterparts have been extensively explored to obtain extra capacity. In this work, P2-type Na 0.67 [Li 0.21 Mn 0.59 Ti 0.2 ] O 2 is introduced, where manganese and oxygen synergistically undergo redox reaction reversibly. In situ X-ray diffraction (XRD) experiments indicate a highly stable lattice structure with an extremely small volume strain of 0.7% during cycles with no sign of phase transitions. The stable crystal structure demonstrates the suppression of manganese disproportionation which is common in the layered Mn-based cathode materials. Thanks to both cationic and anionic redox, this material can deliver a reversible capacity of 231 mA g −1 in the voltage range of 1.5-4.5 V and the high-voltage plateau can be maintained during subsequent cycles with splendid cycling stability.

Anionic Redox Reactions in Cathodes for Sodium‐Ion Batteries

“…Inspired by the fundamental reaction mechanism of Li[Li 1/3 Mn 2/3 ]O 2 and considering that the standard redox potential of Na + /Na is lower than that of Li + /Li (by ∼0.3 V), numerous Na‐based cathode materials exhibiting the oxygen redox reaction have been discovered (both experimentally and computationally) for SIBs [11–16] . The oxygen redox paradigm could be a crucial factor in overcoming the intrinsic energy‐density limits of SIBs and, to a lesser extent, LIBs because pure oxygen redox takes place at a high redox potential of 4.2 V versus Na + /Na and has a more reversible character after the first charge process [3,17,18] . Although oxygen‐redox‐active materials show promising features, the redox activity results in structural instability, resulting in a large irreversible capacity and voltage fading after the first charge process [19–22] .…”

Transition‐Metal Vacancy Manufacturing and Sodium‐Site Doping Enable a High‐Performance Layered Oxide Cathode through Cationic and Anionic Redox Chemistry

“…At the end of charge, the (002) diffraction peak obviously weakens and broadens accompanied by the appearance of a new weak peak located at ≈17.5°, suggesting the coexistence of P2 and OP4 phases. [28,41,42] During the discharge process, the small peak at ≈17.5° gradually disappears and the (002) peak becomes sharp and strong again. Then, all the diffraction peaks can be assigned to the initial P2 phase and a reversible peak shifting process is observed, unraveling the excellent structural reversibility of P2-Na 0.76 Ca 0.05 [Ni 0.23 □ 0.08 Mn 0.69 ]O 2 upon sodiation/desodiation.…”

Unraveling Anionic Redox for Sodium Layered Oxide Cathodes: Breakthroughs and Perspectives

“…[ 69 ] Li incorporation provides a valuable strategy for reversible anionic oxygen redox and the remission of cationic migration, phase transition, voltage fading, and voltage hysteresis, facilitating electrochemical performance. [ 185,187 ]…”