Structural and Thermodynamic Understandings in Mn‐Based Sodium Layered Oxides during Anionic Redox

Abstract

A breakthrough utilizing an anionic redox reaction (O 2− /O n− ) for charge compensation has led to the development of high‐energy cathode materials in sodium‐ion batteries. However, its reaction results in a large voltage hysteresis due to the structural degradation arising from an oxygen loss. Herein, an interesting P2‐type Mn‐based compound exhibits a distinct two‐phase behavior preserving a high‐potential anionic redox (≈4.2 V vs Na + /Na) even during the subsequent cycling. Through a systematic series of experimental characterizations and theoretical calculations, the anionic redox reaction originating from O 2p‐electron and the reversible unmixing of Na‐rich and Na‐poor phases are confirmed in detail. In light of the combined study, a critical role of the anion‐redox‐induced two‐phase reaction in the positive‐negative point of view is demonstrated, suggesting a rational design principle considering the phase separation and lattice mismatch. Furthermore, these results provide an exciting approach for utilizing the high‐voltage feature in Mn‐based layered cathode materials that are charge‐compensated by an anionic redox reaction.

Recent Advances in Electrode Materials with Anion Redox Chemistry for Sodium-Ion Batteries

“…Their thermodynamic investigation demonstrated an energy state of oxygen located close to the Fermi level, which enabled oxidation of oxygen as sodium was extracted from the layered structure [17] proposed that lithium from the TM layers migrates toward the sodium layer when the octahedral environment is present at a highly desodiated state such as the O2 or OP4 phase. In this state, the density of states for O 2p was located at a higher energy state than that for Mn 4+ 3d [25]; however, the net charge of Mn did not vary during the extraction of sodium ions in the structure (Figure 2(c)). In addition, oxygen was not released from the oxide lattice as the oxidation of oxygen progressed (Figure 2(d)).…”

“…for which a third of the TM layers can be filled by mono-or divalent elements such as Li [17][18][19][20][21][22][23][24][25][26][27][28][29], Mg [46][47][48][49][50][51][52][53][54][55][56], and Zn [57][58][59] to induce an average oxidation state of Mn of 4+. Hence, the extraction of sodium ions in the structure is not theoretically possible because of the difficulty of the oxidation of Mn 4+ to higher valence states in an octahedral environment, in which the electrolyte does not decompose.…”

“…Sodium-deficient layered compounds, Na x [A y TM 1-y ]O 2 (A: Li [17][18][19][20][21][22][23][24][25][26][27][28][29], Mg [46][47][48][49][50][51][52][53][54][55][56], Zn [57][58][59][60], Ni [61][62][63][64][65][66], or Cu The prismatic environment for sodium ions enables maintenance of the large interlayer distances during de/sodiation, which is beneficial for facile diffusion of sodium ions and smooth phase transitions. However, in addition to the aforementioned merits, the sodium deficiency in the compounds results in small charge capacities (80-150 mAh g −1 ), thereby resulting in an abnormal CE in the first cycle that should be circumvented for their adoption in practical applications.…”

“…for which a third of the TM layers can be filled by mono-or divalent elements such as Li [17][18][19][20][21][22][23][24][25][26][27][28][29], Mg [46][47][48][49][50][51][52][53][54][55][56], and Zn [57][58][59] to induce an average oxidation state of Mn of 4+.…”

“…The selection of elements in transition metal layers is of great importance in improving the reversibility and controlling the operation voltage of the anionic redox reaction, namely, Na x [A y TM 1-y ]O 2 (A: Li [17][18][19][20][21][22][23][24][25][26][27][28][29], Na [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45], Mg [46][47][48][49][50][51][52][53][54][55][56], Zn [57][58][59][60], Ni [61][62][63][64][65][66]…”