Anionic Redox Activity as a Key Factor in the Performance Degradation of NaFeO₂ Cathodes for Sodium Ion Batteries

Abstract

The origin of the irreversible capacity of O3type NaFeO 2 charged to high voltage is investigated by analyzing the oxidation state of Fe and phase transition of layered NaFeO 2 cathodes for sodium-ion batteries during the charging process. In-situ X-ray absorption spectroscopy results revealed that charge compensation does not occur through the Fe 3+ /Fe 4+ redox reaction during sodium extraction as no significant shift to high energy was observed in the Fe K-edge. These results were reinforced with ex-situ near-edge X-ray absorption spectroscopy, which suggests that oxygen redox activity is responsible for charge compensation. Formation of Fe 3 O 4 product occurs because of oxygen release at high voltage when more than 0.5 Na is extracted from the structure; this is observed by transmission electron microscopy. NaFeO 2 irreversibility is due to the formation of Fe 3 O 4 with oxygen release, which inhibits Na insertion into the structure.

Structural Stabilization of P2‐type Sodium Iron Manganese Oxides by Electrochemically Inactive Mg Substitution: Insights of Redox Behavior and Voltage Decay

“…Considering the gradual and continuous oxidation of Fe is observed in the half-height energy plot ( Figure S9), this pre-edge peak intensity change mostly at the high voltage is likely due to the structural distortion related to the local structural changes associated with high-spin Fe 4 + . [16] As shown in Figure S9, similar to the Mn case, the oxidation state of Fe for the pristine state is also increased after Mg 2 + substitution. The overall trend for Fe redox does not differ in both NMFO and Mg-NMFO, but it reveals that the redox range of Fe is smaller than that of Mn where charge compensation is more dominant at the low voltage region.…”

“…[15] Even though the Fe 3 + /Fe 4 + redox reaction is not well established compared to the Fe 2 + / Fe 3 + redox reaction, it provides the potential for developing improved electrode materials with high energy density. [16] However, its main disadvantage has been identified as poor capacity retention due to structural change at a high voltage above 4.0 V (vs. Na/Na + , hereafter). [16] The structural change is generally understood to be a result of the gliding of the transition-metal oxide slabs, resulting in a rearrangement of the Na sites from prismatic to octahedral and a severe reduction of the unit cell along the c-axis.…”

“…[16] However, its main disadvantage has been identified as poor capacity retention due to structural change at a high voltage above 4.0 V (vs. Na/Na + , hereafter). [16] The structural change is generally understood to be a result of the gliding of the transition-metal oxide slabs, resulting in a rearrangement of the Na sites from prismatic to octahedral and a severe reduction of the unit cell along the c-axis. Yabuuchi et al described the highvoltage phase as an OP4-type structure (O-and P-type mixed stacking layer).…”

Deciphering the oxygen absorption pre-edge: a caveat on its application for probing oxygen redox reactions in batteries

“…As a result, O-K sXAS pre-edge evolution has been used overwhelmingly in a large number of publications as the evidence of the so-called oxygen redox states in battery electrodes based on TM oxides. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] On the contrary, as we pointed out directly in a recent review, 6 The importance of this debate stems from the fact that the oxygen activities in electrochemical systems hold both sides of the dilemma of stability and performance, 32 which are critical topics for materials across a wide range of energy applications, including batteries and catalytic systems. 33 Therefore, many fervent research topics on oxygen activities are in dire need of a correct interpretation of the O-K sXAS pre-edge.…”

Deciphering the Oxygen Absorption Pre-Edge: Universal Map of Transition Metal Redox Potentials in Batteries

“…As a result, O-K sXAS pre-edge evolution has been used overwhelmingly in a large number of publications as the evidence of the so-called oxygen redox states in battery electrodes based on TM oxides. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] On the contrary, as we pointed out directly in a recent review, 6 the O-K sXAS pre-edge displays clear variations in both the intensity and lineshape in systems like olivine LiFePO 4 and spinel LiNiMnO 4 , 6,27,28 which do not involve oxygen redox reactions at all. Although oxidized oxygen feature does show up in the same pre-edge energy range in model systems, 29?…”