Controlled Oxygen Redox for Excellent Power Capability in Layered Sodium‐Based Compounds

Abstract

resources. [1][2][3][4][5] However, because of the slightly higher standard electrode potential of sodium (−2.7 V vs the standard hydrogen electrode (SHE)) than lithium (−3.02 V vs SHE), high-capacity electrode materials are needed for SIBs to compensate for their lower operation voltage and to achieve energy densities comparable to those of LIBs. It can be assumed that the cost of SIBs can be lowered relative to that of LIBs because of the cost-effectiveness of sodium resources. [6] These factors encourage us to explore potential highenergy-density cathode materials for SIBs.Among the various crystal structures of Na-containing compounds, layered structures have been intensively investigated because of their high capacity delivered under moderate operation conditions. The related layered structures are classified as O3, P2, P3, etc., according to the stacking sequence of the transition metals and alkali ions. [7][8][9] Layered sodium transition metal oxides of the form Na x MO 2 (M = Mn, Fe, Co, Ni, Cr, etc.) are stable in the P2 structure for Na contents (x) in the range of 0.3-0.7. This structure enables sodium ion diffusion between the two face-sharing trigonal prismatic sites such that P2-type layered materials generally exhibit higher discharge capacities than other layered materials. [10][11][12][13][14][15][16][17][18] Na 0.67 MnO 2 is one of the most common compounds among P2-type materials. [19][20][21][22][23] It delivers a high discharge capacity of over 175 mAh g −1 but exhibits severe capacity fade during cycling. One of the main reasons for this capacity fade is the structural disintegration caused by the Jahn-Teller effect of Mn 3+ ions in the MnO 6 octahedra, with elongation of the Mn 3+ -O distance along one direction. This Jahn-Teller effect can be mitigated by increasing the overall Mn oxidation state by substituting Mn with other elements, such as Ni, Co, Fe, Mg, and Al. [24][25][26][27][28][29] For example, improved cyclability was achieved in Mg-substituted Na 0.67 Mn 1−x Mg x O 2 (0.0 ≤ x ≤ 0.2) by suppressing the electrochemical activity of the Jahn-Teller Mn 3+ ions; however, this improvement was attained at the expense of the specific capacity. [26] Recently, Yabuuchi et al. [27] introduced a high-capacity P2-type Na 2/3 [Mg 0.28 Mn 0.72 ]O 2 material that exhibited capacities of over 150 mAh g −1 on charge and 220 mAh g −1 on discharge by raising the upper voltage cut-off to 4.6 V. Note that the average oxidation state of Mn in the compound is ≈3.85+, meaning that it cannot exhibit such a high charge capacity. The A high-rate of oxygen redox assisted by cobalt in layered sodium-based compounds is achieved. The rationally designed Na 0.6 [Mg 0.2 Mn 0.6 Co 0.2 ]O 2 exhibits outstanding electrode performance, delivering a discharge capacity of 214 mAh g −1 (26 mA g −1 ) with capacity retention of 87% after 100 cycles. High rate performance is also achieved at 7C (1.82 A g −1 ) with a capacity of 107 mAh g −1 . Surprisingly, the Na 0.6 [Mg 0.2 Mn 0.6 Co 0.2 ]O 2 compound is able to deliver...

Probing the charged state of layered positive electrodes in sodium-ion batteries: reaction pathways, stability and opportunities

“…107 When charged up to 5 V loss of oxygen can be detected from P2-Na 0. 78 110 More recently, increasing the covalency of the TM-O bond has been shown to be effective in suppressing oxygen release, hence improving electrochemical performance. 86 The TM-O distance and TM-O bonding energy have also been identied as important factors in determining the activity and reversibility of the oxygen redox reaction.…”

“…112 Through comparison of X-ray photoelectron spectroscopy (XPS), operando Raman spectroscopy and electrochemical data, Jia et al found that compared to the sodium decient O3-Na 0. 6 110 DFT calculations suggest that the overlapping Co 3d and O 2p orbitals as well as the reduced bandgap of z 0.61 eV (cf. z 2.65 eV for the analogous Co free composition) upon incorporation of Co facilitates facile electron transfer, enabling long term oxygen redox reversibility.…”

High‐Voltage Oxygen‐Redox‐Based Cathode for Rechargeable Sodium‐Ion Batteries

“…Recently, we suggested the occurrence of an oxygen redox reaction in P2‐Na 2/3 [Mn 0.7 Zn 0.3 ]O 2 , for which the combination of O 2−/1− and Mn 4+/3+ contributed to rechargeable capacity. [ 23 ] Kim et al [ 24 ] observed the oxygen redox in P2‐Na 2/3 [Mg 0.2 Mn 0.6 Co 0.2 ]O 2 , which enabled long‐term cycling assisted by the presence of CoO bonds, facilitating electron transfer with the overlapping of orbitals between O 2− 2p and Co 3+/4+ 3d ( t 2g ). Despite these successes of the oxygen redox reaction toward contributing additional capacity in P2‐layered compounds, the main shortcoming is that the resulting operation voltage remains still low because the main redox species of Mn 4+/3+ is achieved in the voltage range of 1.5–2.7 V versus Na + /Na.…”

Stepwise Hollow Prussian Blue Nanoframes/Carbon Nanotubes Composite Film as Ultrahigh Rate Sodium Ion Cathode

“…In order to meet the greatly increased energy consumption demand, the development of sodium ion batteries (SIBs) technologies with low cost, long life, and high electrochemical performance are imminent. [ 1–5 ] In the last decade, researchers have explored many cathode materials used in SIBs, [ 6–8 ] including sodium superionic conductor materials, [ 9–11 ] sodium‐based layered oxides, [ 12–15 ] fluorophosphates, [ 16–18 ] and organic materials. [ 19 ] However, the restricted space of layered oxide for sodium ions diffusing due to the close‐packed oxygen ion array connected to transition metal elements severely hinders the improvement of SIBs performance.…”