Enabling Stable and Nonhysteretic Oxygen Redox Capacity in Li‐Excess Na Layered Oxides

Abstract

high-energy-density cathodes has been assigned to anion redox reaction via oxygen ions beyond the cationic redox reactions for lithium-and sodium-ion batteries (LIBs and SIBs). [1][2][3] Unfortunately, the oxygen redox reaction generally makes the oxide framework very unstable with phase transitions, resulting in various electrochemical drawbacks such as a very large voltage decay and irreversible capacity for most Li-and Na-based oxygen redox cathodes. [4][5][6] For LIBs, anomalous charge capacities delivered by oxygen redox reactions at 4.5 V (vs Li + /Li) were observed in Li 2 MnO 3 or Li 2 MnO 3 -containing layered oxides upon charging. [7][8][9] However, totally varied discharge curves were observed, which was induced by Mnmigration from the M layer to the A layer in the cathodes. [10,11] For SIBs, Na[Li 1/3 Mn 2/3 ]O 2 exhibiting the oxygen redox reaction was theoretically designed by our group based on the inspiration that redox-inactive Mn 4+ in an octahedral complex is very difficult to be further oxidized to Mn 5+ for compensating charge imbalance induced by Na-extraction in Na[Li 1/3 Mn 2/3 ]O 2 . [12,13] However, the charge voltage curve did not maintain upon discharging, indicating the severe electrochemical polarization inThe demands for higher energy density of rechargeable batteries have been continuously increasing recently, and cationic redox based current cathodes have little scope to further increase energy density since they already exhibit near-theoretical specific capacities. In this regard, oxygen redox (OR) reactions have emerged as a promising breakthrough for sodium-ion battery (SIB) cathodes. Most OR-based layered oxides suffer from drastic hystereticoxygen capacities upon discharging after the first charging. In contrast, stable and nonhysteretic oxygen capacities are herein enabled via Al 3+ incorporation into Li-excess Na layered oxide (NLMO). By combining experimental work and first-principles calculations, it is found that there is an additional stable phase during the oxygen redox for Al incorporated NLMO in comparison with bare NLMO, which is a critical factor in extending and stabilizing the discharge capacity in thermodynamics. In addition, the additional redoxinactive Al 3+ leads to heterogeneous oxygen redox rather than homogeneous, which results in stabilization of the oxide framework with sensitively control of the oxygen participation upon cycling.

Physicochemical Screen Effect of Li Ions in Oxygen Redox Cathodes for Advanced Sodium-Ion Batteries

“…Mn migration into the Li layer is a critical factor in triggering electrochemical degradation after activating the OR reaction; it involves a phase transition to the spinel-type structure. − Several experimental and computational methods have been explored to suppress undesired Mn migration upon charging to enable the use of OR-based Li-excess layered oxides. − Although various modifications were tested, such as doping, coating, and incorporating NMC, to generate nonhysteretic charge–discharge curves in the Li-excess Mn oxide, it continued to pose a challenge. − Therefore, unlike the abovementioned 3 d OR-layered Mn oxides for LIBs, several 4d and 5d Li-excess layered oxides (Li 2 RuO 3 and Li 2 IrO 3 ) were investigated to understand the fundamental mechanism of OR reactions without Ru and Ir migration into the Li layer upon charging and discharging. − Unlike LIBs, a comparison based on the Li-to-Mn ratio (R) modulated two-oxide models exhibiting a P2-type structure for SIBs, Na 0.75 [Li 0.25 Mn 0.75 ]­O 2 and Na 0.6 [Li 0.2 Mn 0.8 ]­O 2 ; the latter 3 d cathode featured a nonhysteretic oxygen capacity at ∼4.2 V vs. Na + /Na without the Mn migration into the Na layer during the first cycle . The reversible OR potential was characterized by a distinct biphasic reaction in the thermodynamic perspective, and its reaction could be stabilized by the heterogeneous OR mechanism . Given the importance of superstructure control, it was revealed in Li-excess 3 d Na oxides with R = 4 that the Li ions in the [Li 0.2 Mn 0.8 ]­O 2 layer in the pristine state returned to their initial positions upon discharging after activating the first desodiation.…”

“…39 The reversible OR potential was characterized by a distinct biphasic reaction in the thermodynamic perspective, and its reaction could be stabilized by the heterogeneous OR mechanism. 40 Given the importance of superstructure control, it was revealed in Li-excess 3d Na oxides with R = 4 that the Li ions in the [Li 0.2 Mn 0.8 ]O 2 layer in the pristine state returned to their initial positions upon discharging after activating the first desodiation.…”

Vacancy-enhanced oxygen redox and structural stability of spinel Li2Mn3O7−x

“…2 Oxygen redox has been reported in the Na-deficient Mn-based layered oxide cathode materials for the sodium-ion batteries (SIBs) and results in specific capacities up to >200 mA h g −1 . 3–5…”

An interactive design for sustainable oxygen capacity in alkali-ion batteries

“…That is, the O3–O1′ reversible stacking transition by layer gliding is expected to be a thermodynamic reaction mechanism, similar to the P2–O2 stacking sequence transition in Li-excess Na oxide with respect to the reversible layer gliding upon oxygen oxidation. 26,30 Fig. 1d shows that the stacking type in the Na-rich phase is a P2-type layered structure, whereas that in the Na-poor phase is an O2-type structure.…”

“…27 Along these lines, this conclusion strongly motivated a comparison of Li 1− x Mn 1/2 O 11/8 F 1/8 for LIBs versus Li-excess Na oxides for SIBs in mimicking the latter model, which showed nHR oxygen capacities upon discharge after the first charge process. 26,30…”