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Air-stability is one of the most important considerations for the practical application of electrode materials in energy-harvesting/storage devices, ranging from solar cells to rechargeable batteries. The promising P2-layered sodium transition metal oxides (P2-NaxTmO2) often suffer from structural/chemical transformations when contacted with moist air. However, these elaborate transitions and the evaluation rules towards air-stable P2-NaxTmO2 have not yet been clearly elucidated. Herein, taking P2-Na0.67MnO2 and P2-Na0.67Ni0.33Mn0.67O2 as key examples, we unveil the comprehensive structural/chemical degradation mechanisms of P2-NaxTmO2 in different ambient atmospheres by using various microscopic/spectroscopic characterizations and first-principle calculations. The extent of bulk structural/chemical transformation of P2-NaxTmO2 is determined by the amount of extracted Na+, which is mainly compensated by Na+/H+ exchange. By expanding our study to a series of Mn-based oxides, we reveal that the air-stability of P2-NaxTmO2 is highly related to their oxidation features in the first charge process and further propose a practical evaluating rule associated with redox couples for air-stable NaxTmO2 cathodes., Air-stability is a critical challenge faced by layered sodium transition metal oxide cathodes. Here, the authors depict a general and in-depth model of the structural/chemical evolution of P2-type layered oxides in air and propose an evaluation rule for the air-stability of layered sodium cathodes.

One of the key challenges of sodium ion batteries is to develop sustainable, low-cost and high capacity cathodes, and this is the reason that layered sodium manganese oxides have attracted so much attention. However, the undesired phase transitions and poor electrolyte-electrode interfacial stability facilitate their capacity decay and limit their practical applications. Herein, we design a novel Al2O3@Na0.67Zn0.1Mn0.9O2 electrode to mitigate these problems, by taking the advantages of both structural stabilization and surface passivation via Zn2+ substitution and Al2O3 atomic layered deposition (ALD) coating, respectively. Long-range and local structural analyses during charging/discharging processes indicate that P2–P2’ phase transformation can be suppressed by substituting proper amount of Mn3+ Jahn-Teller centers with Zn2+, whereas excessive Zn2+ leads to P2-OP4 structure transition at low sodium contents and facilitates the electrode degradations. Furthermore, the homogeneous and robust cathode electrolyte interphase (CEI) layers formed on the Al2O3-coated electrodes effectively hinder the organic electrolytes from further decomposition. Therefore, our synergetic strategy of Zn2+ substitution and ALD surface engineering remarkably boosts the cycling performance of P2–Na0.67MnO2 and provides some new insights into the designing of highly stable cathode electrodes for sustainable sodium ion batteries.

The P2-type Na0.66Ni0.33Mn0.67O2 electrode is one of the most promising layered cathodes, which exhibits high average potential of 3.5 V versus Na+/Na. When working at the high voltage range beyond...

Layered transition metal oxides are the most important cathode materials for Li/Na/K ion batteries. Suppressing undesirable phase transformations during charge-discharge processes is a critical and fundamental challenge towards the rational design of high-performance layered oxide cathodes. Here we report a shale-like NaxMnO2 (S-NMO) electrode that is derived from a simple but effective water-mediated strategy. This strategy expands the Na+ layer spacings of P2-type Na0.67MnO2 and transforms the particles into accordion-like morphology. Therefore, the S-NMO electrode exhibits improved Na+ mobility and near-zero-strain property during charge-discharge processes, which leads to outstanding rate capability (100 mAh g−1 at the operation time of 6 min) and cycling stability (>3000 cycles). In addition, the water-mediated strategy is feasible to other layered sodium oxides and the obtained S-NMO electrode has an excellent tolerance to humidity. This work demonstrates that engineering the spacings of alkali-metal layer is an effective strategy to stabilize the structure of layered transition metal oxides., Suppressing phase transitions is crucial for the layered lithium/sodium transition metal oxide cathodes in batteries. Here, the authors report a water-mediated strategy to mitigate the phase transitions and boost electrochemical performances of manganese-based layered cathodes for cost-effective Na-ion batteries.

The capacity of the layered oxide cathode in a sodium ion battery can be increased by harnessing anionic redox. However, the extra capacity induced by anionic redox comes at the expense of reversibility due to the irreversible oxidation and subsequent loss of oxygen. Here, we report a universal strategy of improving the reversibility of oxygen redox in sodium layered oxides by regulating the superstructural motifs. The intrinsic chemical properties of superstructural motifs can be directionally altered by modulating the interionic interactions, and the rational integration of selected superstructural motifs can result in advanced materials with target performance. As a demonstration, a novel cathode comprising both Mg@Mn6 and Li@Mn6 superstructural motifs is designed and synthesized with inherently inhibited oxygen loss and significantly improved cyclic reversibility. Detailed characterizations on the atomic-level structure and chemistry of materials revealed that the pinning effect of the Mg@Mn6 superstructural motif is critical to maintain a stable layered structure. The findings from this work open up new routes for the design and development of next-generation high energy cathodes with target performance.