Decoupling the air sensitivity of Na-layered oxides.

Air sensitivity remains a substantial barrier to the commercialization of sodium (Na)-layered oxides (NLOs). This problem has puzzled the community for decades because of the complexity of interactions between air components and their impact on both bulk and surfaces of NLOs. We show here that water vapor plays a pivotal role in initiating destructive acid and oxidative degradations of NLOs only when coupled with carbon dioxide or oxygen, respectively. Quantification analysis revealed that reducing the defined cation competition coefficient (η), which integrates the effects of ionic potential and sodium content, and increasing the particle size can enhance the resistance to acid attack, whereas using high-potential redox couples can eliminate oxidative degradation. These findings elucidate the underlying air deterioration mechanisms and rationalize the design of air-stable NLOs.

Screening Heteroatom Configurations for Reversible Sloping Capacity Promises High-Power Na-Ion Batteries.

Heteroatom doping has been proved to effectively enhance the sloping capacity, nevertheless, the high sloping capacity almost encounters a conflict with the disappointing initial Coulombic efficiency (ICE). Herein, we propose a heteroatom configuration screening strategy by introducing a secondary carbonization process for the phosphate-treated carbons to remove the irreversible heteroatom configurations but with the reversible ones and free radicals remaining, achieving a simultaneity between the high sloping capacity and ICE (≈250 mAh g-1 and 80 %). The Na storage mechanism was also studied based on this "slope-dominated" carbon to reveal the reason for the absence of the plateau. This work could inspire to distinguish and filter the irreversible heteroatom configurations and facilitate the future design of practical "slope-dominated" carbon anodes towards high-power Na-ion batteries.

Ostwald Ripening Tailoring Hierarchically Porous Na3 V2 (PO4 )2 O2 F Hollow Nanospheres for Superior High-Rate and Ultrastable Sodium Ion Storage.

Sodium-ion batteries (SIBs) are receiving considerable attention as economic candidates for large-scale energy storage applications. Na3 V2 (PO4 )2 O2 F (NVPF) is intensively regarded as one of the most promising cathode materials for SIBs, due to its high energy density, fast ionic conduction, and robust Na+ -super-ionic conductor (NASICON) framework. However, poor rate capability ascribed to the intrinsically low electronic conductivity severely hinders their practical applications. Here, high-rate and highly reversible Na+ storage in NVPF is realized by optimizing nanostructure and rational porosity construction. Hierarchical porous NVPF hollow nanospheres are designed to modify the issues of inconvenient electrolyte transportation and unfavorable charge transfer behavior faced by solid-structured electrode materials. The individual unique nanosphere is assembled from numerous nanoparticles, which shortens the length of Na+ transport in solid state and thus facilites the Na+ migration. Hollow nanostructure hierarchically porous configuration enables adequate electrolyte penetration, continuous electrolyte supplementation, and facile electrolyte transportation, leading to barrier-free Na+ /e- diffusion and high-rate cycling. In addition, the large electrolyte accessible surface area boosts the charge transfer in the whole electrode. Therefore, the present NVPF demonstrates unprecedented rate capability (85.4 mAh g-1 at 50 C) and long-term cyclability (62.2% capacity retention after 2000 cycles at 20 C).