Layered transition-metal oxides have attracted intensive interest for cathode materials of sodium-ion batteries. However, they are hindered by the limited capacity and inferior phase transition due to the gliding of transition-metal layers upon Na+ extraction and insertion in the cathode materials. Here, we report that the large-sized K+ is riveted in the prismatic Na+ sites of P2-Na0.612K0.056MnO2 to enable more thermodynamically favorable Na+ vacancies. The Mn-O bonds are reinforced to reduce phase transition during charge and discharge. 0.901 Na+ per formula are reversibly extracted and inserted, in which only the two-phase transition of P2 ↔ P’2 occurs at low voltages. It exhibits the highest specific capacity of 240.5 mAh g−1 and energy density of 654 Wh kg−1 based on the redox of Mn3+/Mn4+, and a capacity retention of 98.2% after 100 cycles. This investigation will shed lights on the tuneable chemical environments of transition-metal oxides for advanced cathode materials and promote the development of sodium-ion batteries.

The increasing demand to efficiently store and utilize the electricity from renewable energy resources in a sustainable way has boosted the request for sodium-ion battery technology due to the high abundance of sodium sources worldwide. Na superionic conductor (NASICON) structured cathodes with a robust polyanionic framework have been intriguing because of their open 3D structure and superior thermal stability. The ever-increasing demand for higher energy densities with NASICON-structured cathodes motivates us to activate multielectron reactions, thus utilizing the third sodium ion toward higher voltage and larger capacity, both of which have been the bottlenecks for commercializing sodium-ion batteries. A doping strategy with Cr inspired by first-principles calculations enables the activation of multielectron redox reactions of the redox couples V2+/V3+, V3+/V4+, and V4+/V5+, resulting in remarkably improved energy density even in comparison to the layer structured oxides and Prussian blue analogues. This work also comprehensively clarifies the role of the Cr dopant during sodium storage and the valence electron transition process of both V and Cr. Our findings highlight the importance of a broadly applicable doping strategy for achieving multielectron reactions of NASICON-type cathodes with higher energy densities in sodium-ion batteries.

Nanomaterials such as nanowires, carbon nanotubes, and nanoparticles have already led to breakthroughs in the field of biological and medical sensors. The quantum size effects of the nanomaterials and their similarity in size to natural and synthetic nanomaterials are anticipated to improve sensor sensitivity dramatically. Nanowires are considered as key nanomaterials because of their electrical controllability for accurate measurement, and chemical-friendly surface for various sensing applications. This review covers the working principles and fabrication of silicon nanowire sensors. Furthermore, we review their applications for the detection of viruses, biomarkers, and DNA, as well as for drug discovery. Advances in the performance and functionality of nanowire sensors are also surveyed to highlight recent progress in this area. These advances include the improvements in reusability, sensitivity in high ionic strength solvent, long-term stability, and self-powering. Overall, with the advantages of ultra-sensitivity and the ease of fabrication, it is expected that nanowires will contribute significantly to the development of biological and medical sensors in the immediate future.

Layer‐structured oxide cathodes have a lot of phases, which can be varied depending on Na ion contents and finally determine their electrochemical properties. Therefore, the off‐stoichiometry of layer‐structured oxides with the Na ions may differentiate not only their capacities but also the cyclic stabilities, kinetics, and so on, highlighting the importance of Na ion content. However, Na2CO3 tends to be irreversibly formed on surface by making use of the Na ions lost from the lattice. Thereby, the O3 phase with stoichiometric Na content changes into the off‐stoichiometric P2 phase bringing about significant disadvantages. To address this issue, a thermal activation process is suggested to simultaneously decompose Na2CO3 into electrochemically active Na ions and modulate the off‐stoichiometric P2 phase into the stoichiometric O3 phase. This study indicates that minimizing the loss of Na ions and maintaining the lattice framework with higher contents of Na ions during the synthesis of Na‐incorporating layered cathodes should be a key toward attaining electrochemical superiority.

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