Radicals are inevitable intermediates during the charging and discharging of organic redox electrodes. The increase of the reactivity of the radical intermediates is desirable to maximize the capacity and enhance the rate capability but is detrimental to cycling stability. Therefore, it is a great challenge to controllably balance the redox reactivity and stability of radical intermediates to optimize the electrochemical properties with a good combination of high specific capacity, excellent rate capability, and long-term cycle life. Herein, we reported the redox and tunable stability of radical intermediates in covalent organic frameworks (COFs) considered as high capacity and stable anode for sodium-ion batteries. The comprehensive characterizations combined with theoretical simulation confirmed that the redox of C−O• and α-C radical intermediates play an important role in the sodiation/desodiation process. Specifically, the stacking behavior could be feasibly tuned by the thickness of 2D COFs, essentially determining the redox reactivity and stability of the α-C radical intermediates and their contributive capacity. The modulation of reversible redox chemistry and stabilization mechanism of radical intermediates in COFs offers a novel entry to design novel high performance organic electrode materials for energy storage and conversion.

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Phosphorus doping is an effective strategy to simultaneously improve the electronic conductivity and regulate the ionic diffusion kinetics of TiO2 being considered as anode materials for sodium ion batteries. However, efficient phosphorus doping at high concentration in well-crystallized TiO2 nanoparticles is still a big challenge. Herein, we propose a defect-assisted phosphorus doping strategy to selectively engineer the surface structure of TiO2 nanoparticles. The reduced TiO2–x shell layer that is rich in oxygen defects and Ti3+ species precisely triggered a high concentration of phosphorus doping (∼7.8 at. %), and consequently a TiO2@TiO2–x -P core@shell architecture was produced. Comprehensive characterizations and first-principle calculations proved that the surface-functionalized TiO2–x -P thin layer endowed the TiO2@TiO2–x -P with substantially enhanced electronic conductivity and accelerated Na ion transportation, resulting in great rate capability (167 mA h g–1 at 10 000 mA g–1) and stable cycling (99% after 5000 cycles at 10 A g–1). Combining in/ex situ X-ray diffraction with ex situ electron spin resonance clearly demonstrated the high reversibility and robust mechanical behavior of TiO2@TiO2–x -P upon long-term cycling. This work provides an interesting and effective strategy for precise heteroatoms doping to improve the electrochemical performance of nanoparticles.

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The Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode has attracted great interest owing to its low cost, high capacity, and energy density. Nevertheless, rapid capacity fading is a critical problem because of direct contact of NCM811 with electrolytes and hence restrains its wide applications. To prevent the direct contact, the surface inert layer coating becomes a feasible strategy to tackle this problem. However, to achieve a homogeneous surface coating is very challenging. Considering the bonding effect between NCM811, polyvinylpyrrolidone (PVP), and polyaniline (PANI), in this work, we use PVP as an inductive agent to controllably coat a uniform conductive PANI layer on NCM811 (NCM811@PANI–PVP). The coated PANI layer not only serves as a rapid channel for electron conduction, but also prohibits direct contact of the electrode with the electrolyte to effectively hinder side reaction. NCM811@PANI–PVP thus exhibits excellent cyclability (88.7% after 100 cycles at 200 mA g–1) and great rate performance (152 mA h g–1 at 1000 mA g–1). In situ X-ray diffraction and in situ Raman are performed to investigate the charge–discharge mechanism and the cyclability of NCM811@PANI–PVP upon electrochemical reaction. This surfactant-modulated surface uniform coating strategy offers a new modification approach to stabilize Ni-rich cathode materials for lithium-ion batteries.

Summary: A method to measure the Young's modulus of a single electrospun polyacrylonitrile (PAN) fiber is reported. The Young's modulus can be calculated from the force‐displacement curves obtained by the bending of a single fiber attached to an atomic force microscopy (AFM) cantilever. It is suggested that the high modulus of electrospun fibers is caused by the orientation of molecular chains, which is confirmed by wide‐angle X‐ray diffraction (WAXD) measurements. The communication will provide a basic understanding of the relationship between mechanical properties and structures of electrospun fibers.A PAN fiber was attached to a contact mode cantilever to facilitate the measurement of force‐displacement curves and Young's modulus.imageA PAN fiber was attached to a contact mode cantilever to facilitate the measurement of force‐displacement curves and Young's modulus.

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