Red phosphorus (P) has attracted intense attention as promising anode material for high-energy density sodium-ion batteries (NIBs), owing to its high sodium storage theoretical capacity (2595 mAh g ). Nevertheless, natural insulating property and large volume variation of red P during cycling result in extremely low electrochemical activity, leading to poor electrochemical performance. Herein, the authors demonstrate a rational strategy to improve sodium storage performance of red P by confining nanosized amorphous red P into zeolitic imidazolate framework-8 (ZIF-8) -derived nitrogen-doped microporous carbon matrix (denoted as P@N-MPC). When used as anode for NIBs, the P@N-MPC composite displays a high reversible specific capacity of ≈600 mAh g at 0.15 A g and improved rate capacity (≈450 mAh g at 1 A g after 1000 cycles with an extremely low capacity fading rate of 0.02% per cycle). The superior sodium storage performance of the P@N-MPC is mainly attributed to the novel structure. The N-doped porous carbon with sub-1 nm micropore facilitates the rapid diffusion of organic electrolyte ions and improves the conductivity of the encapsulated red P. Furthermore, the porous carbon matrix can buffer the volume change of red P during repeat sodiation/desodiation process, keeping the structure intact after long cycle life.

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Heteroatom doping is regarded as a promising method to enhance the sodium storage performance of carbon materials. In this work, a sulfur-enriched N-doped multichannel hollow carbon nanofiber (denoted as S-NCNF) film is prepared through electrospinning technology and heat treatment with sublimed sulfur as the flexible anode for sodium ion batteries (NIBs). The S-NCNF film displays outstanding electrochemical performance, particularly with a high rate capacity (132 mA h g at the current density of 10 A g ) and remarkable long cycling stability (reversible specific capacity of 187 mA h g at 2 A g over 2000 cycles). The improved sodium storage performance results from the unique 3D structure, abundant defects, and increased interlayer spacing of S-NCNFs. The density functional theory calculations demonstrate that nitrogenous carbon nanofibers doping with sulfur could not only promote the adsorption of sodium but also favor electrons' transfer. This strategy has been demonstrated as a general process to design free-standing carbon-based thin film with other heteroatom doping.

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