Micro-Nano Regulation Of Nitrogen-Doped Hard Carbon Electrodes Towards Sodium Storage

Carbon materials are considered as the most promising anode materials for sodium ion batteries due to their low cost and eco-friendly advantages.The development of advanced carbon anode materials with high rate performance and long cycle stability is accelerating the application of sodium ion batteries in energy storage systems.Unlike graphite,hard carbon has a "card house" structure composed of graphite-like crystallites and amorphous regions.This structure contains various active sites,such as edges,defects,and functional groups.The nanostructure regulation of hard carbon materials is demonstrated as an effective method for the negative electrode of sodium ion batteries.In view of the development of electrode materials for lithium-ion batteries,many micro/nano-structured hard carbon materials have been developed due to kinetics that facilitate the transport of ions and electrons,such as hollow nanostructured carbon materials,porous carbon materials,carbon fibers,carbon nanosheets,etc.In addition,as an effective method to enhance the electrochemical performance of carbon materials by adjusting the electronic and chemical properties of carbon,heteroatom doping(nitrogen,phosphorus,sulfur,etc.)is also used to improve electrochemical sodium storage performance.In this paper,single-layer and multi-layer hollow carbon sphere structures of hard carbon materials are designed,and their micro/nano structures are adjusted to improve the sodium storage electrochemical performance.The details are as follows:1.We synthesized a nitrogen-doped single-layer hollow carbon sphere(SHCS)using a hard template method.Nitrogen-doped hollow carbon spheres with controlled carbon layer thickness and uniform size were obtained by adjusting the size of SiO2 template and the amount of carbon source(DA).We further explored the effects of different carbonization temperatures on their structure and electrochemical performance.By optimizing the synthetic parameters,the as-synthesized SHCS exhibits good long-cycle stability and high rate performance.Typically,the SHCS electrode has negligible attenuation in(500 cycles)cycling capacity at a current density of 0.6 A g-1,and has a discharge capacity of 102 mA h g-1 at a current density of 1.2 A g-1.2.Based on SHCS material control strategy,we prepared double-layer mesoporous hollow carbon sphere(DHCS)material.Compared with SHCS,DHCS possesses higher specific surface area and pore volume,and the carbon layer contains more defects.The difference in material structure affects the electrochemical performance.Compared with SHCS electrodes,DHCS electrodes have excellent rate performance(10 C,113 mA h g-1)and cycle stability(1000 cycles at 0.6 A g-1,143.6 mA h g-1).In addition,the relationship between the microstructure,electrochemical performance and sodium ion storage mechanism of DHCS materials is analyzed in this paper.3.Plasma-enhanced chemical vapor deposition(PECVD)uniformly grew array graphene on the surface of aluminum foil conductive current collector to improve the sodium metal deposition interface,thereby guiding the uniform deposition of metal sodium and inhibiting the formation of sodium dendrites.Experiments showed that under the NaPF6 liquid electrolyte system,the modification of the current collector by the array graphene improves the sodium-philophilic property of the interface,thereby promoting the uniform deposition of metal sodium and inhibiting the formation of dendrites.Therefore,the cycle stability of the metal sodium negative electrode is improved.