Preparation And Electrochemical Properties Of Two-dimensional Carbon Nanosheets As Anode Materials For Sodium-ion Batteries

In recent years,the use of fossil fuels has greatly exacerbated environmental problems.Therefore,people have begun to turn their attention to renewable and clean energy such as wind energy and tidal energy.The key to using clean energy is to find large-scale energy storage devices.Sodium-ion batteries(SIBs)have the advantages of high energy density,abundant resources,safety and stability,and are expected to achieve commercial application in large-scale energy storage devices.Electrode materials are the core of the battery and determine the performance of the battery.Therefore,in order to realize the commercial application of sodium-ion batteries,it is necessary to find high-performance and low-cost cathode and anode materials.Carbon material is a promising anode material due to its wide range of sources,low cost,and controllable structure.Among them,two-dimensional carbon nanosheets are considered to be one of the most promising candidates for anodes in Na-ion batteries due to their large specific surface area and good electrical conductivity.In this paper,two different template methods were used to control the structure and morphology of carbon materials by regulating the carbonization temperature,and prepared two-dimensional carbon nanosheet anode materials with high specific capacity,high rate and excellent cycle performance.The specific research contents are as follows:(1)Using cinder as precursor and molten salt as template,two-dimensional ultrathin carbon nanosheets were controllably prepared by adjusting the carbonization temperature(CTx,x represents temperature).The large specific surface area of 2D carbon nanosheets provides a large number of active sites for sodium storage and its ultrathin structure facilitates electron and charge conduction,so CTx has high reversible capacity,excellent rate capability and cycling stability.After optimization,CT1000 has a reversible specific capacity of 221 m A h g^-1at a current density of 0.1 A g^-1,and a capacity retention of 94.2%after 2000 cycles at a current density of 1 A g^-1,showing excellent cycle stability.The excellent electrochemical performance,simple preparation process,and low cost make CTx a promising energy storage anode in Na-ion batteries.(2)Using polymer polyvinylpyrrolidone(PVP)as precursor and boronic acid as template,B,N co-doped carbon nanosheets(BNCx,x represents temperature)were synthesized.The synergistic effect of heteroatom doping and ultrathin carbon nanosheets provide BNCx with abundant defects,active centers,and short ion/electron transfer distances,which are beneficial to improve their rate performance and cycling stability.The optimized BNC exhibits excellent rate performance and cycling stability,with a reversible specific capacity as high as 295 m Ah g^-1at a current density of 0.05 A g^-1and a capacity retention rate was 95.2%after 2000 cycles at a current density of 1 A g^-1.Meanwhile,the storage mechanism of sodium is shown to be a pseudocapacitively controlled process,which demonstrates the material has fast charge-discharge tolerance,which contributes to the excellent rate performance and cycling stability.This work reveals a simple and efficient templating method to fabricate 2D heteroatom co-doped carbon nanosheets for superior sodium storage performance.