| Potassium has more abundant reserves,similar redox potential and a smaller ion solvation radius than lithium,which gives potassium ion batteries（PIBs）the advantage of low cost,high operating voltage and fast ion transfer rates.PIBs are considered an important power source as an alternative to commercial lithium ion batteries and have attracted strong interest from researchers.However,the large ionic radius of potassium slows down the reaction kinetics and can cause huge volume expansion of the electrode material,which can damage the material structure and affect its electrochemical performance.Therefore,the search for electrode materials with high capacity and fast reaction kinetics remains a pressing issue for the practical application of PIBs.Graphene-based carbon materials are widely used as high-performance potassium storage anode materials due to their high specific surface area and excellent electrical conductivity.At the same time,studies have shown that nitrogen doping of graphene can further improve the electrical conductivity of the material and expand the layer spacing to achieve better potassium storage performance.However,the existing nitrogen doping techniques have problems such as low doping amount and few active sites.Therefore,this paper aims to prepare graphene-based anode materials with high nitrogen doping content,abundant active sites and suitable layer spacing.A series of high-performance nitrogen-doped graphene anode materials were designed and prepared by a simple hydrothermal process using melamine as the nitrogen source.The specific research contents and conclusions are as follows:（1）Nitrogen-doped graphene（NG）materials were produced by hydrothermal method using graphene prepared by mechanical exfoliation method as the substrate.The NG materials showed interconnected flakes with a few defects at the edges and achieved 0.34 at%content of nitrogen doping,which effectively improved the electrical conductivity and electrochemical reactivity of the materials.The NG anode materials exhibited good K+storage performance(1000 m A g-1,800 cycles,78.5 m Ah g-1).（2）In order to further increase the nitrogen doping content in the materials,nitrogen-doped graphene oxide（GO）containing a large number of functional groups was chosen as the substrate and the nitrogen-doped graphene oxide（NGO）materials were prepared by hydrothermal method.The analysis shows that the NGO material retains a large number of folded structures of GO and increases the nitrogen doping amount to 3.74 at%,which improves the reactive area,increases the electrical conductivity and layer spacing,accelerates the reaction kinetics and alleviates the volume expansion.Based on this,the NGO anode material exhibited good cycling stability(100 m A g-1,100 cycles,196 m Ah g-1)and further improved multiplicity performance(1000 m A g-1,1000 cycles,95 m Ah g-1).（3）In order to reduce the oxygen-containing functional groups and continue to increase the nitrogen doping content,the nitrogen-fluorine co-doped graphene oxide（NFGO）anode materials were prepared by a one-step hydrothermal method with the introduction of fluorine.The introduction of fluorine can increase the layer spacing and structural disorder of the material,providing abundant active sites for nitrogen doping.At the same time,the co-doping of nitrogen and fluorine elements can produce a synergistic effect and further improve the electrochemical properties.Compared to NGO,NFGO materials contain up to 7 at%nitrogen and 22.4%less oxygen,which reduces the K+diffusion hindrance and enhances the affinity for potassium.Based on this,the NFGO anode material still has a specific capacity of 235 m Ah g-1 after 100cycles at a current density of 100 m A g-1 and a specific capacity of 131 m Ah g-1 after850 cycles at a high current density of 1000 m A g-1. |
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