| Lithium-ion batteries have been widely used in electric vehicles,portable power supplies and digital products due to their advantages such as high theoretical capacity,high voltage range,long service life,no memory effect and wide operating temperature range.However,the lithium resources in the Earth’s crust only account for 0.0065 w%,and the distribution is uneven.Due to the regional limitation of lithium resources,it is difficult to develop lithium materials,which makes the cost of lithium materials high.In this case,the alkali metals sodium（2.74 w%）and potassium（2.47 w%）elements of the same main group,which are abundant and have similar properties,become one of the candidates to replace lithium.However,the large radius of sodium and potassium ions and the slow diffusion rate in graphite lead to poor reaction kinetics,which makes the carbon anode materials of lithium-ion batteries unable to be directly applied to sodium/potassium ion batteries.Therefore,it is urgent to find anode material that can match its large ionic radius to meet the reversible insertion/extraction process of sodium/potassium ions.For carbon anode materials,it is an effective strategy to obtain carbon materials with abundant active sites,abundant defects and stable structure through heteroatom-doping.In this paper,we construct heteroatom-rich organic compounds as carbon precursors to improve the carbon layer spacing,achieve heteroatom doping and increase the active site,so as to enhance the transport kinetics of sodium/potassium ions in carbon materials.Specific research contents are as follows:1.Using F127 as structure guide agent,quadrilateral prisms were synthesized by hydrothermal self-assembly with tannic acid in alkaline solution,and then sulfur-oxygen doped porous ultra-thin carbon nanosheets（S/O-CNS）were obtained by vulcanization.During the calcination process,the carbon precursor shrinks along the longitudinal direction at low temperature to form a two-dimensionalπ-πstack structure.Elemental sulfur is used as a dopant to form sulfur-doped carbon materials,and the stacked carbon materials are shear into carbon nanosheets.The doped,independent and twisted carbon nanosheets create more active sites and defective structures,thus speeding up the transport of potassium ions.As the PIBs anode,S/O-CNS showed satisfactory specific reversible capacity(323.8 mAh g-1 at 0.1 A g-1after 200 cycles),excellent rate performance(159.9 mAh g-1 under 10 A g-1)and long-cycle stability(198.9 mAh g-1 at1.0 A g-1 after 700 cycles).2.Nitrogen and phosphorus doped carbon materials（N,P-C）were synthesized by a simple liquid-phase mixing reaction using melamine as nitrogen and carbon sources and phytic acid as phosphorus sources.The effects of different calcination temperatures on the microstructure,layer spacing and material size were studied.The prepared nitrogen-phosphorus doped carbon material has 86.23%high edge nitrogen content,which creates more active adsorption sites for the material.The addition of phosphorus atoms expands the layer spacing and increases the electronic conductivity,and achieves good energy storage performance as anode material for sodium-ion batteries.At the current density of0.1 A g-1,the reversible specific capacity is maintained at 285.6 mA h g-1 after 200 cycles,and the Coulomb efficiency is close to 100%.The reversible specific capacity was maintained at 170.7 mAh g-1 for 1400 cycles even at 1.0 A g-1 current density,indicating good long-term cycle stability.3.Using g-C3N4 and 2,2’-diaminodiphenyl disulfides as reactants,the two reactants interact with each other to form hydrogen bonds by liquid phase method,and then the nitrogen and sulfur co-doped carbon nanosheets（GNSC）are prepared by high temperature calcination.This thin nanosheet structure obviously increases the specific surface area of the material,and thus increases the electrochemical properties.Through XRD,Raman and XPS analysis,the effects of temperature and different mixing ratios of g-C3N4 and 2,2’-diaminodiphenyl disulfides on the morphology,structure and composition of the materials were studied.Nitrogen and sulfur co-doped carbon material as potassium-ion batteries anode material,the specific capacity is maintained at 254.8mAh g-1 after 300 cycles under the current density of 0.1 A g-1.It has the capacity of 125.4mAh g-1 after 1000 cycles even at 1.0 A g-1 current density,showing good potassium storage performance. |
