Rational Fabrication And Performances Of Carbon-based Anode Materials For Sodium-ion Batteries

As we all know,lithium-ion batteries（LIBs）have been widely used in commercial technology and varieties of energy storage systems,due to the high energy density.Compared to LIBS,sodium-ion batteries（SIBs）with abundant reserve and lower cost of sodium have been deemed to one of the most promising materials for large-scale energy storage.So it is very meaningful to build the new system for electrode materials of sodium stationary.The ionic radius of Na⁺is much larger in order that one cannot be used as electrode materials for SIBs directly which was from those for LIBs.Recently,there are a lot of reports about the metal oxides and polyatomic compounds in the study for cathode materials of SIBs,such as Na_0.44MnO₂,Na_xCoO₂,and NaVFePO₄.However,conventional graphite anode materials which were suitable for LIBs cannot perform very well because of the smaller interlayer space.When discharged/charged in the cycling process,larger Na ions cannot transmit easily into the nanosheets of graphite.Among all of anode materials,carbonaceous materials have been extensively reported owing to the stable structure,high capacity and environmental friendliness.In this work,we modified the carbonaceous materials by using sol-gel method and high temperature calcination with rational designing thinking,so as to prepare the high-performance anode materials for SIBs.We combined many characterized methods to examine and analyze the electrochemical properties of prepared materials,and study the difference of the materials before modified and after modified.To solve the problem that carbonaceous materials were used for anode materials for SIBs with low first-cycle Coulombic efficiency and poor cycling performance,we used simply sol-gel method and calcination to prepare the N-rich carbon nanosheets（N/C）.Appending a lot of N atoms to the carbonaceous materials,not only can accelerate the electron transport,but also can increase the binding sites and improve the adsorbability in the charging process with the enlargement in specific surface.In this way,N/C performs excellent electrochemical properties.At a current density of50 mA g^-1,the reversible capacity of 80 mAh g^-1.After first initial cycles,N/C keeps cycling with almost no attenuation.Furthermore,the Coulombic efficiency is nearly100%.To further improve the capacity of N/C,we can try to append the S atoms with a larger ionic radius to N/C electrode materials because of the smaller interlayer distance of N/C.By doping S to N/C,it is beneficial for enlargement of interlayer distance and transmission of Na⁺in the discharging/charging process.Compared to N/C,the S doping N/C（S-N/C）expanded interlayer distance of unparallel nanosheets and larger surface area is truly helpful for electrochemical Na storage.At the same time,we found the substitute to special sites of pyrrolic-N by doping S.And it is beneficial for improvement of capacity with the Faradic Reaction.At a current density of 50 mA g^-1,the S-N/C anode materials perform 350 mAh g^-11 and this number is much larger than that with no doping S.Moreover,the first Coulombic efficiency improved a lot.With the excellent rate performance of S-N/C,the electrodes perform 110 mAh g^-1 when the current density of 10 A g^-1.By the way,we use the mixed gas（H₂S:Ar=9:1,v:v）to dope the N/C in this work innovatively.Ultimately S-N/C performs very well with long cycles owing to its stable structure.Also,this work provides great guiding significance for designing potentially N/S-codoped electrode and electrocatalytic matarials.