Preparation And Electrochemical Properties Of 3D Carbon-based Anode Materials For Sodium-ion Batteries

Lithium-ion batteries（LIBs）are widely used in portable electronic products,large-scale power supplies,new energy electric vehicles and energy storage,due to their high voltage and energy density,and excellent cycle stability.However,the continuous consumption of lithium resources and the high cost of lithium-ion batteries have greatly limited theie application in large-scale energy storage.Sodium ion batteries（SIBs）are the ideal alternatives to LIBs,due to the earth abundance and low cost of sodium source as well as the similarities between Na and Li ions such as chemical properties and electrochemical behaviors in electrode materials.Unfortunately,because the radius of Na ion is 70%larger than that of Li ion,greater volume variation and lower specific capacity would occur in SIBs than in LIBs when analogous electrode materials are employed.Thus,it is an essential ingredient for developing novel electrode materials with high specific capacity and cyclability.In view of this,we choose three dimensional（3D）graphene and their hybids with FeP as the research objects,to develop high-performance anode materials for SIBs.Herein,three works is presented.（1）By using melamine as a nitrogen source and sodium hypophosphite as a phosphorus source,N,P dual-doped graphene aerogels（NPGAs）were prepared.The mass ratio of melamine to graphene on the electrochemical properties was investigated.Among them,NPGA prepared with the mass ratio of 1:2 showed the best electrochemical performance.It delivered a high reversible capacity of 352.5 mA h g^-1at 50 mA g^-1.Moreover,it showed excellent rate capability.The discharge capacities were 357.6,329.5,293.6,260.6,228.5 and 189.8 mA h g^-1 at 0.05,0.1,0.2,0.5,1 and3 A g^-1,respectively.More importantly,an ultra-stable capacity of 121.8 mA h g^-1 at 5A g^-1 after 2000 cycles can be obtained.These fascinating results could be attributed to their unique structural features in several aspects:1)N,P codoping,especially the high content of pyridinic-N doping,could introduce defects for storing more Na ions,enlarge the interlayer distance and ehance the diffusion kinetics of Na ion.2)3D structure could induce fast Na ion/electron transfer kinetics and increase the capacitive contribution under high rate charge/charge.（2）N,P-codoped graphene confined iron phosphide nanodots（FeP/NPG）were successfully obtained via a low temperature phosphidation of NH₂-rich reduced graphene oxide decorated Fe₂O₃.High content of graphitic-N in FeP/NPG was achieved through the directional transformation of-NH₂ in the precursor.Moreover,monodispersed FeP nanodots were uniformly embedded into a 3D NPG network.These two merits induced fast Na ions and electron transfer kinetics,and excellent structural stability of FeP/NPG nanocomposite electrodes during repeated cycling.Furthermore,computations illustrated that N,P-codoping could significantly improve the affinity for Na⁺and FeP and the discharge products of Fe and Na₃P,and simultaneously enhance the electronic conductivity.These features result in fascinating sodium storage performance of FeP/NPG:a large reversible capacity of 613 mA h g^-1 at 50 mA g^-1,and good rate capability of 422 mA h g^-1 at 1 A g^-1 and 349 mAh g^-1 at 3 A g^-1.Also,an ultra-stable capacity of 378 mA h g^-1 at 1 A g^-11 after 700 cycles can be obtained.（3）3D dual carbons（graphene and carbon nanotube）confined FeP was fabricated.3D graphene could induce fast Na ions and electron transfer kinetics,and ehance the structural stability under charge/discharge processes,while carbon nanotubes could further facilitate the electron transfer.When the amount of FeSO4·7H2O was 0.5 g,the best sodium storage performance could be obtained.It delivered a high reversible capacity of 466.2 mA h g^-1 at 50 mA g^-1.Moreover,it showed excellent rate capability.The discharge capacities were 522.3,472.1,430.5,390.3,332.7 and 287.6 mA h g^-1at 0.05,0.1,0.2,0.5,1 and 3 A g^-1,respectively.More importantly,an ultra-stable capacity of 325.6 mA h g^-1 at 1 A g^-1 after 1000 cycles can be obtained.