Preparation And Sodium Storge Performance Of Free-Standing Hard Carbon Anode Material

Hard carbon materials are considered to be one of the most promising anode materials for sodium-ion batteries in practical applications due to their good structural stability and high capacity.Hard carbon materials typically contain a large number of disordered structures with defects and voids which help to increase their reversible capacity.But too many disordered structures will hinder the transmission and conduction of electrons,resulting in poor rate performance.An effective method is to use the transition metal nickel as catalyst to make hard carbon materials graphitized,which could enhance its conductivity and thus improve rate performance.Heteroatom doping is another effective strategy to improve the electrochemical performance of hard carbon materials.The main content of this paper is to improve the conductivity of hard carbon electrode through catalytic graphitization of metal nickel foam and increase the active site via nitrogen atom doping strategy to achieve the improvement of rate performance and reversible capacity.?1?Using glucose as carbon source and nickel foam as support,a free-standing hard carbon anode electrode material?HC/NF-1,HC/NF-2 and HC/NF-3?was obtained by a simple hydrothermal and subsequent heat treatment method and used directly in a sodium-ion batteries.The carbon spheres in the obtained HC/NF-2 sample with a diameter of⁷00 nm.After nickel foam catalysis,the pleated layered graphitic carbon comprising of 84layers of single layer graphite is produced which the thickness is about 30 nm.HC/NF-2 shows excellent electrochemical performance.High reversible capacity of 285.6 mAh·g^-1 after 100 cycles at a current density of 0.1 A·g^-1 and excellent rate performance(107.1 mAh·g^-1,5.0 A·g^-1).At a relatively high current density of 1.0 A·g^-1,the reversible capacity was maintained at 166.2mAh·g^-1 after 1000 cycles,and coulomb efficiency exceeded 99%.?2?Nitrogen-doped hard carbon free-standing material?NC/NF?was prepared by introducing nitrogen atom using melamine as nitrogen source.NC/NF shows high reversible capacity(347.5 mAh·g^-1,0.1 A·g^-1)as a free-standing anode for sodium-ion batteries,which is 34%higher than HC/NF.The platform capacity of NC/NF(126.2 mAh·g^-1)is almost 4.2 times that of HC/NF(30.1 mAh·g^-1).Meanwhile,it possesses extremely long cycle durability(217.1 mAh·g^-1,1.0 A·g^-1 after 1000 cycles).Excellent electrochemical performance can be attributed to the synergistic effect of the enlarged layer spacing?0.384 nm?and defect sites brought about by nitrogen doped.?3?When the carbonization temperature is gradually increased,the nitrogen doping amount gradually decreases,which is 4.37%in the product NC/NF-800 at 800°C.NC/NF-800 finally shows excellent electrochemical performance,with the reversible capacity of 340.5 mAh·g^-1,253.7 mAh·g^-1,202.1 mAh·g^-1 and 90.5 mAh·g^-1 at 0.1 A·g^-1,0.5 A·g^-1,1.0 A·g^-1,and 5.0 A·g^-1,respectively.With carbonization temperature increased,the structural order of the NC/NF-x sample increases gradually while the defects are reduced,resulting in a more stable graphite layer spacing,and the sodium ions are more easily inserted into the graphite layer,so that the platform capacity is gradually increased and the slope capacity is decreased.