Preparation Of Transition Metal Sulfides/Carbon Composite And Their Sodium Storage Properties

As an energy storage and conversion systems with great application potential,the sodium ion batteries have attracted much attention in recently years.Transition metal sulfides have the advantages of good redox reversibility,high theoretical capacity and abundant resources.However,when it is used as the anode for sodium ion batteries,it has the disadvantages of poor electrical conductivity and large volume expansion change.This will lead to a low capacity and poor cycle stability.To address these drawbacks,we use different synthesis methods to prepare a composite,which combine transition metal sulfides with carbon-based materials to alleviate the volume expansion,enhancing their electrical conductivity and cycle stability,and study the structure and electrochemical properties of the composite.The research details are as follows:1.The ultrathin sulfur-doped graphene-like carbon?S/C?layer wrapped Ni₃S₂/Ni composite?Ni₃S₂/Ni@S/C?is simply fabricated by one-step synthesis method,which using the Nickel?II?oxalate dihydrate and Poly?sodium-p-styrenesulfonate?as raw materials.The S/C in the Ni₃S₂/Ni@S/C can not only enhance the electrical conductivity of the material but also effectively alleviate the large volume expansion of the electrode during the process of charging/discharging,and make the structure of electrode more stable.The existence of nano-sized nickel in the composite can not only enhance the electrical conductivity but also facilitate the conversion of Ni₃S₂ in the process of charging/discharging,thereby enhancing its structural stability.At a current density of 100 mA g^-1,the Ni₃S₂/Ni@S/C electrode deliver a high discharge specific capacity of 318.2 mA h g^-11 after 120 cycles;in the high current density(1000 mA g^-1),it can exhibit a reversible specific capacity of 212.7 mA h g^-1 after250 cycles.Even at 2000 mA g^-1,it can deliver a reversible specific capacity of223.9 mA h g^-1.2.C@MoS₂ is prepared by solvothermal method using nitrogen-doped hollow carbon nanocubes?C?as precursor.The nitrogen-doped carbon coated C@MoS₂ nanocubes composite?C@MoS₂@C?is prepared by coating with a layer of polydopamine?PDA?on the surface of C@MoS₂ and then carbonizing it at high temperature.The double-layer nitrogen-doped carbon of C@MoS₂@C can realize spatial microscopic limit and restrain Mo atoms action to ensure the microscopic structure of MoS₂ can be restored during the charge/discharge process.The nitrogen-doped carbon layer contains a part of molybdenum oxide,and the Mo atoms are effectively restrained and fixed by the C?O?Mo band during the charge/discharge process.At the end of the discharge stage,the structure of MoS₂ completely collapses,and then will be fully recovered during the charging process,which means it has a memory effect.The nitrogen-doped carbon can not only effectively alleviate the large volume change of the electrode but also facilitate the ions and electrons transporting,which can make the structure of C@MoS₂@C more stable.The specific capacity of 321.4 mA h g^-1can be obtained at 1 A g^-1 after 1800 cycles;when at ultrahigh current density of20 A g^-1,the C@MoS₂@C electrode can deliver a high capacity of 200 mA h g^-1.Even at high current density(10 A g^-1)and ultra-long cycling test?10000 cycles?,it also displays an ultra-stable cyclability.Which can deliver a reversible capacity of 177.8 mA h g^-1 at 1000th and then retain a reversible capacity about 163.9 mA h g^-1 after 10000 cycles.