Modification Of Transition Metal Sulphide Based Sodium Ion Battery Anode Materials And Modulation Of Their Electrochemical Properties

The development and promotion of clean energy and efforts to achieve a sustainable and efficient energy supply is an effective way to achieve the’double carbon goal’.Among energy storage batteries,the widespread use of lithium-ion batteries has led to a severe scarcity of lithium resources,while sodium-ion batteries（SIBs）,with their similar working mechanism and lower cost,are considered to be perfect candidates for replacing lithium-ion batteries in energy storage systems and have good potential for large-scale energy storage applications.Transition metal sulphides possess good magnetic and optical properties,are widely available,low in cost,have excellent safety features and possess excellent electrochemical properties when used as anode materials for SIBs,with a theoretical specific capacity of over 600 m Ah g^-1.However,these electrode materials can suffer from volume expansion,decreasing conductivity and side reactions of the active material during the electrode reaction.To address such problems,certain modification methods are proposed in this paper to solve them.The main results are as follows:Firstly,Ni S₂/MWCNTs composites were synthesised by a one-step hydrothermal method.The 3D conductive network structure formed effectively reduced the volume change during cycling,resulting in stable morphology and uniform distribution of nanomicrospheres with good cycling stability.When cycled for 600 turns at a current of 1 A g^-1,the capacity retention rate was as high as 96.7%,demonstrating its excellent long cycling performance.Based on the analysis of the charge storage mechanism,it is known that Ni S₂/MWCNTs are controlled by both capacitance and diffusion,with the pseudo-capacitance contribution dominating and increasing as the scan rate is continuously increased.Secondly,Ni S₂@NC composites were prepared by PVP-assisted hydrothermal method and subsequent annealing treatment.The Ni S₂ nanospheres were encapsulated by a thin carbon layer,which increased the specific surface area of the composites and increased the reactive sites,slowing down the volume expansion during the electrode reaction,resulting in higher electrical conductivity as well as structural stability of the composites.In addition,the nitrogen-doped carbon layer has a synergistic effect with the nanosphere structure,which can further increase the ionic conductivity,improve the ion diffusion kinetics,and enhance the cycling and multiplicity performance of the battery.After a current density of 0.2 A g^-1,Ni S₂@NC exhibits a considerable capacity retention,with a capacity of 554.3 m Ah g^-1 after 400 cycles.The coulombic efficiency of the first cycle is as high as 92.3%,demonstrating excellent sodium storage performance.At a current density of 1 A g^-1,after 800 cycles,Ni S₂@NC can still maintain a capacity of 436.3 m Ah g^-1 and exhibits good cycling stability.Finally,Zn S@NC composites were obtained by a simple hydrothermal method as well as a carbon cladding treatment process.By regulating the nanosphere structure,the volume expansion can be effectively slowed down,making the composite structure stable.The nitrogen-doped carbon layer derived from dopamine hydrochloride can reduce the contact between the active material and the electrolyte,thus reducing the occurrence of electrode side reactions,accelerating the ion conduction rate and maintaining the volume stability during cycling,and the introduction of nitrogen further enhances the material conductivity and improves the ion diffusion kinetics.When used as an anode material for SIBs,it has a sodium storage capacity of 330 m Ah g^-1 after 50 cycles at low currents and maintains a high cycling stability at high currents,still reaching 289.48 m Ah g^-1 after 250 cycles.