Research On Transition Metal Selenides As Negative Electrode Materials For Sodium Ion Batterie

Non-renewable resources such as traditional fossil energy sources are gradually failing to meet the ever-increasing global demand for energy and with the serious environmental problems caused by the consumption of traditional energy sources increasing,the development of new renewable resources is gaining attention,whether it is electric vehicles,robots,drones or grid storage of intermittent renewable resources,rechargeable electrochemical energy storage is a requirement to meet the future Key technologies.Amongst these,lithium-ion batteries are a more mature technology that is already commonly used in everyday life.However,given the scarcity of lithium resources with mass use,the problem will become insurmountable in the near future.Due to the abundance and low cost of sodium,coupled with good electrochemical properties and similar operating principles,sodium ion batteries hold great promise for large-scale applications.It is worth noting that sodium ion batteries can use metallic aluminium as the collector fluid for the negative electrode,whereas in lithium ion batteries,copper is the only choice for the negative electrode,as lithium reacts with aluminium at a relatively low potential to form a binary alloy.The suitability of aluminium as a collector fluid in sodium ion batteries will further reduce the cost of the technology in large-scale applications,as aluminium is not only cheap and light,but also the most abundant metal element.The search for high-performance performing anode materials for sodium ion batteries has become an important task.Among the various types of anode materials,transition metal selenides are a key focus due to their high theoretical capacity and low cost.However,because of the disadvantages such as huge volume expansion during sodium storage and low electrical conductivity,it has a short cycle life and low multiplicative performance in practical applications.Based on the above problems,this thesis aims to improve its performance by redesigning its microstructure and introducing high conductivity carbon.The specific studies are as follows:（1）A novel bilayer carbon-encapsulated hollow nanoparticle Fe Se₂@NC@rGO was designed by hydrothermal synthesis and calcination.the unique hollow structure can keep its particles from fragmentation and agglomeration during long-term cycling,while increasing the contact area between the material and the electrolyte to reduce the ion shuttle path and increase the diffusion rate.the nitrogen-doped carbon shell can provide more sodium ion shuttle process The nitrogen-doped carbon shell can provide more active sites for the sodium ion shuttle process and protect its structural stability.The modification of double-layer carbon makes Fe Se₂@NC@rGO still have a capacity of 639.1 m Ah g^-1 after100 cycles at a current density of 0.1 A g^-1 and 320.7 m Ah g^-1 at a high current density of10 A g^-1for more than 880 cycles,and the double-layer carbon modified Fe Se₂@NC@rGO has higher sodium diffusion coefficient and higher The Fe Se₂@NC@rGO with higher sodium diffusion coefficient and higher pseudocapacitance contribution maintains excellent performance at different temperatures and provides high capacity and excellent stability in full-cell applications,maintaining a capacity of 252.6 m Ah g^-1after 135 cycles.The excellent electrochemical properties of Fe Se₂@NC@rGO indicate that it has promising applications in the future.（2）Sb₂Se₃ is encapsulated in situ inside redox graphene（rGO）by a facile one-step hydrothermal method,and the reduction reaction of rGO is completed simultaneously with hydrothermal synthesis of Sb₂Se₃.The improvement of Sb₂Se₃ by using the high conductivity and ductility of rGO can effectively alleviate the electrode damage caused by volume expansion during charging and discharging,and maintain the structural stability of Sb₂Se₃.Different graphene oxide contents were optimized to synthesize pure phase Sb₂Se₃.Sb₂Se₃@rGO maintained a specific capacity of 685.6 m Ah g^-1 after 100 cycles at a current density of 0.1 A g^-1in the sodium storage performance test,and 650.3,601.4,600.8,596.9,579.7,561.9,514.0 m Ah g^-1 in the rate performance test at current densities of 0.1-5 A g^-1.The high pseudo-capacitance contribution is an important reason for its excellent performance.