Structural Design Of Graphene And Its Application In Lithium Air Batteries

Lithium air batteries with extremely high theoretical energy density have received great attention in recent years and are expected to replace traditional lithium ion batteries as electric vehicle power batteries or applications in portable electronic devices to promote the resolution of energy and environmental issues.However,lithium air batteries generally have many scientific and technical problems such as high overpotential,insufficient rate performance and poor cycle stability,and it is necessary to develop a cathode having high catalytic activity and matching structure.With its many excellent properties,carbon materials have shown great vitality in the research of lithium air battery cathodes.Among them,graphene with super large specific surface area and ultra-high conductivity is an ideal cathode material.However,the low catalytic activity of graphene limits its further development in lithium air batteries,so it is necessary to make proper structural design based on the excellent use of graphene.In this thesis,the lithium air battery of organic electrolyte system was taken as the research object.Graphene was selected as the basic cathode material.Through the structural design and surface modification of graphene,the performance of lithium air battery was improved,and the high-activity cathode suitable for lithium air battery was explored to promote the further development and application of lithium air batteries.In this thesis,a high-quality three-dimensional graphene is prepared by improved chemical vapor deposition?CVD?method.On the basis of this,the three-dimensional graphene with hierarchical porous structure is prepared by partially oxidizing the nickel foam substrate by exposing air during CVD.The efficient use of the large specific surface area of three-dimensional graphene and the construction of hierarchical porous structure make it rich in catalytic active sites and material transport channels.Used as an independent cathode in lithium air batteries,it shows a high specific capacity of 9559 mAh g^-1 at a current density of 57 mA g^-1 and maintains a specific capacity of 3988 mAh g^-1 at a high current density of 285 mA g^-1.In the current density range of 57-171 mA g^-1,the battery can be stably cycled for150 cycles without significant degradation.Further research found that the high activity of graphene is mainly derived from its boundary carbon suspension.According to this,we use carbon quantum dots with high density boundaries as catalysts in the cathode of lithium air batteries.The carbon quantum dots?supernatant and precipitation carbon quantum dots after centrifugation?were prepared by a hydrothermal method using glucose as carbon source.The composite cathode obtained by immersing it on the surface of carbon paper and drying showed an extremely high specific capacity of 68900 mAh g^-1 and 80040 mAh g^-1,respectively.And the two composite cathodes can be stably cycled for 300 cycles at current densities of 1333 mA g^-1 and 800 mA g^-1,respectively.By comparing and analyzing the two carbon quantum dots,it is found that the carbon quantum dots derived from precipitation have higher catalytic activity,and the performance improvement of the lithium air battery is more remarkable.