Structure-Controlled Carbon As Anode Materials For Sodium Ion Batteries

With the rapid development of technology and our society,energy sources play a more and more important role in people's life.At present,lithium ion batteries,with high energy density and good cycle stability,have been put into commercial use.As a kind of green energy storage system,it has been gradually replacing the traditional batteries,such as lead-acid batteries and nickel-cadmium batteries.However,the price of lithium-containing precursor materials continues to rise with the growing markets of large-scale energy storage and electric vehicles.On the other hand,the lithium resources in the worldwide are not abundant and they distribute unevenly around the world,resulting in that lithium resources can not meet the future needs.So it is extremely important to develop a novel energy storage system to replace lithium ion batteries.Sodium and lithium belong to the same family in the periodic table and they have many similar chemical properties.Sodium ion batteries were originally researched in tandem with lithium batteries in the late 1970's and early 1980's.As lithium ion batteries have the higher specific energy density,sodium ion batteries have not been as extensively studied as lithium ion batteries.In recent years,with the development of large-scale energy storage and electric vehicles,sodium ion batteries have gained more and more attentions because of the abundant and low cost,and have become one of the alternatives to lithium ion batteries.The cathode materials of sodium ion batteries have made a great progress.The results show that the graphite anode materials for lithium ion batteries are not suitable for sodium ion batteries.Therefore,the carbon material with larger interlayer spacing is the choice of the anode materials for sodium ion batteries.In this work,we use simple and easy to industrialized methods for preparing soft carbon,phosphorus doped soft carbon,series of biomass hard carbon,coconut shell hard carbon and graphene and investigate their electrochemical performance as the anode materials for sodium ion batteries.The research contents are as follows:?1?Study on pitch-derived soft carbon:The pitch-derived soft carbon is synthesized via a pyrolysis condensation method using coal tar pitch as raw material.The results show that the soft carbon material has good electrochemical properties.The first discharge capacity is 158 mA h g^-1,and the capacity can still be maintained at 133 mA h g^-11 after 200 cycles at the current density of 100 mA g^-1,the coulombic efficiency reaches 99.9%.In addition,at the high current density,the soft carbon materials show excellent rate performances.?2?Study on the preparation and sodium-storage properties of phosphorus doped soft carbon:The effect of phosphorus doping on the electrochemical properties of soft carbon materials is also studied in this work.The results show that the specific capacity and rate performance of soft carbon materials have been significantly improved after doping phosphorus.The phosphorus doping soft carbon electrode exhibits excellent cycling stability,delivering a capacity of 201 mA h g^-1at the current density of 100 mA g^-11 after 200 cycles,which is much higher than the soft carbon materials without doping phosphorus(133 mA h g^-1).?3?Study on the preparation and sodium-storage properties of series of biomass hard carbon:In order to achieve the purpose of waste utilization,the biomass waste products as the carbon sources have been attracting great attention.Specifically,we use coconut shell,bamboo charcoal,apricot and walnut shell as the raw materials and prepared four kinds of biomass hard carbon materials.The results show that the hard carbon derived from coconut shell,bamboo charcoal,apricot shell and walnut shell have the similar properties.As the anode materials for sodium ion batteries,the hard carbon from the hard fiber biomass can not only realize the recycling of renewable materials,but also reduce the cost of hard carbon materials.?4?Study on the preparation and sodium-storage properties of coconut shell hard carbon:We use coconut shell as raw materials to prepare hard carbon as anode materials for sodium ion batteries.The morphology,structure and electrochemical properties of coconut shell hard carbon have been studied,and the optimal preparation conditions are obtained finally.The results show that the coconut shell hard carbon has an excellent electrochemical performance.The first discharge capacity is 274 mA h g^-1,the capacity can still be maintained at 267.4 mA h g^-1at the current density of 100 mA g^-11 after 200 cycles.In addition,it also shows an excellent rate performance at the high current density.To exemplify the actual performance,a rechargeable device with the coconut shell hard carbon as the anode and Na_0.67Mn_0.65Fe_0.35O₂ as the cathode in a pouch cell was constructed.It exhibits excellent cycling stability with a capacity retention of 90%after 200 cycles at the current rate of 1 C.?5?Study on the preparation and sodium-storage properties of graphene materials:We synthesize the two-dimensional graphene materials with a post oxidation treatment of graphite method at the last part of the paper.The experimental results show that,the cycling stability of graphene is pretty good without the irreversible capacity loss of the first cycle.The specific capacity of graphene materials can be maintained at 274.4 m A h g^-11 after 200 cycles,which shows a good sodium storage capacity.