Preparation And Properties Of Hard Carbon Anode Materials For Sodium Ion Batteries

In the near future,rapidly evolving lithium ion batteries will require lithium sources that are not available from existing resources.Therefore,there is a need to develop the next generation of post-lithium battery technology based on sustainable development to meet the growing demand for energy storage.Sodium is an abundant element in the earth and has similar redox properties to lithium.Therefore,this provides an attractive opportunity for sodium ion batteries to become a strong"candidate"for lithium ion batteries,especially for grid energy storage or low-speed/short-range power transportation.However,the graphite anode that dominates the commercial lithium ion batteries market cannot provide a similar storage capacity for sodium ion batteries as for lithium ion batteries.This means that there is an urgent need to develop an anode material with excellent performance to store sodium ions,and with a more sustainable raw material and preparation process than the current graphite anode.Among all the candidate anode materials,hard carbon has attracted great interest due to its good reversible sodium ion storage capacity.Unlike graphite,hard carbon consists of short-range graphitic domains and long-range amorphous features,and this structure contains many active sites,such as edges,defects and functional groups.However,hard carbon anode still has some problems,such as excellent multiplicative performance and cycling stability at high current densities.Morphology and doping modification are considered as effective methods to improve the sodium storage performance of hard carbon anode.Therefore,in this thesis,doped hard carbon materials with special morphology were synthesized by modulating the morphology of precursors and heteroatom doping,which led to a significant enhancement and improvement of their overall electrochemical performance.The main studies are:（1）Rod precursors were synthesized by self-assembly in a binary solvent system using biomass plant polyphenol tannins as raw materials,followed by direct carbonization to form carbon rods（PTA-700）.When used as the negative electrode of sodium ion batteries,the reversible capacity was 218 m Ah g^-1 after 100 cycles at a current density of 0.1 A g^-1.The reversible specific capacity remained at 114.0 m Ah g^-1 even after 3000 cycles at a high current density of 1 A g^-1.The excellent electrochemical performance was attributed to the abundant mesopores and defects of the carbon rods,which can provide more active sites and space for sodium ion storage.These features can provide more active sites and space for sodium ion storage and facilitate the transport transfer of sodium ions.（2）A high proportion of active nitrogen-doped hard carbon（PTA-Lys-800）was synthesized by the Mannich reaction using amino acids and tannic acid as the nitrogen and carbon sources.For sodium ion batteries,PTA-Lys-800 provides excellent cycling stability and multiplicative performance(reversible specific capacity of 338.8 m Ah g^-1 with 86%capacity retention for 100 cycles at a current density of 100 m A g^-1;capacity of 131.1 m Ah g^-1 at a current density of 4 A g^-1).The outstanding PTA-Lys-800 electrode performance was attributed to the formation of a stable pore structure and a high proportion of N-5 during the carbonization process.the abundant pores facilitated the rapid diffusion of ions,while the high proportion of N-5 introduced more defects and active sites.The sodium storage mechanism of the PTA-Lys-800 electrode was analyzed by combining pseudocapacitance behavior analysis and non-in situ Raman spectroscopy,and the adsorption-dominated capacitive behavior dominated the charging and discharging process.（3）Hollow carbon spheres（PTA-NHCS-700）containing up to 9.58 wt%nitrogen were successfully prepared by rational design of Schiff base chemistry with tannic acid by introducing diethylenetriamine as the nitrogen source.The carbonization process formed a stable graded pore structure,moderate defects and large specific surface area.The unique and robust hollow spherical structure ensures continuous sodium ion transfer and short diffusion distance,and the high content of nitrogen doping improves the surface wettability of the carbon material and accelerates charge transfer by generating additional defects.The application to the negative electrode of sodium ion batteries demonstrates excellent electrochemical performance.The capacity was 154.2 m Ah g^-1 after 10000 cycles at a current density of 5 A g^-1.This study provides a new idea for the preparation of high performance sodium ion battery anode materials by achieving the morphology modulation and doping modification of carbon materials through a simple synthetic method.