title: Design and Synthesis of Graphitic Carbon Nitride (g-C3N4) Based Materials for Rechargeable Batteries
Creators: name: Adekoya, Oluwatobi
Publisher: name: Griffith University
Year: 2020
Subjects: subject: carbon nitride nanofibers
subject: lithium ion battery

description: Carbon nitrides are a unique family of nitrogen-rich carbon materials with multiple beneficial properties for effective alkali metal ion transport/storage. Graphitic carbon nitride (g-C3N4) is considered the most viable member of the carbon nitride family because of its high nitrogen content, wide structure with several nitrogen-defect pore sites, ease of synthesis, affordability, and scalability. Also, g-C3N4 delivers a lithium ion battery (LIBs) theoretical capacity of 524 mAh/g unlike graphite which records only 327 mAh/g. However, due to the ineffective intercalation/deintercalation reaction of Li+ with C3N4 it suffers low capacity, poor conductivity and structural deformation when applied as an anode material for battery application. Due to this problem, the application of g-C3N4 for LIBs has slowed down, and the prospects of g-C3N4 for emerging battery systems such as potassium ion batteries (KIBs) have not been explored. In this thesis, we present unique strategies to resolve the problems of irreversible Li+ intercalation, poor conductivity, and structural destruction, and explore g-C3N4-based composites for KIB system. In the first study, one-dimensional carbon nitride nanofibers were designed and proved to be a more effective and better performing anode material for LIBs than bulk g-C3N4. This work was accomplished by combining theoretical computing and experimental techniques, Density functional theory calculation showed that the edges of the 1D-g- C3N4 nanofibers exhibited a suitable Li adsorption energy for stress-free adsorption and desorption of adsorbed Li-atoms. Moreover, our synthesized 1D-g-C3N4 nanofiber possessed edges and pores, as well as higher pyridinic nitrogen content unlike the bulk g-C3N4. The 1D-g-C3N4 nanofiber delivered a superior specific capacity of 181.7 mAh/g, a specific capacity of 138.6 mAh/g after 5000 cycles when cycled at 10C along with excellent stability and power density. This performance remains the highest amongst reported C3N4 anode materials in literature. Carbon nitride/graphene (C3N4/graphene) heterostructure is commonly reported for lithium ion batteries and this heterostructure design occurs in different configurations of 1D/2D or 2D/2D. However, a clear theoretical understanding of how the configuration of such heterostructure affects battery performance is not established. By using a first principle theory approach we studied the 1D/2D and 2D/2D C3N4/graphene heterostructures with a focus on their conductivity, charge transfer, bond structure and rearrangement/breakage and theoretical reversible capacity. In all our study, the DFT results showed that 1D/2D C3N4/graphene delivers superior charge transfer, electronic conductivity, theoretical capacity, and structural integrity compared to 2D/2D configuration. This work expanded upon the relationship between the heterostructure configuration and the electrochemical performance, this work will encourage the design of effective heterostructures for rechargeable batteries. Motivated by the result of the 1D/2D C3N4/graphene heterostructure for LIBs, we employed it for potassium ion battery application. When the fabricated 1D-g-C3N4 nanofiber was employed in potassium ion batteries, the high nitrogen content facilitated K+ adsorption; however, the K-atom diffusion barrier was too high for effective adsorption/desorption. Therefore, we combined the 1D-g-C3N4 nanofiber with 2D reduced graphene oxide (rGO) to design a 1D/2D C3N4/rGO composite for stable and effective potassium storage. In this work, we also combined the use of Density Functional Theory calculations and experimental battery testing along with high powered characterization techniques to study the storage mechanism of the composite electrode material for potassium ion battery. The 1D/2D composite benefitted from the larger surface area and conductivity of 2D reduced graphene oxide and the nitrogen rich active sites of the 1D-g-C3N4 nanofiber. Additionally, DFT calculations showed that the graphene structure from 2D rGO possessed lower K-atom diffusion barrier and superior conductivity which provided shorter ionic transport distances and boosted electronic conductivity in the composite. Thanks to the synergistic interaction between the 1D-g-C3N4 nanofiber and 2D rGO, the electrode delivered a remarkable specific capacity of 464.9 mAh/g after 200 cycles at 1 A/g and 228.6 mAh/g after 1000 cycles at 10 A/g, which is one of the best potassium ion battery anode material performance reported so far. Another approach to exploring the benefits of the 1D-g-C3N4 nanofiber is to use it as a source of N-doped carbon. Metal oxides such as cobalt oxide (Co3O4) have been widely applied as anode materials in rechargeable LIBs but the small d-spacing limits their application for large-sized metal ion batteries such as potassium ion batteries. Moreover, through DFT calculations we proved that the poor performance of Co3O4 for KIBs is due to poor conductivity, high diffusion barrier, and weak potassium interaction. Thanks to the concept of interfacial engineering, we fabricated a hierarchical composite of Co3O4@N-doped carbon in which the N-doped carbon is derived from 1D-g-C3N4. The material design approach for the composite involved coating the surface of Co3O4 with N-doped carbon such K+ can be effectively transported through the that at the interface both materials via multiple ionic pathways. Furthermore, the structural design of the composite enabled increased Co3O4 spacing for effective K+ diffusion, improved conductivity, and protection of the core structure from damage. Based on the entire composite, a capacity of 448.7 mAh/g was delivered after 40 cycles, and 213 mAh/g was retained after 740 cycles when cycled at 500 mA/g. This work combined the principle of material boundary engineering with theoretical computation to design a composite anode material whose performance exceeded that of most metal-oxide-based KIB anodes reported in literature. In summary, the strategies presented in this thesis show that the morphology and electronic properties of g-C3N4 can be manipulated to resolve the problems of irreversible intercalation/deintercalation, poor conductivity, and structural deformation. Moreover, the application of g-C3N4 has been extended to potassium ion batteries and we are the first research group to demonstrate this in literature. Also, the electrochemical performances recorded from experimental battery testing and theoretical computation (DFT simulation) shows that g-C3N4 and g-C3N4-based materials are promising advanced anode materials for LIBs and KIBs. These strategies can be extended to other members of the carbon nitride family such as CN, C2N, C3N etc. for different metal-ion batteries.