Defect Modulation And Potassium Storage Properties Studies Of Carbon Materials

Lithium-ion batteries（LIBs）have been commercialized on a large scale,the development of new secondary batteries is imminent due to the shortage of lithium resources.Potassium-ion batteries（PIBs）possess abundant raw material resources,low cost,and similar chemical properties to lithium,which provides a wide range of application prospects for PIBs.Graphite,a typical potassium storage anode material,forms KC₈ compounds through an intercalation process.However,the large size of K⁺leads to diffusion kinetic hysteresis between carbon layers and volume expansion of up to 61%,resulting in severe capacity decay,poor cycle life and rate performance of graphite anode,among other problems.Soft carbon possesses a turbine-layered carbon lattice structure,which is more flexible structure.However,potassium storage capacity and rate performance still need to be improved due to the lack of energy storage active sites.Hard carbon with rich defect active sites can significantly improve electrochemical potassium storage performance.However,hard carbon surface exposes a large volume of defective structures,which will react irreversibly with K⁺in the process of electrochemical potassium storage,thus reducing its cyclic stability and initial coulomb efficiency.The sloped region is determined by the surface adsorbed K⁺,while the low potential plateau region is driven by the K⁺intercalation in the charge/discharge curve.The introduction of pore and defective structures can boost the active sites on the carbon surface,which in turn can improve the storage space and fast diffusion kinetics of K⁺.However,defect-dominated capacitive potassium storage behavior results in low initial coulomb efficiency,sloped curves at low potentials,and low materials density.Therefore,balancing the two potassium storage mechanisms through effective carbon microstructure modulation is an important means to improve the electrochemical properties of carbon materials.In this paper,the micro-structure of carbon materials is regulated by defect engineering to achieve excellent electrochemical potassium storage performance.The main contents and results are as follows:（1）The introduction of pore structure and carbon defects improves the performance of potassium storage.Porous carbon nanosheet microspheres（PCNMs）were prepared by chemical deposition on basic zinc carbonate（Zn₅（CO₃）₂（OH）₆）microspheres using pitch as carbon source.Compared with bulk carbon（CMs）,PCNMs showed more abundant mesoporous and carbon defects,resulting in excellent electrochemical properties of potassium and lithium storage.Benefiting from rich energy storage active site and fast ion diffusion kinetics,PCNMs-700 exhibits a superior reversible capacity(323 m Ah g^-1 at 0.05 A g^-1)and a high-rate(157 m Ah g^-1at 1 A g^-1)in potassium-ion half cells.Meanwhile,the PCNMs-700 electrode also maintains high reversible lithium storage capacity and fast lithium storage performance in lithium-ion half cells.In addition,when being applied to potassium-ion hybrid capacitors（PIHCs）,PCNMs-700 electrode exhibits stable and efficient electrochemical potassium storage performance.（2）Pore expansion induces carbon defects and enhances rapid potassium storage.The in-situ pulverization-reaggregation of basic magnesium carbonate realizes the accurate modulation of pore width in carbon with pitch as the carbon precursor.The flake-like basic magnesium carbonate is in-situ pulverized into Mg O nanoparticles during annealing,serving as self-templates to introduce pores in carbon nanosheets（CNS）.Then the nanoparticles reaggregated during further high-temperature annealing to etch the carbon matrix to induce the pore expansion from 5.0 to 9.3 nm.At the same time,the pore expanding strategy successfully realized the modulation of carbon structure and increased the carbon defect density.Benefiting from the in-situ pore expanding strategy,CNS-1100 possesses both rich defect-sites and fast K⁺diffusion kinetics,leading to high reversible capacity(402 m Ah g^-1 at 0.1 A g^-1)and high-rate(217 m Ah g^-1 at 1 A g^-1).Moreover,this strategy can be successfully applied to basic zinc carbonate,basic nickel carbonate and basic cupric carbonate.（3）Carbon defect regulation assists reversible and rapid electrochemical potassium storage.Hard carbon spheres（HCS）with tunable structure and controllable defects were fabricated by carbonizing phenolic resin in the range of 800～2400°C.By adjusting the pyrolysis temperature,the variation of carbon defect density of HCS was analyzed in detail with the help of XRD and Raman,and then the evolution of the microstructure of HCS with the pyrolysis temperature was deduced.The relationship between the microstructure and electrochemical properties of HCS was determined by electrochemical tests.The resulting material is used as anode electrode materials for PIBs,HCS-1100 electrode exhibits a plateau capacity of 225.3 m Ah g^-1 with a 60.5%reversibility of the plateau capacity.The HCS-1100 electrode performs a reversible capacity of up to 202.6 m Ah g^-1 at 1 A g^-1.In further,the relationship between the local carbon structure evolution and the potassium/sodium ion storage mechanism was further deduced by studying the electrochemical profiles of potassium/sodium storage in HCS samples.In this thesis,there are 75 figures,13 tables and 153 reference articles.