Preparation And Performance Of Heteroatoms Dual-doped Hierarchical Porous Carbon Materials In Alkali Metal-selenium Batteries

In the past few decades,under the rapid development of science and technology,mankind consumes fossil fuels without restriction,causing problems such as the greenhouse effect and environmental pollution.For the long-term development of mankind,it is inevitable to move towards to a low-carbon,sustainable and clean energy structure.As an efficient energy storage and conversion devices,secondary batteries undoubtedly play an important role in the transformation of energy consumption structure.With the development of the society,the traditional commercial secondary battery cathode materials are limited by their theoretical specific capacity,and it has been difficult to meet human requirements for higher energy density of secondary batteries.Therefore,it is extremely important to develop cathode materials for secondary batteries with higher specific energy.In recent years,selenium（Se）,as a new type of cathode material with high electrical conductivity and high theoretical energy density(3253 Wh kg^-1),has attracted the attention of researchers and has become a potential cathode material for secondary batteries.However,the serious volume expansion of the Se cathode during the electrochemical cycle,the"shuttle effect"caused by the dissolution of polyselenide,and the occurrence of irreversible side reactions will reduce the utilization of active materials and limit their further development and application.In response to the above problems,this doctoral dissertation mainly focuses on how to anchor Se through the synergistic effect of spatial confinement and dual-doped heteroatoms,and effectively improve the electrochemical performance of electrode materials.The following research results have been achieved:First,the precursor material of polydopamine（PDA）was prepared by hydrothermal method,and potassium hydroxide was used as an etchant to prepare a heteroatoms（N and O）dual-doped hierarchical porous carbon（HDHPC）matrix by a one-step sintering method.X-ray photoelectric spectroscopy test results show that the amino and hydroxyl groups originating from PDA are converted into pyrrolic nitrogen,pyridinic nitrogen and various oxygen-containing groups in HDHPC,and the electrochemical properties of Se/HDHPC electrode materials are improved.First-principles calculations show that the doping of heteroatoms in the composite material effectively strengthen the interaction between HDHDC and the active material Se.Supplemented by the effective load and volume buffer effect of the porous structure,the Se/HDHPC composite exhibits excellent electrochemical performance in the lithium-selenium（Li-Se）battery.At a current density of 0.5 C(1C=675 m Ah g^-1),a reversible electrochemical capacity up to 545 m Ah g^-1 can be maintained after 1500 cycles,and the capacity decay rate is as low as 0.0074%per electrochemical cycle.Subsequently,based on the research in the previous part,we extend the application of the prepared Se/HHPC composite electrode material to the fields of sodium ion batteries（SIBs）and potassium ion batteries（PIBs）.The electrochemical properties of the Se/HHPC cathode in SIBs and PIBs are studied.In sodium-selenium（Na-Se）batteries,Se/HHPC exhibits high cycle performance(402m Ah g^-1 after 500 cycles at 0.5 C),which is better than its performance in potassium-selenium（K-Se）batteries(228 m Ah g^-1 after 200 cycles at 0.2 C).We infer that this may be due to the gradual increase in the radius of Na⁺and K⁺,which not only retard the kinetics of the electrode,but also increass the difficulty of the reaction between the alkali metal ions and the active material in the porous structure,leading to a reduction in reversible capacity.At last,adjust the ratio of poly-3,4-ethylenedioxythiophene to potassium hydroxide,and obtain sulfur,oxygen co-doped hierarchical porous carbon（SO-HPC）with the best pore structure through one-step sintering and etching.The abundant ultra-micropores in SO-HPC not only ensure the sufficient contact between Se and electrolyte,but also suppress the shuttle effect and side reactions of selenide during redox process.In addition,the uniform distribution of in-situ S,O co-doped carbon material introduces more active sites and improves the ionic conductivity of the electrode material and enhances of the chemical adsorption between the carbon matrix and Li₂Se.When used as a cathode electrode of an alkali metal-Se battery,Se/SO-HPC exhibits excellent cycle performance.In high and low temperature tests,the outstanding electrochemical performance of the Se/SO-HPC cathode materials in alkali metal-Se batteries,especially Li-Se batteries,also illustrates the great potential of Se as a high-performance and practical secondary battery cathode material.To sum up,the relevant research in this paper mainly focuses on the application of electrode material composed of heteroatom dual-doped hierarchical porous carbon and Se in alkali metal-selenium batteries in a wide temperature range.We select suitable precursors to obtain in-situ heteroatom-doped hierarchical porous carbon with good pore structure through carbonization/activation method,and improve the electrochemical performance of the composite material.The works provide new direction for advancing the further research of Se cathode materials and moving towards practical application.