The Application Research Of Doped Modified Carbon In Alkali Metal-ion Batteries

The effective utilization of intermittent power and efficient transmission of power need more excellent large-scale energy storage systems（ESSs）technology.Due to the scarcity and uneven distribution of lithium resources in the earth’s crust,the feasibility of large-scale energy storage of lithium ion batteries has been controversial.Therefore,there is great interest in developing alternative energy storage systems based on the earth’s abundant materials.Sodium ion batteries（NIBs）have abundant sodium reserves and good availability,which can meet the material supply and cost demand of large-scale smart energy storage.Double-ion batteries（DIBs）system has attracted extensive attention due to its advantages of low cost,high energy and environmental protection,etc.The high-performance and low-cost sodium-based double-ion battery makes NIBs and large-scale ESSs have good compatibility,which determines their effectiveness in practical applications.One of the main challenges in developing NIBs was the anode material,as graphite could not be reversibly inserted（or reacted with）sodium at sufficient rate,in contrast to lithium-ion batteries,which were reversibly inserted in C to form LiC₆.Therefore,NIBs needs substituted anode materials.In addition,in recent years,due to the abundance of potassium resources and potassium ion can well intercalate with commercial graphite anode of lithium ion battery,potassium ion batteries（KIBs）cause wide attention in the field of electrochemical energy storage,and have achieved continuously improved battery performance,making KIBs a kind of promising energy storage system.It is well known that heteroatom-doped carbon materials usually have good electrochemical properties,such as enhanced electrical conductivity and capacity.The enhanced electrochemical performance is mostly due to the heteroatoms in the carbonaceous materials which could generate extrinsic defects and provide additional reaction sites.Up to now,carbonaceous materials doped with nitrogen atom（N）,phosphorus atom（P）or sulfur atom（S）are mostly investigated and have achieved significant progress.Compared with other heteroatoms,the oxygen element is very common in organic materials,especially for the carbohydrate and carbonyl compounds.More importantly,the reactive oxygen groups could afford extra capacity through the transform of carboxide and enolate.Combining the advantages of the oxygen and non-oxygen atoms,it will effectively improve the electron ion transfer rate,increase the reaction sites,and promote the penetration of electrolyte,so as to obtain carbon nanomaterials with excellent electrochemical properties.In this thesis,based on the organic precursor,different soft carbon materials were synthesized by a simple and reproducible thermal polymerization method and used in double sodium ion,potassium ion battery and potassium-sulfur battery,and also explored the electrochemical reaction mechanism of different battery systems.The main contents and innovative points are as follows:（1）Phosphorus-doped soft carbon（P-SC）and soft carbon（SC）were prepared by slow thermal polymerization of PTCDA.It can be seen from the morphologies that doped with phosphorus the size of P-SC becomes smaller,and phosphorus and oxygen in P-SC are homogeneously doped into the carbonaceous substrate.Combined with the calculation results,the presence of phosphorus atom or phospho-oxygen incorporated into SC would improve the metallic behavior,improving the electrical conductivity of the electrode materials,thus facilitating the rapid storage of sodium ions.On the other hand,a solid-electrolyte interphase（SEI）could pre-form in the anode during the pre-sodiation process,thus the anode has no need to consume large amount Na⁺to form SEI during the cycling of the NDIBs.Consequently,the NDIBs with P-SC anode after pre-sodiation process exhibit high Coulombic efficiency（over 90%）and long cycle stability(81 mA h g^–1at 1000 mA g^–1 after900 cycles with capacity retention of 81.8%),far more superior to the un-sodiated NDIBs.（2）Phosphorus-doped minisize soft carbon（P-mSC）was prepared by simple slow thermal polymerization of triphenylphosphine and PTCDA precursor.After doped with phosphorus,instead of the rod structure of soft carbon a small size pieces containing PTCDA characteristics was formed.At the same time,phosphorus and oxygen in P-mSC are also uniformly doped into the carbon matrix.Similar to P-SC,P doping is conducive to improving the conductivity of the material and the rapid storage of potassium ions.In addition,the small size increases the electrochemical active site,shortens the K⁺transmission distance,and makes P-mSC exhibit excellent potassium ion storage performance.At the current density of100 mA g^–1,the initial discharge capacities of P-mSC and SC were 576 mA h g^–1 and 449 mA h g^–1,respectively.After 4000 cycles at 500 mA g^–1 current density,P-mSC electrode can also show a reversible capacity of 177 mA h g^–1,while the capacity of SC is relatively low.（3）Nitrogen doped porous carbon was prepared by a simple and repeatable thermal polymerization of Phthalocyanine iron（II）precursor with SiO₂ as template.According to our speculation,at a temperature of 1000oC,SiO₂ particles under 50 nanometers may be in molten,guaranteed the carbon matrix is uniform and hollow structure,which could buffer volume expansion during insertion of K⁺.The appropriate degree of graphitization ensures the stability of electrode materials during the electrochemical cycles.Combined with theoretical calculation,both N doping and carbon defects are beneficial to K⁺storage.Due to the presence of N doping and defects,large specific surface area,special mesoporous structure,excellent electrical conductivity and graphitization,the electrode,as apotassium ions anode,exhibits long cycle life and good rate.After 2000 cycles at 1000 mA g^–1 current density,a reversible capacity of the electrode was maintained at 160 mAh g^–1.（4）Confined and covalent sulfur cathode（CCS）was prepared by a simple slow thermal polymerization of acrylonitrile and sulfur precursor and applied to potassium sulfur battery.Compared the electrode performance at different temperatures,and proved the sulfur in the CCS is confined and covalent,combined with the ester based electrolyte,effectively inhibite shuttling effect.The 450oC annealed CCS shows the highest capacity,possibly due to its suitable sulfur content and high conductivity,making it a promising next-generation cathode material for potassium sulfur batteries.In addition,the changes of CCS cathode under different voltage states and the best voltage range of the K-S battery were explored.Herein,a high energy and stable room temperature K-S battery was developed with a confined and covalent sulfur cathode,which could deliver an energy density as high as⁴45 Wh Kg^–1,with a Coulombic efficiency close to 100%,and a superior cycle stability with a capacity retention of 86.3%over 300 cycles at a voltage cut-off of 0.8-3.0 V.