Structural Design And Regulation Of Carbon-based Materials And Their Sodium/potassium Storage Properties

The application scope of lithium-ion secondary batteries is constantly expanding and has entered various fields of social life.However,the limited and unevenly distributed lithium resources can hardly meet the rapidly growing market demand.Although the energy density of sodium-ion batteries and potassium-ion batteries is lower than that of lithium-ion batteries,they have the advantages of abundant resources,low cost and high low-temperature performance,and are expected to replace lithium-ion batteries in some application scenarios,such as low-speed electric vehicles and grid energy storage.The ionic radius of sodium and potassium ions is larger than that of lithium,thus offering advantages such as low desolvation energy,high ionic conductivity,and high carrier mobility;however,the larger ionic radius also leads to problems such as slow reaction kinetics and large volume changes during discharge/charge.This dissertation focuses on the structural modulation of carbon based anode materials for sodium/potassium ion batteries,starting from the aspects of the heteroatom doping,defect concentration,carbon interlayer spacing,micro-nano structure,heterostructure,etc.,and is committed to improving the comprehensive electrochemical performance of electrodes.The reaction mechanism and electrochemical kinetics of related electrode materials are systematically investigated,and the evolutionary relationship between structure-performance is clarified.The main research contents and results are as follows:（1）Sulfur-rich N-doped porous carbon framework（SRNDC）was prepared by introducing C-S_x-C（x≤3）bonds into the subsurface of three-dimensional porous carbon through the cyclization dehydrogenation process of porous precursor with the assistance of sublimated sulfur.The subsurface interlayer spacing of SRNDC-700 was expanded to 0.40 nm under the synergistic effects of surface tensile stress,heteroatom size effect and C-S_x-C bond angle effect,which significantly increased the thickness of the accessible subsurface layer at high current densities.SRNDC-700 electrodes display a high discharge specific capacities of 160.6 and 69.5 m Ah g^-1 at 20 and 50 A g^-1,respectively.Due to the high reversibility of C-S_x-C（x≤3）bonds,the average capacity decay rate of SRNDC-700 is only 0.0025%after 6000 cycles at 10 A g^-1.When the scan rate reaches 10 m V s^-1,the pseudocapacitance-controlled capacity retention is close to 100%,while that of the diffusion-controlled capacity is less than20%.（2）Few-layer exfoliated graphite is coated with amorphous carbon（EG@NC）.By changing the properties of the amorphous carbon layer,the potassium ion concentration in the coating layer is increased,and the transport rate of potassium ions in the graphitized carbon interlayer is accelerated by taking advantage of the inner and outer ion concentration gradient.The calculated results of the interfacial concentration difference show that the concentration of potassium ions at the EG/NC interface is 8times that of the electrolyte in the optimized sample（EG@NC-200）.Research shows that a bottleneck section occurs during the transport of potassium ions in pure exfoliated graphite（EG）,and the apparent diffusion coefficient（D_A）of this section is4×10^-12 cm² s^-1;but in the corresponding phase transition stage,the D_A value of EG@NC-200 is about 1.4×10^-9 cm² s^-1,an improvement of nearly three orders of magnitude.In situ XRD measurements show that increasing the potassium ion concentration gradient at the interface can promote the formation of high stage potassium-graphite intercalation compounds（K-GICs）and accelerate the transition from stageⅢK-GIC to stageⅡK-GIC.Even at a current density of 1.6 A g^-1,the discharge specific capacity of EG@NC-200 below 0.5 V is as high as 150 m Ah g^-1,which is nearly 20 times that of EG.This suggests that the strategy of concentration gradient-dirven rapid potassium ion diffusion can improve the power density of the full cell without sacrificing its energy density.In addition,the ion-enrichment effect of the outer amorphous carbon layer enables EG@NC-200 to also exhibit excellent potassium storage performance in 0.4 M KPF₆ carbonate-based electrolyte,greatly reducing the need for electrolyte solute concentration.（3）The complexes were obtained by the chelation of amino trimethylene phosphonic acid with divalent cations(Co²⁺);subsequently,the composite containing hierarchical carbon and cobalt pyrophosphate,denoted as Co₂P₂O₇/C@C,was prepared by coating dopamine on the surface of the complexes and carbonizing them at 700°C.In this composite,hierarchical carbon acts as an electron transport channel,and pyrophosphate provides an ion transport channel,thereby constructing an electron/ion dual transport channel and improving the reaction kinetics of the composite.The specific capacity of Co₂P₂O₇/C@C-700 is 346.4 m Ah g^-1 at 0.1 A g^-1and 190.3 m Ah g^-1 when the current density is increased to 5 A g^-1,with a capacity retention of 55%.Usually,the pseudocapacitance ratio of the conversion negative electrode is less than 70%at a low scan rate of 0.1 m V s^-1,but that of Co₂P₂O₇/C@C-700 is as high as 85%and reaches 95%at 1.0 m V s^-1.Benefiting from the stability of the hierarchical carbon structure and the high reversibility of the cobalt redox active centers,Co₂P₂O₇/C@C-700 exhibits a capacity retention of 71%after1000 cycles at 2 A g^-1.Furthermore,this strategy is versatile and can be extended to the preparation of other M₂P₂O₇/C@C（M=Fe,Ni,Mn,Zn,etc.）.（4）The ferric trichloride-graphite interlayer compounds（Fe Cl₃-GICs）were studied to form ternary intercalation compounds of Fe Cl₃-LA-GICs by inserting dodecylamine（LA）into Fe Cl₃-GICs through the coordination of Fe³⁺with amine groups.The ternary GICs can be converted into composite with stacked structure of iron sulfide and low-defect-concentration few-layer graphene（Fe S_x@DFG）by a first oxidation-then-sulfuration strategy.The low-defect-concentration of graphene not only reduces the occurrence of irreversible reactions,but also improves the reversibility of Fe S_x between graphene layers,resulting in an average initial coulomb efficiency（ICE）of 91%for Fe S_x@DFG,while the ICE of the same type of composite is around 60%.In the ether electrolyte system,the average diffusion coefficient of Fe S_x@DFG reaches 10^-9 cm² s^-1,which is comparable to the kinetics of co-intercalation of Na⁺-ether solvent complexes into graphite,thus exhibiting excellent rate capability.The discharge specific capacity of Fe S_x@DFG is as high as391 m Ah g^-1 at a current density of 10 A g^-1,which is 70%of the specific capacity at0.1 A g^-1.