Structure Design And Calcium Storage Properties Of Organic Electrode Materials

The contradiction between the increasing demand for sustainable energy and the increasingly depleted fossil oil resources has prompted the rapid development of low-cost,high-security energy storage devices.In recent years,lithium-ion batteries,as the most successful commercial energy storage devices,have been widely used in various portable electronic devices and electric vehicles.However,due to the limited abundance and uneven distribution of lithium resources,the increasing demand for lithium resources leads to a sharp increase in its price.So it is considered to be difficult to satisfy the demand for scale energy storage.In contrast,as the fifth most abundant element in the Earth’s crust,calcium has the advantage of low price.Compared with monovalent ions（Li,Na,K）,divalent Ca²⁺can transfer twice the charge number when the same number of intercalation ions leading to higher capacity.In addition,Ca²⁺has a lower polarization force than other multivalent ions（Zn,Mg,Al）,leading to faster kinetics.Therefore,calcium ion batteries are expected to be one of the candidates for the next generation of scale energy storage systems.However,the current calcium ion battery system is still infant.Among the reported calcium storage materials,most inorganic materials show problems such as slow kinetics and poor rate performance,which is due to the strong electrostatic interaction between calcium ions and the inorganic material lattice.In contrast,the high reactivity and fast reaction kinetics of organic materials can effectively solve the problem of sluggish Ca²⁺kinetics.However,the currently reported organic electrode materials still face challenges such as material dissolution,poor electronic conductivity,and low capacity.Therefore,the development of high-performance organic electrode materials is the key to promote the development of calcium ion battery systems.In view of the above problems,this paper started from the improvement of polymer electrode materials,through the structure design,using conductive carbon network to improve the conductivity and inhibit dissolution of material.To avoid the reduction of energy density caused by the introduction of excessive carbon sources,covalent organic framework materials（COFs）,which are intrinsically difficult to dissolve and facilitate to transport electrons,were selected to replace the polymer materials.Through structural design and molecular regulation,various COFs electrode materials with high capacity,high rate performance and long cycle stability were prepared,and electrochemical performance and mechanism were systematically studied.The main research contents are as follows:First,we report in situ formed poly（anthraquinonyl sulfide）（PAQS）@CNT composite as non-aqueous calcium ion battery cathode.The rapid diffusion of Ca²⁺is facilitated by the fast kinetics of the enolization redox chemistry of organic compounds and the accelerated electron transport by the introduction of CNT to form a complete conductive network.Moreover,the electronic conductivity of PAQS increases with the increase of the CNT content introduced,and the voltage gap between oxidation and reduction state decreases.The PAQS@CNT electrode exhibited a specific capacity of 116 m Ah g^-1 at a current density of0.05 A g^-1.It also exhibits excellent rate performance with a specific capacity of 60 m Ah g^-1at 4 A g^-1 current density.The demonstrated rate performance exceeds that of inorganic anode materials reported so far in non-aqueous organic calcium ion batteries.In addition,PAQS@CNT also demonstrated excellent cycle stability,with a capacity retention rate of 83%after 500 cycles.The electrochemical mechanism is proved to be that the PAQS undergo reduction reaction of their carbonyl bond during discharge and become coordinated by Ca²⁺and Ca（TFSI）⁺species.Computational simulation also suggests that the construction of Ca²⁺and Ca（TFSI）⁺co-intercalation in the PAQS is the most reasonable pathway.Secondly,we synthesized a nitrogen-and oxygen-rich quinone functionalized1,4,5,8,9,12-hexaazatriphenylene-based COF（TB-COF）.TB-COF was firstly investigated as anode material in aqueous calcium ion battery.Due to the abundant active sites and N atoms,TB-COF has good electronic conductivity and high capacity.At a current density of1 A g^-1,TB-COF exhibits a specific capacity of 275 m Ah g^-1.At a high current density of 30A g^-1,TB-COF still shows a capacity of 109 m Ah g^-1.The demonstrated rate performance exceeds that of most of the currently reported anode materials for calcium ion batteries.Due to the stable COF structure,the capacity decay rate of TB-COF is only 0.01%per cycle after3000 cycles.Mechanism studies showed that Ca²⁺/H⁺co-intercalate in TB-COF and coordinate C=O and C=N active sites.In addition,we have detected a novel C=C active site.Based on theoretical calculation and experimental analysis,TB-COF can store calcium in three steps and store up to 9 Ca²⁺,and the C=C active site is located in the third step.The evolution of the radical intermediate can explain the mechanism of the C=C active site in detail.To verify the practicality of the material,we also assembled the TB-COF with Prussian blue cathode as an aqueous full cell.The full cell exhibited an energy density of 141 Wh kg^-1 and a power density of 4172 W kg^-1,as well as long-cycle stability with 74%capacity retention after 3000 cycles.Finally,we replaced the ligand on the basis of TB-COF and increased the pore size based on COFs modular structure feature and synthesized a new hexaazatriphenylene-based COF（TP-COF）.TP-COF has similar theoretical capacity as TB-COF as well as larger pore size,which facilitates faster Ca²⁺transport and thus further improves the rate performance.At a current density of 30 A g^-1,TP-COF successfully exhibited a better rate performance than TB-COF with a specific capacity of 145 m Ah g^-1.The effect of pore size on ion transport is thus proved.TP-COF also exhibited excellent cycling stability with a capacity retention rate of 80%after 1000 cycles.The mechanism study showed that Ca²⁺/H⁺was co-intercalate in the TP-COF material and combined with C=O/C=N.The theoretical calculation shows that the reaction is divided into two steps of calcium intercalation.The results obtained in this doctoral dissertation show significant research ideas for the construction of organic electrode materials for high-performance calcium ion batteries,and also provide new insights into calcium storage mechanism.