Design And Mechanism Studies Of Porous Carbon-selenium Composites For Lithium-selenium Battery Cathodes

Li-Se batteries,characterized by high theoretical energy density,are one of the ideal choices for distributed energy storage systems.Combining porous carbons and elemental Se to form coating structure of the Se/C composites can effectively alleviate the volume expansion of Se and inhibit the shuttle effect of polyselenide,which is one of the most effective cathode design strategies for Li-Se batteries.Due to the lack of efficient characterization,the structural properties,material physical properties,and discharge mechanisms of Se/C composites are still unclear,which severely restrict the development of Li-Se batteries and Alkalis-Se batteries systems.In the thesis,we focus our reserches on the Se/C composites.Firstly,novel hierarchical porous carbons are designed and initially applied to Li-Se batteries.Then,the confined structure of Se and the corresponding discharge mechanism were clarified by various in-situ and ex-situ techniques such as NMR and EPR.Finally,the material physical properties of the Se/C composite were discussed,and the relationship between the structural properties and the electrochemical performance was preliminarily established.The main contents and results of the thesis are as follows:(1)A series of hierarchical porous carbon materials were designed and synthesized and their electrochemical properties were preliminarily studied.Taking chitosan as carbon source and tin dioxide as sacrificial template,A series of hierarchically porous carbons CISC-x(x=29/58/87)with adjustable mesopore contents were prepared by removing the template in a one-step high-temperature method.When studying the electrochemical properties of hierarchical porous carbons,it is found that the CISC-29 porous carbon electrode can take into account both the energy density and high power density of electrochemical electric double layer capacitors(EDLC).Especially,the symmetric cell assembled with CISC-29 electrode exhibited a specific capacitance of 40 F/g and a capacitance retention of 98.4%after 20000 cycles.Taking 19F and 11B as NMR probes,solid-state NMR experiments combined with NLDFT pore volume-pore size distribution model show that CISC-29 has suitable meso/micropore ratio,abundant ultramicropore volume and good interpore connectivity.These advantages may facilitate the occurrence of desolvation effects,which may be the reason why CISC-29 exhibits excellent electrochemical performance.Finally,Se@CISC-29 were applied as the cathodes for Li-Se batteries.The specific capacity of Se@CISC-29 hardly decayed after 50 cycles at a low current density of 0.1 A/g,and it still maintained a reversible specific capacity of 590.8 mA/g after 500 cycles at a high current density of 1A/g.(2)Multiple characterization techniques to clarify the confined structure and discharge mechanism of Se in Se/C composites.On the basis of the above material design scheme,a hierarchical porous carbon CISC-58 containing equal pore volume of micropores and mesopores was successfully synthesized and applied as the host of Se cathode for Li-Se batteries.The Se@CISC-58 composite cathodes provide an initial specific capacity of up to 689.5 mAh/g at 1 C,and maintain a reversible capacity of 420.5 mAh/g after 1000 cycles,while achieving high Initial Coulombic Efficiency(ICE)as 72.6%.SsNMR shows that confined Se in Se@CISC-58 belongs to an amorphous chain state.EPR experiments further found that the confined Se and Li2Se exhibit characteristic radical signals,respectively.This proves that the confined Se exists as disordered coiled Sex chains in the CISC-58,while Li2Se exists as crystallites containing a large number of defects.Based on the experimental results of ex-situ EPR,in-situ NMR and MAS NMR,we have proposed the multi-step discharge mechanism of the Se@CISC-58 composite cathodes in the carbonate electrolytes:During the discharge process,the Sex chains undergoes electrophilic lithiation and cracking firstly to generate long-chain polyselenides,then the long-chain polyselenides transform to the low-order polyselenides and Li2Se2,and transform to Li2Se finally.The discharge mechanism reveals that the Sex chains undergoe irreversible structural transformation after the first cycle,which may contribute to the low initial Coulombic efficiency(ICE)of Li-Se batteries.This work provides some new ideas for the design of cathode materials for high-performance Li-Se batteries.(3)The glass transition and chain dispersibility of confined Se enhance the electrochemical performance of Se/C composites.We discovered the glass transition phenomenon of confined Sex chains(clusters)in Se/C composites at 37℃ by DSC and ssNMR,and the confined Se in different pore structures show different glass transition behaviors.The structure-dependent diffusion kinetics of ions can be regulated by using glass transition and changing the dispersion of Sex chains.Electrochemical test show that increasing the test temperature to around Tg and increasing the melting diffusion temperature to 500℃ can improve the utilization of Se and the battery performance to varying degrees.When the test temperature is above Tg,the segmental motion of the confined Se chain is enhanced.When the melting diffusion temperature of the Se/C composite is increased,the length of the Sex chain decreases and the dispersibility of the chain increases.Both provide a larger free volume favorable for ion migration in the confined Sex chain,resulting in multidirectional Li+channels and accelerating the structure-dependent Li+diffusion kinetics in the electrode material,thereby promoting the complete delithiation of chain-like Li2Sex and the deep lithiation of Sex chains,and improving the reversible capacity.This work provides some important references for the study of the relationship between the material physical properties and electrochemical performance of Se/C composites.(4)Structural properties,failure mechanisms and coping strategies of Se/C cathodes with high Se content.We first systematically investigated the structural characteristics of confined Se in Se@CMK-3 composites with different Se contents by ssNMR,EPR and DSC.Studies have shown that with the increase of Se content,the length of confined Sex chain increases.It is accompanied by more severe chain coiling,stacking or winding,and even large-sized and dense Se clusters are formed in nanopores.Ex-situ EPR experiments show the formation of dense Se clusters seriously hindered the transition of chain-like Li2Sen to lower-order polyselenides,resulting in serious voltage hysteresis in the formation of Li2Se.Finally,based on the above failure mechanism,we propose a deep discharge strategy.In the deep discharge mode,solvent molecules penetrate into the selenium clusters through the electric double layer reaction and participate in the mass transfer process,which improves the structure-dependent lithium ion diffusion kinetics of the electrode material.Electrochemical test results show that the deep discharge strategy at a current density of 0.1 C enables the specific capacity improvement rate of the cathode with high Se content to reach 67%.This work further deepens the research on the relationship between the structure and electrochemical performance of cathodes with high Se content,and provides important theoretical support for the design of cathodes with high energy density.