| Based on the rapid development of the times,traditional fossil energy as the main mode of consumption has caused environmental pollution and global warming.Lithium-ion batteries(LIBs)have gradually occupied the market as a new energy source due to their high energy density and wide application range.However,in recent years,owing to the distribution and reserves of lithium resources,the high cost of LIBs has begun to limit their development.Potassium has attracted extensive attention from researchers with its abundant resource reserves and low electrode potential.Therefore,potassium ion batteries(KIBs)are expected to become the next generation battery system and are widely used in the market.Among them,optimizing the design of electrode materials is the prerequisite for the development of high-performance potassium ion batteries.Because selenium has excellent electronic conductivity(1×10-3 S m-1)and high volumetric specific capacity(3250 m A h cm-3)as well as good compatibility with low-cost ester electrolytes,it possesses great potential among the KIBs cathode materials with.It is also very important to find suitable negative electrode materials for potassium ion batteries.Selenide as the negative electrode material of the conversion mechanism exhibits excellent theoretical specific capacity and rate performance.However,the volume expansion that occurs during charging and discharging is problem that needs to be solved urgently.At the same time,selenium cathode materials also face the challenges of low utilization rate and unclear reaction mechanism.Based on the above problems,this paper studies the performance exploration and mechanism analysis of selenium-based composite carbon as an electrode material in potassium ion batteries.The research content is as follows:(1)We chose a mesoporous MOFs-based material(Fe-MIL)as the carbon template to prepare porous iron selenide coated with carbon(NFC).The MOFs-based material has a unique microporous window(5.5-8.6(?))to restrict selenium and mesoporous cages(25-29(?))to form mesoporous channels.NFC exhibited stable long-cycle performance and excellent rate performance.At a current density of 500 m A g-1,the capacity of 310 m A h g-1 was still maintained after 1000 cycles.This was due to its large specific surface area and mesoporous channels providing more active sites and speeding up the transmission of potassium ions.At the same time,the unique carbon coating structure also effectively inhibits volume expansion during charging and discharging.The in-situ XRD characterization technique also confirmed that the entire reaction process is a reversible process of producing potassium selenide from iron selenide in one step.(2)Combining the work of mesoporous iron selenide,we chose mesoporous MOFs-based material(Zn-BTC)as the carbon template to confine selenium into its pores.According to the difference in selenium loading,we explored porous fully loaded selenium materials(Se/C FSC)and the electrochemical performance of porous carbon partly loaded selenium material(Se/C PSC).PSC materials exhibited excellent electrochemical performance,including stable cycling performance(at a current density of 1 A g-1,a capacity of 366.5 m A h g-1 after 1000 cycles is maintained)and high rate performance(20 A g-1at a current density of,there is still a capacity of 113.1m A h g-1).Because the selenium was partially filled into the pores,a part of the space reserved can effectively alleviate the volume expansion,resulting in different electrochemical properties.At the same time,the porous carbon structure and the mesoporous channel improve the electronic conductivity and shorten the ion transmission path.Se/C PSC material exhibited impressive electrochemical performance.(3)In order to explore the effect of selenium with different pore size limits on the reaction of potassium-selenium batteries in ester electrolytes,we limited selenium to Ketjen black mesoporous carbon(Se@Mes)and commercial microporous carbon(Se@Mic).The in-situ XRD characterization technique found that the reaction of selenium in different pore diameters the mechanism is also different.In microporous carbon,selenium is directly converted to potassium selenide,while in mesoporous carbon,selenium is firstly converted to intermediate phase polyselenides,and then converted it is potassium selenide.And through the theoretical calculation results,it was confirmed that the micropores have stronger adsorption energy for potassium selenide.Thus,the electrochemical reaction is a one-step reaction in micropores while the electrochemical reaction is a multi-step transformation reaction accompanied by the production of higher-order polyselenides in mesopores.In the process of electrochemical cycling,Se@Mic showed more excellent electrochemical performance.This result also confirmed that the micropores can effectively inhibit the production and side reactions of polyselenides,increasing the utilization rate of selenium and improving the electrochemical performance. |
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