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Study On The Electrochemical Potassium Storage Performance Of Two-dimensional Materials And Carbon Composite Materials

Posted on:2022-02-24Degree:MasterType:Thesis
Country:ChinaCandidate:S T GengFull Text:PDF
GTID:2492306554953539Subject:Master of Engineering
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Metal ion capacitors integrate the advantages of high energy density of batteries and high power density of supercapacitors,and thus become a new hot spot in the research of energy storage devices.Compared with lithium,potassium has abundant reserves and low cost,so potassium ion capacitors have great potential in large-scale energy storage applications.However,the kinetic process of K+is relatively slow during the insertion and extraction process of the battery-type negative electrode,which cannot match the faster charge storage process of the capacitive positive electrode material.Therefore,the realization of a negative electrode material with high rate performance is very important for the improvement of the performance of the potassium ion capacitor.Compared with carbon anode materials,transition metal chalcogenides(TMDs)have a generally higher theoretical potassium storage capacity,which has attracted widespread attention.Tungsten disulfide(WS2)is a typical layered TMDs with a layer spacing of 0.618nm,which is conducive to the rapid insertion and extraction of K+.The oxidation-reduction potential corresponding to the K+intercalation is lower than that of Li+and Na+,which is conducive to the improvement of the energy density of the potassium ion capacitor.The electron mobility of WS2(1103 cm2V-1s-1)is better than other TMDs(Mo S2:340 cm2V-1s-1;Mo Se2:cm2V-1s-1),making it a highly potential Anode material with high rate performance.According to four-electron conversion process of WS2to W and K2S,the theoretical capacity of WS2is approximately 431.6 m Ah g-1.The nano-preparation of WS2will effectively promote the conversion reaction,thereby improving the potassium storage performance of WS2.However,the structural deformation caused by the conversion reaction during the discharge/charge process will be very significant,which will cause the electrode capacity to drop rapidly.The main work here is:1.We designed a freestanding anode of the carbon-coated WS2nanosheets supported on carbon nanofibers(denoted as C-WS2@CNFs)for PIBs.The CNFs as supporting skeleton regulate the uniform growth of WS2nanosheets,the few-layered structure of WS2nanosheets promotes the conversion reaction,and carbon coating accommodates volume changes and enhances the electrical conductivity of electrodes.As a result,the C-WS2@CNFs exhibited high reversible capacity of 320 m Ah g-1at 50 m A g-1and good rate capability of 169 m Ah g-1at 10 A g-1.To sum up,the C-WS2@CNFs exhibited high capacities and rate capability in K+storage.In addition,this work may also broaden the way to design better electrode materials for other battery systems.2.We further revealed the reaction mechanism comprising initial intercalation(WS2→KxWS2)and subsequent two-step conversion via intermediate phase of K2S5to finally produce K2S and W(KxWS2→K2S5→W+K2S)by in situ Raman and ex situ XRD analysis.Density functional theory(DFT)calculations confirmed the fast K+diffusion kinetics and high K+adsorption capacity of C-WS2@CNFs.These studies can provide advanced design strategies for the construction of high-performance electrodes for PICs and provide an in-depth understanding of the energy storage mechanisms of other TMDs series.3.The self-supporting activated carbon fiber electrode material with ultra-high specific surface area was successfully prepared by chemical activation method,and its potassium storage characteristics when used as the positive electrode of potassium ion battery were studied in detail.The potassium ion capacitor with C-WS2@CNFs as the anode and activated carbon nanofibers(ACNFs)as the cathode,can work stably at voltages up to 4.0 V,and can output a high energy density of 180.4 Wh kg-1at a power density of 399.6 W kg-1,and a high energy density of 12.6 k W kg-1The power density can output an energy density of 42 Wh kg-1,and the capacity retention rate reaches 100%after 5500 cycles at a current density of 4 A g-1.We believe that this work can provide inspiration for the development and practical application of potassium ion capacitors.
Keywords/Search Tags:potassium ion hybrid capacitor, anode, tungsten disulfide, electrospinning, conversion reaction mechanism
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