Constructing 3D Freestanding Dual Carbon Composites For Potassium Ion Storage

To implement the strategy of sustainable development and solve various pollution issues,the renewable energy resources(solar,wind,and water energies)have been rapidly developed in the past decades.However,their regional and intermittent characteristics result in severe energy wastage.Thus,it's essential to develop low-cost large-scale electrical energy storage system(ESS)to utilize the intermittent energies.Potassium ion batteries(KIBs)as alternative of lithium ion batteries(LIBs)have been considered as one of the most promising EES owing to their abundant resources(2.4 wt.%in the Earth crust,nearly 1000 times than Li resources),and a low standard electrode potential of K(-2.93 V vs.standard hydrogen electrode,SHE).Graphitic carbon materials are considered as vital anode materials of LIBs because of their low-cost,stability and low plateau.Thus,carbon materials as one of the most important anode materials of KIBs have drawn extensive interest of researchers.And the K+can also insert into graphite,and form a graphite intercalation compound of KCs,corresponding to a theoretical capacity of 289 mAh/g.However,graphitic carbon suffers from expansion(61%volume expansion)and collapse of crystal structure due to the large radius of K ion(1.38 A),resulting in a fast decay of capacity.And the narrow interlayer(3.36 A)of graphitic carbon is also unfavorable for rapid intercalation/extraction of K ions,thus leading to a poor rate capability.To overcome the bottleneck of carbon materials in KIBs,this thesis focuses on the preparation of high-performance dual carbon composites anodes for KIBs.Chapter 1 demonstrates the background and mechanism of KIBs,and introduces the research progresses of cathode and anode materials for KIBs.The bottleneck and improving strategies of carbon materials in KIBs are expounded in detail.And then,the research ideas and methods are developed based on these researches.Chapter 2 presents the experimental methods and materials in the experimental section,material characterization methods and electrochemical measurements.Chapter 3 presents a carbon nanotubes-modified graphitic carbon foam(CNTs/GCF)structure to get over bottlenecks of graphitic carbon materials in KIBs.The flexible film shows excellent capacity of 226 mAh/g after 800 cycles at the rate of 100 mA/g with a retention of 98%.The CNTs/GCF shows a higher diffusion coefficient of K ion owing to the three-dimensional(3D)interconnected structures and CNTs.Based on the Bruce-Dunn method,the CNTs/GCF delivers hybrid K ion storage mechanisms of intercalation and surface adsorption.The hybrid K ion storage mechanisms of CNTs/GCF are also proven by the in-situ Raman spectra.Chapter 4 demonstrates a strategy to improve the K ion storage performance of hard carbon.The nitrogen(N)and oxygen(O)dual-doped carbon@graphene foam(NOC@GF)composite films have been constructed by CVD and electrochemical polymerization methods,which combined graphitic carbon,heteroatoms doped hard carbon,flexible and freestanding characteristics.The NOC@GF delivers an excellent long-term cycling stability of 281 mAh/g at 1 A/g with a capacity retention of 98.1%after 5500 cycles.This outstanding performance is attributed to the hybrid K ion storage mechanisms of intercalation and surface adsorption.The density functional theory(DFT)calculations prove that N and O doping sites are benefit to facilitating K ion adsorption and thus promoting K ion storage.Chapter 5 presents a CNTs modified 3D interconnected N and O dual-doped carbon framework(CNTs/NOCF).The CNTs can build an excellent electrically conducting network that can promote the transformation of electron and K ion among CNTs and NOCF,consequently gaining much better electrochemical behavior and better rate performance.Chapter 6 presents the summary,novelty,shortage and perspective of this thesis.