Rational Synthesis And Property Investigation On The Anode Materials For High Performance Hybrid Capacitors

Lithium-ion batteries(LIBs)lead the market for portable smart electronic devices due to their high energy density,good cycle stability and environmental friendness.However,limited resources and uneven distribution make lithium ion resources too expensive,which hinders the further development of lithium-ion batteries in the large-scale energy storage.Due to the abundant sodium/potassium resources,low cost and similar electrochemical properties to lithium,sodium/potassium-based energy storage devices as a next-generation energy storage system with broad application prospects,have become the focus of attention.Sodium/potassium-ion hybrid capacitors(SIHCs/PIHCs),which can combine the advantages of high energy density of ion batteries as well as high power density and long life of supercapacitors,have developed rapidly in recent years.However,the relatively large ion radiuses of Na+and K+cause the slow kinetics of redox reactions in most anode electrodes,which could not match well with the rapid capacitance behavior in the cathode,seriously affecting the electrochemical properties of SIHCs and PIHCs.Therefore,it is very important to make substantial progress in the design of anode materials to realize the wide application of SIHCs and PIHCs.In this thesis,we carried out the reasonable structure optimization and improvement of performance for metal compound and carbon anode materials through the design of nanostructures,the combination with carbon-based materials and heteroatom doping to obtain high-performance SIHCs and PIHCs.Simultaneously,the relationship between the structure and the properties and the energy storage mechanisms of electrode materials were deeply studied and analyzed by a series of in situ and ex situ testing techniques.The main research contents are as follows:1.In Chapter 3,we developed a simple ethylene glycol reflux and subsequent vapor phase conversion strategy to synthesis a series of bowl-like hollow iron-based nanoparticles(C@FeX,X=O/S/Se/P)and study the sodium storage properties of as-prepared samples simultaneously.Taking the C@FeSe2 as an example,the results of morphology and structure characterizations show that the ultra-small FeSe2 nanoparticles coated with few carbon layers are uniformly embedded into the hollow carbon bowls,forming a "dual-carbon" protected mechanism.Thanks to the unique structure of hollow bowls,ultra-small size of nanoparticles and "dual-carbon"protected mechanism,C@FeSe2,C@Fe3O4 and C@FeP samples as anode in sodium-ion batteries(SIBs)could display excellent cycling stability and outstanding rate performance.Besides,the SIHCs based on the C@FeP anode and commercial activated carbon(AC)cathode could also exhibit high energy density and satisfactory cycling stability.2.In Chapter 4,we developed simple oxidation polymerization and annealing treatment under high temperature with Sb2S3 nanowires as bifunctional template to facricate Se-doped MoS2/carbon hybrid hollow nanotubes(MoS2-xSex/C-HNTs)and study the corresponding sodium storage performance.The results of structure characterization and performance test show that the optimal MoS2/3Se4/3/C-HNTs sample could be obtained by changing the mass ratio of Se powder in the annealing process,and the doping of selenium in MoS2/3Se4/3/C-HNTs can not only enlarge the lattice spacing of the layered structure,but also produce more defects and active sites,leading to a significant increment of capacity and more stable cycling for KIBs compared to that of pure MoS2.In addition,the results of in situ X-ray diffraction(XRD),Raman spectroscopy,and galvanostatic intermittent titration technique(GITT)test reveal that the energy storage mechanism of K+in MoS2/3Se4/3/C-HNTs,and display that the doping of Se could lead to the higher diffusion coefficient,excellent structural robustness and disordered layer structure,thus improving the storage performance of K+.In addition,the excellent electrochemical performance of PIHCs assembled with MoS2/3Se4/3/C-HNTs anode and AC cathode also indicates that the doping of Se is efficient and feasible for the modification of MoS2 based anode materials.3.In Chapter 5,we developed a nanoscale confined in-situ oxidation polymerization process using MnO2 hollow spheres as the sacrificing template and oxidation agent to prepare the phosphorus and nitrogen co-doped hollow carbon spheres(PNHCS),and the sodium/potassium storage performance of PNHCS were studied systematacially.The results of morphology characterization show that the porous shell of PNHCS is composed of the cross-linked nanosheets.The analysis of X-ray photoelectron spectroscopy(XPS)manifest that the doping of P would facilitate the formation of highly active pyridinic-N,which is beneficial to improve the electrochemical performance of PNHCS anode.According to the electrochemical test,PNHCS anode could exhibit outstanding electrochemical performance in both sodium ion batteries and potassium ion batteries,and the potassium ion hybrid capacitor assembled with PNHCS anode and AC cathode also shows high energy density and power density,as well as long cycle stability.A combined analysis including in-situ Raman spectrum and ex-situ XPS characterization reveal the electrochemical reactions of Na+/K+ in PNHCS anode.GITT results also show that P doping could increase the diffusion rate of Na+/K+in the PNHCS anode.Finally,the density function theory(DFT)calculations demonstrate that the dual-doping of P and N could enhance the electron transport kinetics and ion adsorption capacity of the PNHCS anode,thereby improving its electrochemical performance.Therefore,the improvement of Na+/K+storage performance of PNHCS anode is mainly attributed to the synergistic effect of the unique hollow structure and dual-doping of P/N.4.In Chapter 6,we analyze the innovation and shortcomings of the research work in the thesis and look forward to the future research direction.