Preparation And Structural Optimization Of Carbon-Based Composites For Hybrid Capacitors

Hybrid capacitor,integrated the advantages of high energy density of batteries,high power density and long cycle stability of supercapacitors,has great application potential in regenerative braking of vehicles and high-power driving of construction machinery.Achieving the capacity and reaction kinetics matching of positive and negative electrodes is the key to improve the electrochemical performance of hybrid capacitors.In this thesis,we have prepared a series of carbon-based battery nanocomposites by using a variety of surface-/interface-regulating methods.Consequently,the capacitance,rate and cycle performance of electrodes have been enhanced by precisely optimizing the active particle size,the distribution density of active sites,porosity and coordination environment of materials.The structural evolution mechanism of electrodes and its correlation with energy storage activity have been deeply revealed by advanced characterization and in-situ measuring techniques.Moreover,the hybrid capacitors are also assembled by employing the as-fabricated nanocomposites and capacitive-type materials as electrodes to verify the practicability.The main results are as follows:The carbon-based composites loading with untrafine Ni₂P nanoparticles were prepared by in-situ modulating the surface/interface properties of graphene oxide（GO）with the coprecipitation and phosphating technologies.We have investigated the effects of GO surface/interface properties,paraffin introduction and Ni₂P mass loading on particle growth,materials structure and properties.The Ni（OH）₂ generated on GO modulated the hydrophilic properties of solid phase,thus enabling the interface inverting from oil-water-solid to water-oil-solid,and resulting in oil-enveloped Ni（OH）₂/GO structure that can function to modulate the growth and aggregation of adjacent nanoparticles.The phosphorized Ni₂P@C/G composites with highly-density distributed active sites（47 wt.%,～11 nm）can deliver excellent charge transfer and mass transport characteristics,thus exhibiting a high pseudocapacitive contributions of 95.7%at 5 m V s^-1 as anode for sodium storage.The sodium-ion capacitor assembled with the Ni₂P@C/G anode and B,P-codoped carbon sheets cathode delivers a high energy density of 54 Wh kg^-1 at the power density of 23 k W kg^-1.The fast preparation of stage-1 dominated FeCl₃-graphite intercalation compounds was achieved by a microwave heating strategy,where the graphite was used as raw material.We have investigated the irradiation conditions and the mechanism of microwave-accelerated intercalation of FeCl₃ towards graphite.The interaction between the electric field component and the conjugatedπ-electron in graphite induced the generation of Joule heat and electron transition,thus promoting the ionization of FeCl₃ molecules.As a result,reaction kinetics have been strengthened due to the accumulated internal energy of intercalation molecules.The stage-1 dominated graphite intercalation compounds with FeCl₃ content of 57 wt.%（AP-GICs-1）can be synthesized in 190 s.When evaluating as the anode for lithium storage,the compounds delivered a superior rate performance and high volumetric capacity up to 1650m Ah cm^-3 at 1 A g^-1 due to the large layer spacing（0.93 nm）and high FeCl₃ content.The lithium-ion capacitor assembled with the AP-GICs-1 anode and YP-50 cathode delivers a good energy density of 28 Wh kg^-1 at a power density of 24 k W kg^-1.With the assistance of operando Raman technique,we have clarified the electronic decoupling effect among graphite and FeCl₃layers and excessive lithium storage behavior of graphite layer,confirming that the formation of FeCl₃ clusters in layers is responsible for the increasement of capacity.The densified and ordered LDH-80/rGO monolithic composite films were prepared by using GO and exfoliated NiMn layered double hydroxides（NiMn LDHs）as components.We have investigated the oxidation conditions and confirmed that HCl O₄ can selectively oxidate the Mn sites in LDHs laminates,leading the stretching of Mn-O bond in LDHs laminates,the responsive deformation as well as exfoliation of nanosheets.The as-exfoliated LDH-80 are composed of wrinkled nanosheets with the thickness of 0.9 nm.Benefiting to the weak electrostatic interaction between wrinkled LDH-80 and GO nanosheets,LDH-80/rGO monolithic composite films can be prepared by vacuum filtration and low temperature reduction.In the films,the large layer spacing of LDH-80 nanosheets can be maintained to a large extent,which can guarantee the sufficient exposure of active sites and promote the ions transport inner films.Moreover,the densified and ordered structure can improve the stability of electrode.The hybrid capacitor assembled with the film and YP-50 delivers a high power density of 24 k W kg^-1 at the energy density of 33 Wh kg^-1 and a retention of 88%after 5000cycles at 10 A g^-1.