Pitch-based Porous Carbon Electrodes For EDLCs With High Rate Capacity And Competitive Energy Density

As a kind of new-type electrochemical energy storage device,electric double layer capacitors（EDLCs）have broad application prospects in the field of energy and urban rail transit system on account of high power density.However,there is a fatal shortboard that the energy density of EDLC device is only 5-10 Wh kg^-1,much lower than that of lithium-ion batteries.In this paper,a series of research works have been carried out about the rational synthesis of porous carbons as high-performance EDLC electrode materials.Moreover,the assembly mechanism of nano-sized carbon precursor was illuminated,showing important scientific significance and practical value.Coal tar pitch-derived amphiphilic carbonaceous material CP-A5 and graphite oxide were particularly chosen to be carbon precursors.Using chemical-recombination method,three-dimensional frame-filling structural C/C porous composites were constructed to solve the contradictory issue between high electrical conductivity and over-developed porous structure.The key factor in the formation of C/C porous composite lies in the KOH-catalytic nucleophilic addition between hydroxyl on CP-A5 and epoxy on graphite oxide,which can make CP-A5layers chemically combine with the outer surface of graphite oxide and thus avoid the re-stacking of graphene layers under heat treatment.By well-optimizing the KOH dosage and the content of graphite oxide,a series of highly conductive C/C composites were constructed.They can change ternary solid-solid interfaces（with porous carbon/binder/conductive agent）to binary solid-solid interfaces（with conductive porous carbon/binder）,to solve the problem of unstable electrode structure under multiple high-voltage charge/discharge processes.In addition to enhancing the overall energy density of EDLCs,they ensure excellent energy storage features of EDLCs in the whole lifespan.More importantly,when using conductive agent-free EDLC electrode,the rate performance of EDLC is synergistically affected by the electrical conductivity and porous structure;while the lifespan of EDLC is obviously decided by electrical conductivity.With the voltage above 2.7 V,the lifespans of most porous carbon-based EDLCs drop rapidly due to the undesired parasitic processes.By constructing relatively perfect carbon network and achieving the passivation of the carbon material surface,a C/C composite with high withstanding voltage was prepared,which exhibited high sp²-bonding carbon content and low oxygen-containing group content.The thermal treatment on the one hand can rearrange the carbon skeleton and reduce the carbon defects to some extent;on the other hand can eliminate the oxygen-containing groups to a certain degree and increase the relative contents of C-O group and C=O group.This C/C composite can successfully operate at voltage of 3 V in TEABF₄/PC electrolyte and at voltage of 3.5 V in pure EMIMBF₄ electrolyte.In EMIMBF₄electrolyte,it can obtain high specific capacitance of 156 F g^-1 at 50 m A g^-1 and energy density of 66.3 Wh kg^-1.Two-dimensional pitch-based porous carbon nanosheets（PCNs）were constructed from sulfonated pitch carbon precursor with polyaromatic hydrocarbons by interface self-assembly method.The robust hydrogen-bonding networks between SP and F127can induce the orientation of tiny 2D pieces in SP and thus form steady lamellar lyotropic liquid crystals.And high temperature-induced reactions of free CH₂·radicals in adjacent tiny pieces can form new C-C covalent bonds and chemically combine with each other to generate micron-sized intermediate carbon materials.When used as EDLC electrode materials,they demonstrated ultrahigh specific capacitance of 157.8F g^-1 at 20 A g^-1,holding top-level rate capacity C_20/0.05 of 86.7%in EMIMBF₄electrolyte.Simultaneously,the energy density can retain up to 67.1 Wh kg^-1 at high power density of 17.5 k W kg^-1,which is comparable to graphene-based porous carbons.