| The sustainable development of society gives further requirements to electrochemical energy storage devices?EESDs?in energy filed,i.e.green,safe and high performance.So far,based on the studies of the pseudocapacitance of oxygen containing functional groups?OCFGs?in carbon materials and the intercalation of anion into carbon cathode,the concept of all-graphene EESDs using organic electrolytes has been proved.However,the performance of graphene electrode still need to be improved.As green solvent,ionic liquids?ILs?are capable to be used in EESDs to enhance its safety,but interactions?ionic adsorptions and faradaic reactions?at carbon/IL interface and their effects on electrochemical current-potential response still need in-depth studies.Accordingly,we tuned the chemical structure of graphene electrodes to enhance the pseudocapacitance of OCFGs and the electrochemical performance of anion intercalation/de-intercalation.Besides,we studied IL/graphite interface by electrochemical scanning tunneling microscopy?EC-STM?.The electrochemical oxidation approach can effectively increase the number of OCFGs on/in reduced graphene oxide?rGO?surface and bulk.The content of total OCFGs increases more as the oxidation voltage being elevated,resulting in 290.5 m Ah g-1 for the oxidized electrode at 5.0 V?vs Li/Li+?,much higher than 170 m Ah g-1 for the pristine rGO electrode.Oxidations at moderate voltages?such as 4.7 V?can lower ohmic and electrochemical polarizaitons,significantly improving the rate capability.The carboxyl groups of rGO are not active for pseudocapacitive reaction in the potential of 1.5 4.5 V.The electrochemical reduction approach can activate carboxyl groups into carbon-oxygen double bonds,thus the specific capapcity was improved from 170.4 m Ah g-1 to 346.1 m Ah g-1(0.05 A g-1).An advantage of the electrochemical reduction approach is that no extra defect can be induced,thus maintaining the electric conductivity as much as possible.The thin solid electrolyte interphase?SEI?layer formed during the electrochemical reduction at lower potentials undergoes self-dissolution at high potentials,then alleviating detrimental influence of SEI layer.These two aspects favor the excellent rate--capability,e.g.,41.4 m Ah g-1 at the current density of 50 A g-1.We studied the electrochemcial intercalation/de-intercalation behavior of PF6 anion for graphene material as cathode in carbonate-based electrolyte.It was found that the intercalation/de-intercalation behavior of less-defective graphene is similar to graphite.The specific capacity of the less-defective graphene is 75.3 m Ah g-1(0.05 A g-1).The nitrogen-doped graphene?NG?has been synthesized by thermally treated graphene oxide with nitrogen sources at high temperature.The nitrogen-doping can properly decrease the voltage plateau of anion intercalation and improve the specific capacity to 121.8 m Ah g-1.The doping also improves energy density from 347.4 Wh kg-1 to 463.9 Wh kg-1.X-ray photoelectron spectroscopy?XPS?and X-ray absorption spectroscopy?XAS?studies reveal the mechanism for the Faradaic charge of the anion intercalation is that,the acceptortype doped graphite intercalation compound in is formed after PF6 intercalation and the Faradaic charge is origining from the loss of ? orbital electrons?N 2p and C 2p electrons?.We utilized density functional theory?DFT?calculation and EC-STM to predict the binding energy and to study the stability of self-assemble periodic structure for imidazolium cations with various alkyl chain length on graphite,respectively.It was revealed that,as lengthening the alkyl chain,the binding energy from van der Waals dispersive force increases,the adhesive strength between cation and graphite enhances and the electrochemical erosion is suppressed.C8MIm+ is able to intercalate into graphite steps at lower potentials and a stable C8MIm/graphite intercalation compound is formed. |
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