Charge Storage Mechanisms Of Polyatomic Ions In Carbon Electrode Materials

Carbon materials have been playing an important role in the history of human energy utilization by virtue of multiple advantages.Modern society calls for advanced energy devices which are safer,efficient and environmentally benign.Dual-carbon cells are considered as one of promising solutions and have attracted extensive attention from researchers.Unlike traditional symmetrical supercapacitors,which are mainly based on the ion adsorption at both activated carbon(AC)electrodes,the novel dual-carbon cells take advantages of other carbonous materials to improve their overall performance.Thus,polyatomic ions in the original electrolyte systems will follow the new energy storage mechanisms.In this paper,storage behaviors of polyatomic ions in various carbon electrodes are investigated by in situ and even combined in situ electrochemical techniques.A more comprehensive and thorough understanding of relevant mechanisms has been obtained,which guides the better application of carbon electrode materials.The work can be listed as follows;1.Triethylmethyl ammonium/Tetraethyl ammonium tetrafluoroborates(TEMABF4/TEABF4)dissolved in propylene carbonate are the most common non-aqueous electrolyte solutions for supercapacitors.They are applied to graphite/AC capacitors,in which the charge storage at graphite negative electrode is based on the cationic intercalation.The capacitor containing TEMA+ shows a much higher capacity and superior cycle-ability.Theoretical calculation results show that it doesn't stem from the stronger resistance to reductive decomposition.In situ X-ray diffraction(XRD),electrochemical dilatometry(ECD)and electrochemical quartz crystal microbalance(EQCM)are combined to verify the co-intercalation solvent molecules together with TEMA+,which bring a large recoverable expansion in according electrodes.A benign solvation effect is proposed for facilitating the transportation and storage of TEMA+inside graphite.The decreased charge density overrules the side-effect of bigger intercalant(steric hindrance)on the graphitic structure.2.The storage behavior of three isomeric quaternary alkyl ammonium cations(isopropyltrimethyl ammonium,iPTMA+;diethyldimethyl ammonium,DEDMA+;propyltrimethyl ammonium,PTiA+)in graphite electrodes is studied from graphite/AC asymmetric capacitors.In situ XRD and Raman spectroscopy(Raman)prove that the different cycle performance among them is mainly caused by the gap of kinetics.iPTMA+moves slowly in graphite electrodes compared with DEDMA+and PTMA+,and this gap is more obvious in their de-intercalation process.Poorly effective interactions between iPTMA+ and graphite generate more defects in graphite electrodes and inferior rate capability in capacitors.Theoretical calculation results indicate that it may come from different shapes under the same volume.For the cation containing branched alkyl groups,it is difficult to convert into a slim conformation,which ensures the unhindered shuttle in graphite electrodes.3.An asymmetric dual-carbon cell with non-metal electrolytes is proposed.Two kinds of mesocarbon microbeads(MCMB)samples with different treatments are served as electrode materials.Charge storage behaviors of polyatomic anions/cations in different MCMB electrodes are studied by in situ XRD and ECD synergistically.Potassium hydroxide calcined MCMB electrodes exhibit a significant volumetric expansion in the initial polarization,which enlarges the total storage capacity(so called the electrochemical activation).A quantitative correlation between the two in situ techniques is established in graphitized MCMB electrodes,the acceptor of ion-intercalation behavior.These findings reveal the optimal working condition of electrodes,which needs a trade-off between the sufficient utilization for charge storage and structural stability.4.The structural change of different graphite electrodes(a series of graphite samples with different particle size distributions)during the intercalation/de-intercalation of BF4-is systematically investigated.In situ XRD,Raman and ECD provide a panoramic view to this dynamic revolution,which has been associated with the performance of corresponding AC/graphite capacitors.The graphite with miinimal size(4-7 pun)exhibits more anion accumulation in the shallow region.It happens before the charge storage in the bulk graphite,which is disclosed by a less crystal lattice expansion.The extra ion storage leads to a higher capacity at the normal current density,however,also drags down the de-intercalation process and causes a more capacity fade at high rates.The sample with medium size(9-18 ?m)may be a more suitable choice to ensure both energy and power densities.