| Dual-ion battery is a new type of energy storage device whose working mechanism is based on the synchronous electrochemical storage of anions and cations at the positive and negative electrodes,respectively.One of the most commonly applied positive electrode materials in dual-ion battery is anion-graphite intercalation compound(AGIC),but the efficiency and reversibility of the anion storage process is still to be improved.As the anion stored in graphite positive electrode is completely supplied from the electrolyte solution,the electrochemical performance of the device is highly dependent on the electrolyte solution.In this paper,the electrochemical storage behavior of tetrafluoroborate anion(BF4-)in graphite positive electrode from some typical carbonate-based organic solutions is studied,and the influences of the solvation and the ion-pair effects are discussed:1?Compared to traditional cyclic carbonate solvents such as ethylene carbonate(EC)and propylene carbonate(PC),butylene carbonate(BC)has the same cyclic framework but a longer substituent group(ethyl group)and hence the larger molecular size,higher molecular weight and oxidative stability.Therefore,using BC as the solvent prevents the electrolyte solution from decomposition at elevated potential that guarantees the high columbic efficiency of the anion storage process,and the co-intercalation of BC into graphite electrode with the anion will be notably reflected by both the mass change and the lamellar expansion of graphite electrode that makes the analysis of anion solvation legible.In BC solution,the intercalation of BF4-,ClO4-and PF6-into graphite electrode are systematically compared.Results indicate that the number of BC co-intercalated into graphite is related to the type of anion.Because the molecular weight of BC(116.114 g mol-1)is close to that of triethyl methyl ammonium cation(TEMA+)(116.224 g mol-1),the intercalation of TEMA+into graphite electrode is employed as the internal for the quantitative analysis of the anion solvation ability,and the solvation ability order among three anions is BF4->ClO4->PF6-.2?In classic electrochemistry,the intercalation and de-intercalation of ion are considered as entirely opposite reactions,corresponding to the inverse current response of electrodes.However,during the de-intercalation of EC-solvated BF4-(EC-BF4-)from graphite electrode(cathodic current),transient anodic current arises when the potential of graphite electrode drops to 1.7 V(vs.AC-QRE),during which the crystal structure change is hard to be captured by existing in situ XRD technique.Combining the results of EQCM and electrochemical dilatometer(ECD),the abnormal current response is identified as the back flow of BF4-into graphite electrode during the discharge section.This phenomenon does not take place in graphitized mesophase carbon microbeads(MCMB)positive electrode until the surface layer of MCMB is get rid of by a mild oxidation,indicating the direct correlation between the inverse current and the stripped EC molecules from intercalated EC-BF4-in graphite.3?To accurately adjust the solvation state of the anion during its storage process in graphite,the use of multi-solvent electrolyte solution is more effective compared to the single solvent electrolyte solution.Taking the solution of BF4--EC/GBL(y-butyrolactone)as an example,when the differences of kinetic rate and electrochemical stability between the two "pure" solvation states(EC-BF4-and GBL-BF4-)are determined,the gradual change of the anion behavior can be observed through in situ XRD and in situ Raman measurements during the intercalation of BF4-into graphite from the electrolyte solutions of different solvent compositions,indicating that the two solvents have similar affinity in solvating BF4-and the intercalated BF4-is solvated by both EC and GBL solvents.4?Unlike cyclic carbonate,linear carbonate such as ethyl methyl carbonate(EMC,?=2.9 C V-1 m-1)has much lower permittivity.Compared to those from the electrolyte solutions of LiClO4-EMC(specifc capacity of graphite:42 mAh g-1)and LiPF6-EMC(specifc capacity of graphite:80 mAh g-1),the anion intercalation from LiBF4-EMC is especially sluggish that causes sever polarization in the lithium/graphite dual-ion battery(specifc capacity of graphite:6 mAh g-1),which largely results from the incompetence of EMC solvent in separating the Li+-BF4-ion pairs(ion-pair effect).By replacing Li+ with large-sized cation(eg.tetrabutyl ammonium cation)or adding solvent with higher permittivity(eg.PC,?=65 C V-1 m-1)into the solution,the intercalation of BF4-into graphite electrode can be observably facilitated.On the other hand,the ion-pair effect influnences the intercalation of Li+ into lithium titanate(LTO)cathode,as well as the electrochemical performance of LTO/graphite dual-ion battery.Even in the solution with super high permittivity(EC,?=96 C V-1 m-1),the ion-pair effect of Li+-BF4-shows considerable impact that delays the intercalation process of the anion and suppresses the back flow behavior consequently. |
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