Storage Behavior Of Solvated Hexafluorophosphate Anion In Graphite Electrode

Dual-ion battery is a new type of electric energy storage device whose operation mechanism is based on the synchronous reversible storage of anions and cations,in positive and negative electrodes,respectively.The compatibility between graphite anode and electrolyte is the key issue in its technology.The electrolyte salts of LiPF₆ has been widely used in commercial non-aqueous electric energy storage devices such as lithium-ion batteries and supercapacitors.Therefore,it is of great academic and applicational values to study the electrochemical behavior of graphite anode for storing anions in LiPF₆ solutions.In this paper,LiPF₆ is used as the electrolyte salt dissolved in several representative pure solvents or binary solvent mixtures to prepare the electrolyte solutions.To explore the electrochemical intercalation behavior of PF₆-into graphite electrode in the presence of these solutions,by using a variety of in situ electrochemical characterization methods,we can analyze the influence of PF₆-solvation state on its intercalation behavior from different viewpoints.This dissertation includes the following five aspects:1.LiPF₆-EMC(ethylmethyl carbonate)solutions demonstrate an excellent compatibility with graphite positive electrode.By means of altering the concentration of LiPF₆,the performance of graphite positive electrode can be further optimized.The traditional electrochemical tests,including galvanostatic charge-discharge and cyclic voltammetry,were carried out on Li/graphite batteries with LiPF₆-EMC solutions.The results are at first verified that the solution of 2M LiPF₆-EMC is the best choice.In situ X-ray diffraction(XRD)and electrochemical quartz crystal microbalance(EQCM)techniques are used to trace the crystal structure evolution of anion-graphite intercalation compounds and the mass change of graphite electrode during charge and discharge process,respectively.For the first time it was discovered that the solvent and anion transport in opposite directions across the graphite/solution interface.This fact means if the concentration of the electrolyte salt is too high,the solution will be short of EMC solvent,and then there is no adequate solvent molecules supply from the solution phase to help the anions de-intercalate from the graphite electrode in the discharge process,which have a negative effect on the performance in the terms of both long cycle and rate capability.Therefore,the appropriate concentration of LiPF₆-EMC electrolyte is very important to improve the performance of graphite positive electrode.On the other hand,both NMR and IR analysis provided evidence for the solvation state of the anions in the solution.2.In order to accelerate the ion transport in LiPF₆-EMC electrolyte,the addition of y-butyrolactone(GBL)as a co-solvent can significantly enhance the ionic conductivity of these electrolytes.In 1 M LiPF₆-GBL/EMC solutions with varied contents of GBL and EMC,storage behavior of PF₆-in graphite electrode has been investigated.At first,the conventional electrochemical results(including galvanostatic charge-discharge and cyclic voltammetry tests)indicated that the addition of GBL can decrease the anion storage capacity of graphite electrode.And then in situ XRD and EQCM were utilized to characterize the crystal structure and mass changes of the graphite anode,respectively,during the anion storage.The corresponding results demonstrated that as compared with GBL,EMC preferentially binds to the anion and is more actively involved in the electrochemical intercalation process.In the solutions with low EMC contents,EMC and GBL will synergetically solvate PF₆-anion and co-intercalate into graphite electrode.At last,the ionic conductivity and NMR results of the solutions provided evidence for the interaction between ions and solvents.3.Among LiPF₆-cyclic carbonate solutions,LiPF₆-propylene carbonate(PC)can match graphite positive electrode very well.The galvanostatic charge-discharge test reveals the relationship between the anion storage capacity of graphite electrode and the concentration of LiPF₆.Ex situ XRD tests of the graphite electrode at the end of charge showed that the intercalation gallery height value between anions-graphite intercalation compounds(AGICs)decreases with the increase of LiPF₆ concentration,which fact indicates that the average number of PC molecules accompanying with each PF₆-anion co-intercalated into graphite electrode decreases.The in situ XRD tests of the graphite electrodes indicates that as LiPF₆ gets dense,the anion-graphite intercalation compounds formed in the same graphite electrode get quite diversified.And then EQCM of graphite electrode demonstrated that PC and the anion transport in the opposite directions in the presence of solutions with higher LiPF₆ concentrations,which derives from the role of Li+-PF₆-ion pair.The NMR and IR spectra of the solution supported the interaction between the ions and PC solvent.4.In order to improve the application of LiPF₆-PC electrolyte in low-temperature environments,the addition of a co-solvent of with low melting point and low viscosity such as methyl acetate(MA)in these solutions may be an available choice.By adjusting the relative contents of PC and MA in 1M LiPF₆-PC/MA solution,the electrochemical storage behavior of PF₆-in graphite electrode was investigated by galvanostatic charge-discharge,in situ and ex situ XRD tests.The relationship between PF₆-anion storage capacity in graphite electrode and MA content in solution conveys a "W" shape.In the solutions with specific compositions,it was discovered for the first time that the ratio of the electrolyte volume to the mass of graphite electrode has a decisive effect on the type of solvent co-inserted into the graphite electrode.On the other hand,the infrared spectra and NMR data of the solutions provided evidence for the interaction between ions and solvents.5.To further understand the electrochemical behavior of PF₆-anions stored in the graphite positive electrode in LiPF₆ based on the mixtures composed of cyclic carboxylate,LiPF₆-PC/GBL electrolyte was prepared.Conventional electrochemical test(including cyclic voltammetry and galvanostatic charge-discharge)disclosed that with the increase of GBL content in the solution,PF₆-anion storage capacity in graphite electrode exhibits an abnormal fluctuation.Ex situ and in situ XRD measurements were carried out on the graphite electrode to determine the formation and crystal structure evolution of AGICs.In the solutions with lower GBL contents,the co-solvation of the anion by both GBL and PC solvent molecules and their co-intercalation into graphite electrode can be observed.Infrared spectra and NMR data of the solution confirmed the interaction between ions and solvents and the solvation environment of the anions.