Quartz Crystal Microbalance Analysis At Electrochemical Energy Storage Electrode/Electrolyte Interface

China has the largest energy storage market all over the world,and the scale of electrochemical energy storage in China will exceed 10 GW in 2022.Faced with such a huge energy storage market,it is urgent for people to deeply understand the energy storage mechanism of different electrochemical energy storage devices,so as to develop new energy storage materials and energy storage systems.Although there are many different electrochemical energy storage devices,the basic energy storage process in them can be divided into two types:ion adsorption-desorption behavior and chemical reaction.Therefore,the study at electrode/electrolyte interface is the key to understand the energy storage mechanism.For interface research,various testing methods including infrared,nuclear magnetic resonance,neutron scattering,etc.are used to analyse the morphology,composition and structure of electrode materials in situ to promote the design optimization of electrode materials.However,there is still a lack of in-situ analysis methods for the mass and mechanical properties of the electrode,the former is related to the specific charge storage process,and the latter determines the cycling stability of the electrode materials.In recent years,the electrochemical quartz crystal microbalance(EQCM)is gradually used to analyse the electrochemical energy storage electrode/electrolyte interface because it can monitor the change of electrode mass,viscosity and shear modulus in-situ.The challenges of its application are how to use it to explain the interfacial process in a more complex system.For example,it is still lack of effective methods to study the ion adsorption-desorption behavior in carbon materials with complex pore size distribution by EQCM.Then most studies on EQCM focus on capacitors but few on much more complex battery systems.Third,using EQCM-D(EQCM with dissipation monitoring)to measure the mechanical properties change of electrode is still a difficult problem.Therefore,in this paper,EQCM-D was applied to achieve in-situ analysis of the electrochemical energy storage electrode/electrolyte interface,and then constructive suggestions are proposed to improve the performance of energy storage materials.(1)EQCM was used to analyze ion adsorption/desorption behaviors at porous carbon electrode/electrolyte interface.Experimental results show that adsorption/desorption behaviors of anion and cation are affected by the pore size distribution.For RGO with only pores?1 nm,its ion adsorption/desorption behavior is shown as a standard hydrated ions exchange model,while charge storage mechanisms in AMEGO with both subnanopores(<1 nm)and mesopores(>1 nm)are quite different.Further analysis indicated that the presence of subnanopores leads to the counter-ion desorption.In addition,dissolvation of hydrated ions can even occur in these subnanopores.The combined effects of the ion diameter and the pore size of the carbon material leads to the different energy storage mechanism in AMEGO and RGO.(2)EQCM was used to analyze solid electrolyte interphase(SEI)formation process on copper electrode in different electrolytes.Experimental results show that the type of electrolyte has a significant effect on the structure and composition of SEI.The SEI formation in PC and EC/DEC-based electrolytes involves the growth of a rigid layer followed by a viscoelastic layer,whereas a distinct"one-layer" rigid model is applicable to the SEI formulated in TEGDME-based electrolyte.Moreover,SEI masses reached 20.9 and 4.7 ?g/cm2 in PC-and EC/DEC-based electrolytes,respectively,compared to 2.4 ?g/cm2 of SEI in TEGDME-based electrolyte when discharged to 0 V.The electrode shear modulus decreased and viscosity increased continuously during the formation of the SEI in ester-based electrolytes.We attribute different SEI properties to different SEI chemical compositions,which have been revealed by tracking the mass-change-per-mole-of-electron-transferred using EQCM-D and further confirmed by X-ray photoelectron spectroscopy.As an effective in-situ interface testing method,EQCM-D will help us to further understand the association between SEI formation process and the long-term stability of lithium ion batteries,and provide new ideas for future electrolyte design.(3)EQCM-D was used to focus on the SEI formation on graphene-coated copper collector in the first cycle discharge process.We found that the coating of graphene on copper collector caused the SEI to change from a rigid layer to a viscoelastic layer at a lower potential.Although the overall mass of the SEI generated on graphene coated copper collector was close to that of uncoated copper collector,SEM and AFM results showed that the thickness of the SEI was smaller than that obtained on bare copper current collector,indicating the formation of a "thinner and denser" SEI.These research results are expected to provide a clear direction for obtaining stable SEI and improving the electrochemical performance of lithium metal batteries.