Fundamental Research On High Energy Density Supercapacitors Based On Water-in-salt And Concentrated Organic Electrolytes

Energy storage technology is the key technology of energy development and utilization.Supercapacitors have attracted enormous attentions due to their unique advantages,such as high power density,fast charge/discharge rate,long cycle life and high safety.They have broad applications in the fields of new energy power generation,new energy vehicle,aerospace and national defense,etc.The energy storage capacity of supercapacitors is still lower than that of conventional secondary batteries,so the development of supercapacitors with high energy density is a current research hotspot.Concentrated electrolytes(e.g.,water-in-salt)exhibit excellent basic physical and chemical properties.For example,their high concentration enables a wider potential window,thereby achieving high energy density.The energy storage mechanism can provide the guideline for the optimization of electrode morphology and electrolyte composition.In classical electric double layer theoretical model,the electrode surface is assumed as an infinite plate.The electric double layer structure is described by mean field theory,continuum hypothesis and Debye-Huckel theory.Classical electric double layer theoretical model has been applied to conventional low-concentration electrolytes and macroporous electrodes.However,with the increasing demand for large-capacity energy storage devices,concentrated electrolytes with wide potential window(i.e.,water-in-salt)and nanomaterials with large adsorption area are developed for the new electric double-layer capacitors.There are strong electrostatic ion-ion interactions in water-in-salt electrolytes,leading to the tight combination of ions and formation of large ion clusters.These phenomena make the ion electrostatic adsorption process in the nanochannel particularly complicated.Based on current research status,numerical simulation and microstructure characterization are employed to systematically explore the potential window enhancement mechanism and ion adsorption process process of concentrated aqueous electrolytes(water-in-salt)in the nanochannel.In addition,to overcome the low conductivity of water-in-salt electrolyte,we explore the ion transport enhancement mechanism of water-in-salt/organic solvent hybrid electrolyte within nanochannels,guiding the optimization of supercapacitor internal resistance.Based on potential window enhancement mechanism,concentrated organic gel polymer electrolyte is developed to achieve the organic-electrolyte-based supercapacitors with ultrahigh energy density.In this paper,cyclic voltammetry test is used to explore the voltage window of different concentrations of lithium bis(trifluoromethanesulphonyl)imide(Li TFSI)electrolyte.When the concentration increases from 1 mol kg^-1 to 21 mol kg^-1,the potential window is widened to 2.7 V.Furthermore,the potential window enhancement mechanism of concentrated electrolytes is revealed using the methods combining molecular dynamics simulation and Raman spectroscopy.97.26%of the water molecules are more stable when they are coordinated with cations,beneficial for the enhanced electrolyte decomposition stability.By analyzing number density distribution,radial distribution function,and coordination number,we report that ion desolvation in the nanochannel promotes the formation of the inner Helmholtz electric double-layer structure,which is absent in the planar electrode based on the classic electric double-layer theory.By analyzing the coordination structure between cations and anions,we found that the ion electrostatic interactions in the nanochannel are more significantly weakened compared with bulk system,leading to the higher ion transport characteristics.The weakened electrostatic interaction is because the nanoconfinement effect of nanochannel restricts the number of ion adsorption peak.In addition,it is proved that the concentrated Li TFSI electrolytes have a potential application in the field of high-capacity aqueous supercapacitors.To improve the ionic conductivity of water-in-salt electrolyte,molecular dynamics simulation is used to reveal the ion transport enhancement mechanism of water-in-salt/organic-solvent mixed electrolytes in the nanochannel.By analyzing the coordination structure between cations and anions,we found that the ion transport characteristics are mainly dominated by the molecular size of the organic solvent in the1 nm channel,different from the traditional view of optimizing only by selecting the organic solvent with high kinetic characteristics.According to the concentration coefficient and number density distribution,we found that the formation of electric double layer structure in the nanochannel is significantly controlled by ion electrostatic interactions.The strong electrostatic interactions make cations and anions difficult to separate,and they enter the nanochannel together.Furthermore,molecular dynamics simulation is carried out for a wider nanochannel.This paper finds that the 1.84 nm channel has a similar trend of ion transport to the 1 nm channel,and the weakened nanoconfinement effect causes the ion adsorption amount significantly higher than that of 1 nm channel.According to the obtained mechanisms,we optimized the composition and ratio of water-in-salt/organic-solvent mixed electrolytes to achieve the excellent rate performance.The pore size of the electrode material(0.95 nm and 1.656 nm)was close to that of simulation system,and Warburg resistances of supercapacitors were reduced by 80.8%and 44.9%,internal resistances were reduced by 33.5% and 36.6%,respectively.Finally,based on the potential window enhancement mechanism for the concentrated electrolytes,an organic gel polymer electrolyte with high electrochemical stability is proposed,and the potential window is widened to 4 V.4.2 mol L^-1electrolyte system has a higher ion concentration than 1 mol L^-1 electrolyte,leading to the specific capacitance increased by 48.5%.Depending on the high potential window and specific capacitance,the energy density of 4.2 mol L^-1 electrolyte-based supercapacitor reaches to 80.6 Wh kg^-1,higher than that of currently proposed organic gel supercapacitors(20-60 Wh kg^-1).By analzying the microstructure using molecular dynamics simulation and Raman spectroscopy characterization,we revealed the potential window enhancement mechanism of the concentrated organogel electrolyte.Due to 99.473%of solvent molecules coordinated with Li⁺ions,the decomposition of solvent molecules is inhibited,leading to the improved electrolyte decomposition stability.According to the concentration coefficient and number density distribution,it is found that the ion adsorption behavior of concentrated organogel electrolyte in the nanochannel is significantly affected by the ion electrostatic interactions.The strong interacton between cations and anions make them to enter the nanochannel together.Thus,the nanochannel is partially occupied with the identical-sign ions,resulting in the lower opposite-sign ion concentration than the low-concentration electrolyte.The concentrated organogel electrolyte proposed in this paper is suitable for the field of high-capacity energy storage devices.