Micromechanisms On The Energy Density And Power Density Of Capacitors By Structured Thermodynamics

Renewable energy is a key component in stabilizing energy supply and protecting ecological environment,relying on high-performance electrochemical energy storage devices(i.e.,batteries and supercapacitors).However,the competitive relationship between energy density and power density of electrochemical energy storage devices hinders its large-scale application.The interfacial electrolyte microstructure is a key factor in regulating energy density and power density.At present,the regulation mechanism of electrolyte microstructure on thermodynamic properties is not clear,and the difficulty lies in the lack of quantitative theoretical model.In this paper,the statistical thermodynamic model coupling with fluid structure is expanded to study the regulation mechanism of interfacial electrolyte microstructure for the equilibrium and non-equilibrium thermodynamic properties,which provides a theoretical basis for the design of high-performance supercapacitors.The main contents of this paper are as follows:(1)The contribution of ion desolvation to the capacitance of microporous electrode materials was clarified.With the help of the multi-scale method combining MDFT and CD FT,the structure model of solvation diameter was determined by coupling the molecular structure of solvent,which builds the bridge of theoretical prediction and experimental measurement.It is found that as the ion desolvation effect enhanced,the solvation diameter of ions decreases and the corresponding capacitance increases.This multi-scale method not only quantitatively reveals the solvation microstructure in confined space,but also provides theoretical guidance for the design of microporous electrode materials.(2)The regulation mechanism of interfacial electrolyte microstructure on surface charge density was clarified.The intuitive physical expression of surface charge density was derived in terms of the electrolyte microstructure,which effectively distinguishes the charge polarization effect from the dielectric polarization effect.It is found that there is charge inversion in concentrated electrolyte solution,and the maximum energy density is obtained under medium electrolyte concentration and high voltage difference.The direct connection between electrolyte microstructure and energy storage properties is established,which provides theoretical guidance for the design of high-performance supercapacitors.(3)The combination of experimental and theoretical approaches demonstrated the regulation mechanism of interfacial electrolyte microstructure for capacitance.Based on the variation of interfacial free energy during the charging/discharging process of supercapacitor,the regulation mechanism of electrolyte microstructure was clarified,and further was verified by experiments.It is found that the enhanced ordering of interfacial electrolyte microstructure(i.e.,multilayer distribution structure)can increase the capacitance,and the maximum capacitance is obtained at 3.0M electrolyte concentration.The combination of theoretical calculation and experimental measurement establishes a direct connection between electrolyte microstructure and macroscopic properties,which provides a feasible scheme for experimental design of high-performance supercapacitors.(4)The extraction of salinity energy was optimized by using the regulation mechanism of electrolyte microstructure.Based on the interfacial potential difference between seawater and freshwater in response to electrolyte microstructure,the thermodynamic cycle curves of seawater and freshwater can be directly regulated.It is found that the electrolyte microstructure formed by non-static attraction of counterions at the interface is beneficial to expand the distance between the thermodynamic cycle curves,resulting in the increase of salinity energy and the decrease of operating voltage,which verifies the regulation mechanism of the interface electrolyte microstructure.(5)A complete physical model was constructed to capture the non-equilibrium thermodynamic properties of double-layer capacitors accurately and effectively.The combination of electrolyte microstructure and dielectric decrement effects(or overlapping effects)can construct a complete physical model,which expands the DDFT to accurately reveal the adsorption and diffusion of ions.It is found that the dielectric decrement effect can lead to the anomalous inhibition of interfacial adsorption,and the enhanced this effect will inhibit the energy storage of capacitors.It is further found that the strong overlapping effect can promote the complete separation of cation and anion,and then break through the competitive relationship between adsorption amount and adsorption rate.A complete physical model not only expands DDFT,but also provides a feasible scheme to break the competition relationship between energy density and power density.In summary,this paper determines the quantitative prediction model of ion desolvation effect,and builds an effective bridge for theoretical prediction and experimental measurement.Additionally,the intuitive physical connection between the interfacial electrolyte microstructure and thermodynamic properties is revealed.And the interfacial free energy is proposed to evaluate the regulatory effect of electrolyte microstructure,which is successfully applied to optimize the extraction of salinity energy.Furthemore,the electrolyte microstructure coupling overlapping effect constructs a complete physical model,which breaks the competitive relationship between adsorption amount and adsorption rate,providing a theoretical basis for optimizing the energy density and power density of supercapacitors.