Theoretical And Computational Studies Of Electrochemical Behavior In Ultra-concentrated System

Ionic liquids,solid electrolytes,ion-intercalation materials,etc.are typical ultra-concentrated systems that are widely used in today’s high energy density energy devices.Therefore,researches on the electrochemical properties of ultra-concentrated ion systems are also increasing.Interfacial ion distribution（electric double layer structure）,ion transport and charge transfer are the key factors determining the performance of electrochemical devices.In these respects,ultra-concentration electrochemical systems are fundamentally different from traditional electrochemical systems.This paper explores this,and the main results are summarized as follows,1.Effect of ion size on the electric double layer and redox reaction on nanodiskelectrodeThe nanocrystallization of materials has become the development direction of current energy devices,and the importance of research on nanoelectrochemical interfaces is followed.In addition,the demand for energy density of energy devices is increasing,and the use of ultra-concentration systems is increasing.The volume of ions in ultra-concentrated systems is much larger than that in conventional dilute solutions.Then the ion size in the concentrated system is close to the nano-interface size,which will have an impact on the nano-device.Using the disk electrode as the research model,the symmetric electrolyte MN solution was used as the research system with a radius of 0.38 nm and 0.68 nm.The ion transport behavior and electrostatic potential distribution in the electrolyte system are described by the Nernst-Planck equation and the Poisson equation considering the ion volume effect.The results show that this ion size effect becomes more pronounced at smaller electrodes for electrodes above 10 nm;while it’s opposite for electrodes smaller than10 nm due to the electrode edge effect.In addition,we also investigated the effect of the ion size effect on the cyclic voltammetric response of the cations and anions reduction reactions on the nanodisk electrode.The results show that the ion size effect exacerbates the electric double layer effect and further suppresses the anion reduction reaction.However,as described above,the degree of inhibition on the electrode of 10 nm or less is attenuated,while attenuate the EDL effect for cation reduction.The EDL effect on the voltammetric responses for cation reduction could be significantly overestimated when ignoring the finite sizes of redox ions,while it will be underestimated for anion reduction.2.Ion-vacancy coupled charge transfer model for ion transport in concentratedsolutionsThe strong interactions between ions in the ultra-concentrated solution and the vacancies have an important influence on the ion transport.Traditional cognition generally treats ion transport as diffusion,and we deal with ion transport problems with a random walk model.Finally,it was found that the ion transport process can be considered as a chemical reaction between ions and vacancies.The random walk equation is then subjected to coarse graining to obtain a continuous kinetic equation.In the processes of obtaining rate constant,we statistically process the free energy of the system through lattice-gas model,and finally obtain the chemical potential of the occupied ions and the transition ions.The expression of the rate constant is obtained by the transition state theory and the BEP relationship.The subtlety is that the chemical potential of ions that were thought to be is actually the chemical potential of the ion-vacancy couple.With this new concept,the driving force of ion transport is actually the chemical affinity of the ion-vacancy coupled charge transfer reaction,which is a very valuable concept for combing two basic processes in transport and reaction.The phenomenological model is parameterized for a particular material by means of a first principle calculation.In addition,ion transport equations away from equilibrium and extensions to multi-component systems are discussed.And applying this model to the properties of the electric double layer that simulates and predicts ionic liquids as well as solid electrolytes.From the simulation results,the ionic liquid exhibits an oscillating distribution on the surface of the electrode due to the short-range interaction between ions.The solid electrolyte has a much larger thickness of the electric double layer than the ionic liquid and the electric double layer in the conventional electrolyte due to the single carrier,and the electric double layer exhibits an asymmetric distribution.3.Ion-coupled electron transfer in Li MPO₄On the basis of previewing the ion transport in concentrated system as ion-coupled vacancy transfer reaction,we consider the reaction in the bulk phase of Li MPO₄ during charge and discharge as ion-coupled electron transfer reaction,i.e.,Li M²⁺PO₄（4）)+M³⁺PO₄（4）+1)?M³⁺PO₄（4）)+Li M²⁺PO₄（4）+1).Then we can get the factors affecting Li transport in Li MPO₄ not just only the activation energy of ion transport that determines the value of diffusion coefficient of Li transport in the previous literature.In addition,it can be seen from the results of our calculation that the electron transfer coupling matrix element,,and the value of the configuration reorganization energyλrequired for the change of the environment around the metal M required for electron and ion transfer are important factors for the ion transport.Especially,our calculation results show that the coupling matrix element,,in the electron transfer reaction has an important influence on the ion-coupled electron transfer reaction.In summary,for the ultra-concentrated electrolyte system and electrode active materials,we use a unified charge（ion/electron）transfer model to deal with the electron/ion transfer problem.The difference is that for ultra-concentrated electrolyte systems,the ion-vacancy coupling reaction rate is mainly affected by the strong interaction between ions with the concentration of vacancies.For electrode active materials,the ion-electron coupling transfer reaction is not only related to the interaction between ions,but also the key factors affecting the electronic factors.