Studies On The Electrochemical Reaction Kinetics And Performance Optimization Of Redox Electrolyte Enhanced Electrochemical Energy Storage Devices

Electrochemical energy storage devices play a significant role in the field of energy storage due to their high efficiency,convenience,flexibility and adaptability.By introducing ions/ion pairs that can undergo redox reactions in the electrolyte,it is possible to provide a significant portion of additional capacity without destroying the electrode structure,and also promote the electrochemical kinetics of the reaction system based on liquid-phase reactions,thus achieving a simultaneous improvement of energy density and power density.Nevertheless,the research on the energy storage mechanism of this redox electrolyte-enhanced electrochemical energy storage device is still immature at present,which makes the performance optimization strategy for this system somewhat blind,and also cannot fundamentally solve its problems of poor reversibility and serious self-discharge.Therefore,the development and commercialization of redox electrolyte-enhanced electrochemical energy storage devices still face huge challenges.In view of the above problems,this paper takes a typical redox electrolyte reaction system as the research object.Combines a new quasi-static electrochemical measurement and in situ characterization,its electrochemical behavior at the electrode/electrolyte interface can be systematically analyzed,and an electrochemical reaction kinetics model has been established.Under the guidance of the reaction mechanism,certain strategies are adopted to optimize the electrochemical performance of the system and we provide a theoretical explanation for the problem of poor reversibility of the reaction.On this basis,by adjusting the electrolyte composition,electrode/electrolyte configuration and ion diffusion coefficient,improving the reversibility of the reaction and suppressing self-discharge,and summarizing and dividing the electrochemical behavior of different reaction systems,further guiding the design and performance optimization of redox electrolyte enhanced electrochemical energy storage devices.The specific work is as follows:（1）Taking[Fe（CN）₆]^4-/[Fe（CN）₆]^3-redox electrolyte couples as the example,we use multi-potential steps measurement（MPSM）to semi-quantitatively analyze its electrochemical behavior,combined with in situ Raman characterization of ion distribution state on electrode surface,constructing an electrochemical kinetics evolution model.The model shows that there is diffusion of reacted ions toward the bulk of electrolyte under reaction potential,as well as diffusion of unreacted ions toward electrode/electrolyte interface.This kind of ionic exchange in electric double layer increases with increasing concentration of redox electrolyte ions,accelerating reaction kinetics process while transferring more charge,thus achieving simultaneous improvement of power density and energy density.However,diffused reacted ions cannot return to reaction interface for charge transfer in next stage,resulting in low coulombic efficiency.This phenomenon is more significant at high concentration.（2）Aiming at low coulombic efficiency problem of above system,we improve reversibility of reaction by adjusting initial concentration ratio[Fe（CN）₆]^4-/[Fe（CN）₆]^3-in electrolyte.Systematic test results show that when concentration ratio is 1:1 system can achieve 100%coulombic efficiency and 97.8%capacity retention after 2000charge/discharge cycles.Further MPSM analysis combined with in situ Raman analysis results show that the introduction of[Fe（CN）₆]^3-cannot inhibit ionic exchange and reaction of[Fe（CN）₆]^4-at high potential,but through reverse reaction process at low potential to offset charge loss.Based on this phenomenon we systematically analyzed distribution and movement redox electrolyte ions under different electrolyte components different charge/discharge stages further improved electrochemical kinetics evolution model.（3）In regard to the above system’s unable to inhibit the diffusion of reaction ions,by fixing reacted ions on electrode surface,the charge loss can be reduced during reaction process,thus improving reversibility and suppressing self-discharge.Taking Mn²⁺/MnO₂ deposition/dissolution as example.Based on MSPM analysis,the electrochemical behavior between different electrode/electrolyte configurations under different voltage windows can be distinguished.Results show Mn²⁺/MnO₂deposition/dissolution occurs at higher potential accompanied by proton intercalation/deintercalation,thus the diffusion of protons under neutral conditions prevents the complete dissolution of MnO₂ and also causes a decrease in the reversibility of the system,an effect that is more pronounced in the electrode-free system.In addition,the intercalation/deintercalation of metal cations occurs in the system at low potentials,and such process is affected by the full cell potential window as well as the charge/discharge rate,and slows down the overall reaction kinetic process.（4）By introducing eutectic redox electrolytes reducing diffusion coefficient ions,suppressing diffusion reacted ions improving reversibility.From the results of electrochemical measurements with different electrodes in aqueous and eutectic electrolytes,it can be concluded that When the diffusion coefficient of ions is small,the electrochemical process will be interrupted after the depletion of interfacial ions or electrons,rather than maintaining dynamic equilibrium by ion exchange as in aqueous electrolytes.Based on the difference in the number of adsorbed ions at the electrode/electrolyte interface,the process can be divided into electrolyte depletion state as well as electron depletion state.In aqueous electrolytes,the differences at the reaction interface are smoothed out by ion exchange outside the electric double layer,eventually exhibiting similar electrochemical behavior.Based on the above conclusions,the kinetic model of electrochemical reactions in redox electrolytes is further extended,which can guide the application and development of redox electrolyte-enhanced electrochemical energy storage devices.