Research And Synthesis Of Aqueous Sodium Ion Colloid Electrolyte And Its Application In Batteries

Due to the characteristics of abundant raw material sources,environment-friendly and high safety,aqueous sodium ion batteries currently receive an increased research attention.The electrolyte plays a key role on the electrochemical performance of aqueous sodium ion batteries.This study aims to develop a colloidal electrolyte that is suitable for aqueous sodium ion batteries.The electrolyte has quasi-solid characteristics and can solve problems such as liquid leakage,salt creep,and electrode powdering in water cells.At the same time,it can theoretically increase electrochemical window of the aqueous electrolyte.Main research content and results have been obtained as follows:1.Design and preparation of high stability aqueous sodium ion colloidal electrolytes.Based on the design of different colloidal electrolytes,the XRD and SEM were used to characterize the phases and surface morphology of the fumed silica,and the gelation time,gel strength,and aging characteristics of the colloidal electrolytes were studied.Colloidal electrolytes were prepared by different dispersion conditions,and the effects of dispersion rate and dispersion time on colloidal stability were studied.It was found that with the increase of fumed silica content,the gelation rate of colloidal electrolytes continuously increased.The gel strength was continuously enhanced,and the water loss rate was continuously reduced.On the premise of ensuring the colloidal strength without destroying its stability,the content of fumed silica is preferably 4wt.⁶wt.%.Increasing the dispersion speed and increasing the dispersion time can effectively prolong the gelation time,but the too long dispersion time can also cause rapid gelation of the colloidal electrolyte.The optimization of the reasonable dispersion rate can be controlled between 18000r/min²2000r/min,and the dispersion time can be controlled between 3min⁴.5min.2.The preparation and characterization of the pH-controllable colloidal electrolytes.In this study,we explained the gelation process of the colloidal electrolytes microscopically.We characterized the maintenance properties of the colloidal electrolytes on water macroscopically,and found the way of pH regulation method suitable for the colloidal electrolytes.It was found that there are two main intervals of colloidal particle size distribution,and the standing time has a significant effect on the size of colloidal particles.Compared with the electrolyte solution,the colloidal electrolyte can improve the water retention ability under both medium and high temperature conditions.pH regulator（NH₂）₂CO content,holding temperature and holding time have significant effects on the pH adjustment effect.After optimization of the above factors,a reasonable（NH₂）₂CO content can be controlled at 0.5wt.¹wt.%,and holding temperature can be controlled at70°C⁸0°C.After the pH adjustment,the gas evolution intensity of the material can be reduced.The retention rate of the 390 cycles capacity is increased from 16.9%to92.1%,and the charge-discharge polarization is reduced.3.Electrochemical behavior of different electrode systems in colloidal electrolytes.In this study,we found that different current collector materials had a very different gas evolution potential in the positive and negative electrode,and through a comprehensive characterization found the collector suitable for the colloidal electrolyte.Compared with the electrolyte solution,the colloidal electrolyte can effectively inhibit water decomposition under the environment of weak acid and weak base（pH between 6 and 8）.Through the study of the self-discharge characteristics,a separator suitable for a gel electrolyte was found.In the study of ion transport characteristics,we found that the conductivity and diffusion resistance of the colloidal gel before gelation were basically the same as those of the electrolyte solution,and the conductivity and diffusion resistance after gelation slightly decreased.The measurement of the ion diffusion coefficient also confirmed the existence of this phenomenon.The colloidal electrolyte gel has a lower interface impedance than the electrolyte solution.After optimization of the above factors,through the assembly and characterization of the battery,we found that the sodium-incorporated compound NaTi₂（PO₄）₃ was circulated 10000 cycles in the sodium ion colloidal electrolyte and the capacity retention rate was as high as80%.The pH test results showed that the pH in the negative electrode area gradually increases,the pH in the positive electrode area gradually decreases after many cycles.A pH gradient is formed between the positive electrode and the negative electrode to inhibit water decomposition.The colloidal electrolyte developed in the present study has the characteristics of quasi-solid state,controllable pH,and strong water retention capability,which can prevent problems such as leakage of electrolyte in aqueous batteries and salt leaching in electrolytes.It provides an important guiding significance for the design of new aqueous batteries.