| The structure and physical properties of highly concentrated aqueous solutions are one of the important research branches of electrolytes involved in partial energy storage.At present,researchers mainly focus on the phase separation of the system,the formation of anion network structure,the diffusion mechanism of cations such as Li+,and the desolvation of cations before they enter the solid-liquid interface layer.This paper focuses on revealing the structure and property information of water in aqueous solutions to achieve a more comprehensive understanding of aqueous solutions.With a particular focus on how water molecules affect or modulate the interactions between anions and cations in different concentration ranges of several highly regarded lithium salt water solutions,as well as in bulk and nanospace.By measuring the diffusion behavior of the bound water near the ion,it is revealed whether there is a coupling phenomenon between cation diffusion and the diffusion of the bound water near the ion that is traditionally assumed.In order to achieve the above purpose,based on the differential scanning calorimeter,Raman spectroscopy,infrared spectroscopy,nuclear magnetic resonance,electron paramagnetic resonance and other experimental methods,the structure and some physical properties of a series of lithium salt aqueous solutions and AlCl3 aqueous solutions were studied in this paper.The main research results are as follows:(1)Based on the previous accumulation of the research group,the concept of a new state diagram of aqueous solution is explicitly proposed in this paper.The state diagram generally divides the aqueous solution into three concentration zones.The water molecular content in the two boundary concentration points is the number of hydration water of solute and the multiple number of the hydration water of solute that is insensitive to the solute type.The division of the three concentration ranges relates the low concentration,medium concentration and high concentration of the original ambiguous definition of the aqueous solution to the solute bound water number.(2)For the zone Ⅲwith high water content of LiTFSI(Bis(trifluoromethane)sulfonimide lithium),the water molecule consists of the hydration water of solute and bulk-like free water far away from the solute.The hydration water is easy to vitrify when it is cooled.Based on the glass transition temperature of the hydration water,the hydration water number of LiTFSI is 7,which is very close to LiCl with 6 hydration water.This result emphasizes the similarity of the TFSI-—H2O interaction in Zone Ⅲ compare with Cl"—H2O.LiTFSI·18H2O and LiTFSI·7H2O are the dividing points of the three concentration intervals of the aqueous solution.In Zone Ⅱ,water molecules are composed of hydration water and confined water located in the gap between closely packed hydrated solutes.LiTFSI aqueous solutions exhibit properties that are significantly different from simple electrolytes or other lithium salt solutions in Zone II.The anti-plasticization of water molecules and the absence of cold crystallization of water in the system are the most significant characteristics.The water molecules preferentially form the hydration water around Li+,and then the added water molecules help to form the specific network structure of Li+H2O-TFSI-,which is considered as the original dominant factor of the above phenomenon.In zone I,the number of water molecules is less than the hydration number of solute,and the abnormal concentration dependence of the vibration properties of water molecules reveals that phase separation occurs in the zone I of the system.This paper clearly points out the concentration range characteristics of phase separation in the LiTFSI·nH2O system for the first time,especially the relationship with hydration number of solute.(3)A detailed discussion was conducted on the relationship between the glass transition temperature and water content in three concentration ranges of LiTFSI·nH2O system in nanospace.Especially for Zone Ⅲ with high water content,partial crystallization during the cooling process results in the remaining non crystallized concentrated phase(LiTFSI·7H2O)being secondary confined between the pore wall and the ice at the pore center.The change in water content can adjust the thickness of the liquid film located in the secondary confined space,highlighting the influence of the pore wall on the glass transition and relaxation behavior of the aqueous solution.The experiment indicated that the glass transition temperature of LiTFSI·7H2O significantly increases in the nanopores with the decrease of liquid film thickness.Under the same conditions,the glass transition temperature of LiFSI·7H2O and LiCl·6H2O is independent of the liquid film thickness.This result emphasizes that the hydrophilic silica surface significantly modulates the interactions between different components of the LiTFSI·7H2O system and their interactions with hydroxyl groups of pore wall,and this modulation effect has a significant anion dependence.(4)In this paper,the 1H diffusion coefficient of water molecules in the first hydration shell of high valence cation A13+and its variation with concentration and temperature are revealed for the first time.At 298K,1H translation diffusion coefficient is higher than Al3+,and they both have the same diffusion coefficient only when the temperature is reduced to 243K.The conclusion that water molecules and ion in the first hydration shell of the cation have different diffusion coefficients at room temperature is also applicable to Li+.This result shows that the iceberg model or the vehicular mechanism of the diffusion of the apparent ions in cooperation with the adjacent bound water is not a microscopic description of the translational diffusion of metal cations with high charge density.In this paper,we found that the Stokes Einstein relationship with the ionic hydration radius as the diffusion radius can quantitatively establish the relationship between the ionic diffusion coefficient and the macro viscosity in the entire concentration range when analyzing the diffusion coefficient of the published aqueous solution containing high valence cations.The above paradox emphasizes that the SE relation is only an apparent equivalent description of the effect of strong interaction on ion diffusion between the ion with high charge density and water.(5)In this paper,a series of aqueous solution systems with abnormally increased specific heat from cooling to before glass transition were found outside the aqueous solution of hydrazine trifluoroacetate.The analysis results emphasize the importance of CF3-as a hydrophobic functional group to the occurrence of the abnormal specific heat of deeply supercooled water solution.At the same time,the discovery of abnormal specific heat of Li+ electrolyte aqueous solution expands the requirement for the original properties of anions and cations to form hydrogen bonds with surrounding water molecules. |
PDF Full Text Request