Study On Infrared Spectra And System Structures Of LiBF₄-based Electrolyte Solutions

The properties of electrolyte solutions for lithium battery are statistical behavior of complex microscopic interactions, such as ion-molecule and molecule-molecule interactions in solutions. These interactions determine the performance of the electrolyte solutions, even of the lithium battery. In this paper, lithium tetrafluoroborate(Li BF4) was selected as lithium salt and propylene carbonate(PC), diethyl carbonate(DEC) and N, N-dimethylformamide(DMF) were selected as organic solvents, respectively. Li BF4/PC, Li BF4/DEC, Li BF4/DMF, Li BF4/PC+DEC and Li BF4/PC+DMF electrolyte solutions with different concentration gradient were prepared and analysed. According to the analysis of FTIR spectra and quantum chemistry calculation, the ion-molecule interaction, molecule-molecule interaction, preferential solvation, structural model etc. inside the electrolyte solutions have been studied. This research work is of great significance and academic value in the field of electrolyte modification, development of low temperature new electrolyte system and so on. The main research work and results are as follows:In Li BF4/PC, Li BF4/DEC and Li BF4/DMF unitary electrolyte solutions, strong interactions of Li+-PC, Li+-DEC and Li+-DMF were observed. These interactions occur through the oxygen atom of carbonyl group of PC molecule, through oxygen atom of carbonyl or ether oxygen of DEC molecule, through oxygen atom of O=C-N group of DMF molecule, respectively.In Li BF4/PC+DEC binary electrolyte solutions, strong interactions exist between PC and DEC molecules, by which the C=O stretch of PC molecule shifts to the higher wavenumber side with the increasing of volume ratio VDEC/VPC. Simultaneously, strong interactions of Li+-PC and Li+-DEC were observed in Li BF4/PC+DEC binary system, which through the oxygen atom of carbonyl group of PC molecule and oxygen atom of carbonyl or ether oxygen of DEC molecule. No obvious preferential selective solvation of Li+ was found in Li BF4/PC+DEC electrolyte solutions due to their little difference between PC and DEC in DN values.In Li BF4/PC+DMF binary electrolyte solutions, the interaction between PC and DMF molecules in PC+DMF mixtures is too weak to be detected by IR spectroscopy. However, strong interactions of Li+-PC and Li+-DMF were observed in Li BF4/PC+DMF binary system, which benefited from the oxygen atom of carbonyl group of PC molecule and the oxygen atom of O=C-N group of DMF molecule. The specific solvation of Li+ was distinguished in Li BF4/PC+DMF binary system, where DMF preferentially coordinates with Li+. This can be explained by their large difference of PC and DMF in DN values.