| Understanding the molecular interactions within electrolyte mixtures is essential for designing next generation electrolyte materials for high-voltage lithium ion (Li-ion) battery applications. Despite significant advancements in Li-ion battery electrode materials, which have theoretically enabled cell operation in excess of 5 V (vs. Li/Li+), the state-of-the-art electrolyte formulation has remained largely unchanged over two decades after its initial commercialization. To optimize the electrolyte properties, it is crucial to understand and relate the molecular-level interactions to the measured bulk properties. In the present study, these interactions have been explored through the use of the following techniques: phase diagrams (DSC analysis), X-ray single crystal structural determination, spectroscopic vibrational analysis (Raman) of the solvent and anion bands, and other techniques for determining electrolyte physical and electrochemical properties (density, viscosity and ionic conductivity).;The primary focus of the present work is on the difluoro(oxalato)borate (DFOB--) anion and how the properties of this anion differ from other anions used in Li-ion battery electrolyte mixtures. The synthesis of highly pure LiDFOB is reported, along with the X-ray single crystal structural analysis of the neat salt and its dihydrate (LiDFOB·2H2O). The ion coordination behavior of the DFOB-- anion is compared with the structurally similar BF4 . and lithium bis(oxalato)borate (BOB --) anions. The decomposition mechanism and Raman vibrational band assignments for the LiDFOB salt are also compared with those for LiBF 4 and LiBOB.;The carbonate-based solvents ethylene carbonate (EC) and propylene carbonate (PC) are of extraordinary importance due to their applications in state-of-the-art electrolytes. The Raman analysis of EC- and PC-based electrolyte mixtures to determine solvation numbers, without appropriate correction factors, is inherently flawed due to varying Raman scattering activities of the coordinated and uncoordinated solvent bands. In this study, correction factors are identified through joint quantum chemistry (QC) and experimental analysis techniques that enable the accurate analysis of EC and PC-based mixtures with lithium salts. From this analysis, the ionic association strength of the DFOB. anion was determined and compared to various lithium salt anions (PF6 --, TFSI--, BOB--, ClO 4 --, BF4--, CF 3SO3--, CF3CO2 --).;Understanding the ionic association (Li+...anion) and ion solvation (Li+...solvent) interactions is crucial for an informed design process (i.e., electrolytes-on-demand) if these interactions can be directly linked to the physical and electrochemical properties of electrolyte mixtures. Thus, the solvation behavior of carbonate and lactone-based electrolytes with three lithium salts (LiBF4, LiDFOB and LiBOB) was investigated. Based upon the analysis of the solution structure of these mixtures, the electrolyte transport properties (viscosity and conductivity) were readily explainable. A direct link was therefore made between the solution structure of the electrolyte mixtures and the physicochemical properties they possess.;LiDFOB has proven to be a highly versatile lithium salt, displaying many favorable battery properties including the formation of amorphous mixtures (even in mixtures with EC), relatively high ionic conductivity, and favorable electrode passivation layer properties when mixed with appropriate electrolyte solvents. Thus, alternative applications of the DFOB- anion have been explored, such as its use as an ionic liquid (IL) anion. N-Alkyl- N-methylpyrrolidinium difluoro(oxalato)borate (PY1RDFOB) ILs have been synthesized and characterized. These ILs are liquid at room temperature and have favorable Al corrosion properties, as well as a large electrochemical stability window.;The present study provides a useful set of tools for scrutinizing a new electrolyte material; from the initial synthesis and purity evaluation, to examining ionic association and ion solvation interactions in electrolyte mixtures, and investigating alternative uses of the material for utilization in a myriad of applications. With the use of such techniques, the guided development of electrolyte materials can be employed instead of the purely empirical approach that is commonly used in the battery electrolyte research community to date. |
