Engineered Fluorine-Free Electrolytes for Next-Generation Batteries

Due to the successful commercialization of lithium-ion batteries (LIBs), there is a growing interest in developing new battery materials with improved properties. The uneven distribution of natural resources, the low abundance of battery materials in the Earth’s crust, and the growing geopolitical concerns should also be considered and addressed. In this context, alternative battery technologies, such as sodium-ion batteries (SIBs) and lithium metal batteries (LMBs), are getting attention by researchers, due to the low cost of readily available sodium resources and the very high capacity of a lithium metal anode, etc. Conventional electrolytes of any battery technology are today heavily based on fluorinated salts and volatile organic solvents, posing serious safety issues all the way from synthesis to application and recycling. Additionally, the increasing concerns of per- and polyfluoroalkyl substances (PFAS) highlight the urgent demand to explore performant fluorine-free electrolytes, ideally also non-flammable. In this study, novel fluorine-free ionic materials and electrolytes have been designed and their physical and electrochemical properties thoroughly investigated. In the first part (Paper I), fluorine-free “solvent-in-salt” (SIS) sodium electrolytes based on sodium bis(2-(2-ethoxyethoxy)ethyl) phosphate (NaDEEP) salt and tris(2-(2-ethoxyethoxy)ethyl) phosphate (TEOP) solvent are presented. The addition of TEOP increased the electrochemical oxidation stability of the SIS electrolytes and an unusual ionic conductivity behavior is observed – the ionic conductivities of the electrolytes increase with increasing salt concentration. In the second paper (Paper II), a series of new orthoborate-based ionic materials, containing the bis(glycolato)borate (BGB) anion and phosphonium/ammonium cations are prepared and compared with the popular bis(oxalato)borate (BOB) salts. Some of these ionic materials are room temperature ionic liquids (RTILs), while others are organic ionic plastic crystals (OIPCs). The tetrabutylphosphonium bis(glycolato)borate ([P4444][BGB]) OIPC displays much higher decomposition temperature than the structural analogous [P4444][BOB] IL, and multinuclear solid-state NMR spectroscopy indicated weaker cation-anion interactions in phosphonium-based salts than the ammonium-based ones. Given the excellent moisture and thermal stabilities brought by the BGB anion, a family of BGB-based alkali and alkaline metal salts were synthesized and characterized (Paper III). The LiBGB-based electrolytes using dimethyl sulfoxide (DMSO), triethyl phosphate (TEP) and trimethyl phosphate (TMP) have excellent moisture stability, optimal ionic conductivity, better aluminum (Al) passivation and long-term Li plating-stripping performance. Sequentially, the next study (Paper IV) is focused on investigating the effect of additives on the performance of these electrolytes, such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), etc. Finally, in the fifth paper (Paper V), two- and three-component eutectic electrolytes based on pyrrolidinium saccharinate [Pyrr][Sac], lithium saccharinate Li[Sac] and/or [P4444][BGB] salts were created. The physicochemical properties of these salts as well as the Li compatibility and cell performance are thoroughly investigated. Overall, these studies identified several new fluorine-free salts and electrolytes with beneficial properties that can potentially be used in next-generation batteries.