Nonaqueous Electrolytes with Advances in Solvents

Abstract

Most of the liquid electrolytes used in commercial lithium-ion (Li-ion) cells are nonaqueous solutions, in which roughly 1 mol dm−3 of lithium hexafluorophosphate (LiPF6) salt is dissolved in a mixture of carbonate solvents selected from cyclic carbonates—ethylene carbonate and propylene carbonate—and linear carbonates—dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. In Sect. 2.1, the physicochemical properties of these carbonate solvents are listed and the phase diagrams and electrolytic conductivity data of mixed carbonate solvent systems are given. However, recent market demands for Li-ion cells with higher energy, higher power, and higher safety requires new solvents to improve the performance of cells in electrolytes based on carbonate solvents only. New heteroatom-containing organic solvents including fluorine, boron, phosphorous, and sulfur, which have been applied to lithium cells in recent years, are reviewed from the viewpoints of synthesis, physicochemical properties, and cell performance by four authors. Section 2.2 mainly reviews the papers on novel fluorinated organic solvents, which include fluorinated lactones, fluorinated linear carboxylates, fluorinated cyclic carbonates, fluorinated linear carbonates, fluorinated monoethers, fluorinated diethers, and others. The physicochemical properties of typical fluorinated compounds are summarized in comparison with nonfluorinated counterparts. Section 2.3 summarizes the recent promising progress of electrolyte solvents that contain boron atoms, particularly borate esters and cyclic borate esters. The authors also introduce some boron compounds acting as additives and supporting salts in electrolytes. Section 2.4 reviews organophosphorous compounds as nonflammable or flame-retardant electrolytes for lithium-ion batteries. These include organic phosphates, phosphites, phosphonates, or phosphazenes, and a phosphonamidate as co-solvents or additives. The author introduces polymeric gel electrolytes containing these flame-retardant components. Section 2.5 reviews papers on lithium and lithium-ion cells using sulfur-containing organic solvents, including sulfide, sulfoxide, sulfone, sulfite, sulfonate, and sulfate. Particularly, the performance of sulfones such as ethyl methyl sulfone and sulfolane as electrolyte solvents for high-voltage cells is introduced.