Organic Ice Electrolytes for Lithium Batteries

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Solid-state ionic conductors are being actively developed for batteries employing lithium electrochemistry. Lithium battery cells based on solid electrolytes are believed to be free from concerns found in conventional lithium ion batteries (LIBs) based on liquid electrolytes. Solid electrolytes are expected to be non-volatile and nonflammable without electrolyte leakage, suppressing dendritic growth of lithium metal. The benefits of solid electrolytes come from their immobile and mechanically hard state distinguished from the mobile and fluidic state of liquid electrolytes.Solid electrolytes popularly employed for lithium batteries, including inorganic oxides and sulfides as well as organic polymers, are classified as network solids (e.g., Li7La3Zr2O12, garnet in oxides and Li6PS5Cl, argyrodite in sulfides) based on covalent and/or ionic bonds. A building unit of network solids, the atomic ratio of which is described by chemical formula, is repeatedly extended to form a continuous network throughout the material. On the other hand, the possibility of molecular solid electrolytes, the phases of which are determined by the inter-molecular interactions, has rarely been suggested.Recently, an example of molecular solid electrolyte was presented by Guo, Z. et al. (2019). When 1 m Li2SO4 (aq) was frozen, the ionic conductivity of the solid ice electrolyte was 0.1 mS cm-1 at -17 oC. Such an ionically conductive ice electrolyte was not easily expected from the practical wisdom in LIB field: LIBs do not work when their electrolytes are frozen. For example, carbonate-based mixture electrolyte, ED(=1 M LiPF6 in ethylene carbonate/dimethyl carbonate) was frozen at -30oC and the cell used that electrolyte could not delivered any charging/discharging capacity at all.In contrast, each solvent of components of ED, a cyclic carbonate (ethylene carbonate, EC) and a linear carbonate (dimethyl carbonate, DMC) were theoretically expected to have Li+-conductive channels in their frozen crystal structures. Experimentally, the high-t Li+ (Li+ transference number) frozen-solid electrolytes (EC or DMC based single-solvent electrolyte) successfully drove lithium metal batteries below their freezing points (fp) even if their mixture did not work as an ionic conductor in its frozen state. Interestingly, the t Li+ of the electrolytes sharply increased below fp, declaring that the conduction mechanism changed from vehicular conduction to hopping conduction of Li+ through crystal structure with low diffusion energy barrier (0.28 eV at -20 oC vs. 0.32 eV of LLZO at RT). From the lessons from the carbonates, we proposed a cyclic sulfone (sulfolane, SL) as another solvent for molecular-solid electrolytes. The frozen SL electrolyte at -30 oC allowed its LMB cells to deliver the capacity equivalent to that of a conventional carbonate liquid electrolyte that is not frozen at the same -30 oC (EC+EMC where EMC = ethylmethyl carbonate). More importantly, the LMB cells with the frozen SL was longer-lasting than the liquid cells. The frozen organic ice electrolyte effectively and totally suppressed dendritic growth of lithium metal by utilizing its own mechanical hardness, high t Li+ and interface-specific anion-derived SEI layer.