Ionic liquid electrolytes and their mixtures for lithium batteries

2017-02-09T05:26:40Z (GMT) by Bayley, Paul Morgan
The insatiable apatite humanity has for energy provides a great desire for portable battery technology, with large growth and increased development sure to become one of the big achievements of this century. Lithium batteries provide the energy density required by modern devices, however, the flammability and toxicity of the electrolytes currently employed leaves much to be desired in the way of safety. In particular, lithium metal anodes, that take full advantage of the electrochemical properties of lithium and are the focus of this research, are considered too unsafe for commercial applications. Ionic Liquids (ILs) provide the safety and electrochemical stability to facilitate better lithium batteries, however, they still suffer from the disadvantage of typically having a viscosity 1-2 orders of magnitude above conventional solvents. Through a greater understanding of the affect of additives and blending on the transport properties and speciation within the electrolyte, a commercially practical material is closer to development. Screening a range of molecular additives, from common organic solvents to short chain oligoethers, has shown that the viscosity of the additive is not important and that selection of the right structure can dramatically enhance the transport properties and alter the speciation of the lithium ions. Although the carbonates increase the lithium ion diffusivity, their main mode of enhancement in battery electrolytes is their ability to polymerise into suitable Solid Electrolyte Interphase (SEI) layers. Even though tetrahydrofuran (THF) increased lithium diffusion more than the carbonates, its electrochemical instability negates any positive enhancement. Short chain oligoethers, particularly tetraglyme (TG), are an extremely effective additive that essentially removes the detrimental effect on the transport properties of the Ionic Liquid (IL) when lithium salt is added, enhancing the lithium ion diffusivity by more than a factor of 4 at 274 K. Interestingly, the diffusivity and molecular motions of the pyrrolidinium cation remain largely unchanged regardless of the additive that is present. Variable temperature Nuclear Magnetic Resonance Spectroscopy (NMR) spin-lattice relaxation experiments display a clear trend in lithium environment based on the complexing ability of the additive, with the order from the almost unchanged to dramatically different, respectively; Toluene, THF, 1NM3 and TG. Purely ionic electrolytes maintain the full safety aspect that ILs provide, however, the difficulties in maintaining reasonable transport properties while improving lithium electrochemistry are highlighted. The Nmethyl- N-butyl-morpholinium cation exhibited much lower conductivity than analogous structures without an ether oxygen yet its favourable lithium electrochemistry and high temperature cycling performance is promising for high temperature applications or for use as an additive. Binary and ternary mixtures of ILs with lithium salts provide a viable avenue to increase the transport properties and extend the operating temperature of the electrolyte. Using a common cation, N-methyl-N-propylpyrrolidinium, and mixed anions, the binary blend with an anion concentration at a ratio of 2:1, bis(fluorosulfonyl)imide:bis(trifluoromethanesulfonyl)imide, produced a material with a melting temperature more than 20 K lower than either pure component.