NMR Studies of Ion Transport in Ultra-Concentrated Aqueous Lithium Electrolytes

Abstract

The electrolytes currently used in lithium ion batteries are a source of many cost and safety issues, which can be mitigated by a switch to aqueous electrolyte. This was, until recently, precluded by the narrow electrochemical stability window of the aqueous electrolyte, a problem which severely limited the practical voltage and energy density of the battery. However, recent developments have shown that it is possible to open up this stability window by increasing concentration of the salt (LiTFSI), enabling the formation of an interphase. This opens the door for a fresh focus on the properties of aqueous electrolytes. (1) Nuclear Magnetic Resonance (NMR) is a powerful tool for exploring ionic and molecular transport in electrolytes. The nuclei in question (7Li , 19F, and1H) are particularly well-suited to measurement by NMR, due to their gyromagnetic ratios and abundances. We performed NMR measurements on a range of concentrations of LiTFSI aqueous electrolyte, varying from 1 m to 21 m. In particular, pulsed gradient spin-echo self-diffusion experiments were performed on a 300 MHz spectrometer. Self-diffusion coefficients for both ions and water molecules were obtained over a range of temperatures from 20°C to -60°C. The diffusion results are displayed in Figs. 1 and 2. As expected, we note a general trend of smaller diffusion coefficients with lower temperature, as well as smaller diffusion coefficients with higher salt concentration. Cation transference numbers were determined directly from the diffusion measurements and those results are listed in Table 1. The cation transference number increases as the temperature falls and the salt concentration rises, as well. Although this investigation focuses on the transport properties of the liquid phase, ongoing work will address SEI formation on a variety of anodes in contact with this unusual aqueous system.