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NMR Studies of Local Structure and Ion Transport in Lithium Battery Electrolytes - Steve Greenbaum

Hunter College of CUNY, New York, USA

Nuclear magnetic resonance (NMR) is a unique method of probing local structure and dynamics in a wide variety of materials owing to the short range nature of most of the interactions that produce spectral features and govern relaxation behavior. Among its advantages, besides not requiring crystalline samples, are elemental (nuclear) specificity and its reliably quantitative nature in that the integrated intensity of a particular spectral component is directly proportional to the number of nuclei in the corresponding material phase. Modern day NMR has grown into an enormously diverse array of sophisticated experimental techniques in studies ranging from complex biochemical systems in the solution phase to a wide selection of solid-state compounds, with often negligible overlap in methodology. As a counterexample of the increasing divergence between the liquids and solids NMR communities, we present here several ongoing NMR investigations of lithium ion battery materials, utilizing both liquid state and solid state NMR.

The prototypical lithium ion battery electrolyte consists of LiPF6 dissolved in a mixture of cyclic and linear carbonates, ethylene carbonate (EC) and dimethyl carbonate (DMC) as examples of the former and latter, respectively. The specific solvation shell of the Li+ ions affects transport properties of the solution as well as charge transfer processes at the electrodes. With Army Research Lab collaborators Kang Xu and Arthur von Cresce, natural abundance 17O nuclear magnetic resonance (NMR) spectra have been obtained for a series of EC/DMC solutions ranging from 0 to 100% EC with fixed 1M LiPF6 concentration. Using a simple model of dynamic equilibrium between associated and non-associated solvent molecules, an average primary Li+ solvation structure as a function of EC/DMC concentration can be conjectured.

In the solid electrolyte realm, in collaboration with Chengdu Liang,, Oak Ridge National Lab, we investigated the phase evolution of nanoporous β-Li3PS4 prepared by wet chemical synthesis and subsequent thermal treatment by 7Li and 31P magic angle spinning NMR. The β phase exhibits ionic conductivity over two orders of magnitude higher than that of the corresponding bulk crystalline compound, γ -Li3PS4.

Finally, we describe a novel solvent-free Li polymer electrolyte based on a new semicrystalline polymer, in collaboration with Mike Zimmerman of Ionic Materials, Inc. Structural and transport aspects were investigated by 1H, 7Li, 13C, and 19F NMR, and by NMR diffusion and relaxation measurements. What emerges is an ion-conducting polymer unlike those reported to date, with a transport process that is decoupled from the host polymer dynamics. Of principal interest are ionic conductivity as high as 1.5 X 10-3 S cm-1 at 25oC, comparable to or exceeding the value of a standard liquid electrolyte in a polyolefin separator, and a cation transference number of 0.7. This electrolyte has been successfully tested in Li metal/LiCoO2 under conditions that suggest safe operation of rechargeable lithium batteries with considerably higher power and energy densities than today’s Li ion batteries.


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