Lithium Environment in Dilute Poly(ethylene oxide)/Lithium Triflate Polymer Electrolyte from REDOR NMR Spectroscopy Jason R. Wickham, † Shawna S. York, ‡ Nathalie M. Rocher, † and Charles V. Rice* ,† Department of Chemistry and Biochemistry, UniVersity of Oklahoma, 620 Parrington OVal, Room 208, Norman, Oklahoma 73019, and Department of Chemistry, Oklahoma Baptist UniVersity, Box 61772, Shawnee, Oklahoma 74804. ReceiVed: January 30, 2006 The role of the lithium ion environment is of fundamental interest regarding transport and conductivity in lithium polymer electrolytes. X-ray crystallography has been used to characterize the lithium environment in completely crystalline poly(ethylene oxide) (PEO) electrolytes, but this approach cannot be used with dilute PEO electrolytes. Here, using solid-state NMR data collected with the rotational-echo double-resonance 13 C{ 7 Li} (REDOR) pulse sequence, we have been able to characterize the crystalline microdomains of a PEO-lithium triflate sample with an oxygen/lithium ratio of 20:1. Our data clearly demonstrates that the lithium crystalline microdomains are nearly identical to those of a completely crystalline 3:1 sample, for which the crystal structure is known. Introduction Rechargeable lithium ion batteries provide a convenient, portable source of electricity. 1-3 The successful commercializa- tion of this technology resulted from the discovery and development of novel cathode and anode intercalation materials. The electrodes are separated by an inert porous polymer separator, usually poly(propylene), impregnated with low mo- lecular weight organic liquids containing a dissolved salt. However, these organic liquids present a potential fire hazard and may react with the electrode materials. Major efforts to develop solvent-free ion-conducting polymer electrolytes have been underway for 30 years. 2-9 Critical to these efforts is the need to identify the chemical interactions that hinder or facilitate ion transport. Many laboratories have investigated solid-polymer electro- lytes based on poly(ethylene oxide) (PEO), due to its low glass transition temperature (T g ) and its ability to dissolve metal salts. 10-13 Lithium provides a high energy density, yet the room- temperature conductivities of PEO/Li salt systems are too low to replace current liquid or hybrid batteries. Developing a solid polymer electrolyte capable of replacing current electrolytes requires knowledge of lithium transport between the electrodes. Lithium cations are coordinated to the polymer electrolyte through weak chemical interactions. Since these polymer electrolytes have T g ’s that are well below room temperature, they have a fair amount of segmental motion. Thus, a lithium cation that is five coordinate will eventually be forced into a four-coordinate conformation due to the segmental motion of the polymer electrolyte. This four-coordinate conformation is less stable than the five-coordinate one, which causes the lithium cation to hop to a neighboring binding site, facilitating lithium transport. 14 For PEO/Li salt systems, impedance measurements can be used to evaluate ion transport, 15 but these measurements do not provide fundamental chemical insight as to the PEO- lithium chemical structure. Low temperature X-ray crystal- lography data of completely crystalline PEO electrolytes revealed a PEO helix around a central core of lithium cations. 16 Triflate anions, CF 3 SO 3 - , participate in lithium coordination by forming bridges between two lithium cations. Dilute PEO electrolytes are heterogeneous at room temper- ature, consisting of a pure PEO phase, a crystalline PEO-salt phase, and an amorphous phase containing some dissolved salt. 7,13,17-19 It has been observed in numerous PEO-salt systems that ionic conductivity occurs mainly in the amorphous phase. 13 Heating the polymer near 100 °C significantly increases conductivity, which is attributed to a melting of the crystalline phase. 12 There have been various efforts made to reduce the amount of the crystalline phase at room temperature, thereby increasing conductivity, using plasticizers, copolymers, and fillers. 4,20-25 These efforts would benefit from the ability to characterize the lithium environment in dilute polymer electro- lytes. X-ray crystallography cannot be employed to study dilute polymer electrolytes because the samples are heterogeneous and do not possess the long-range structural order necessary for diffraction. However, solid-state NMR is ideally suited to provide structural data for the various domains within dilute polymer electrolytes. Previous solid-state NMR studies of PEO or PEO-based polymer electrolytes have examined the connec- tion between ion diffusion and polymer segmental motion. Spin-lattice relaxation and pulse field gradient measurements of 7 Li and 19 F yield ion diffusion rates, which are used to gauge the success of polymer electrolyte modifications. 26-34 The 7 Li spectrum can also be used to identify the number of lithium binding sites. 35,36 Carbon-13 magic angle spinning (MAS) NMR * Corresponding author. E-mail: rice@ou.edu. † University of Oklahoma. ‡ Oklahoma Baptist University. 4538 2006, 110, 4538-4541 Published on Web 02/23/2006 10.1021/jp060643m CCC: $33.50 © 2006 American Chemical Society