Li ion transport in an intercalated polymer electrolyte

Solid-state NMR studies of the crystalline and amorphous domains within PEO and PEO: LiTf systems

Solid polymer electrolytes (SPEs) contain amorphous and crystalline regions, each of which have unique contributions to the (13)C NMR spectrum. Understanding and assigning the (13)C NMR signals are vital to interpreting the NMR data collected for each phase. The (13)C CPMAS solid-state NMR spectrum of poly(ethylene oxide), a common polymer electrolyte host material, has superimposed broad and narrow components. Previously, the narrow component has been assigned to the amorphous region and the broad component to the crystalline PEO fraction. These assignments for pure PEO have been applied to various PEO:salt systems. Using lithium triflate salt dissolved in PEO, we revisit the spectral assignments and discover that the narrow component is due to crystalline PEO:LiTf component, which is reversed from the previous pure PEO assignment. This paradigm shift is based on data collected from a 100% crystalline PEO:LiTf with a 3:1 oxygen:lithium ratio sample, which exhibited only the narrow peak. For dilute electrolytes, such as 20:1 PEO:LiTf, the (13)C CPMAS spectra contain the narrow peak superimposed on a broad peak as seen with pure PEO. As dilute electrolytes are heterogeneous with crystalline and amorphous regions of both pure PEO and PEO:LiTf complex, peak assignments for pure PEO and PEO:LiTf are important. Thus, we reexamine the previous assignment for pure PEO using samples of pure powdered PEO, thermally treated pure powdered PEO, and a thin film PEO cast from an acetonitrile solution. With these different samples, we observed the growth of the narrow peak under conditions that favor crystallization. Therefore, for pure PEO, we have reassigned the narrow peak to the crystalline region and the broad peak to the amorphous region. In light of our observations, previous NMR studies of pure PEO and PEO SPEs should be reinvestigated. We also use rotational echo double resonance (REDOR) to study the 20:1 PEO:LiTf created from 2 and 100 kDa PEO. We find that the lithium environment is similar in the respective microcrystalline domains. However, the 100 kDa samples have a larger fraction of pure crystalline PEO.

Montmorillonite−Poly(ethylene oxide) Nanocomposites: Interlayer Alkali Metal Behavior

Clay-PEO nanocomposites can have large electrical conductivities that make them potential electrolyte materials for rechargeable lithium batteries, but the origin of these large conductivities, especially for Li-containing materials, is poorly understood. This paper presents X-ray diffraction (XRD), TGA-DTA, and (7)Li and (23)Na NMR data for PEO nanocomposites made with natural (SWy-1) and synthetic (MNTS) montmorillonite clays that provide new insight into interlayer structure. An increase in basal d(001)-spacings demonstrates successful intercalation of PEO in all samples, and X-ray line narrowing shows that this intercalation improves the layer stacking order. The basal spacings of 17.9-19.4 A are consistent with a helical or bilayer structure of PEO in the interlayer. TGA-DTA provides quantitative results for the hydration state of the nanocomposites, demonstrates PEO intercalation, and shows that the composites prepared from the synthetic montmorillonite are less stable than those made with SWy-1. (7)Li NMR shows that the nearest neighbor hydration state of Li(+) is unaffected by PEO intercalation and suggests weak interaction of Li(+) with PEO. (23)Na NMR shows that PEO intercalation results in the conversion of the multiple Na(+) hydration states observed for the pristine clay into inner sphere sites most likely formed through coordination with the basal oxygens of the clay. These differences between lithium and sodium suggested that tighter binding of the Na to the clay may be the origin of the conductivity of Li-montmorillonite-PEO nanocomposites being as much as 2 orders of magnitude larger than those of Na-montmorillonite-PEO nanocomposites. The results confirm the idea that polymer oxygen atoms do not participate in sequestering the exchangeable cations and agree with the jump process for cation migration advanced by Kuppa and Manias (Kuppa, V.; Manias, E. Chem. Mater. 2002, 14, 2171).