Steve Greenbaum

Interpreting the structural and electrochemical complexity of 0.5Li2MnO3·0.5LiMO2 electrodes for lithium batteries (M = Mn0.5−xNi0.5−xCo2x, 0 ≤x≤ 0.5)

The structural and electrochemical features of layered 0.5Li2MnO3·0.5LiMO2 electrodes, in which M = Mn0.5−xNi0.5−xCo2x (0 ≤ x ≤ 0.5), have been studied by powder X-ray diffraction, electrochemical differential-capacity measurements, 7Li magic-angle-spinning nuclear magnetic resonance, and X-ray absorption near-edge spectroscopy. Li2MnO3-like regions in the as-prepared samples were observed for all values of x, with transition-metal cation disorder between the LiMO2 and Li2MnO3 components increasing with cobalt content (i.e., the value of x). The structural disorder and complexity of the electrochemical redox reactions increase when the Li2MnO3-like regions within the electrode are activated to 4.6 V in lithium cells; interpretations of structural and electrochemical phenomena are provided.

Enhancement of ion transport in polymer electrolytes by addition of nanoscale inorganic oxides

Abstract The effect of addition of nanoparticle inorganic oxides to poly(ethylene oxide) (PEO) complexed with LiClO 4 on cation transport properties has been explored by electrochemical and 7 Li nuclear magnetic resonance (NMR) methods. The presence of the nanoparticles generally increases the ionic conductivity and the cation transference number, the effect being greatest for TiO 2 . The enhancement in cation transference number is directly correlated with increased Li diffusivity measured by NMR. The NMR results also demonstrate that the increased ionic conductivity is not attributable to a corresponding increase in polymer segmental motion, but more likely a weakening of the polyether-cation association induced by the nanoparticles.

Irreversible Capacities of Graphite in Low‐Temperature Electrolytes for Lithium‐Ion Batteries

Carbonaceous anode materials in lithium-ion rechargeable cells exhibit irreversible capacity, mainly due to reaction of lithium during the formation of passive surface films. The stability and kinetics of lithium intercalation into the carbon anodes are determined by these films. The nature, thickness, and morphology of these films are in turn affected by the electrolyte components, primarily the solvent constituents. In this work, the films formed on graphite anodes in low-temperature electrolytes, i.e., solutions with different mixtures of alkyl carbonates and low-viscosity solvent additives, are examined using electrochemical impedance spectroscopy (EIS) and solid-state ^(7)Li nuclear magnetic resonance techniques. In addition, other ex situ studies such as X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy were carried out on the graphite anodes to understand their microstructures.

Lithium-7 NMR and ionic conductivity studies of gel electrolytes based on poly(methylmethacrylate)

Gel electrolytes synthesized from poly(methylmethacrylate), ethylene carbonate, propylene carbonate and various lithium salts [LiClO4, LiAsF6, or LiN(CF3SO2)2] have been investigated by differential scanning calorimetry, electrical conductivity, and 7Li, 19F and 75As NMR spectroscopy. Although the ionic conductivities of the gels approach those of liquid electrolytes above room temperature, the NMR results indicate that the immediate environments of both the cations and anions differ significantly in the gel and in the liquid. Thus the presence of microscopic regions of pure liquid electrolyte in the gel can be ruled out.

A basic investigation of anhydrous proton conducting gel electrolytes

Abstract The aim of this paper is to summarize recent trends in the studies on nonaqueous proton conducting gel electrolytes. The physical–chemical properties of these systems are described and their relation to the type and concentration of polymer matrix, proton donor as well as the solvent used is discussed. This analysis is performed on the basis of electrochemical (impedance spectroscopy, cyclic voltammetry), spectroscopic (NMR, PFG NMR, FT-IR), and thermogravimetric (DSC) experiments. The proton conduction mechanism is postulated and its dependence on the sample composition is discussed. The possibility of application of proton conducting gels in electrochromic devices is presented.

Electrical impedance studies of acid form NAFION® membranes

Electrical conductivity/dielectric relaxation studies of acid form NAFION-117 have been carried out at frequencies from 10 to 108 Hz. By direct measurement, it is shown that when “standard” two terminal measurements are made across the thickness of a 0.18 mm film, it is necessary to use frequencies in excess of 107 Hz in order to observe the bulk conductivity of the sample. As a consequence, previous reports of a power law dependence for the electrical conductivity are not associated with the bulk electrical conductivity but rather are due to electrode effects and space charge. As confirmation, it is shown that by changing the geometry of the electrodes, the low frequency electrical response of the material is significantly changed.

Lithium-7 NMR studies of concentrated LiI/PEO-based solid electrolytes

Abstract Highly concentrated polymer electrolytes based on poly(ethylene oxide) (PEO) and LiI, with EO/Li ratio ≤3, were investigated by differential scanning calorimetry (DSC), powder X-ray diffraction (XRD) and 7Li solid state nuclear magnetic resonance (NMR) methods. The effect of 15-nm particle size Al2O3 additives and in several cases, other constituents ethylene carbonate and poly(methylmethacrylate) on structure and Li+ ion environment was explored. The addition of Al2O3 suppresses the formation of crystalline phases, including free LiI, which is present in EO/Li=1.5 samples without Al2O3. The conductivity jump observed in these concentrated electrolytes at around 80°C is correlated with an NMR-observed transition to a Li+ environment which is similar to that of free ions in a molten phase.

Dielectric relaxation and deuteron NMR of water in polyimide films

Polyimide films containing up to 3.2 wt. % water (H2 O or D2 O) have been studied by dielectric relaxation (DR) and 2H nuclear magnetic resonance (NMR) spectroscopies. Dielectric loss measurements show the presence of a double peak near 200 K due to water, with two components designated as γ1 and γ2 . The γ2 peak, which appears at the lower temperature, is only present for the interior portion of thick (>25 μm) films and rapidly decreases as the moisture content decreases. Correspondingly, deuteron NMR measurements show the presence of narrow and broad lines, which are correlated, respectively, with the γ1 and γ2 DR peaks. The spin‐spin relaxation time T2, is much longer for the narrow line than for the broad line. The γ1 DR peak and the narrow NMR line are attributed to sites where isolated water molecules are only loosely coupled to the polymer chains. The γ2 and broad NMR lines involve small clusters of water molecules with associated nuclear magnetic dipole‐dipole coupling. Additional measurements on ...

New Generation of Ordered Polymer Electrolytes for Lithium Batteries

Department of Physics, Hunter College of City University of New York, New York, New York 10021, USAIn poly~ethylene oxide!-based solid electrolytes, ionic conduction can occur by cations moving inside the helix~along the helixaxis! and by anions moving on its envelope. This particular mode of ion transport can be enhanced by alignment of the polymericstructural units. We describe a procedure for orienting the helices in the perpendicular direction, the result of which is a one-order-of-magnitude increase in polymer electrolyte~PE! conductivity and a similar decrease in PE/electrode interphase resistance.This procedure could also be of importance in the orientation of polymers in the nanoscale for various applications.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1803434# All rights reserved.Manuscript submitted February 27, 2004; revised manuscript received May 19, 2004. Available electronically October 6, 2004.

Sodium-23 NMR and Complex Impedance Studies of Gel Electrolytes Based on Poly(acrylonitrile).

Highly conducting gel electrolytes prepared from mixtures of poly(acrylonitrile) (PAN), ethylene carbonate (EC), propylene carbonate (PC) and NaClO4 have been studied by complex impedance, differential scanning calorimetry (DSC) and 23Na nuclear magnetic resonance (NMR) spectroscopy. Ionic motional correlation times spanning nearly five orders of magnitude, probed by temperature-dependent dynamical features of the 23Na quadrupole-broadened NMR lineshape, are strongly influenced by the single glass transition temperature observed in the gel electrolyte. Dielectric measurements of mixtures of PAN, EC and PC suggest that the polar nitrile group in PAN may interact with Na+ ions in the gel. All results are consistent with the conclusion that PAN provides stability to the gel network down to the immediate vicinity of the Na+ ions, rather than acting as a rigid framework for regions of liquid electrolyte.