An apparatus, near-electrode imager, for employing nuclear magnetic resonance imaging to provide in situ measurements of electrochemical properties of a sample as a function of distance from a working electrode. The near-electrode imager use the radio frequency field gradient within a cylindrical toroid cavity resonator to provide high-resolution nuclear magnetic resonance spectral information on electrolyte materials

Gerald, Rex E.,II

Klingler, Robert J.

Rathke, Jerome W.

Woelk, Klaus

Gerald, Rex E., II

UNT Digital Library

NEAR-ELECTRODE IMAGER“~f3gJ’y-S-90,928jt+f7EM’&u$-A93K?3rJ_EROMEW=THKEROBERT J. KLINGLERKLAUS WOELKREX E. GERALD II.DISCLAIMERThis repoti was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily “constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or refiect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.5CONTRACTUAL ORIGIN OF THE INVENTIONThe United States Government has rights in this inventionpursuant toContract No. W-31-I 09-ENG-38 between the U.S. Department of Energy and theUniversity of Chicago..-. -BACKGROUND OF THE INVENTIONField of InventionThis invention, a near electrode imager, relates to a near-electrode imagerwith nuclear magnetic resonance (NMR) sensitivitiesfor providinga direct in-situanalysis of an electrochemical process in order tojdetermine such phenomena as10 the compositionof an electrode-electrolyte interface, ion penetration depths within2..advanced synthetic electrode materials, and ion concentrationgradients within solidstate batteries.Description of Related ArtThe near-electrode imager provides a direct in situ spectroscopicmethod to5 probe the fundamental-ion transport process in advanced battery materials and thedetailed chemical compositionof the electrode-electrolyte interface. The near-electrode imager uses the radio frequency (RF) gradientwithin a cylindricaltoroidcavity to provide high-resolutionnuclear magndtic resonance (NMR) spectralinformationon electrolyte materials as a function of radial distance from the working10 electrode of a battery or fuel cell. With a spatial resolutionnear 1.0 micron, the newimager has resolving power nearly two orders of magnitude higherthanconventional MRI while still retaining NMR chemical shift information,which is lost inconventional magnetic resonance imaging (MRI) measurements. The result is athree dimensional plot (or image) of NMR signal intensityand chemical shift versus15 radial distance from the central working electrode. Significantly,the chemical-shiftinformation allows chemical species to be distinguishedfrom one another. Thus, itbecomes possible to follow reaction pathways that may exist in different parts of thecell, most n~ably at the importantelectrode-electrolyte interface. In addition,thetheoretical spatial resolutionof the near-electrode imager is better than possible20 with MRI because of the stronger gradient that is intrinsicto the torus. Further,spatial resolutionwith the near-electrode imager is less dependent on the linewidths of the NMR signals used in the measurements. This factor is particularlysignificantfor polymer electrolytes (such as those used in lithium-polymerbatteries),whose broad NMR lines would limitthe resolutionof conventionalMRI techniques to3.,> 50pm. The highspatial resolutionand the abilityto retain NMR spectralinformationcombine to make the near-electrode imager nearly ideal for theinvestigationof fundamental processes in battery and fuel cells. The near-electrodeimager providesfor the systematic investigationof the fundamental reaction5 chemistry that has not been previouslypossiblewith alternative analysis techniques.The near-electrode imager providesa direct i? situ spectroscopicmeans toprobe the compositionof the electrode-electrolyteinterface and ion penetrationdepths within advanced syntheticelectrode materials. In addition,the devicebe used to investigate the fundamental chemical and physicalpropertiesthat10 influence the ion transport mechanismsin the currentgeneration of batterymaymaterials. The imaging technique iswell suited to measure ion concentrationgradients within solid state batteries, diffusioncoefficientsat different locationsin aworking cell, confirmational dynamicsof polymer electrolytes, and changes inphase composition.This inventionrelates to an improvedmethod and apparatus for NMRimaging, and in particular, to an apparatus that provides in situ NMR chemical shift,distance, and mobility informationfor species that are located on, within,and aroundthe electrod= in electrochemical processes. The near-electrode imager providescomplete high-resolutionnuclear magnetic resonance (NMR) spectra as a function20 of distance away from the workingelectrode of a battery or fuel cell. The near-electrode imager utilizes the rotatingframe imaging (RF]) method of data collectionand the intrinsicallystrong radio frequency (RF) field gradients producedwithintoroid cavitythis imagingresonators to providespatial resolutionnear 1.0 micron. Importantly,technique retains the chemical shift informationthat is typically4.510sacrificed in conventionalMRI measurements. Consequently, the near-electrodeimager has the abilityto followreactionpathways because it allows individualchemical species to be distinguishedon the basis of their NMR chemical shiftsandcouplingconstants. In addition,the near-electrode imager has been demonstratedto retain the capabilityto determine spin relaxation mechanisms. This lattercapabilitymakes it possibleto probe nobilities and the phase compositionofpolymer electrolytes in the electrolytedepletionzones that form adjacent to theelectrodes during normal battery operation. The near-electrode imager can examineion concentrationgradients in workingelectrochemicalcells.The imager system providesfor a toroidcavity nuclear magnetic resonance(NMR) detector capable of quantitativelyrecordingradial concentrationprofiles,diffusionconstants,displacementsof charge carriers, and radial profilesof spin-Iattice relaxation time constants.It is the object of this inventionto providean apparatus for employing nuclear15 magnetic resonance techniques to materialsto providean analytical profileofphenomena associated with electrochemicaland other processes on a real-timebasis...-.It is~urther object of this inventionto provide a method of employingtheimaging apparatus to investigatethe fundamental chemical and physical properties20 that influence ion transport mechanisms.It is another object of this inventionto providea method for probingthecompositionof the electrode-electrolyteinterface altd ion penetrationdepths withinsyntheticelectrode materials.5.,.Additionaladvantages, objects and novel features of the inventionwillbecome apparent to those skilled in the art upon examination of the followingand bypractice of the invention.SUMMARY OF THE INVENTION5 To achieve the foregoing and other advantages, this invention, a near-electrode imager, is an apparatus and method for employing nuclear magneticresonance (NMR) imaging to provide in situ NMR chemical shift, distance andmobilityinformationfor species that are Iocatecfon, within, and around electrodes inelectrochemical processes. The near-electrode imager uses the radio frequency10 field gradient withina cylindricaltoroid cavity to provide high-resolutionnuclearmagnetic resonance spectral informationon electrolyte materials as a functionof15radial distance from the working electrode of a battery or fuel cell. The near-electrode imager has a resolutionof approximately 1.0 micron.BRIEF DESCRIPTION OF THE DRAWINGSThe present invention is illustratedin the accompanying drawings where:Figure 1 is an exploded view of the near-electrode imager.Figure 2 is an exploded view of the electrochemical cell compartmentincludingth=ssociated electrical connections.Figure 3 is an alternate embodiment of the sample containingcell used in the20 imager.Figure 4 is a plot of the signal intensityfrom the sample versus the radialdistance from the central electrode.Figure 5 is a plot of the spin-latticerelaxationa functionof distance from the workingelectrode.6time constant of the sample asDETAILED DESCRIPTION OF THE INVENTIONFigure 1 is an exploded view of the near-electrode imager 10. The toroidcavity 12 is constructedfrom a material that is a good electrical conductor. In the5 current embodiment, the toroid cavity is constructedof copper in the form of a hollowcylinder open at one end and closed at the other. The closed end 14 is drilled andtapped to receive two compressionfittings 16 and 18 which serve to pass electrodes20 and 22 to the potentiostat42 of figure 2. Ttle open end of the toroid cavity 24 istapped to receive a similarlythreaded toroid cavity end cap 26. The toroid cavity10 end cap 26 is drilled to receive a single compressionfitting28 which serves to passthe working etectrode 20 from the NMR spectrometer 44, figure 2 through the toroidcavity end cap to the base 31 of the electrochemical cell compartment 32. Thecentral workingelectrode 20 of the electrochemical cell 32 then passes through the15electrochemical cell 32 to compressionfitting 18 and onto the potentiostat42.Figure 2 illustrates an exploded view of the electrochemical cell inconjunctionwith the electrical components used to operate the cell and record theresponse of the sample containedwithinthe cell. The electrochemical cell 32 ispositionedWin and electrically integratedwith the toroid cavity resonator 12, figure1, whereby the central conductor20 is shared by both the NMR and electrochemical20 circuitsas shown in figures 1 and 2. In one embodiment, the electrochemical cell 32consists of a cylindricallyshaped glass container, as shown in figures 1 and 2,closed at one end except for a openingto allow fopthe passage of the workingelectrode 20. In another embodiment, figure 3, the cell is formed from a glass tube52 which is sealed at both ends by means of rubber septa, 54. The working7. .electrode 20 passes through holes in both septa. In both illustratedembodiments,the working electrode 20 is coincidentwith the major axis of the electrochemical cellcompartment represented by the closed-endglass cylinder32 and the tube 56. Acounter electrode 34 which consistsof a cylindricalsolenoidcoilwhich is positioned5 so that it is symmetrical about the centralworkingelectrode 20, interiorto the cell,and positionedeither at the base of the cylinderor expanded alongthe walls of thecell as is shown in figures 1,2 and 3. The compressionfitting18 throughwhich thecentral working electrode 20 passes as it exits the closed end of the toroidcavityresonator 12 employs a an electricallyconductingbrass ferrule46 in conjunction10 with a hollowthreaded fastener 48 which screws into a similarlythreaded receptor inthe closed end of the toroid cavity resonator 12. The compressionfitting28 for theportionof the central working electrode which passescap employs a Vespel@ ferrule 50 in conjunctionwithwhich screws into a similarly threaded receptor in the15 cap 26. The Vespel@ ferrules are used to electricallythroughthe toroidcavity enda hollow,threaded fastener 52base of the toroidcavity endinsulatethe electrodes fromthe conductingtoroid cavity resonatorand the toroid cavity end cap whereas thebrass ferrule electrically couples the electrode to toroid cavity resonator. Thecbunter ele=ode connector 22 electricallycouples the counterelectrode coil 34 tothe potentiostat42; in doing so, the connector22 transverses the closed end of the20 toroid cavity resonator 12 througha compressionfitting 16. The compressionfittingemploys an electrically insulatingVespel@ Ferrule 54 and a hollowthreadedfastener 56 which screws into a similarlythreaded receptor in the resonator 12. Theconnector 22 is connected to a potentiostat42.815The combinationof an electrochemicalcell compartment32 with the toroidcavity NMR detector allowsfor the measurement of macroscopictransportproperties, local dynamics, and chemistryof ions as a functionof distance from theelectrode. In operation the electrochemicalcell compartment32 is filled to a height5 so as to cover the counter electrode 34 with a sample electrolytecontaininganelectroactive substrate. Voltages applied across the workingand counter electrodesby means of the external potentiostat42 effect changes in the chemical compositionadjacent to the working electrode withinthe electrochemicalcell compartment 32.The NMR spectrometer 44 recordsthe response of the sample to the applied10 magnetic field generated by the toroidcavity 12. The near-electrode imager isexternally tuned to the NMR frequency of interest usingcircuitrywithinthe NMRspectrometer 44. The resultingdata is processed usingthe rotatingframe imaging(RFI) method to produce plotsof the NMR signal intensityand chemical shiftversusthe radial distance from the central workingelectrode.In one example the one-dimensionalradial concentrationprofilesof ionssolvated in a polyethyleneoxide matrixwere recorded by ‘9Ffor several cells. Asequence of ‘9F NMR images, recordedat differentstages of cell polarization,revealed th=volution of a regionof the polymer depleted of charge carriers. In thiscase, polyethylene oxide and lithiumtriflatewere dissolvedin hot tetrehydrofuran,20 which was subsequently removed by heating under vacuum. A white solid monolithremained after complete removal of the solvent. The white solidwas heated forseveral minutes at 174°C in a closedcontainer untilthe solidmelted into amoderately viscous, clear liquidthat could be easily handled. The plasticizedpolymer electrolytewas prepared by mixingthe hot liquidployehtyleneoxide9polymer electrolytewith tetraglyme, propylenecarbonate, and additional lithiumtriflate salt. The mole ratio of the electrolyteused was 1.000 LiCF&O~: 0.043CH~(OCH2CH2)Q10CH~:0.840 CHJOCHzCHJqOCHa: 1.682 propylene carbonate.A quantityof 1.255 grams of the polymer electrolyte mixture was placed in5 the electrochemical cell of the toroid cavity NMR detector. The cylindricalcell usedwas a glass tube having an insidediameter of 10.4mm and a length of 19.6mm.Circular rubbersepta of diameter 11 mm and thickness of 3mm were used to sealthe tube at bothends. A 0.80mm diameter goldwire electrode was formed into a 5turn helix and placed against the innerwall of the glass tube. One end of the wire10 protrudedthroughthe top septum. A 0.62mm diameter copper wire passed throughthe center axis of the cell, piercingbothsepta. A potentialwas applied across thecell with the negative and positiveterminals of the potentiostatconnected to thecentral workingelectrode and the outer counterelectrode respectively. The workingelectrode of the electrochemical cell assembly also function as the central conductor15 of the toroidcavity NMR probe.Fluorine-19 NMR rotatingframe images were recorded at room temperatureby usinga Varian UNITY INOVA-300WB spectrometer at the followingsettings:quency, 282.224 MHz; spectralwidth, 6kHz; 1024 data points;4-srecycle delay; four transients per spectrum;128 spectra recorded with a pulse width20 increment of 4ps (spectral width, 125 kHz). The two-dimensional data sets wereprocessed with 50-Hz line broadeningin the F2 and F1 dimensions, respectively.One-dimensional T, spatial images, which record the spin-lattice relaxation as afunctionof radial position,requiredthat a compositethe acquisitionof the normal rotatingframe image.10180° pulse be executed pn!orto.The current vs. time profileof an electrochemical cell subjected to an appliedpotential is determined by the cell geometry, the equivalents of charge carried by thecharge carriers, and the mobilityof the charge carriers. Electrochemical cellpolarization is the deleterious result of the transport of nonelectroactive ions. In a5 polarized cell, the charge carriers near the working electrode are depleted to zero,thereby effectively forming an open circuit. In such a case, the cell ceases tofunctionuntilthe charge carriers near the working electrode are replenished bydiffusionor other means of mass transport. -The conductionof charge throughsolid polymer electrolytes in an10 electrochemical cell can proceed very slowly, requiringan extended period of time toobtain sufficientcell polarization. The electrolyte is used to investigatevariousaspects of cell polarization includingthe physical dimensions and profileof thedepletion zone, figure 4, and the moleculardynamics of the anions throughmeasurements of spatially resolved spin-lattice relaxation time constants, figure 415 and 5. In figure 4 the dark rectangle on the left side of the plot represents theworking electrode having a radius of 0.31 mm. The (a) curve represents the initialprofilewith (b), (c), and (d) representingthe profiles after various periodsofcharging. T’f’Echarging currentversus time insert occurred between curves (a) and(b). Curve (c) represents the curve after 3.5h charging’at a currentwhich decreases20 as the square root of the charging time, with the typical charging current varyingbetween 10-40 pA. Curve (d) was the result of several days charging at the samerate as curve (c). Figure 5 depicts the spin-latticerelaxation time constants plottedas a functionof the radial distance from the working electrode. The curve suggeststhat the triflate anions are not entirely depleted in the volume around the working11.electrode. The solid and dashed lines in curve (a) are the radial profilesof curves (a)and (d) in figure 4. The spin relation curve in (b) of figure 4 is for the dashed line in(a) of figure 5.The foregoing descriptionof a preferred embodiment of the invention has5 been presented for purposes of illustrationand description. It is not intended to beexhaustive or to limit the inventionto the precise form disclosed, and obviouslymany modifications and variations are possible in light of the above teaching. Theembodiments described explain the principlesof the inventionand practical,applications and should enable others skilled in the ati to utilizethe invention in10 various embodiments and with various modifications as are suitedto the particularuse contemplated. It is intended that the scope of the inventionbe defined by theclaim3 appended hereto.15..-.2012.Abstract.An apparatus, near-electrode imager, for employing nuclear magneticresonance imaging to provide in situ measurements of electrochemical propertiesofa sample as a function of distance from a working electrode. The near-electrodeimager usetie radio frequency field gradientwithin a cylindricaltoroid cavityresonator to provide high-resolutionnuclear magnetic resonance spectralinformationon electrolyte materials.16..--- ---- ---3Toroid CavResonatorEIectrochemicCompartmentvEnd CapT..,15E.T‘v- 3s.7.06.05.0$ 3.0.-.2.01.00.0I// 200180I II aa 140.“ ●/“””s#..“/.9600 100 200 300 400 500Time (s)0.3 0.4 0.5 0.6 0.7 0.8 0.9Radial Position (mm).-.%“1.00.8z- 0.6b-0.4020.3 0.4 0.5 0.6 0.7 oaRadial Position(mm)

Near-Electrode Imager

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