,. Magnetic Resonance Imaging of Polymer Electrolytes and Insertion Electrodes* Robert J. Klingler, Rex E. Gerald II, and Jerome W. Rathke s em * s -0 Chemical Technology Division mz~ Electrochemical Technology Program 4 Argonne National Laboratory -?!?g Argonne, Illinois 60439 to be presented at Advanced Lithium Solid State Batteries Workshop Burkshire Guest Suites and Conference Center Towson, MD July 13-15, 1999 The submitted manuscript has been created by the University of Chfcago as Operator of Argonne National Laboratory ( Argonne ) under Contract No. W-3I-109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains foritaelf, and others acfing on ifs behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, dk.- tribute copies to the pubfic, and perform publicly and display publicly, by or on behalf of the Government. work supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, and Chemical Technology Division, Argonne National Laboratory, under contract W-31-109-ENG-38.....,,.,,. ------,..
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. MAGNETIC RESONANCE IMAGING OF POLYMER ELECTROLYTES AND INSERTION ELECTRODES Robert J. Klingler, Rex E. Gerald II, and Jerome W. Rathlw Chemical Technology Division, Argonne National Laboratory 9700 South Cass Ave Argonne, IL 60439 (630) 252-9960 -- Fax (630) 252-9373 E-mail:klingler@cmt. anl.gov OBJECTIVE This program seeks to better define electrode-electrolyteinterfacesand solid-stateion transportmechanismsthat are a central feature of fbel cells and advanced electrochemical systems. The goal is to develop a new generation of materials with enhanced energy efficiency and reduced tendency toward dendrite or passive film formation at the electrode-electrolyte interface. APPROACH This program uses in situ magnetic resonance imaging (h4ri) to probe the microenvironment immediately adjacent to the electrode, not simply the properties of the bulk materials. Work is conducted with the nearelectrode imager in Fig. 1, which was designed to simultaneously provide 2-~m distance resolution and NMR chemical shill information (l). In addition, a related NMR method has been developed to map the chemical concentration profiles within the electrical conduction layer of a metal electrode (2). Together, these in situ spectroscopic methods are capable of resolving structure, mobility, redox chemistry, and spatial distribution of electroactive species located on, within, and adjacent to the electrodes in a varie~ of electrochemical devices. ACCOMPLISHMENTS We have demonstrated a new method of collecting images with our electrochemical-nmr cell, which allows complete concentration profiles to be obtained on a millisecond time scale. For comparison, atypical 7Liprofile using the original rotating-frame-imaging (RFI) technique required several hours (3). The new flash data collection method makes it possible to follow fast millisecond kinetic processes that occur on or near the working electrode by rapidly collecting sequential images. Significantly, the new flash data collection achieves the same distance resolution, 2 pm, as the original RFI method. The shorter data collection time for the new flash method results from monitoring only a single value for the M chemical shift. Accordingly, this method is limited to following only one chemical species at a time. However, this is not a serious restriction as each species in the system can be monitored in a separate set of images. The lithium concentration profile in the surface conduction layer, 20 ym, of a lithiumaluminurn alloy has been determined for the first time by an in situ NMR imaging technique (2). The method employed is a variant of the RFI method used in our electrochemical-nmr cell. Alloys of this nature are of interest for lithium storage at the anode of rechargeable lithium ion devices. In addition, these same alloys are used in the aerospace industry, where the surface composition markedly affects the strength and corrosion resistance. The new NMR imaging technique makes it possible to directly monitor the alkali metal concentration profiles within the critically important surface layer by a nondestructive analysis technique and should be particularly powerfi.d for following alloy formation in lithium-aluminum anode storage materials. Using a distance selective NMR imaging technique, we have demonstrated that it is possible to resolve the crystalline phase of a polymer electrolyte from the more abundant amorphous component. It has been known for some time that dispersions of lithium salts in polyethylene oxide exhibit complex phase behavior, and that the ionic mobility varies dramatically between the various crystalline and
.. amorphous phases. However, it has not been previously possible to monitor the changes in phase composition near the electrode under in situ current load conditions. The ability to image the crystalline phase is a notable achievement because of the large 7Li NMR linewidth, ca. 80 I&Iz, for this phase. This accomplishment was possible due to the exceptionally large B, field gradients that are attainable as a result of employing a toroid cavity resonator as the basic NMR component in the near electrode imager. MAJOR TECHNICAL BARRIERS Does the ionic mobility vary across the salt gradients that form in polymer battery materials? Does the phase composition of polymer electrolytes change with battery recycle history? What are the factors that influence lithium dendrite formation? FUTURE DIRECTIONS We have found that the *?Flongitudinal spin relaxation time for CF$O~Li varies in direct proportion to the salt concentration within the electrolyte depletion zones that form next to the cathode (3). These measurements of NMR spin relaxation time will be extended using the new flash method of image collection. This new method makes it possible to probe for changes in the ion solvation sphere on the millisecond time scale following the passage of current. In addition, we will extend the range of electrolytes under study. Currently, we are working with the imide salt LiN(SOzCF~)z and have some promising composite electrolyte materials that are available to us from our collaborators. We have shown that it is possible to image the amorphous and crystalline phases of a LiN(SOzCF~)z/(polyethylene oxide) electrolyte. We will investigate this system more closely to look for changes in the phase composition following the passage of current and the establishment of an electrolyte depletion zone. In addition, the cell will be cycled multiple times to establish the effect of recycle history on the phase composition. A major objective is to measure the relative lithium ion mobility in the various phases under in situ current load conditions and to determine the rate of lithium ion exchange between the various polymer electrolyte phases. We are working with carbon insertion eiectrode materials. Our previous 7Li NMR imaging results have shown that solvated lithium ions are preferentially concentrated within the pores of the carbonaceous material (4). More recent work has demonstrated that it is possible to monitor lithium dendrite formation at the surface of carbon working electrodes as demonstrated in Fig. 2. The electrochemical-nmr imaging cell exhibits exceptionally high sensitivi~ to the metallic lithium dendrites that are unequivocally characterized in terms of location by the NMR imaging feature and in terms of the composition by the distinctive Knight shift for metallic lithium. We will extend these studies to include several types and sources of carbonaceous material in an attempt to better define the factors that influence lithium dendrite formation on these electrodes. In addition, we will look more closely at the carbon-electrolyte interface during lithium intercalation. We are also interested in defining the location and chemical composition of any irreversibly bound lithium in the carbonaceous materials themselves. ACKNOWLEDGMENTS Support forthis work was provided by the Office of Basic En:rgy Sciences, Division of Chemical Sciences, U.S. Department of Energy. REFERENCES (1) J. W. Rathke, R. J. Klingler, R. E. Gerald II, K. W. Kramarz, and K. Woelk, Prog. A?MR Spectro.x., 30, 209(1997). (2) R. E. Gerald, R. J. Klingler, J. W. Rathke, and L. H. Nuiiez, l 1 zechemist, 76,45-50 (1999). (3) R. E. Gerald, R. J. Klingler, J. W. Rathke, G. Sandi, and K. Woel~ In Situ Imaging of Charge Carriers in an Electrochemical Cell, in Spatially Resolved Magnetic Resonance, Eds., P. B1umleret al., Wiley- VCH, Weinheim, 103-110 (1998). (4) G. Sandi, R. E. Gerald, L. G. Scanlon, K. A. Carrado, and R. E. Winnans, Mater. Res. Sot. Symp. Proc., Materials for Electrochemical Energy Storage and Conversion II-Batteries, Capacitors, and Fuel Cells, 496,95-101 (1998)......--.-.-
...........,.. -. :..,.l J Carbon (working Electrode) Figure 1. Schematic of Near-Electrode Imager.. Lithium Dendrite Lithium Counter Electrode Figure 2. 7LiNMR Images of Metallic Lithium before (left) and after (right) Electrodeposition of Dendritic Lithium at the Surface of the Working Electrode. -..,...-,.,$,.... ~;~ -.,..,..........,,+.,.,- -.. ---- --