CAMP December Newsletter: Page 3
The techniques of galvanostatic cycling, slow scan cyclic voltammetry, EIS, PITT and GITT are being combined together to study the solid-phase diffusion characteristics of lithium ions in the cathode material, as well as in the SEI, at different operating temperatures. The SEI conducts lithium ions, but blocks electrons, and this minimizes electrolyte degradation through redox reactions with the electrode. However, the SEI gradually loses its electron-blocking property with repeated cycling, and thus exposes the underlying electrode to progressive corrosion due to reactions with the electrolyte, especially at elevated temperatures. Professor Roy’s group is testing different electrolyte designs that can alleviate this problem, and in a collaborative effort with CAMP Professor Sitaraman Krishnan, they have shown that, certain ionic liquid electrolytes could be quite effective in this regard. This work involving ionic liquid electrolytes is now being expanded to a range of systems designed for high temperature applications of electrochemical energy storage devices.
FIGURE 2: Electro-analytical examination of the cycle-dependent performance of a lithium manganese oxide battery cathode. A solid electrolyte interphase (SEI) layer is formed on the cathode surface by cycling the cell in a lithium perchlorate based electrolyte of ethylene carbonate (EC) and diethyl carbonate (DEC). (A) Galvanostatic discharge of the cathode at a rate of 5-C in a half-cell configuration using a Li metal anode, displaying multiple cycles after SEI formation. (B) Successive charge and discharge cycles of the cathode in a voltage controlled mode of slow-scan cyclic voltammetry (SSCV), performed using the same cell at a rate of 100 mV/s. The voltage features detected in the plots in (A) represent phase transitions in the Li host lattice of the cathode as a result of progressive intercalation. In (B), these features appear as distinct current peaks. The net rate of Li intercalation/de-intercalation is affected by diffusion limited transport of lithium ions within the surface film of the cathode. Additional measurements involving the techniques of electrochemical impedance spectroscopy (EIS) galvanostatic intermittent titration technique (GITT) and potentiostatic intermittent titration technique (PITT) are used to study the detailed nature of this charge transport process.Professor Roy’s group is also studying materials for the next generation of sodium metal halide batteries. These batteries are being developed by an industry /academic consortium of several universities and research laboratories, including Clarkson, Alfred, Columbia, Stony Brook and BNL. The consortium, led by GE Global Research, was awarded a $2.5M grant by the New York State Energy Research and Development Authority (NYSERDA) to enhance battery reliability, cycle life, and performance. The new batteries are designed to fuel hybridized long haul trains and augment the electric grid. Dr. Roy (Professor and Chair of Clarkson’s Department of Physics) is participating in this project through a research grant received from General Electric. More information about Professor Roy’s research can be found at: http://people.clarkson.edu/%7edroy/cr_projects.htm.
Modeling of Gradient Copolymers
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The plan of the project is to design gradient polymers that will permit a few charged mers to be present on the surface of a polymeric coating that is otherwise nonionic and of lower surface energy. There are several issues in designing such polymer surfaces. For example, microphase separation is a well-known phenomenon that occurs in block copolymers involving chemically dissimilar mers. In a microphase-segregated polymer coating, mers of the same type tend to congregate with one another and form structures. The resulting structures can be lamellar, gyroid, cylindrical, or body-centered cubic, depending on the composition. See Figure 1. It shows an example of a phase-segregated gradient polymer. Hofler’s research is directed at determining the structures that form when the polymer chains have a smooth gradient in composition from one end to the other and the extent to which one can ensure that a few charged groups will be present on the surface.