Electrochemical and Spectroscopie Evaluation of Lithium Intercalation in Tailored Polymethacrylonitrile Carbons
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Abstract Disordered polymethacrylonitrile (PMAN) carbon monoliths have been studied as potential tailored electrodes for lithium ion batteries. A combination of electrochemical and surface spectroscopic probes have been used to investigate irreversible loss mechanisms. Voltammetric measurements show that Li intercalates readily into the carbon at potentials IV positive of the reversible Li potential. The coulometric efficiency rises rapidly from 50% for the first potential cycle to greater than 85% for the third cycle, indicating that solvent decomposition is a self-limiting process. Surface film composition and thickness, as measured by x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS), does not vary substantially when compared to more ordered carbon surfaces. Li+ profiles are particularly useful in discriminating between the bound states of Li at the surface of solution permeable PMAN carbons. Introduction A primary goal in the development of Li ion batteries is to minimize the irreversible losses in the intercalating host matrix to maximize energy density. Mechanisms of irreversible loss include solvent decomposition and subsequent solid electrolyte interphase (SEI) formation which contains bound ionic lithium, surface deposition of lithium followed by solvent reactions and entrapment with the carbon matrix. Focus is currently being placed on synthetically derived carbon materials as potential anodes (13). The goal is to either take advantage of intrinsic properties in a given carbon or to try to tailor the properties by altering the synthesis process. The overall degree of disorder, as well as other microstructure features like open porosity, can be altered with the potential for achieving capacities in excess of that found for ordered graphite. Polymer derived carbonized polymethacrylonitrile represents one material that appears well suited for use as an optimized, tailored carbon (4). Successful development of PMAN electrodes requires an understanding of how irreversible loss processes correlate with carbon surface activity and structure. Voltammetry, x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) have been used to study the process of solvent decomposition, Li intercalation and Li deposition on monolithic PMAN electrodes. A contiguous method has been developed to couple solution and vacuum environments to minimize the contamination and secondary chemistry that is intrinsic to ex situ analysis methods. The basal plane of highly ordered pyrolytic graphite (HOPG) and glassy carbon (GC) have been included as examples of more highly ordered, non-intercalating, non-porous carbons for comparison purposes. Experimental The carbon electrodes were electrochemically treated in a contiguous cell attached directly to the ultrahigh vacuum system. This cell, shown in cross-section in Figure 1, is mounted in a separate vacuum cell that is pressurized with UHP Ar during the electrochemical experiments. Carbon electrodes are transported on an ele
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