A Comparative Study of Coordination in Gel Electrolytes Quantitative Analysis

Abstract

Gel electrolytes, created for use in modern lithium-ion batteries, 1 are complex materials with respect to their composition and to their behavior. The gels consist of a polymer mixed with a lithium salt and a plasticizer. The plasticizer, being an organic solvent able to dissolve the salt, maintains a liquid-like state within the material. At the same time, the polymer gives the gel a mechanical strength approaching that of a solid. Thus, the resulting gel electrolyte can successfully separate the electrodes in a battery and minimize leakage problems, while at the same time it will retain the ionic conduction properties of a liquid, such as low internal impedance and high mobility of the charge-carrying species.In practice, the choice of the individual components in the electrolyte material is crucial. Their chemistry and compatibility with each other and with the electrodes, will affect both the mechanical and electrochemical properties of the material. 2-4 By a systematic investigation of the changes in the properties of the gels, introduced by variation of components and their relative amounts, questions about the interactions between the components and about transport mechanisms within the systems can be answered. One particularly important property in this context is lithium ion coordination. The cation is a most important species for the battery function since it is responsible for the transfer of the electric charge between the electrodes, and the way this ion moves through the electrolyte will be a decisive factor for the efficiency and workability of the whole system. In gel materials, the lithium ion can, in principle, choose to coordinate with the anion, the plasticizer, or the polymer chain. Each of these coordination types would be of different strength and would involve different amounts of the various species, creating a wide range of properties for the complete electrolyte.Fourier transform infrared spectroscopy (FTIR) is an appropriate method for monitoring the coordination of the lithium ion. Certain bands resulting from specific vibrations of each of the potentially coordinating species have been observed to split or shift to different frequencies on cation coordination. 5-7 As a first step in analyzing the coordination, these "coordination bands" can be found and identified. Next, by comparing these bands in spectra obtained from samples with different composition, the changes in coordination occurring can be explored, systematized, and ultimately understood.In a previous paper, 8 the coordination ability of two plasticizers with different cation coordinating properties, namely, propylene carbonate (PC) and dimethyl sulfoxide (DMSO), has been studied qualitatively. Although clear differences could be observed between FTIR spectra from different samples, a quantitative estimate of the variations of the spectral bands was not possible by mere visual inspection and, therefore, a quantitative band-shape analysis is required.In the present study such a quantitative band-shape analysis of the spect...

Plasticized microporous poly(vinylidene fluoride) separators for lithium‐ion batteries. I. Swelling behavior of dense membranes with respect to a liquid electrolyte—Characterization of the swelling equilibrium

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Ion conductive electrolyte membranes based on co-continuous polymer blends

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