Effects of the Chemical Structure of Thickening Agent on the Dispersion and Electrochemistry of Li-Ion Batteries

To make electrode sheets for lithium-ion (Li-ion) cathodes or anodes with appropriate mechanical properties, the electrode-active material and conductive agent generally have to be blended with specific polymeric binders. To mix all the inorganic electrode materials homogeneously with polymeric binder, appropriate solvent that can dissolve the binder is required. According to the type of solvent used, the as-prepared electrode slurry can be classified as two systems – a water-based system and an organic solvent-based system. For environmental consistency and cost considerations, the water-based system, which uses environmentally benign binders and solvents, has attracted increasing attention and has become a new trend in the manufacture of Li-ion electrodes. Among water-compatible polymers, the aqueous emulsion of styrene-butadiene rubber (SBR) is the commonly used binder for electrodes of Li-ion batteries. It is known that SBR provides not only good binding for electrode powders but also the required adhesion of electrode sheets to the current collector. Nevertheless, SBR comes in the form of an aqueous emulsion, which has very low viscosity and, therefore, does not facilitate the wetting of its fabricated electrode slurry on the current collector during casting. To improve the wetting, SBR is generally blended with the thickening agent CMC. Many studies have shown that using CMC as a thickening agent is beneficial to the rate capability of the as-prepared cathodes but too much addition may cause the corresponding electrode slurries to gel, which the gelled slurries are unavailable for casting. Since CMC contains two kinds of functional groups, carboxyl and hydroxyl groups, it would be interesting to clarify which one of the functional groups causes the gelation of the electrode slurries. Based on the experimental results of replacing CMC by other reagent, such as poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA) and guar gum (GG), it was found that the hydroxyl group rather than the carboxyl group dominates the gelation of CMC in the slurry. Besides to the gelling function, the presence of hydroxyl group in the chemical structure of the thickening agent is also detrimental to the electrochemical performances of as-constructed electrodes and Li-ion cells.