Mechanical and electrical properties of poly(vinylidene fluoride-tetrafluoroethylene-propylene)/super-S carbon black swelled in liquid solvent as an electrode binder for lithium-ion batteries

The mechanical and electrical properties of poly(vinylidene fluoride-tetrafluoroethylene-propylene) (PVDF-TFE-P) and carbon black-filled PVDF-TFE-P composites were investigated. The carbon black was used for its electrolyte absorption properties in addition to boosting the conductivity. This elastomeric binder system may have application to tin- or silicon-based electrode materials for Li-ion batteries, which undergo huge volumetric changes during charge/discharge cycling. The mechanical and electrical properties were measured while film samples were immersed in a liquid solvent (ethylene carbonate: diethyl carbonate I : 2) commonly used in the battery electrolyte. Uncrosslinked PVDF-TFE-P uptakes about 140% solvent by mass and swells significantly. The amount of solvent absorbed can be reduced and the mechanical properties improved by crosslinking the polymer. Two crosslinking recipes, based on bisphenol and triethylenetetramine (TETA), were investigated carefully. Compared to the bisphenol-based crosslinking recipe, the proposed TETA-based crosslinking recipe gave films with a higher degree of crosslinking and better mechanical properties. The TETA-crosslinked composites had very good mechanical and electrical reversibility even during cyclic deformation to 100% strain. The cycling results of amorphous Si0.64Sn0.36 electrodes show that the capacity retention of the electrodes can be significantly improved by using the proposed elastomeric binder. (C) 2004 Wiley Periodicals, Inc.