Comparison of PVDF and PVDF-TFE-P as binders for electrode materials showing large volume changes in lithium-ion batteries

The mechanical and electrical properties of a terpolymer, poly(vinylidene fluoride-tetrafluoroethylene-propylene) (PVDF-TFE-P, BRE-7131X, Dyneon Corp.) and its carbon black-filled composites (without active anode material) have been investigated carefully and are compared to those of PVDF (Solef 1008). High capacity anode materials such as a-Si and a-Si0.64Sn0.36 have up to 250% volumetric changes during charge/discharge cycling which challenges the mechanical properties of standard binders used in Li-ion battery electrodes. The measurements were carried out on dry polymer films and on films immersed in a nonaqueous solvent commonly used in Li-ion cells (ethylene carbonate/ diethyl carbonate, EC/DEC, 1: 2 by volume). PVDF and its carbon-filled composites show a maximum elongation before break of less than 10%. However, triethylenetetramine crosslinked BRE-7131X and its carbon-filled composites can be stretched to more than 100% strain before breaking in air and in EC/DEC (1: 2 by volume). Furthermore, the stress and the resistivity of the carbon-filled crosslinked BRE-7131X films changes reversibly during elongation/contraction cycles. (C) 2003 The Electrochemical Society.