Effect of poly(vinylidene fluoride) binder crystallinity and graphite structure on the mechanical strength of the composite anode in a lithium ion battery

We have evaluated the mechanical strength of a series of composites consisting of carbon particles bound together by poly(vinylidene fluoride) (PVDF), which is closely related to the carbonaceous anode in a lithium ion battery. We used a balanced beam scrape adhesion tester and evaluated the influence of carbon particle structure, the chemical properties of PVDF, and the processing parameters of annealing temperature and casting solvent on the adhesion of the composite film to a copper substrate. The composite prepared with amorphous carbon shows over 10 times higher adhesion strength than those fabricated from other graphite materials. This results from chemical binding that is intermediate between semi-ionic and covalent C-F bonds, as detected by X-ray photoelectron spectroscopy. To address the effect of the crystalline phase of the binder on the adhesion strength, we investigated PVDF crystallinity in the composite films using differential scanning calorimetry. Samples with higher crystallinity show higher adhesion strength, independent of annealing temperature and casting solvent. The scratch adhesion was also measured for swollen electrodes immersed in 3:7 volume ratio of ethylene carbonate:ethyl methyl carbonate (EC:EMC) at different temperatures. After being swollen, the composite films prepared from PVDF modified with hydroxyl functional groups show higher adhesion strengths than the others due to their low uptake of the electrolyte solvent.