Diss. University of Akron, 2018.
Advanced active materials, such as Si, for lithium ion battery electrodes are becoming highly engineered, but their cycling performance can be significantly impacted by the mechanical, transport, and electrochemical properties of the polymeric binder in the electrode. The swelling and plasticization of most widely used binder poly(vinylidene difluoride) (PVDF), poly(acrylic acid) (PAA), branched polyethylenimine (BPEI) and potential binder sulfonated ethylene propylene rubber (SEPDM) by common carbonate-based electrolytes is probed using quartz crystal microbalance with dissipation (QCM-D). The swelling of the PVDF was significantly greater than the other polymers, while addition of Li salt only marginally deceased the swelling. The composition of ethylene and propylene carbonate in the electrolyte more significantly impacts the swelling with the 50:50 mixture exhibiting the greatest swelling. The shear modulus of the PVDF remains on the order of 10 MPa for all conditions examined. This mechanical invariance is attributed to the semi-crystalline structure of PVDF that provides a robust network. These measurements provide insight into the electrolyte-binder interactions and can be used to help select pairs for emerging high-performance electrodes from the aspect of swelling of binder by carbonate electrolyte and interaction between binder and active material.
As QCM-D is a very sensitive and accurate method for in-situ real time analysis on thin film swelling and mechanical properties, it can also be used to measure salts effect on the swelling of physical crosslink hydrogel. In this study, the Hofmeister series effect on swelling of 2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate (FOSA)/N,N-dimethylacrylamide (DMA) physical crosslink hydrogel which containing 9.7 mol% FOSA (DF10) is introduced. The influence of temperature and different salts species (Na2SO4 and NaClO4) are investigated. The DF10 swollen in aqueous solution dissipates significant energy to enable determine of the mechanical properties. The swelling of DF10 is reversible with temperature increases and salts concentration. Na2SO4 decreases the swelling of DF10 and increases elastic modulus. Conversely, NaClO4 increases the swelling ratio, softens the DF10 hydrogel and increases viscosity in low concentration NaClO4 solutions (from 0.003M to 0.1M). At higher NaClO4 concentration (3M), the swelling decreases slightly when compared with the swelling at 0.3M. This study demonstrated that salts have dramatic effect on swelling of DF10 in aqueous solution, resulting in the change on swelling ratio and mechanical properties of DF10 hydrogel. The efficiency of the anions in increasing swelling was found to be consistent with the Hofmeister anion sequence with the ability of destabilize the hydrophobic aggregates.
