Conductivity Degradation of Polyvinylidene Fluoride Binder during Cycling: Measurements and Simulations for Lithium-Ion Batteries

Battery electrodes are complex multiphase composites which must provide efficient bicontinuous networks for transport of electrons (through the particle phase) and positive lithium ions (through the electrolyte filled pores of the electrode). A crucial but often neglected element of battery electrodes is the binder, typically a mixture of polyvinylidene fluoride (PVDF) and carbon black. The binder has two primary roles – to provide mechanical integrity and to improve electrical conduction of the electrodes. Migration of the binder has also been implicated as a potential mechanism of capacity fade in rechargeable lithium ion batteries.

We will present experimental characterization of the polymer binder for battery applications. Mechanical properties of the composite binder will be shown for both dry films and also binder swollen with carbonate electrolytes used in rechargeable lithium batteries. The electrical properties are strongly dependent on the applied stress. The evolution of mechanical and electrical properties of the binder after repeated cycling will be shown.

Then we will examine the impact of cyclic mechanical stresses on the performance of lithium cobalt oxide cathodes. Experimental results will be presented on the evolution of electrical conductivity as a function of mechanical cycling. Mechanisms for electronic conductivity in these complex porous composite electrodes will be investigated using mesoscale simulations using experimentally determined three dimensional structures of battery cathodes. Implications for battery internal resistance and cycling stability will be discussed.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND2016-4371 A