The Role of Composite Binder on Mechanics and Performance of Lithium Ion Battery Electrodes

Abstract

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 and more modestly on the strain rate. The evolution of mechanical and electrical properties of the binder after repeated cycling will be shown. Mesoscale simulations will be presented using experimentally determined three dimensional structures of battery cathodes. Using these realistic microstructures, the role of polymer binder properties on the effective modulus of the electrode will be examined. The complex particle scale geometry results in a heterogeneous stress distribution with the binder between particle contacts experiencing high localized stresses. 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. SAND2015-10738 A