The polymer-composite binder used in lithium-ion battery electrodes must both hold the electrodes together and augment their electrical conductivity while subjected to mechanical stresses caused by active material volume changes due to lithiation and delithiation. We have discovered that cyclic mechanical stresses cause significant degradation in the binder electrical conductivity. After just 160 mechanical cycles, the conductivity of polyvinylidene fluoride (PVDF):carbon black binder dropped between 45-75%. This degradation in binder conductivity has been shown to be quite general, occurring over a range of carbon black concentrations, with and without absorbed electrolyte solvent and for different polymer manufacturers. Mechanical cycling of lithium cobalt oxide (LiCoO 2 ) cathodes caused a similar degradation, reducing the effective electrical conductivity by 30-40%. Mesoscale simulations on a reconstructed experimental cathode geometry predicted the binder conductivity degradation will have a proportional impact on cathode electrical conductivity, in qualitative agreement with the experimental measurements. Finally, ohmic resistance measurements were made on complete batteries. Direct comparisons between electrochemical cycling and mechanical cycling show consistent trends in the conductivity decline. This evidence supports a new mechanism for performance decline of rechargeable lithium-ion batteries during operation -electrochemically-induced mechanical stresses that degrade binder conductivity, increasing the internal resistance of the battery with cycling. Lithium-ion batteries (LIB) are an enabling energy storage technology for portable consumer electronics, electric vehicles and renewable power generation in part due to their high energy densities. The energy density is driven by not only the relatively large potential of lithium-ion chemistries, but also the ability of active materials to store large amounts of lithium.1 The most common graphitic carbon anode can absorb up to one lithium for every carbon atom. Recent research on higher capacity anodes such as silicon has highlighted an increased need for understanding the mechanics of lithium-ion batteries. As the lithium is shuttled between the anode and cathode, the active materials expand and contract to accommodate the lithium. The resulting volume changes are accentuated for high capacity materials such as silicon which can increase in volume by up to 400% during lithiation. 2Because most LIB electrodes are porous multicomponent composites, understanding the generation and impact of mechanical stresses on batteries can be difficult. The electrode is generally 50-75 vol% solid fraction with active material consisting of micron-sized particles held together by an active binder, which is itself a composite of conductive carbon particles and polymer. The performance of the battery is highly dependent on this complex structure which must allow efficient ion and electron transport through the electrode. The void space in the porous structure allows lith...

Physical property measurements including viscosity, density, thermal conductivity, and heat capacity of low-molecular weight polydimethylsiloxane (PDMS) fluids were measured over a wide temperature range (˗50 o C to 150 o C when possible). Properties of blends of 1 cSt and 20 cSt PDMS fluids were also investigated. Uncertainties in the measurements are cited. These measurements will provide greater fidelity predictions of environmental sensing device behavior in hot and cold environments. NOMENCLATURE CO 2 Carbon dioxide ESD Environmental sensing device LA Launch accelerometer NSC National Security Campus N 2 Nitrogen PDMS Polydimethylsiloxane VTC Viscosity temperature coefficient Figure 16. Room temperature viscosity of PDMS fluids measured in this report, by S. Wells [2] and predicted using a simple mixing rule [12].

The transient transport of electrolytes in thermally-activated batteries is studied using electron probe micro-analysis (EPMA), demonstrating the robust capability of EPMA as a useful tool for studying and quantifying mass transport within porous materials, particularly in difficult environments where classical flow measurements are challenging. By tracking the mobility of bromine and potassium ions from the electrolyte stored within the separator into the lithium silicon anode and iron disulfide cathode, we are able to quantify the transport mechanisms and physical properties of the electrodes including permeability and tortuosity. Due to the micron to submicron scale porous structure of the initially dry anode, a fast capillary pressure driven flow is observed into the anode from which we are able to set a lower bound on the permeability of 10-1 mDarcy. The transport into the cathode is diffusion-limited because the cathode originally contained some electrolyte before activation. Using a transient one-dimensional diffusion model, we estimate the tortuosity of the cathode electrode to be 2.8 ± 0.8.

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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