An Elastic, Conductive, Electroactive Nanocomposite Binder for Lithium-Sulfur Batteries

One of the principal challenges for powering next-generation electronics is adapting batteries to substrate platforms used in non-planar, flexible, or wearable devices. Commercial Li-ion batteries contain brittle active materials and packaging, and are based on low energy-density chemistries, which impede the ability to create miniature power sources required for mobile and flexible devices. Lithium-sulfur (Li-S) batteries have high theoretical capacity (1672 mA h g^-1 S) and energy density (2600 Wh kg^-1), and commonly employ conductive organic materials that are compatible with flexible substrates. However, like commercial lithium-ion systems, Li-S batteries utilize electrochemically inactive binders that represent up to 10 – 20% of the total cathode mass. An even higher weight-fraction of electrochemically inactive base material is typically used to create flexible or stretchable electrodes, which substantially lowers energy density. Therefore, an electrochemically active elastomeric matrix that contributes to charge/discharge capacity would substantially improve the energy-density of flexible and stretchable batteries.

To this end, we describe a conductive, elastic, and electroactive nanocomposite material composed of polypyrrole and polyurethane (PPyPU) that can be used in lithium-sulfur electrode slurries as a binder, or as a carrier material for directly dispensing or printing slurries on flexible substrates. The benefits of this nanocomposite for flexible energy storage devices are two-fold. First, the optimized synthesis protocol produces highly-conjugated polypyrrole (PPy) nanoparticles that form an electrically percolating network within the polyurethane matrix and participate in electrochemical charge/discharge over a wide potential window. Second, the elastomeric polyurethane matrix endows the composite with mechanical pliability and accommodates the severe volume expansion of sulfur that is known to compromise the structural integrity of Li-S electrodes during cycling and negatively impact extended cycle performance.

Cyclic voltammetry (CV) demonstrated that PPyPU contributed to charge and discharge capacities between 1.8 and 4.3 V vs. Li/Li⁺, which encompasses the entire charge/discharge window traditionally used in Li-S batteries (1.8 – 2.8 V), and also contributes capacity in the potential ranges where sulfur is not electrochemically active (i.e.in the anodic sweep above 2.6 V, and between 2.3 and 2 V in the cathodic sweep). Galvanostatic charge/discharge testing revealed that PPyPU also effectively eliminated the activation overpotential typically observed at the beginning of charge cycles in electrodes with insulating binders such as polyvinylidene difluoride (PVdF). Furthermore, electrodes formulated with PPyPU as a binder displayed low electrode polarization that remained stable over one-hundred full discharge cycles.

In summary, we have synthesized a low-cost, conductive, and elastic polymeric nanocomposite that may be used as an electrochemically active binder for lithium-sulfur batteries. Electrically insulating binders such as PVdF, carboxymethylcellulose, gelatin, and others used in slurry-cast electrodes represent 10 - 20 wt. % of electrochemically inactive electrode mass, and the weight-fraction of elastic base material is even higher in flexible/stretchable batteries. Therefore, the conductive PPyPU binder increases the weight fraction of active material and the gravimetric energy density of lithium-sulfur cathodes. The conductive binder eliminates activation overpotential, and the elastic nature of PPyPU accommodates the volume expansion associated with sulfur during charge and discharge, and provides a matrix platform for flexible and stretchable electrodes.