Cycling performance for a Li-S cell with the new binder (PEB-1) compared to a cell with a conventional PVDF binder. Rate of C/5. In all cases, the composite sulfur electrode was cast onto an aluminum current collector. Li et al. Click to enlarge.

Active layers in electrochemical energy storage devices typically incorporate polymer binders to aid in processing composite electrodes with well-controlled architecture and compliant mechanical integrity. Polymer binders also dictate the extent of electrode swelling with electrolyte and help mitigate cracking on drying or swelling, or on large volume changes experienced using certain electrode chemistries between their extremes in state-of-charge.

Often overlooked is whether a polymer binder is an active or a passive component in the composite electrode, a distinction that denotes whether or not it participates in charge or mass transport; it can also be adaptive if it can be made to switch between passive and active states, e.g., using thermal excursions or redox chemistry.

Whereas the chemical constitution of a polymer binder should dictate whether it is passive, active, or adaptive in the electrode, it remains a challenge to reveal the molecular basis by which these behaviors manifest. Without this information, rational design principles for polymer binders remain obscure.

—Li et al.

When a lithium-sulfur battery stores and releases energy, the chemical reaction produces mobile molecules of sulfur that become disconnected from the electrode, causing it to degrade and ultimately lowering the battery’s capacity over time. To make these batteries more stable, researchers have traditionally worked to develop protective coatings for their electrodes, and to develop new polymer binders that act as the glue holding battery components together. These binders are intended to control or mitigate the electrode’s swelling and cracking.

When a lithium-sulfur battery stores and releases energy, the chemical reaction produces mobile molecules of sulfur that become disconnected from the electrode, causing it to degrade and ultimately lowering the battery’s capacity over time. To make these batteries more stable, researchers have traditionally worked to develop protective coatings for their electrodes, and to develop new polymer binders that act as the glue holding battery components together. These binders are intended to control or mitigate the electrode’s swelling and cracking.

The new binder goes a step further. Researchers from the Organic Synthesis Facility at Berkeley Lab’s Molecular Foundry, a research center specializing in nanoscale science, designed a polymer to keep the sulfur in close proximity to the electrode by selectively binding the sulfur molecules, counteracting its migratory tendencies.

The new polymer acts as a wall. The sulfur is loaded into the pores of a carbon host, which are then sealed by our polymer. As sulfur participates in the battery’s chemical reactions, the polymer prevents the negatively charged sulfur compounds from wandering out. The battery has great promise for enabling the next generation of EVs.

—Brett Helms, corresponding author

The next step was to understand the dynamic structural changes that are likely to occur during charging and discharging as well as at different states of charge. David Prendergast, who directs the Foundry’s Theory Facility, and Tod Pascal, a project scientist in the Theory Facility, built a simulation to test the researchers’ hypotheses about the polymer’s behavior.

Their large-scale molecular dynamics simulations, conducted on supercomputing resources at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), confirmed that the polymer has an affinity for binding the mobile sulfur molecules, and also predicted that the polymer would likely show a preference for binding different sulfur species at different states of charge for the battery.

Experiments conducted at Berkeley Lab’s Advanced Light Source and Argonne National Laboratory’s Electrochemistry Discovery Lab confirmed these predictions.

The research team took their study one step further by also examining the performance of lithium-sulfur cells made with the new polymer binder. Through a set of experiments, they were able to analyze and quantify how the polymer affects the chemical reaction rate in the sulfur cathode, which is key to achieving high current density and high power with these cells.

By nearly doubling the battery’s electrical capacity over long-term cycling, the new polymer raises the bar on the capacity and power of lithium-sulfur batteries.

The combined understanding of the synthesis, theory, and characteristics of the new polymer have made it a key component in the prototype lithium-sulfur cell at DOE’s Joint Center for Energy Storage Research (JCESR).

Researchers from JCESR at Berkeley Lab and Argonne National Lab comprised the team, together with scientists from the Massachusetts Institute of Technology, and UC Berkeley. Funding for the project was provided by JCESR, a Department of Energy Innovation Hub that is supported by the DOE Office of Science.

Berkeley Lab’s Molecular Foundry, Advanced Light Source, and NERSC are DOE Office of Science User Facilities that are open to visiting researchers from around the nation and world.

Resources

• Longjun Li, Tod A. Pascal, Justin G. Connell, Frank Y. Fan, Stephen M. Meckler, Lin Ma, Yet-Ming Chiang, David Prendergast & Brett A. Helms (2017) “Molecular understanding of polyelectrolyte binders that actively regulate ion transport in sulfur cathodes” Nature Communications 8, Article number: 2277 doi: 10.1038/s41467-017-02410-6