[Illustration] This figure shows the formation of complex ion clusters during the cycling of lithium-sulfur battery cells. These ion clusters consist of a cationic polymer binder, a battery electrolyte and an anionic sulfur active material.
Lithium-sulfur batteries are promising candidates for replacing ordinary lithium-ion batteries in electric vehicles because they are cheaper, lighter, and can store nearly twice the energy for the same quality conditions. However, as time goes by, lithium-sulfur batteries become unstable and the electrodes deteriorate, which limits their widespread adoption.
Recently, a team led by scientists at the US Department of Energy's Lawrence Berkeley National Laboratory found that the capacity of the new lithium-sulfur battery module doubled compared to conventional lithium-sulfur batteries, and the charging cycle at high current densities exceeded 100 times. This is a key performance indicator for electric vehicles (EV) and aerospace. They designed a new polymer binder to actively regulate the critical ion transport process in lithium-sulfur batteries and demonstrated how it works at the molecular level.
Brett Helms, a scientist at the Institute of Molecular Casting at the Lawrence Berkeley Laboratory, said: "The new polymer is like a wall. Sulfur is supported in the pores of the carbon body and then sealed by the polymer. The polymer blocks the reaction due to the sulfur's involvement in the chemical reaction of the battery. The negatively charged sulfur compounds are released, which in turn produces the next generation of electric vehicles."
When a lithium-sulfur battery stores and releases energy, the chemical reaction produces movable sulfur molecules that are disconnected from the electrode, causing decomposition and ultimately reducing the capacity of the battery. In order to make these batteries more stable, researchers have been working hard to develop protective coatings for electrodes and to develop new polymer adhesives to bond battery components together. These traditional adhesives are designed to control or mitigate the expansion and cracking of the electrodes, and new adhesives go one step further. Researchers from the molecular foundry research center at Lawrence Berkeley Labs designed a polymer that counteracts its tendency to migrate by selectively binding sulfur molecules to hold sulfur close to the electrode.
The next step is to understand the dynamic structural changes that can occur during charging and discharging and under different charging conditions. David Prendergast, who directs the casting theory facility, and Tod Pascal, a scientist in the theoretical facilities project, have established a hypothesis that simulates the behavior of the polymer. Prendergast said: "We can now reliably and efficiently model the sulfur chemistry in these binders based on detailed quantum mechanical simulations from dissolved sulfur-containing products.
Their large-scale molecular dynamics simulations on the supercomputing resources of the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley Laboratories confirmed that the polymer has affinity for binding to moving sulfur molecules and is also predictive of the polymer's usefulness. Different sulfur species are combined under different charging states of the battery. These predictions were confirmed using experiments with advanced sources from Lawrence Berkeley Laboratories and the Electrochemical Laboratory at Argonne National Laboratory.
The team further studied the performance of lithium-sulfur batteries prepared with new polymer binders. Through a series of experiments, they were able to analyze and quantify how the polymer affects the rate of chemical reactions in the sulfur cathode, which is the key to achieving high current density and high power in these cells. By long-term cycling, the battery's capacitance is nearly doubled, and the new polymer increases the capacity and power of the lithium-sulfur battery. The United States Department of Energy's Joint Center for Energy Storage Research (JCESR) has made the synthesis of new polymers a comprehensive understanding of the theory and properties that make it a key component of prototype lithium-sulfur batteries.
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