Energy Focus: Bifunctional poly(vinylpyrrolidone) binders enhance lithium sulfide cathode performance

Intercalation cathodes used in current lithium-ion batteries have a theoretical capacity of approximately 300 mA h g^–1. While this is sufficient for most commercial handheld electronics, it limits the widespread use of lithium-ion batteries in large-scale applications such as zero-emission electric vehicles. Although alternative next-generation cathode materials such as sulfur and fully lithiated sulfur (Li₂S) demonstrate impressive theoretical capacities of 1673 mA h g^–1 and 1166 mA h g^–1, respectively, the structural stability and long-term cycling performance of these electrodes are still unexplored.

In the September issue of Chemical Science (DOI: 10.1039/c3sc51476e; p. 3673), a group of researchers from Stanford University, California, and Beihang University, Beijing, report improved structural stability and cycling of Li₂S cathodes through the use of a bifunctional binder. Since previous studies have shown that compatible binders can significantly impair the overall performance of the electrode, Yi Cui’s group at Stanford and their collaborators focused on finding a new binder. This would replace the commonly used poly(vinylidene fluor-ide) (PVDF) for sulfur cathodes that displays strong affinity to Li₂S and lithium polysulfides (Li₂S_n, 4 ≤ n ≤ 8). By using ab initio simulations to screen various functional groups often found in macromolecular binders and their binding energies with Li₂S, the researchers selected poly(vinylpyrrolidone) (PVP) as a promising choice due to the high affinity between the >C=O groups in PVP and Li₂S and Li-S· species. Subsequent electrochemical testing with coin cells assembled with Li₂S cathodes using PVP binders confirmed their hypothesis.

The high binding energies of PVP with Li₂S and Li₂S_n allowed for uniform dispersion of the active material and carbon within the electrode and minimized loss of polysulfides into the electrolytes during cycling. These batteries also exhibited an initial specific capacity of 760 mA h g^–1 and 94% capacity retention in the first 100 cycles. Even after 500 cycles, the PVP cells retained up to 69% of their initial capacity. The researchers envisage that their simple strategy of rational binder selection could be extended to the identification of new binders for other promising high-capacity electrode materials.