Pore Choices: The Delicate Balance of Porosity and External Surface Area in Zirconium Metal–Organic Frameworks for Lithium–Sulfur Batteries

Lithium-sulfur batteries are a promising energy storage system with a high theoretical energy density. A significant challenge in realizing this energy density is the polysulfide shuttle effect, where soluble polysulfides migrate between the two electrodes during cycling, which ultimately leads to reduced sulfur utilization and poor capacity retention. To mitigate these issues, metal-organic frameworks (MOFs) have been utilized as cathode additives to create tunable pore environments for capturing polysulfides and facilitating their redox reactions. However, an often-overlooked aspect of MOFs beyond pore dimensions is the confluence of particle size and porosity on the Li-S battery performance. Herein, we investigate two isoreticular Zr-based MOFs, MOF-801 and UiO-66, with varied particle sizes to understand the interplay of these parameters. At lower charge rates, we found that the high external surface area of small particles provided the best battery performance. However, at higher charge rates, porosity tips the scale as UiO-66 outperforms MOF-801 even at the smallest particle sizes. This study highlights the critical balance between the external surface area and internal porosity, where optimizing particle size can be as crucial as tailoring pore dimensions for effective polysulfide confinement and redox.