Understanding and Mitigating Mechanical Degradation in Lithium–Sulfur Batteries—Additive Manufacturing of Li₂S Composites and Nanomechanical Particle Compressions

Lithium–sulfur batteries are key energy storage enablers in sectors such as transportation and grid storage due to the low cost and high theoretical energy density of sulfur as a cathode material. However, significant challenges exist with cathode degradation during cycling, including mechanical failure via cracking or detachment of insulating active material lithium sulfide (Li₂S) from the conductive matrix in the cathode, causing irreversible capacity fade. Additive manufacturing is a promising route towards the development of mechanically resilient 3D electrodes with complex architectures, high active material loadings, and large areal capacities relative to conventional 2D film electrodes. Additive manufacturing processes for fabricating lithium-sulfur (Li-S) cathode materials are beginning to be explored; currently only extrusion methods have been reported, which are limited in resolution to &gt;150 µm and offer limited control over the final morphology and microstructure of the printed material.<br/><br/>We report an additive manufacturing process which enables fabrication of 3D architected lithium sulfide (Li₂S) carbon composite cathodes for Li-S batteries via digital light processing stereolithography and subsequent pyrolysis. The composites have feature sizes of ~50 µm and are comprised of a porous glassy carbon matrix containing crystalline Li₂S deposits within the pores. We 3D print porous polymer composites using water-in-oil emulsions of aqueous lithium sulfate dispersed in a UV-curable photopolymer resin. Pyrolysis then converts the porous polymer matrix to glassy carbon and the lithium sulfate deposited within the pores to lithium sulfide via carbothermal reduction. We investigate the effects of surfactant concentration and emulsion composition on pore size, pore connectivity, and Li₂S morphology via SEM image analysis and show that pore size and Li₂S crystal size can vary from ~10 nm to ~5 µm and are related to the size of aqueous domains in the emulsion.<br/>Such architected cathode materials have promise for mitigating mechanical degradation in high volume-change battery materials such as the sulfur cathode. We additionally performed nanomechanical compression experiments, making use of a Hertzian elastic contact model, on lithium sulfide powders to determine how this material yields, deforms, and fails due to volume-change-induced stress during battery cycling. We place these measurements into the context of in-situ stress evolution measurements in literature and discuss design considerations for 3D-architected battery materials to enable the rational design of high energy density, long-cycling, and mechanically robust sulfur cathodes.