Understanding Lithium Dendrite Suppression by Hybrid Composite Separators: Indentation Measurements Informed by Operando X-ray Computed Tomography

Abstract

This study investigates the significance of the mechanics of hybrid particle−polymer separators in the stabilization of lithium metal interfaces by probing these properties in realistic conditions informed by X-ray microcomputed tomography (micro-CT). Elastic properties and viscoelastic behavior of inorganic microparticle-filled poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) films are characterized using a nanoindentation experiment whose displacement simulates the interfacial response seen in operando micro-CT. It is determined that the dominating mechanical behavior in this hybrid separator relevant to lithium metal cell conditions is comprised of viscoelasticity. Consistent with this finding, along with correlations across other physicochemical properties, a mechanism describing the improvement of lithium metal cycling performance according to inorganic filler type and content is proposed.

Hybrid Particle Brush Coatings with Tailored Design for Enhanced Dendrite Prevention and Cycle Life in Lithium Metal Batteries

“…It remains to be seen whether further improvement may be possible by designing the system to also be effective at mechanically blocking dendrite growth. It was found that for particle–polymer separators, both ion transport homogeneity and mechanical enhancements should be key design considerations to achieve better lithium metal cycling . An additional coating was synthesized that consisted of an in situ cross-linked matrix of poly­(ethylene glycol) diacrylate (PEGDA, M n = 575 g mol –1 ), scaffolding the swollen particle brushes.…”

“…The remaining THF in the drop cast coatings was left to evaporate with gentle heating on a hot plate for an hour. The cell design was adapted from our previous work, which enabled high-quality microcomputed tomograms of lithium metal batteries . In this case, two 17 mm diameter Celgard 3501 separators (Celgard) were sandwiched by electrodes that achieved the minimum electrode separation requirement (50 μm) for optimal micro-CT resolution of lithium metal in its various morphologies.…”

“…10,12,13 The separators with larger amorphous polymer fractions had good interfacial compatibility with the particles and maximal interfacial area across those domains owing to fewer agglomerations. 10 One composite nanostructure in particular has the potential to completely avoid polymer crystallinity, particle agglomerations, and interface heterogeneity. 14 This comprises polymer chains covalently attached, or grafted, onto/from nanoparticles.…”

“…Combining ceramic and polymeric materials into a composite electrolyte is thought to reap the benefits of both materials, as ceramics have the mechanical properties necessary to push back against lithium protrusions, while plasticized polymer viscoelasticity helps to maintain contact with interfacing components . It was found that in the dendrite boundary condition of particle–polymer composite separators, sufficient stiffness to push back against lithium metal dendrites as well as flowability around these protrusions correlated with the most stable cycling . Another finding was that homogeneity of the dispersion of particles in the polymer is necessary to prevent severe mossy lithium .…”

“…It was found that in the dendrite boundary condition of particle–polymer composite separators, sufficient stiffness to push back against lithium metal dendrites as well as flowability around these protrusions correlated with the most stable cycling . Another finding was that homogeneity of the dispersion of particles in the polymer is necessary to prevent severe mossy lithium . Even in plain liquid or solid-state electrolyte systems, agglomeration of ions or defects in the ceramic are segues to cell failure by means of lithium plating and stripping “hot spots.” Thus, homogeneity is a critical prerequisite that can also be assumed for hybrid coating designs.…”