Technological breakthroughs

Compiled by Bao Hien

Solid-state batteries have long been regarded as the next step in battery technology, promising higher energy density and greater safety than today’s lithium-ion batteries using liquid electrolytes. However, a persistent technical obstacle has hindered their commercialization for years: small branches of metallic lithium, known as dendrites, can form during charging and penetrate the ceramic solid electrolyte layer—which is considered a hard and durable material—causing internal short circuits. A newly published international collaborative study has provided an answer to a long-standing question: how can a soft metal such as lithium break through a ceramic material that is many times harder than itself?

The old hypotheses do not hold up

Previously, battery materials researchers generally believed that lithium dendrites penetrated ceramics through the accumulation of mechanical stress at the tips of the dendrites, similar to the way a sharp object gradually presses through a hard surface. Another hypothesis proposed that lithium “nucleated” at grain boundaries within the ceramic structure—microscopic weak points between adjacent crystal grains—and then gradually propagated from there.

The research team, involving the Max Planck Institute for Sustainable Materials in Germany and Shanghai Jiao Tong University in China, reexamined both hypotheses using advanced microstructural analysis methods, including cryogenic scanning and transmission electron microscopy (cryo-SEM and cryo-TEM), electron energy-loss spectroscopy (EELS), and three-dimensional nanoscale image reconstruction. The results showed no evidence of lithium “nucleating” separately ahead of the dendrite tips under actual battery operating conditions, ruling out the grain-boundary nucleation hypothesis.

The actual mechanism: hydrostatic pressure, like a jet of water penetrating rock

Instead, the researchers identified hydrostatic pressure as the primary mechanism. When metallic lithium becomes confined within pre-existing microcracks in the ceramic, it generates enormous pressure that compresses the material from all directions, sufficient to cause the brittle ceramic to crack and the fractures to continue propagating. The researchers compared this mechanism to water being forced into cracks in rock and creating new fractures, rather than a sharp object gradually drilling through a hard surface.

Three-dimensional nanoscale reconstructions showed that lithium completely filled the tips of microscopic cracks and continued to propagate into cracks at the micrometer scale. Analysis of crystallographic orientations also showed that approximately 20% of the cracks propagated directly through the ceramic crystal grains (transgranular), rather than only along the boundaries between grains (intergranular) as commonly believed. This indicates that dendrites can fracture the ceramic structure through both mechanisms simultaneously.

From mechanism to solution: using cracks to “guide” dendrites

Understanding the actual fracture mechanism has enabled the researchers to propose a somewhat counterintuitive solution: rather than simply trying to prevent cracks from forming altogether, they developed a “defect engineering” strategy that deliberately introduces predefined cracks into the ceramic structure to guide dendrite propagation, preventing dendrites from developing in a straight path through the entire electrolyte layer and causing a short circuit.

The researchers also proposed two complementary approaches: increasing the toughness of the ceramic material to delay the formation of initial cracks, and applying a protective coating to the surface of the lithium electrode to limit dendrite initiation from the outset.

Implications for the commercialization of solid-state batteries

The research team leader emphasized the importance of understanding the underlying physics before optimizing materials: a fundamental understanding of material behavior is crucial to turning promising technologies into practical applications. The finding is considered an important step forward because dendrite-induced short circuits have long been one of the major technical barriers preventing solid-state batteries—which are expected to offer higher energy density and greater safety than current lithium-ion batteries—from reaching commercial-scale production for electric vehicles and other energy-storage applications.