Solid-state EV battery cathodes need at least 2.5 nanometer protective coatings

May 15, 2026

Researchers identified 2.5 nanometers as the minimum coating thickness that suppresses side reactions in sulfide solid-state batteries.

(Nanowerk News) Researchers at Hanyang University in South Korea have pinned down the minimum thickness a protective solid-state battery cathode coating must reach to do its job. A lithium niobium oxide layer of 2.5 nanometers, applied through powder atomic layer deposition, is the smallest film that effectively blocks unwanted reactions between cathode particles and sulfide solid electrolytes. The finding (Energy Storage Materials, “Minimum effective thickness of cathode protective layers for sulfide-based all-solid-state batteries via powder-atomic layer deposition”) gives battery developers a concrete lower bound for cathode-electrolyte interface design in next-generation electric vehicle batteries.

Key Findings

Sulfide-based all-solid-state batteries, often shortened to ASSBs, replace the flammable liquid electrolyte found in conventional lithium-ion cells with a solid conducting material. The switch promises higher safety and greater energy density, two qualities that matter most for electric vehicle applications. Commercialization has been held back by a chemistry problem at the cathode-electrolyte interface, where unwanted reactions between the cathode active materials and the sulfide electrolyte degrade performance. Coating the cathode particles with a thin protective film is the standard workaround, since the layer keeps the two reactive surfaces apart and limits parasitic chemistry. Earlier work had narrowed the useful range to below 5 nanometers, since thicker coatings impede lithium-ion transport and undermine the cell’s interfacial stability. The lower bound, however, the point at which a film becomes too thin to protect the cathode at all, had not been quantified. A group led by Tae Joo Park, a professor in the Department of Materials Science and Chemical Engineering at Hanyang University, carried out the work. “Our study moves the field beyond the long-standing ‘optimal thickness’ concept by providing a quantitative basis for thickness-dependent interface design,” Park said. The team published its findings online in Volume 86 of Energy Storage Materials on March 8, 2026. The researchers chose lithium niobium oxide, abbreviated LNO, as their model coating and deposited it onto NCM811 cathode powders, a nickel-rich material widely paired with sulfide electrolytes. A rotary-type powder atomic layer deposition (ALD) reactor handled the deposition. The team introduced lithium and niobium precursors in alternating cycles together with ozone, a supercycle approach that gave them precise control over both film thickness and composition. Park’s group prepared three coating thicknesses on the NCM811 powders: 1.0 nanometer (LNO-1), 2.5 nanometers (LNO-2.5), and 5.0 nanometers (LNO-5). They assembled the coated powders into torque-cell ASSBs for electrochemical testing, with an uncoated cell serving as the reference. Initial discharge capacities tracked inversely with coating thickness: LNO-1 delivered 229 milliamp-hours per gram, LNO-2.5 reached 216, and LNO-5 reached 207. Cycle life moved in the opposite direction. The LNO-2.5 and LNO-5 cells sustained roughly 28 percent more cycles than the LNO-1 cell. The thinnest coating also showed 59 percent higher interfacial resistance to ion transport than the thicker pair, suggesting that a 1.0 nanometer film is too thin to fully suppress the degradation reactions despite delivering a high initial capacity. The uncoated reference made the role of the protective layer plain. That bare cell ran for 43 percent fewer cycles and showed about 145 percent higher interfacial resistance than the LNO-2.5 cell. Spectroscopic and microscopic analyses traced the difference to interfacial side reactions, which the data showed were suppressed only once the coating reached at least 2.5 nanometers. “Our results show that the minimum effective thickness of the LNO protective layer to suppress side reactions in sulfide-based ASSBs is 2.5 nm,” Park said. “This provides a practical guideline for cathode–electrolyte interface optimization in next-generation solid-state batteries.” A defined floor for coating thickness gives battery developers a concrete target rather than a trial-and-error range. The authors suggest the design rule could support more durable ASSBs for electric vehicles, potentially extending battery lifespan and driving range. They note that the powder-ALD process used here is compatible with scalable manufacturing, although full integration into gigafactory-scale production still faces engineering challenges. The work converts a qualitative design preference into a measurable specification, narrowing the useful design window for sulfide cathode-electrolyte interfaces from anywhere below 5 nanometers to a defined band of 2.5 to 5 nanometers. Engineers now have a quantitative target for the thinnest film that still protects the cathode, preserving as much of the initial discharge capacity as possible without sacrificing cycle life.