Hanyang University Researchers Identify 2.5 Nanometers as the Minimum Effective Coating Thickness for Longer-Lasting Solid-State EV Batteries

GYUNGGI-DO, South Korea, May 15, 2026 /PRNewswire/ -- Sulfide-based all-solid-state batteries (ASSBs), which use a solid electrolyte instead of a liquid one, are emerging as a promising way to overcome the safety and energy-density limitations of conventional lithium-ion batteries. However, a major obstacle to their practical use is the poor chemical compatibility at the interface between the cathode active materials (CAMs) and sulfide-based solid electrolytes.

One widely studied solution is to coat the surface of cathode materials with a thin protective layer. This layer can prevent direct contact between the cathode and the electrolyte, reducing harmful side reactions. Previous studies have shown that precisely controlling the thickness of such protective layers below 5 nanometers (nm) is essential to maintain effective lithium-ion transport and interfacial stability. However, the minimum thickness required for the coating to work effectively has remained unclear.

To address this question, a research team led by Professor Tae Joo Park from the Department of Materials Science and Chemical Engineering at Hanyang University in South Korea, systematically examined the minimum effective thickness of cathode protective layers required for sulfide-based ASSBs. "Our study moves the field beyond the long-standing 'optimal thickness' concept by providing a quantitative basis for thickness-dependent interface design," explains Prof. Park. Their study was made available online and published in Volume 86 of Energy Storage Materials on March 08, 2026.

The researchers employed lithium niobium oxide (LNO) as a model protective layer in the study. Using a rotary-type powder atomic layer deposition (ALD) system, they deposited LNO protective layers with controlled thicknesses onto NCM811 powders, a widely used CAM for sulfide-based ASSBs.

To precisely control the composition and thickness of the layers, the team employed a supercycle method, where lithium and niobium were deposited in alternate cycles, along with ozone (O₃). Using this technique, they fabricated torque-cell type ASSBs using NCM811 powders coated with LNO protective layers of 1.0 nm (LNO-1), 2.5 nm (LNO-2.5), and 5.0 nm (LNO-5) thicknesses.

Electrochemical performance analysis showed clear thickness-based trends. The LNO-1 cell exhibited the highest initial discharge capacity of 229 mAh g^-1, compared to 216 mAh g⁻¹ for LNO-2.5 and 207 mAh g⁻¹ for LNO-5 nm, indicating a gradual decrease with increasing coating thickness. On the other hand, the LNO-2.5 and LNO-5 cells showed approximately 28% longer cycle life than that of the LNO-1 cell. In addition, the LNO-1 cell demonstrated 59% higher interfacial resistance to ion transport compared to LNO-2.5 and LNO-5 cells.

In comparison, the bare cell showed a 43% shorter cycle life and about 145% higher interfacial resistance than the LNO-2.5 cell. Spectroscopic and microscopic studies further showed that interfacial side reactions were effectively suppressed only when the coating thickness reached at least 2.5 nm.

"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," remarks Prof. Park. "This provides a practical guideline for cathode–electrolyte interface optimization in next-generation solid-state batteries."

This design guideline could enable more durable ASSBs for electric vehicles, potentially extending battery lifespan for longer driving ranges. The precise powder-ALD process shows promise for scalable manufacturing toward commercialization, despite remaining challenges in full gigafactory integration.

Overall, the study offers an important design rule that could help accelerate the development of longer-lasting, high-energy solid-state batteries.

Reference
Title of original paper: Minimum effective thickness of cathode protective layers for sulfide-based all-solid-state batteries via powder-atomic layer deposition
Journal: Energy Storage Materials
DOI: https://doi.org/10.1016/j.ensm.2026.105027

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