Researchers at Hanyang University in South Korea have identified 2.5 nanometers as the minimum effective thickness for lithium niobium oxide cathode coatings in sulfide-based all-solid-state batteries. Applied via powder atomic layer deposition to NCM811 cathodes, the threshold improves cycle life and lowers interfacial resistance, supporting longer-lasting EV battery designs.
A research team in South Korea has identified 2.5 nanometers as the minimum effective coating thickness for cathode protective layers in sulfide-based all-solid-state batteries (ASSBs), a finding that could inform the design of longer-lasting cells for electric vehicles.
The study, led by Tae Joo Park, professor in the department of materials science and chemical engineering at Hanyang University in Ansan, South Korea, tested lithium niobium oxide (LNO) protective layers deposited at thicknesses of 1.0nm, 2.5nm and 5.0nm onto NCM811 cathode powders, a cathode active material widely used in sulfide-based ASSBs.
ASSBs, which replace the liquid electrolyte of conventional lithium-ion cells with a solid one, are seen as a route to higher energy density and improved safety for EV batteries. However, poor chemical compatibility between cathode active materials and sulfide solid electrolytes has slowed commercialization. Coating the cathode with a thin protective layer reduces side reactions at the interface, with earlier work establishing that thickness must stay below 5nm to preserve lithium-ion transport. The minimum effective thickness, however, had not been quantified.
“Our study moves the field beyond the long-standing ‘optimal thickness’ concept by providing a quantitative basis for thickness-dependent interface design,” says Park.
Using a rotary-type powder atomic layer deposition (ALD) system, the team applied the LNO coatings via a supercycle method, in which lithium and niobium were deposited in alternating cycles with ozone. The coated powders were then assembled into torque-cell ASSBs for electrochemical testing.
The 1.0nm coating (LNO-1) delivered the highest initial discharge capacity at 229mAh g⁻¹, compared with 216mAh g⁻¹ for LNO-2.5 and 207mAh g⁻¹ for LNO-5. However, the LNO-2.5 and LNO-5 cells showed approximately 28% longer cycle life than the LNO-1 cell, and the LNO-1 cell exhibited 59% higher interfacial resistance to ion transport than the thicker variants.
A bare, uncoated cell showed 43% shorter cycle life and around 145% higher interfacial resistance than the LNO-2.5 cell. Spectroscopic and microscopic analysis confirmed that interfacial side reactions were effectively suppressed only once coating thickness reached at least 2.5nm.
“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,” says Park. “This provides a practical guideline for cathode–electrolyte interface optimization in next-generation solid-state batteries.”
The findings, published in Volume 86 of Energy Storage Materials on March 8, 2026, could support development of more durable ASSBs for EV applications, with the potential to extend pack lifespan and driving range. The powder-ALD process is positioned as compatible with scalable manufacturing, though the team notes challenges remain for full gigafactory integration.
