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Researchers establish minimum effective coating thickness for longer-lasting solid-state EV batteries
Researchers at Hanyang University have identified 2.5 nanometers as the minimum coating thickness required to effectively protect cathode materials in sulfide-based all-solid-state batteries—giving the field a quantitative lower bound it has been missing.
The study focused on lithium niobium oxide (LNO) coatings applied to NCM811 cathode powders via rotary powder atomic layer deposition. Sulfide-based solid electrolytes are chemically reactive at the cathode interface—they generate resistive degradation products on contact with cathode active materials, shortening cycle life. Thin LNO coatings act as a diffusion barrier, but the minimum thickness required to actually suppress those side reactions hadn’t been established.
The Hanyang team deposited LNO at three thicknesses—1.0 nm, 2.5 nm and 5.0 nm—using a supercycle ALD method that alternates lithium and niobium deposition with ozone for precise composition control. The resulting cells showed a clear trade-off. The 1.0 nm coating delivered the highest initial discharge capacity at 229 mAh g⁻¹, but cycle life ran 28% shorter than the 2.5 nm cells and interfacial resistance was 59% higher. Spectroscopic analysis confirmed that side reactions were effectively suppressed at 2.5 nm but not at 1.0 nm—the coating was simply too thin to prevent electrolyte contact. The 2.5 nm cells achieved 216 mAh g⁻¹ initial capacity; going to 5.0 nm dropped that further to 207 mAh g⁻¹ with no meaningful cycle life gain.
Against an uncoated cell, the 2.5 nm coating extended cycle life by 43% and cut interfacial resistance to less than half.
“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,” said Prof. Tae Joo Park, who led the research. “This provides a practical guideline for cathode–electrolyte interface optimization in next-generation solid-state batteries.”
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.
Why ultra-thin coatings matter...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.
How the protective layers were engineered...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.
What the performance tests revealed...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 an approximately 28% longer cycle life than that of the LNO-1 cell. In addition, the LNO-1 cell demonstrated a 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.
Design rule and future implications..."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," said 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.
Provided by Hanyang University
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