Atomic Layer Deposition of Molybdenum Oxide for Interface-Stabilized Single-Crystal NMC

Next-generation lithium-ion batteries demand cathode materials that not only deliver high energy density but also maintain electrochemical reversibility over prolonged cycling. Among the various cathode candidates, single-crystalline lithium nickel manganese cobalt oxide (SC-NMC) has emerged as a promising material due to its superior structural integrity and resistance to grain boundary-related degradation compared to polycrystalline NMC (PC-NMC). However, despite the enhanced bulk stability of SC-NMC, it remains susceptible to surface degradation, particularly under high-voltage conditions. These surface-related issues primarily stem from parasitic side reactions between the active material and the liquid electrolyte, resulting in the formation of an unstable and resistive cathode–electrolyte interphase (CEI) layer. This non-uniform CEI hinders lithium-ion transport, induces localized volume changes, and promotes the formation of internal microcracks during repeated cycling. Ultimately, such degradation mechanisms severely compromise the electrochemical performance and long-term stability of SC-NMC cathodes, limiting their practical application in high-energy-density battery systems.

To overcome these challenges, we employed a surface engineering strategy utilizing powder-based atomic layer deposition (Powder-ALD) to form a conformal, nanometer-scale protective layer on the SC-NMC surface. Specifically, molybdenum oxide (MoOₓ) was selected as the coating material due to its favorable properties, including high electronic and lithium-ion conductivity, as well as excellent electrochemical and chemical stability. The Powder-ALD process was conducted using a custom-built rotary-type ALD reactor designed for powder samples, enabling uniform coating coverage on all surfaces of the SC-NMC particles. The coating thickness was systematically varied from 1 to 5 nm (corresponding to 10 to 50 ALD cycles) to investigate the impact of layer thickness on electrochemical performance and to identify the optimal coating conditions.

The MoOₓ layer functions as an artificial CEI that effectively passivates the reactive surface of the cathode while preserving fast lithium-ion kinetics. Owing to its conformal nature, the ALD-derived coating accommodates mechanical strain and minimizes structural deterioration during charge–discharge cycling. Electrochemical evaluations conducted under various operating conditions demonstrated the effectiveness of the MoOₓ coating. Under a cut-off voltage of 4.3 V and a 1C rate for 100 cycles, the MoOₓ-coated SC-NMC electrode showed an approximately 5% increase in initial discharge capacity compared to the uncoated sample, while achieving 97% capacity retention. Furthermore, under a more aggressive condition of 4.5 V cut-off voltage at a 2C rate for 100 cycles, the coated cathodes retained about 90% of their capacity, highlighting their robustness under high-voltage, high-rate cycling.

To further validate the effectiveness of MoOₓ as a cathode coating material, a comparative study was conducted with SC-NMC particles coated with zirconium oxide (ZrO₂), a material known for its surface passivation and moderate lithium-ion conductivity. Although ZrO₂ effectively suppresses electrolyte decomposition, the MoOₓ-coated samples outperformed ZrO₂-coated ones in both capacity retention and rate capability, suggesting that MoOₓ offers superior interfacial conductivity and structural stabilization.

Our findings underscore the critical role of nanoscale surface modification in enhancing the electrochemical durability of SC-NMC cathodes. The Powder-ALD approach ensures precise thickness control and uniform deposition, even on complex three-dimensional morphologies typical of battery powder materials. Moreover, the application of a conductive and electrochemically stable oxide such as MoOₓ introduces additional advantages, including improved charge transport and prolonged cycling stability under harsh operating conditions. This study demonstrates that MoOₓ-based ALD coatings represent an effective strategy for mitigating surface-driven degradation in high-voltage lithium-ion batteries and provide a viable path toward the commercial viability of SC-NMC materials. Future work will extend this surface modification strategy to solid-state battery systems, enabling comparative electrochemical analysis across multiple battery platforms to further define the essential properties required for advanced cathode coatings.