Early-Stage Growth of LiNbO₃ on NMC811: Substrate-Induced Challenges and In Situ QCM Insights for Optimized ALD-Based Artificial CEIs

Atomic layer deposition (ALD) has emerged as a cutting-edge technique for fabricating cathode-electrolyte interphase (CEI) layers on cathodic materials, offering precise thickness control and excellent conformality to enhance the stability of highenergy-density lithium-ion batteries. However, despite widespread ALD applications, the early-stage growth dynamics of battery materials remain poorly understood and present unexpected challenges. This study provides novel insights into the nucleation dynamics and early growth behavior of lithium niobium oxide (LiNbO 3 , LNO) on physical vapor deposition (PVD)-sputtered LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathode materials using in situ quartz crystal microbalance (QCM) and complementary electron microscopy techniques. The results reveal significant substrate inhibition during initial ALD cycles, leading to island growth, poor surface coverage at low thicknesses, and chemical inhomogeneity. These substrate-induced effects create an interdependence between film thickness and substrate coverage, which compromises artificial CEI effectiveness, particularly at thicknesses below 10 nm. A critical thickness threshold of approximately 13 nm is identified for complete film closure, highlighting a deviation from the ideal layer-by-layer ALD growth and underscoring the strong influence of precursor−surface interactions. Furthermore, this work demonstrates that pulsing sequences play a decisive role in film composition, density, and uniformity. A 1Li:4Nb ratio effectively mitigates compositional gradients caused by the preferential adsorption of lithium tert-butoxide (LiO t Bu) over niobium penta-ethoxide (Nb(OEt) 5) on the NMC811 surface. This approach yields nearstoichiometric LiNbO 3 films with enhanced chemical homogeneity and density, whereas a 1Li:1Nb sequence results in chemically graded and porous films. These findings highlight the critical need for optimized ALD strategies to address substrate-induced growth limitations and advance the artificial CEI performance.