# Early-Stage Growth of LiNbO 3 on NMC811: Substrate-Induced Challenges and In Situ QCM Insights for Optimized ALD-Based Artificial CEIs

Léo Lapeyre, Amit Sharma, Arnold Müller, Camilla Minzoni, Laszlo Pethö, Patrice Raynaud, Johann Michler, Ivo Utke

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## To cite this version:

Léo Lapeyre, Amit Sharma, Arnold Müller, Camilla Minzoni, Laszlo Pethö, et al., Early-Stage Growth of LiNbO 3 on NMC811: Substrate-Induced Challenges and In Situ QCM Insights for Optimized ALD-Based Artificial CEIs. Journal of Physical Chemistry C, In press, 10.1021/acs.jpcc.5c04406. [hal-05236399](https://hal.science/hal-05236399v1)

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Submitted on 2 Sep 2025

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# Early-Stage Growth of LiNbO$_3$ on NMC811: Substrate-Induced Challenges and In Situ QCM Insights for Optimized ALD-Based Artificial CEIs

Léo Lapeyre,* Amit Sharma, Arnold Müller, Camilla Minzoni, Laszlo Pethö, Patrice Raynaud, Johann Michler, and Ivo Utke

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Cite This: https://doi.org/10.1021/acs.jpcc.5c04406

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Supporting Information

**ABSTRACT:** 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 high-energy-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 near-stoichiometric 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.

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## 1. INTRODUCTION

The current development of high-performance lithium-ion batteries for electric vehicles has placed Ni-rich cathode active materials (CAMs), like NMC811 (LiNi$_{0.8}$Mn$_{0.1}$Co$_{0.1}$O$_2$), as serious candidates for next-generation high-energy-density systems¹,² including solid-state batteries (SSBs).³ These materials are particularly attractive due to their superior specific energy capacity (>275 mAh g⁻¹) when operating at high voltages (up to 4.6 V vs Li⁺/Li), surpassing other cathode alternatives.⁴⁻⁶ However, operating at such high potentials introduces critical stability challenges at the cathode-electrolyte interface (CEI). Particularly, electrochemical interactions such as electrolyte decomposition and transition metal dissolution during cycling lead to structural degradation and the formation of a thick, resistive CEI layer. This heterogeneous interphase significantly increases impedance and compromises both cycling stability and battery lifespan.⁷⁻¹⁰

To address these limitations, the concept of artificial CEIs has gained attention as a promising strategy. These engineered protective layers, applied directly to the cathode surface before the battery assembly, aim to stabilize the interface and enhance battery durability.¹¹⁻¹³ While protective coatings have been extensively explored in conventional liquid electrolyte systems, they are also increasingly recognized as essential SSBs.¹⁴⁻¹⁶

For maximum effectiveness, artificial CEIs must be ultrathin to facilitate Li⁺ conduction to the cathode, highly uniform, and

Received: June 26, 2025

Revised: August 11, 2025

Accepted: August 25, 2025
cover as much cathode surface as possible for maximizing material protection.<sup>12,17,18</sup>

Considering those requirements, atomic layer deposition (ALD) is recognized as a state-of-the-art technique for artificial CEIs processing due to its self-limiting and surface saturation nature, providing atomic-level thickness precision and outstanding uniformity and conformity, even on 3D-shaped substrates like NMC811 powder.<sup>19,20</sup>

Among the materials studied for artificial CEIs, lithium niobium oxide (LNO) has emerged as a promising candidate due to its high $Li^+$ conductivity,<sup>21</sup> favorable mechanical properties for withstanding CAM volumetric changes during cycling,<sup>22,23</sup> and wide electrochemical stability window, making it compatible with high-voltage cathodes such as NMC811. LNO coatings deposited by ALD have shown potential to improve cycling stability and rate capability of NMC811 cathodes, suppressing unwanted reactions and mitigating transition metal dissolution.<sup>24,25</sup>

However, despite ALD's advantages, early-stage film growth can involve undesired phenomena that significantly compromise film quality and effectiveness. These include nucleation delays, island growth, and preferential adsorption of precursors,<sup>26,27</sup> leading, respectively, to wrong thickness estimations, poor cathode coverage, and compositional variations, being particularly critical for artificial CEI purposes. Currently, a significant knowledge gap exists regarding the growth dynamics of ALD films on the complex NMC811 surface. Although numerous studies report the electrochemical performances of coated CAMs, the fundamental precursor-surface interactions and actual growth mechanisms (such as the desired layer-by-layer growth)<sup>28</sup> remain unknown, specifically for ternary alloys like lithium niobate.<sup>29</sup>

To gain insights into the initial growth of LNO on NMC811, this study focuses on the deposition of $Li_xNb_yO_3$ using the commonly employed lithium-tert-butoxide (LiO'Bu) and niobium penta-ethoxide (Nb(OEt)$_5$), monitored on NMC811 precoated quartz crystal microbalance (QCM). By observing the mass gain for each individual subcycle, this method reveals that LNO is first deposited on NMC811 as islands, with poor substrate coverage at low thicknesses. A preferential adsorption of LiO'Bu compared to that of Nb(OEt)$_5$ is highlighted, creating a chemical gradient at the NMC811 interface, where Nb is depleted. This phenomenon can be countered by adapting the Li:Nb pulsing ratio, where 1Li:4Nb shows no Nb depletion. The QCM predictions were verified by transmission electron microscope (TEM) using energy-dispersive X-ray spectroscopy (EDS) area mapping for surface coverage estimation, and Rutherford backscattering spectrometry (RBS) combined with elastic recoil detection analysis (ERDA) for quantification of compositional depth profiles. Overall, this study shows unexpected substrate inhibition of LNO on NMC811, resulting in a far-from-ideal ALD process. By monitoring the initial growth of artificial CEIs using the in situ method proposed in this study, it becomes possible to evaluate precursor-surface interactions in order to select the most suitable chemical systems and optimize ALD recipes to meet the requirements of artificial CEI design, ultimately allowing for unprecedented battery performances.

## 2. EXPERIMENTAL METHODS

### 2.1. ALD Deposition and In Situ QCM Monitoring.

LNO films were deposited in a hot-wall SC-Qube ALD system (Swiss Cluster) at 230 °C using alternating subcycles of

lithium and niobium precursors in 1Li:1Nb and 1Li:4Nb ratios. The corresponding mass vs cycle plots and estimated growth per cycle (GPC) values for both pulsing sequences are provided in Figure S6 of the Supporting Information. Lithium tert-butoxide (LiO'Bu, 97%, Aldrich) and niobium ethoxide (Nb(OEt)$_5$, 99.99%, Ereztech) were used as Li and Nb sources, respectively, with deionized water as the coreactant. Precursors were delivered using 40 sccm Ar through heated stainless-steel bubblers (180 °C for LiO'Bu, 170 °C for Nb(OEt)$_5$) and pulsed for 2 s (metal precursors) and 0.15 s (H$_2$O), followed by a 5 s exposure and 60 s Ar purge (200 sccm). The related QCM signal for the 1Li:1Nb pulsing sequence under steady-state conditions can be observed in Supporting Information. An Al$_2$O$_3$ film (>100 nm) was deposited on QCM crystals using TMA and H$_2$O to create a hydroxylated -OH surface for standardized LNO deposition studies.<sup>30,31</sup>

QCM monitoring was performed using a 6 MHz RC-cut quartz crystal with a temperature-stable setup (baked overnight in the ALD reactor at 230 °C) and backside purging, with mass changes ($\Delta m$/A, in ng/cm$^2$) calculated from frequency shifts via Sauerbrey's equation

$$ \Delta f = - \frac{2f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m \quad (1) $$

where $f_0$ is the initial resonance frequency (6 MHz), A is the piezoelectrically active crystal area (in cm$^2$), $\rho_q$ is the quartz density (2.648 g/cm$^3$), and $\mu_q$ is the shear modulus of quartz for the Rc-cut crystal (2.947 × 10$^{11}$ g/cm·s$^2$). A conservative relative uncertainty of ±3% was applied to all QCM mass measurements to account for instrumental noise, thermal drift, and long-term baseline fluctuations.

### 2.2. NMC811 Substrate Preparation and Characterization.
NMC811 thin films (~300 nm) were deposited by RF magnetron sputtering (100 W, 50 sccm Ar + 1 sccm O$_2$, 0.01 mbar) using a 2" commercial target in a Korvus HexL chamber. Films on QCM and Si substrates were postannealed at 400 °C for 6 h in 1 mbar O$_2$. Thin ~50 nm films were also sputtered and postannealed on SiN$_x$ TEM grids.

Elemental composition was assessed via EDS (Ni, Mn, Co) and ERDA (Li, O), benchmarked against commercial NMC811 powder (MSE Supplies, USA), which was also annealed at 400 °C under an O$_2$ atmosphere at a pressure of 1 mbar for 6 h. Surface morphology was analyzed via SEM (Hitachi S-4800) and structural information by XRD (Bruker D8 Discover, Cu K$_{\alpha1/\alpha2}$ radiations, 20–90° θ–2θ, 0.02° step).

Ion beam analysis (IBA) measurements were performed at the Laboratory of Ion Beam Physics at ETH Zurich using Rutherford backscattering spectrometry (RBS, 2 MeV He$^4$) and heavy ion elastic recoil detection analysis (HI-ERDA, 13 MeV $^{127}$I). The collected data were evaluated using the SIMNRA program.<sup>32</sup> For the ERDA measurements, recoil ions under a scattering angle of 36° are identified by the combination of a time-of-flight (TOF) spectrometer and a gas ionization chamber (GIC).<sup>33</sup> Elemental depth profiles in the mass range from H to Nb were extracted by the POTKU code.<sup>34</sup> A correction of the atomic concentrations to t = 0, being the start of the measurement, was performed by extrapolating the measured time-dependent relative signal intensities to t = 0. The concentration of Li was corrected for the isotopic abundance of $^7$Li. The error of the elemental ratios is estimated to be on the order of 7% and 15% for H.
<figure><img src="image_10.png" /><figcaption>Figure 1. (a) SEM surface and (b) cross-sectional morphology of the annealed NMC811 films on the QCM crystal. (c) The X-ray diffractogram of the NMC811 film with the corresponding ICSD reference (8362). XRD characterization was performed on the Si substrate due to overlapping of diffraction peaks between NMC811 and quartz from QCM crystals.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 2. HR-TEM images of the NMC811 PVD film coated with 15 super cycles of LNO. (a) HAADF-STEM image exhibiting morphology with nanoscale grains of approximately 5–10 nm in size. (b) High-resolution STEM image highlighting the layered atomic structure of NMC811 with a d$_{10}$$_{2}$ spacing of 2.89 Å, consistent with the (003) plane of the R3m structure. Yellow boxes indicate regions analyzed for fast Fourier transform (FFT), confirming the crystallographic symmetry along [221] and [241] zone axes. A potential LNO nucleation cluster is indicated, suggesting early-stage ALD growth.</figcaption></figure>

Density (ρ) was computed using the RBS-derived areal atomic density and SEM thickness

$$ \rho = (\delta_{at} \cdot M)/(A \cdot t \cdot N) \qquad (2) $$

where A is the Avogadro number, t is the sample thickness, and N is the total number of atoms in the compound and the molecular mass M = $\sum_{i}^{n} C_{i}m_{i}$ (with C$_{i}$ corresponding to the stoichiometric coefficient as derived from the RBS fit and mi being the atomic mass of the i$^{th}$ element).

The effective film thicknesses t$_{e}$ (in nm) were derived from QCM mass changes Δm/A (in ng/cm$^{2}$) and RBS-estimated LiNbO$_{3}$ densities ρ (in g/cm$^{3}$) using

$$ t_e = \frac{(\Delta m/A)}{\rho} \times 10^{-2} \qquad (3) $$

This calculation assumes a uniform deposition across the entire surface area of the QCM crystal.

High-resolution transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) were captured with a probe-corrected Titan Themis 200 G3

(ThermoFisher Scientific, USA) equipped with a Super-X EDS detector for elemental area mapping.

### 3. RESULTS AND DISCUSSION

#### 3.1. NMC811 Substrate Properties.
The chemical composition of the postannealed NMC811 sputtered film was characterized by two complementary techniques: Hi-ERDA for light elements (Li, O) and EDX for heavier elements (Ni, Mn, and Co). Figure S1 presents the obtained elemental concentrations and depth profiles. The final film composition, Li$_{0.88}$Ni$_{0.8}$Mn$_{0.06}$Co$_{0.14}$O$_{2}$, reveals a slight Li deficiency and minor deviations in the Mn and Co contents compared to the nominal Ni:Mn:Co ratio of 8:1:1. Although these deviations fall within the 7% error margins of the employed analytical techniques, they are likely a result of differences in the sputter yield—a known effect in the PVD deposition of cathode materials.35

It is important to emphasize that this study does not aim to fabricate an ideal or electrochemically optimized NMC811 thin film but rather to produce a surface sufficiently representative of commercial NMC811 for investigating ALD nucleation. Despite some differences in composition and
<figure><img src="image_12.png" /><figcaption>**Figure 3.** (a) Deposited mass of 1Li:1Nb LNO vs ALD super cycles for -OH-terminated and NMC811 surfaces. Displayed MGPC values were calculated by dividing the final mass by the total number of cycles. MGPC stands here for mass gain per super cycle, i.e., a full LNO deposition cycle. (b) Magnified view of the first 5 LNO super cycles for -OH and NMC811 surfaces. An offset of -100 ng/cm² is applied to the NMC811 surface scale for visibility purposes.</figcaption></figure>

crystallinity, the PVD film provides a comparable surface for this purpose. This setup enables in situ monitoring of ALD LiNbO₃ growth via QCM, offering highly sensitive, real-time insights into growth behavior and surface coverage—critical parameters for understanding artificial CEI formation.

The SEM images from Figure 1a,b reveal a polycrystalline microstructure characterized by a rough, irregular surface texture with a granular or porous morphology. The X-ray diffractogram from Figure 1c further confirms the film's polycrystallinity, presenting a series of well-defined diffraction peaks that match the reference ICSD pattern for the desired NMC811 crystalline phase. The close agreement between the measured diffractogram and the reference pattern confirms the successful synthesis of the target NMC811 phase through the sputtering and annealing process.

The PVD-deposited NMC811 films were characterized by HR-TEM to further verify their structural relevance as model surfaces for studying precursor–substrate interactions. While these thin films are not intended to replicate the full electrochemical functionality of bulk NMC811 materials, HR-TEM analysis reveals key morphology and microstructure characteristics that make them suitable substrates for

<figure><img src="image_13.png" /><figcaption>**Figure 4.** Mass gain per super cycle for the two subreactions on (a) -OH surface and (b) NMC811 surface, extracted from mass evolution displayed in Figure 3. The bar plot describes the estimated cationic ratio averaged from intervals of 5 super cycles, calculated from eq 3.</figcaption></figure>

investigating the growth dynamics of ALD-deposited LNO on NMC811.

The film exhibits a uniform granular surface morphology with well-defined crystalline grains ranging from 5 to 10 nm (Figure 2a). At the atomic scale, HR-TEM imaging reveals the characteristic layered structure with $R\bar{3}m$ symmetry, as confirmed by the FFT patterns (Figure 2b), which match the expected crystallographic structure of NMC811. These structural similarities at both the nanoscale morphology and atomic arrangement levels would suggest that surface reactions and growth mechanisms observed on these model films would be representative of those occurring on practical NMC811 cathode materials.

## 3.2. LNO Early-Stage Growth Dynamics on NMC811.

### 3.2.1. In Situ QCM Comparison: LNO on -OH vs NMC811 Surfaces.

#### 3.2.1.1. Surface Influence on Initial Growth.
The QCM recordings for LNO deposition using a 1Li:1Nb pulsing sequence on -OH and NMC811 surfaces are displayed in Figure 3a. A significant difference in growth rate in the linear part of the plot is observed between the two substrates, where NMC811 provides a mass gain per cycle (MGPC) of 39 ± 1.5 ng/cm² cycle—approximately 30% lower than the -OH-
<figure><img src="image_14.png" /><figcaption>**Figure 5.** MGPC and cationic ratio evolution for 1Li:4Nb pulsing sequence on the NMC811 surface. The estimated cationic ratio is averaged from intervals of 5 super cycles, calculated from eq 3. For the 1Li:4Nb sequence, the $\Delta m(Nb_2O_5)$ in eq 3 represents the sum of the $4*\Delta m(Nb_2O_5)$ per super cycle.</figcaption></figure>

terminated surface, which exhibits an MGPC of 56 ± 1.5 ng/cm²-cycle. The origins of this discrepancy are discussed in the following sections.

In addition, the NMC811 surface induces a pronounced nucleation delay compared to the hydroxylated surface, for which a linear growth regime is reached after 30 super cycles, against 75 super cycles on the NMC811 surface. These initial observations suggest that the NMC811 surface strongly influences the growth dynamics of LNO compared to standard -OH surfaces. A closer examination of the first five ALD super cycles, shown in Figure 3b, reveals unexpected precursor-substrate interactions at the NMC cathode material surface.

While the $Al_2O_3$-OH-terminated surface shows a clear step-like growth with stable plateaus between precursor pulses—indicative of well-defined self-limiting reactions—the NMC811 surface exhibits significant mass loss after each precursor pulse. This mass decay reflects weak, nonpermanent interactions, likely due to poor surface reactivity and limited sticking of $LiO^tBu$ and $Nb(OEt)_5$. The consistent desorption pattern during each purge suggests a physisorption-like behavior, resulting in undersaturated growth and low net mass gain per super cycle.

To further investigate precursor-substrate interactions, saturation experiments for $Nb(OEt)_5$ were performed on both -OH-terminated and NMC811 surfaces, as shown in Figure S3a. On the hydroxylated surface, saturation is reached rapidly with a pulse of 0.3 s, yielding a stable MGPC of approximately ~50 ± 1.5 ng/cm² cycle. In contrast, the NMC811 surface requires a significantly longer pulse time of 10 s to reach a comparable MGPC, clearly indicating lower surface reactivity and the need for prolonged precursor exposure. These findings underscore that saturation conditions established on ideal surfaces such as Si- or $Al_2O_3$-OH-terminated surfaces cannot be directly transferred to complex, application-relevant substrates such as NMC811.

The $LiNbO_3$ deposition on NMC811 shown in Figure 3a was intentionally conducted using a 2 s $Nb(OEt)_5$ pulse—originally optimized for -OH surfaces—to reveal this mismatch. The resulting low GPC and incomplete surface

coverage demonstrate that the deposition proceeded under nonsaturating conditions. This is further confirmed in Figure S3c, where the net mass loss upon $H_2O$ exposure—associated with ethoxide ligand removal—continues until approximately super cycle 50 and only stabilizes around super cycle 75, indicating that complete $Nb(OEt)_5$-$H_2O$ surface reactions were only achieved later in the growth process.

Additionally, $H_2O$ exposures to the NMC811 surface reveal reversible mass uptake, contrasting with the expected mass drop from ligand release, as seen in the -OH surface case. This behavior, typical of hygroscopic substrates³⁰ like NMC811, suggests incomplete surface coverage during the early growth, as it indicates that the NMC811 surface is still exposed to the reactor environment instead of being encapsulated by the LNO layer, as it would be expected from a layer-by-layer deposition. Based on this principle, the mass patterns observed during $H_2O$ pulses can be employed as a qualitative coverage indicator of the NMC811 surface (see Section 3.3). Once the mass uptake during water pulses is canceled, full coverage of the NMC811 surface is achieved. As shown in Figure S4, this transition occurs around super cycle 100, marking the point of complete film closure. Note that this principle is only valid for water pulses occurring after $Nb(OEt)_5$, as $LiOH$ is also known to be hygroscopic,³⁰ which can interfere with the hygro-scopicity from the NMC811 substrate. This is well observed in Figure 7, where $H_2O$ exposure after the $LiO^tBu$ pulse at super cycle 55 provides a positive net mass gain, while the following $H_2O$ exposures after $Nb(OEt)_5$ provide a negative mass gain, in direct correlation with the high surface coverage observed by TEM characterization.

A key factor contributing to the differences in precursor adsorption between NMC811 and OH substrates may be the presence of native carbonate species, such as $Li_2CO_3$, which are known to naturally form on the NMC811 surface when exposed to air.³⁶⁻³⁹ These carbonates likely reduce the availability of hydroxyl surface groups required for precursor chemisorption, thereby inhibiting their adsorption and generating the observed nucleation delay and a reduced MGPC on the NMC811 surface during early-stage growth. Future studies should explore predeposition treatments, such as $O_2$ plasma exposure, to activate the NMC811 surface by removing the carbonate layer. This approach could provide a more oxidized and therefore reactive surface, enhancing ALD precursor adsorption and mitigating the inhomogeneities observed in this study.

### 3.2.1.2. Film Composition and Density.
Another potential explanation for the lower MGPC from the NMC811 substrate is a variation in the chemical composition of the LNO film due to the poor Nb precursor adsorption mentioned earlier, which would in turn influence film density compared to films grown on the -OH surface. To estimate the density of LNO films, the final QCM mass after deposition (see Figure S6) was divided by the film thickness, determined by postdeposition SEM analysis (see Figure S7). On the -OH surface, the QCM measurements indicated a final deposited mass of 22′248 ± 667 ng/cm² for a thickness of 51 ± 1.5 nm, corresponding to a 1Li:1Nb growth per cycle (GPC) of 1.28 Å/cy—consistent with previous studies on Si substrates²¹—and a density of approximately 4.36 ± 0.18 g/cm³, slightly lower than the $LiNbO_3$ bulk density of 4.65 g/cm³ and in agreement with reported values for LNO deposited by ALD on the Si substrate.⁴⁰ In contrast, the final mass on the NMC811 substrate was 15620 ± 469 ng/cm² for a thickness of 48 ± 2
<figure><img src="image_15.png" /><figcaption>**Figure 6.** ERDA elemental depth profile of (a) 1Li:1Nb and (b) 1Li:4Nb LNO on NMC811. Elemental ratios are averaged over the LNO thicknesses, highlighted by the blue and red dashed lines. The obtained stoichiometry for the 1Li:1Nb measurement is $Li_{2.13}Nb_{0.66}O_3$ and $Li_{0.90}Nb_{0.96}O_3$ for the 1Li:4Nb measurement, i.e., closer to ideal stoichiometry.</figcaption></figure>

nm, yielding an estimated density of 3.25 ± 0.20 g/cm³, i.e., 25% lower than the -OH surface, in line with the 30% lower MGPC from Figure 3a. RBS-ERDA measurements (Table S1) confirmed these trends, providing a density of 4.63 ± 0.35 g/cm³ on the -OH surface and 3.95 ± 0.29 g/cm³ on NMC811. To verify that this density difference emerges from a compositional variation, QCM mass data from Figure 3a were further analyzed using a Python script to extract the mass variations during $LiO^tBu-H_2O$ and $Nb(OEt)_5-H_2O$ sub-reactions for each super cycle (respectively, the $\Delta m(LiOH)$ and $\Delta m(Nb_2O_5)$ values represented in Figure S2). Figure 4 presents the evolution of MGPC vs ALD super cycles calculated from data displayed in Figure 3a. Furthermore, the molar cationic fraction of niobium (x) is estimated and displayed in the bar plot. The calculation was performed using, in accordance with methodologies reported in prior studies$^{41,42}$

$$x_{Li_{1-x}Nb_xO_y} = \frac{n_{Nb}}{n_{Nb} + n_{Li}} = \frac{1}{1 + 0.5 \left( \frac{M_{Nb_2O_5}}{M_{LiOH}} \right) \left( \frac{\Delta m(LiOH)}{\Delta m(Nb_2O_5)} \right)} \quad (4)$$

where n is the number of moles, M is the molar masses, and $\Delta m$ is the mass gain for each individual subcycle. The molar masses for the two deposited materials, $Nb_2O_5$ and $LiOH$, were chosen based on their reported deposition mechanisms at a working temperature of 230 °C.$^{43,44}$

Figure 4 reveals significant differences in precursor adsorption behavior, growth dynamics, and resulting film composition between the two substrate types. On both surfaces, the MGPC curves exhibit nonlinear deposition behavior, with typical evolution for substrate-inhibited growth (type 2) or island growth mechanisms.$^{26}$ This is evidenced by an initial nucleation phase with a gradual increase in MGPC, followed by a peak marking the transition from isolated nuclei to coalescing islands and, finally, stabilization as film growth reaches a steady-state regime. A similar growth evolution has been reported for ALD on cathode materials, where in situ QCM measurements revealed comparable island coalescence behavior during early-stage film formation.$^{45}$

On the -OH surface, the MGPC from the $Nb_2O_5$ subcycle stabilizes at 42 ± 1.5 ng/cm² against 31 ± 1.5 ng/cm² for the
<figure><img src="image_16.png" /><figcaption>**Figure 7.** Evolution of the LNO surface coverage and growth behavior during 1Li:4Nb ALD on NMC811 at different deposition stages. The top row shows TEM-EDS elemental maps showing Nb (green) and Ni (red) distribution at 15, 25, and 55 super cycles, corresponding to surface coverage fractions (Θ) of 11%, 42%, and 81%, respectively. The EDS contrast was obtained from atomic % concentrations. The bottom row displays the related QCM mass signals for each deposition stage. Reversible adsorption, substrate hygroscopicity, and LiOH hygroscopicity are observed along the three different stages of the deposition.</figcaption></figure>

NMC811 surface. The LiOH subcycle also shows a lowered MGPC on the NMC811 surface, reaching 9 ± 0.3 ng/cm² against 17 ± 0.5 ng/cm² for the -OH surface. On the growth dynamics, the nucleation regime (I) extends up to super cycle 25, during which isolated islands form and begin to coalesce. Between super cycles 25 and 50, a transition regime (II) is observed, leading to the onset of the steady-state growth regime (III), which begins at super cycle 50 and corresponds to the formation of a continuous film. For the NMC811 surface, island formation (I) persists until super cycle 50, followed by the transition regime (II) lasting until super cycle 100. Beyond this stage, the steady-state regime (III) is reached, indicating a full film closure. This is consistent with the larger nucleation delay on NMC811 and mass patterns from NMC811 hygroscopicity discussed earlier. These differences further underline the lower reactivity of the NMC811 surface, in line with the physisorption behavior and poor surface saturation highlighted in Figure 3b.

Both surfaces experience a chemical gradient during the initial nucleation phase, where the Nb ratio (eq 3, represented by the blue bars in Figure 4) increases to steady-state values. The RBS signal qualitatively highlights a deflection at the LNO/NMC811 interface attributed to the observed Nb depletion. A higher Nb ratio is observed on the -OH surface, stabilizing at 0.3, compared to the NMC811 substrate, where the Nb ratio stabilizes at 0.15. This suggests a preferential adsorption from LiO¹Bu, being even more significant on the NMC811 surface, on which the Nb rate is 30% lower from

super cycles 10–75 compared to its steady-state value. As LiOH is significantly less dense (1.5 g/cm³) than Nb₂O₅ (4.65 g/cm³), films with a higher lithium content exhibit lower density, which further explains the lower steady-state MGPC from Figure 3a highlighted by QCM. The presence of Li at the NMC811 surface could play a role in enhancing the preferential adsorption of LiO¹Bu, compared to the -OH surface. The Li-rich composition was confirmed by RBS-ERDA measurements on NMC811, by providing an Nb ratio of 0.25 (Figure 6a), close to the previously reported value obtained via XPS.²¹

A key factor contributing to the observed nonstoichiometric film composition (expected molar Nb ratio x = 0.5 in LiNbO₃) is the difference in growth per cycle between the two types of subcycles. LiOH grows at a significantly higher rate (between 0.8 and 1.5 Å/cy)⁴⁶ compared to Nb₂O₅ (~0.4 Å/cy)⁴⁷ on SiO₂ substrates with the chemistries employed here, likely due to the lower steric hindrance of LiO¹Bu. As a result, a 1Li:1Nb pulsing sequence inherently produces a Li-rich LNO composition, deviating from the expected LiNbO₃ stoichiometry. Also, these substrate-dependent differences in growth dynamics directly influence the film's final density and composition. The -OH surface promotes denser LNO films (although still not reaching the desired LiNbO₃ stoichiometry), while deposition on NMC811 results in lower-density, high lithium content coatings. The intrinsic surface roughness and porosity usually displayed by NMC811 surfaces could play a role in the final properties of the ALD-LNO coatings.
While these findings show the role of substrate on film
composition and density, an important aspect remains
unresolved: even after full surface coverage and transition to
steady-state growth, the MGPC on NMC811 remains lower
than that on hydroxylated surfaces, suggesting that early-stage
growth dictates a long-range film structure. This may be due to
precursor adsorption and surface interactions during nuclea-
tion, which set the atomic packing density, influencing the
overall film density. The lower density of LNO on NMC811
implies that an initially less compact structure persists
throughout deposition. Differences in the polarity or crystallo-
graphic nature of NMC811 could alter precursor binding and
mobility, affecting film growth. This needs further investigation
by atomistic simulations like DFT to examine precursor-
surface interactions and densification mechanisms, which is
beyond the scope of this study.

By employing the estimated LNO densities on NMC811
from RBS-ERDA results, the QCM mass can be translated into
an effective thickness using eq 3, allowing for the
determination of the critical point at which island coalescence
occurs, forming a continuous film. On NMC811, the QCM
mass at super cycle 100 being 4496 ± 135 ng/cm², the critical
thickness for film closure is estimated at 11.4 ± 1.2 nm. This
estimation is particularly significant in the context of artificial
CEIs, where the film must fully cover the NMC811 surface
while remaining thin enough to conduct Li ions effectively.
However, this minimum thickness for full coverage may exceed
the optimal range for artificial CEIs, potentially hindering
battery performance. These findings suggest that previous
studies using LiO'Bu and Nb(OEt)₅ precursors for LNO
deposition on NMC811 likely encountered poor surface
coverage when targeting ultrathin films, as the surface coverage
is directly interdependent with film thickness, as we further
detail in Section 3.3.

Additionally, the Nb depletion observed within the critical thickness range for artificial CEIs can negatively affect battery performance by reducing the Li⁺ conductivity and creating inconsistent ionic transport properties across the artificial CEI layer. To achieve the desired stoichiometry for LiNbO₃, the pulsing sequence must be optimized, as previously demonstrated on SiO₂ substrates,²¹ to mitigate preferential adsorption of LiO'Bu and ensure a more uniform composition.

### 3.2.2. Optimization of Pulsing Sequence for Enhanced LNO Growth on NMC811.
The MGPC evolution and cationic ratio for the 1Li:4Nb pulsing sequence are presented in Figure 5. Similar to the 1Li:1Nb sequence (Figure 4), the characteristic MGPC profile indicates island growth behavior. A peak in MGPC is observed at approximately 25 super cycles, marking the transition from nucleation to island coalescence, followed by steady-state deposition around super cycle 55. These key growth stages were further analyzed via TEM-EDX mapping to assess the evolution of LNO surface coverage (see Section 3.3).

The cationic ratio for the 1Li:4Nb sequence stabilizes at an average of 0.48, indicating a near-stoichiometric LiNbO₃ composition. RBS-ERDA provided an Nb rate of 0.52 (Figure 6b), which is in line with QCM predictions. In contrast to the pronounced lithium enrichment observed for the 1Li:1Nb sequence, no significant chemical gradient is detected during nucleation. The increased number of Nb pulses per super cycle effectively compensates for the preferential adsorption of LiO'Bu on the NMC811 surface, ensuring a chemically uniform film throughout growth. This highlights the

importance of optimizing the pulsing sequence to mitigate
substrate-specific precursor interactions and achieve a con-
trolled film composition.

To further analyze the effect of 1Li:4Nb pulsing on film
density, we again estimated the LNO density from the QCM
mass. Postdeposition SEM measurements (Figure S6) showed
a final LNO thickness of 56.5 ± 4 nm after 250 super cycles,
with a corresponding QCM mass of 25250 ± 757 ng/cm².
Using these values, the estimated LNO density is approx-
imately 4.48 ± 0.51 g/cm³—higher than the 3.25 ± 0.20 g/
cm³ estimated for the 1Li:1Nb sequence. RBS-ERDA analysis
provided a density estimate of 4.40 ± 0.56 g/cm³ (Table S1),
further confirming the QCM-based value and in line with
previous studies.⁴⁰ This result suggests that while the 1Li:4Nb
pulsing sequence produces a denser film than 1Li:1Nb, some
porosity may still be present, most likely arising from the
NMC811 substrate. Transforming the mass at super cycle 55
(5'770 ± 173 ng/cm²) into an effective thickness (eq 3) yields
a critical film closure thickness of 13.1 ± 2.1 nm.

### 3.3. Island Growth and Surface Coverage Analysis by TEM-EDX.
Figure 7 illustrates the evolution of film coverage and deposition behavior during the growth of 1Li:4Nb LNO on NMC811 over 15, 25, and 55 ALD super cycles, corresponding to surface coverages (Θ) of 11%, 42%, and 81%, respectively. The top row displays EDX elemental maps showing the spatial distribution of Nb (green, representing the ALD coating) and Ni (red, representing the NMC811 substrate). The bottom row shows the corresponding QCM mass signals for each deposition stage, providing insight into the growth dynamics.

At 15 super cycles, the Nb signal in the EDX map is weak,
indicating the presence of isolated Nb nuclei forming on the
NMC811 surface with low surface coverage. The QCM signal
corroborates this, showing a mass decay during reactant
purging, as previously discussed. This behavior indicates poor
precursor adhesion and highlights the hygroscopic nature of
the NMC811 surface, evidenced by the significant mass
uptakes during H₂O pulses.

By 25 super cycles, the Nb coverage increases notably, with a
clear reduction in Ni (substrate) exposure, suggesting island
growth and partial coalescence. Correspondingly, the QCM
mass signal shows greater mass uptake during Li and Nb
pulses, reflecting the ongoing film growth. While the NMC811
hygroscopic response to H₂O pulses remains visible, the
magnitude of these mass peaks diminishes, consistent with the
progressive surface coverage by the ALD coating.

At 55 super cycles, the Nb signal becomes dominant in the EDX maps, indicating a near-complete surface coverage and film closure, consistent with a coverage fraction of Θ = 81%. The QCM signal exhibits a near-steady-state growth pattern (Figure S2), indicating the transition to full coverage and more uniform film deposition. A net increase in mass following H₂O exposure after the LiO'tBu pulse, indicative of LiOH hygroscopicity, is clearly observed and must be distinguished from the intrinsic hygroscopicity of NMC811.

Variations in battery performance reported in the literature with different ALD cycle counts are often attributed solely to changes in the film thickness. However, as demonstrated in Figure 7, these differences are also closely linked to variations in the cathode surface coverage. For LNO films deposited with LiO'tBu, Nb(OEt)₅, and H₂O, early ALD stages involve nucleation and island growth, leading to incomplete surface coverage that leaves the cathode vulnerable to degradation. As
the number of ALD cycles increases, the coverage improves, transitioning to full coalescence and film closure. Electro-chemical performance is governed not only by film thickness but also by how uniformly the cathode is coated. Inadequate coverage compromises protection, while excessive thickness hinders lithium-ion conductivity. This underscores the need for carefully optimized ALD processes and precursor combinations to achieve artificial CEIs that offer high surface coverage at ultrathin thickness—ensuring high lithium-ion conductivity, minimized interfacial resistance, and enhanced battery performance. In this context, in situ QCM emerges as a powerful and versatile screening method to probe precursor–surface interactions and nucleation behavior, enabling rapid identification of optimal ALD chemistries for battery interface engineering.

## 4. CONCLUSIONS

This work, employing surface-modified QCM with NMC811 to simulate real cathode substrate conditions, provides new insights into the early-stage growth of $LiNbO_3$ by ALD. The findings reveal that ALD does not conform to the expected layer-by-layer growth at early growth stages but reveal instead a nucleation delay, island growth, and chemical inhomogeneity, leading to incomplete surface coverage at low thicknesses. These factors present significant challenges for artificial CEI design, where uniform, ultrathin films are essential for optimal cathode protection and lithium-ion transport. The pulsing sequence is shown to critically influence film stoichiometry, density, and homogeneity, where a 1Li:4Nb ratio enables the deposition of near-stoichiometric, chemically uniform, and denser $LiNbO_3$, whereas a 1Li:1Nb ratio results in lithium-rich, chemically graded, and under-dense films. This disparity arises from preferential $LiO^tBu$ adsorption on NMC811, causing Nb depletion during the initial growth stages.

A critical film closure threshold of ~13 nm marks the transition to full film closure, underscoring the challenge of balancing surface coverage for cathode protection with thickness constraints for optimal lithium-ion conductivity. These findings highlight the need for optimized ALD strategies to overcome substrate-induced growth limitations. Future research will focus on improving early-stage growth through preposition treatments, such as $O_2$ plasma exposures, to eliminate carbonate layers from the NMC811 surface and enhance nucleation. Additionally, exploring more reactive ALD precursors could help transition from island growth toward a more uniform, layer-by-layer deposition, achieving full substrate coverage at lower thicknesses and further refining the CEI design. By addressing these early growth challenges, ALD can be further optimized to unlock its full potential for next-generation lithium-ion batteries.

## ASSOCIATED CONTENT

### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.5c04406.

EDX and ERDA elemental analysis of PVD-sputtered NMC811; precursor pulsing sequence and QCM mass signals for 1Li:1Nb ALD; QCM saturation curves and hygroscopic response analysis on –OH and NMC811 surfaces; QCM-based estimation of film closure point; SEM cross sections for thickness and density of LNO films; QCM mass gain comparison for 1Li:1Nb and

1Li:4Nb sequences; RBS and ERDA depth profiles showing Nb depletion or uniformity; and calculated LNO film densities from RBS-ERDA data (PDF)

## AUTHOR INFORMATION

### Corresponding Author

Léo Lapeyre – Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun, Switzerland; Laboratoire Plasma et Conversion d'Energie (LAPLACE), Université de Toulouse, CNRS, INPT, UPS, 31062 Toulouse, France; orcid.org/0000-0002-6982-3131; Phone: +41587656358; Email: leo.lapeyre@empa.ch

### Authors

Amit Sharma – Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun, Switzerland; Swiss Cluster, 3700 Spiez, Switzerland

Arnold Müller – Laboratory of Ion Beam Physics, ETH Zürich, 8093 Zürich, Switzerland

Camilla Minzoni – Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun, Switzerland; Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland; orcid.org/0009-0004-2433-9532

Laszlo Pethö – Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun, Switzerland

Patrice Raynaud – Laboratoire Plasma et Conversion d'Energie (LAPLACE), Université de Toulouse, CNRS, INPT, UPS, 31062 Toulouse, France

Johann Michler – Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun, Switzerland; orcid.org/0000-0001-8860-4068

Ivo Utke – Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun, Switzerland; orcid.org/0000-0002-9877-2601

Complete contact information is available at: https://pubs.acs.org/10.1021/acs.jpcc.5c04406

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work was supported by the Innosuisse-Swiss Innovation Agency as part of the Cathode Coatings for Safe High-Energy-Density-Batteries—CONFINED, application number 57874.1 IP-EE.

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