In situ lithiated ALD niobium oxide for improved long term cycling of layered oxide cathodes: A thin-film model study

Protective coatings applied to cathodes help to overcome interface stability issues and extend the cycle life of Li-ion batteries. However, within 3D cathode composites it 15 is difficult to isolate the effect of the coating because of the additives and non-ideal interfaces. In this study we investigate niobium oxide (NbO x ) as cathode coating in a thin-film model system, which provides simple access to the cathode-coating-electrolyte interface. The conformal NbO x coating was applied by atomic layer deposition (ALD) onto thin-film LiCoO 2 cathodes. The cathode/coating stacks were 20 annealed to lithiate the NbO x and ensure sufficient ionic conductivity. A range of different coating thicknesses were investigated to improve the electrochemical cycling with respect to the uncoated cathode. At a NbO x thickness of 30 nm, the cells retained 80% of the initial capacity after 493 cycles at 10 C, more than doubling the cycle life of the uncoated cathode film. At the same thickness, the coating also showed a positive 25 impact on the rate performance of the cathode: 47% of the initial capacity was accessible even at ultrahigh charge-discharge rates of 100 C. Using impedance spectroscopy measurements, we found that the enhanced performance is due to suppressed interfacial resistance growth during cycling. Elemental analysis using TOF-SIMS and XPS further revealed a bulk and surface contribution of the NbO x coating. 30 These results show that in situ lithiated ALD NbO x can significantly improve the performance of layered oxide cathodes by enhancing interfacial charge transfer and inhibiting surface degradation of the cathode, resulting in better rate performance and cycle life.


Introduction
Engineering of artificial solid-electrolyte interfaces in lithium ion batteries are a part of common strategy to improve a wide span of characteristics, including capacity, rate 40 capability and longer cycle life. [1][2][3] By using an artificial solid-electrolyte interface for layered intercalation cathodes such as LiCoO2 and LiNi1-y-zMnyCozO2, the decay of the electrochemical performance can be decelerated and the power performance enhanced. [4,5] This is possible due to the suppression of redox-active metal dissolution from the cathode into the electrolyte and prevention of the formation of a resistive 45 native layer at the cathode-electrolyte interface. [6,7] The employed coatings for this purpose are usually either organic polymers (e.g. polypyrrole[8]) or inorganic materials.
Inorganic coatings, such as metal oxides, are thermally and mechanically stable, nonflammable and normally display little to no reactivity with the cathode itself. [9] The function of the coating depends on the type: Li + conducting materials elevate the ionic 50 diffusion coefficient and thus enhance the rate performance, electronic conductors like graphene allow for improved electronic conductivity across the interface and lastly, oxides, fluorides and other inert materials construct a protective layer and reduce interactions between cathode and electrolyte. [10] The family of niobium oxides (e.g. Nb2O5) and their lithiated form (e.g. LiNbO3) 55 have found several applications in the field of lithium ion batteries. This type of material, which will be denoted as NbOx in the rest of the manuscript, can function as electrode [11] and electrolyte [12] but its most interesting application is as cathode coating. [13,14] Lithiated NbOx processed by atomic layer deposition (ALD) possesses an ionic conductivity of 10 -9 S cm -1 [12] and is electrochemically stable in combination 60 with layered oxide cathode materials, such as LiCoO2 (LCO) [15,16]. Deposited on a cathode material, NbOx can play a role as protective layer, surface modifier and bulk dopant. As a protection coating, it can reduce side reaction and improve structural stability. Interdiffusion on the other hand can increase the stability at the interface by decreasing defects and oxygen loss.
[5] 65 NbOx is well known in the solid-state battery field where it is commonly used to prevent electrochemical decomposition of sulfide solid electrolytes or prevent interdiffusion during high temperature processing of oxide electrolytes. [13,17] Sol-gel coating of LiNbOx on commercial LCO powder, followed by high temperature treatment at 800 °C, was previously used to increase cycle ability [18] but it was difficult 70 to distinctly identify the interface effects. Therefore, to investigate the effects of NbOx coating on the cathode-electrolyte interface, a thin-film approach is advantageous as it offers a simplified and pure model system without the need for binders and additives.
Kato et al. [14] demonstrated this approach by coating a thin-film LCO cathode with 350 nm LiNbOx by pulsed-laser deposition (PLD) and observed that the coating could 75 enhance the rate performance, obtaining 20 % remaining capacity at 30 C. However, since the coating was thicker than the cathode itself, a non-negligible increase in the interfacial impedance is expected as well as difficulties in deconvoluting the effect of the coating from the active material.
The goal of this work is to further deepen the understanding of NbOx as a cathode 80 coating with the goal of overcoming current limitations in rate and power performance.
Atomic layer deposition as a coating method has the distinct advantage of producing conformal coatings with Å-level thickness control. [9] This allows the investigation of ultra-thin coatings of NbOx for cathodes with different thicknesses, ensuring negligible contribution as active material in the electrochemical investigations. In this work, LCO 85 was employed as a representative layered oxide cathode material as it is a well-studied cathode commonly used in Li-

LCO electrode Preparation
The thin-film half cells were prepared on single-side polished sapphire (0001) wafers (University Wafers). The close thermal expansion coefficient of sapphire and the deposited films make it more favorable as a substrate than other materials such as Si. 95 First, the sapphire wafers were coated with a 20 nm Ti adhesion promoter layer and a 300 nm Pt current collector by RF magnetron sputtering. The sputtering deposition was performed from Ti and Pt targets (Plasmaterials Inc.) in an Orion sputtering system (AJA International Inc.) applying a sputtering power of 3 W cm -2 at a pressure of 0.3 Pa and 50 sccm Ar gas flow. In the same system, 300 nm LCO films were deposited 100 from stoichiometric LiCoO2 targets (Toshima Manufacturing Co.) at room temperature with a power of 5.9 W cm -2 and a bias of 70 V applied to the substrate. Details regarding the LCO thin film preparation are provided by Filippin et al. [20] NbOx coating by ALD The as-deposited LCO films were coated with an amorphous NbOx layer by ALD at 105 a substrate temperature of 175 °C with argon as carrier gas at a base pressure of 19 Pa in a Fiji G2 system (Veeco Instruments Inc.). The precursors were niobium(V) ethoxide (NbOEt5) (Sigma Aldrich) and H2O. NbOEt5 was kept at 160 °C while H2O was unheated. The growth rate was determined by ellipsometry on Si (100) reference substrates (Fig. S1) and linear growth was observed with a growth rate of 0.42 Å cycle -110 1 . X-ray diffractometry (XRD) confirmed that the as-deposited layers were amorphous.
The post-annealing treatment for the NbOx coated LCO was carried out at 700 °C for one hour with a heating rate of 10 °C min -1 in a tube furnace (Carbolite Gero GmbH & Co.) at atmospheric pressure under O2 flow.

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The microstructure of the cathode stack was studied by scanning electron microscopy (SEM) (Hitachi FEG-SEM S-4800). The cross section was prepared by scribing and cleaving the substrate with cleaving pliers. To prevent charging the sample was covered with a 2 nm Pt coating. XRD was performed in grazing-incidence mode using Cu Kα1 radiation at an incident angle of ω = 2° in the range of 2θ = 10° -80°. After equilibration, electrochemical impedance spectra (EIS) were acquired by charging the cells to 4 V at 1 C (= 22.0 µA cm -2 ) and then stabilizing the potential until 145 the current was below 1.1 µA cm -2 . Following, a cyclic voltammetry (CV) scan was performed between 3.0 V and 4.25 V at a rate of 0.25 mV s -1 . The cells were then cycled by applying constant charge and discharge currents at 1 C, 2 C, 5 C, 10 C, 50 C, 100 C and 1 C for five times respectively in the range of 3 V to 4.25 V. For lifetime assessment, the cells were cycled at 10 C for 1000 cycles. At the beginning and after 150 each 100 cycles, EIS was measured as described above.

Results and Discussion
Atomic layer deposition of NbOx   The schematic of the investigated thin-film cathode structure is depicted in Fig. 2a. compared to as-deposited NbOx (left). A direct comparison of the peak ratio is possible, since in Nb4s and Li1s the electrons originate in both cases from the S shell [26]. The increased ratio indicates a higher lithium content in the case where the NbOx layer is annealed with the cathode. As discussed earlier, this results in a strong decrease in 240 interface resistance compared to as-deposited NbOx on annealed LCO (Fig. S2). In Fig. 3e a shift to lower binding energies is observed for the Nb3d doublet peak, in the case where the coating is annealed together with the cathode. This indicates an overall lower oxidation state in the Nb-O system due to a higher lithium content and presence of Libounded NbOx species within the sputtered layer thickness. The Nb3d detailed spectra 245 for the as-deposited NbOx (Fig. 3e, left) displays an emerging peak shoulder at lower binding energy (~205 eV) with increasing sputter time, i.e. closer distance to the lithium containing cathode. Fig. 3f shows at the surface (top) a peak at 532 eV, which vanishes after sputtering and corresponds to adsorbed oxygen. The structural oxygen peak (left -531 eV and right -529 eV) remains through the whole layer, with a shift towards 250 lower binding energy in the case of the NbOx layer annealed with the cathode due to the above-mentioned lithiation.  thickness. This might be due to lithiation of the film coating taking place in the first cycles, using up more lithium with increasing film thickness. Additionally, it is possible that the coating and surface doping binds lithium and forms non-active phases, which again would lead to a decreased capacity. Simultaneously, decreased columbic 285 efficiency is observed for samples having a thicker NbOx coating, supporting lithium loss as explanation for the lower discharge capacities (Fig. S4). is commonly used to model coated LCO cathodes. [28]. Fig. 5a presents the impedance spectra of bare and NbOx-modified LCO thin film electrodes. In the high and medium 300 frequency range two semicircles can be observed for all the samples. The first semicircle in the high frequency region is attributed to the lithium ion diffusion across the electrode-electrolyte interphase. The second circle is related to the charge transfer region between the surface film and the active material interphase, as discussed in other publications. [29] Fig. 5b displays the total resistance of the cells. Of note is that the 30 nm coated sample possesses the lowest total resistance, but this is due to the series resistance having the main contribution to Rtot. The initial charge-transfer resistance for the 30 nm coated sample is the highest of all the samples, but due to its low absolute value this is not reflected in the total resistance. 310 Rate performance of the half-cells  The Nyquist plots of the impedance spectra measured after each 100 cycles depicted in Fig. 7b and c visualize the effect of the NbOx coating on mitigating the resistance increase. While the uncoated sample demonstrates impedance growth over two orders 365 of magnitude, the 30 nm NbOx-coated cathode film presents a significantly slower degradation pace. To quantify the degradation extent, the equivalent circuit parameters (see Fig. 5a) were extracted from the impedance spectra measured every 100 cycles. As depicted in Fig. 7d, the charge transfer resistance decreases with increasing coating thickness. However, one needs to carefully balance the decreased capacity with the gain 370 from charge-transfer reduction. In our measurements 30 nm NbOx seems to be optimal, whereas 15 nm coated sample displays behavior close to uncoated samples, and 60 nm samples suffer too much from decreased capacity. 375 Table 1 presents different inorganic coatings on thin-film LCO, with transition-metal oxides being the most common material. The thickness of the coating material differs widely among different publications, ranging from an order of magnitude thinner (such as in this work) up to thicknesses above the cathode itself. [14] The employed LCO 380 thickness of 300 nm is comparable to other publications with exception of the 18 nm epitaxially grown film [31] and the 4 μm thick cathode employed by Lee et al. [32] The capacity at 1 C of the 30 nm NbOx coated LCO cathode is average with 45 µAh/cm 2µm and lower than the theoretical capacity of LCO which lies above 60 µAh/cm 2 -µm, as earlier discussed. However, the observed 47 % remaining capacity at 100 C is 385 superior to other work except for the epitaxially grown LCO for evident reasons. The charge transfer resistance of 20 Ω cm 2 in our work appears to be the lowest reported value. The low initial resistance and the inhibition of the growth thereof results in superior cycle life of almost 500 cycles at 10 C.

Discussion
Comparing NbOx to other inorganic coating materials is not straightforward since 390 the LCO thickness and initial resistance plays an important role. This is expressed through the performance of the uncoated LCO employed in this work, which performs close to some of the coated LCO cathodes reported in Table 1. Since the cathode coating often leads to an increase in charge transfer resistance[5], it is desirable to keep the coating as thin as possible. The coating should also be conformal to protect the rough 395 cathode-electrolyte interface, thus favoring the use of ALD as a deposition technique.
A better coverage of the cathode surface by ALD NbOx in this work can probably explain the superior electro-chemical performance as compared to PLD coatings.

Conclusions
An ALD process for NbOx coatings on layered cathode materials was developed and investigated in a thin-film model system, without binder and conductive additives. 405 Annealing the as-deposited sputtered LCO cathode together with the NbOx coating results in Nb diffusion into the cathode with simultaneous in-situ lithiation of the NbOx coating. This cross-diffusion improves the ionic conductivity of the coating but reduces the initial capacity of the LCO cathode. The lithiated coating mitigates interface degradation and consequently it slows down the growth of the charge-transfer 410 resistance at the cathode-electrolyte interface. Both cycle life and rate performance of the coated cathode films showed a strong dependence on the coating thickness, with the optimal balance between the two found for a thickness of 30 nm. These optimized films showed 80% remaining initial capacity after 500 cycles at 10 C, more than double that of the uncoated LCO, while maintaining an initial capacity close to 100 mAh/g at 10 C. 415 The conformal nature of binary ALD coatings, together with precise thickness control, is an attractive method for improving cycle life and high-rate performance of layered oxide cathode materials.

Acknowledgments
This work was supported by the the Empa internal project "SUISSE-battery", the 420 Swiss National ScienceFoundation [grant number 200021_172764] and the joint Empa-Fraunhofer ISC project "IE4B"under the ICON funding line. We acknowledge the Laboratory for Nanoscale Materials Science for the access to ToF-SIMS equipment.