Tin Oxynitride Anodes by Atomic Layer Deposition ... - ACS Publications

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Tin Oxynitride Anodes by Atomic Layer Deposition for Solid State Batteries David M Stewart, Alexander J Pearse, Nam S Kim, Elliot J. Fuller, A. Alec Talin, Keith Gregorczyk, Sang Bok Lee, and Gary W. Rubloff Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04666 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Chemistry of Materials

Tin Oxynitride Anodes by Atomic Layer Deposition for Solid State Batteries David M. Stewart1*, Alexander J. Pearse2, Nam S. Kim3, Elliot J. Fuller4, A. Alec Talin4, Keith Gregorczyk2, Sang Bok Lee3, Gary W. Rubloff 1,2,5

1. Institute for Systems Research, University of Maryland, College Park, MD

20740

2. Department of Materials Science and Engineering, University of Maryland, College Park, MD

20740

3. Department of Chemistry, University of Maryland, College Park, MD 20740

4. Materials Physics Department, Sandia National Laboratories, Livermore, CA 94550

5. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD

20740

Abstract Major advances in thin film solid state batteries (TFSSBs) may capitalize on 3D structuring using highaspect ratio substrates such as nanoscale pits, pores, trenches, flexible polymers, and textiles. This will require conformal processes such as atomic layer deposition (ALD) for every active functional

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component of the battery. Here we explore the deposition and electrochemical properties of SnO2, SnNy, and SnOxNy thin films as TFSSB anode materials, grown by ALD using tetrakisdimethylamido(tin), H2O, and N2 plasma as precursors. By controlling the dose ratio between H2O and N2 the N-O fraction can be tuned between 0% N to 95% N. The electrochemical properties of these materials were tested across a composition range varying from pure SnO2, to SnON intermediates, and pure SnNy. In TFSSBs, the SnNy anodes are found to be more stable during cycling than the SnO2 or SnOxNy films, with an initial reversible capacity beyond that of Li-Sn alloying, and retaining 75% of their capacity over 200 cycles compared to only 50% for SnO2. Furthermore, the performance of the SnOxNy anodes indicates that SnNy anodes should not be negatively impacted by small levels of O contamination.

Introduction Solid-state Li-ion batteries offer many potential advantages over liquid-electrolyte based systems in terms of safety, improved reliability, and longer cycle life due to their inorganic, non-flammable solid electrolyte.1–3 Commercial thin film solid-state batteries (TFSSBs) are available for niche micropower applications.4 However, improvements in TFSSBs areal energy density and power are necessary for emerging applications such as on-board chip power for devices that integrate sensing, information processing, and communication functions.5,6 One of the challenges to realizing high power and high energy density TFSSBs is the anode. Metallic Li, already employed in commercial TFSSBs, is attractive for its high theoretical capacity (3,860 mAh/g) and low voltage vs. standard hydrogen electrode (-3.040 V). However, its low melting temperature, high reactivity, and propensity for metallic Li to penetrate through solid state electrolyte at current densities above 1 mA/cm2 due to dendrite formation,7 all suggest that an alternative anode material may be crucial. Furthermore, it is desirable to find anode materials compatible with three-dimensional (3D) TFSSBs, which use high aspect ratio geometries to increase the areal power and energy densities of TFSSBs


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systems.8–10 For these applications, existing physical vapor deposition techniques are not sufficiently conformal, resulting in large structural inhomogeneity that leads to poor power performance.9 Atomic layer deposition (ALD) can be highly conformal even over high aspect ratios, is already compatible with industrial microelectronic fabrication techniques, and can grow thin films with highly controlled composition and thickness.11 Here we report results for SnO2, SnNy, and SnOxNy as alternative anode materials prepared by highly conformal ALD. The low deposition temperature required for these ALD processes makes the anodes widely compatible with current integrated chip manufacturing. We show electrochemical performance of the resulting anode materials, culminating in their incorporation and operation in TFSSB half cells. The high reversible capacity and rate capabilities of the nitrogen rich anode materials makes them attractive for energy storage applications requiring conformal thin film deposition.

Sn Oxide, Nitride, and Oxynitride Anode Materials With a theoretical reversible capacity of 782 mAh/g SnO2,12 SnO2 has been well studied in the literature as a low voltage anode with improved cycle stability over pure Sn.13–18 During electrochemical cycling

with

Li,

there

are

two

primary

reactions

which

can

occur

in

SnO2:

SnOଶ + 4Liା + 4݁ ି ⇄ 2Liଶ O + Sn ሺ1ሻ ሺ2ሻ Sn + ‫ݖ‬Liା + ‫ ⇄ ି ݁ݖ‬Li௭ Sn Reaction 1 is an initial conversion reaction which consumes a large amount of Li on the first discharge, after which reversible Li/Sn alloying can begin via Reaction 2 up to the solution limit of 4.4 Li/Sn. The formation of Li2O is beneficial to overall battery cycle performance, as it constrains the volume expansion of the encapsulated Sn particles during alloying, and limits their aggregation into larger particles.17,18 However, the large amount of Li consumed during Reaction 1 results in a significant first cycle capacity loss due to the large negative formation energy for the forward reaction (Δ‫ܪ‬௙∘ = 620 kJ/mol Sn19) and thus high irreversibility of the reaction. This first cycle capacity loss exists for all



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metal oxide/nitride conversion electrodes, but some reports have shown that it can be reduced in SnO2 by forming the anode from nanoparticles of SnO2 with a coating to limit aggregation and maintain small particle size, which may improve reversibility of the conversion reaction.14,20 The growth of SnO2 thin films by ALD has been reported several times in the literature.21–24 Similar to the growth of TiO2 and TiN by ALD, where tetrakisdimethylamido(titanium) (TDMAT) is used as the metal organic precursor,25,26 tetrakisdimethylamido(tin) (TDMAS) can be used as the Sn precursor, with H2O, O2, or O3 as the oxidant. Compared to anodes produced from a slurry of SnO2 powder, carbon black, and polymeric binders, as-deposited ALD SnO2 films are typically amorphous. The resulting Sn particles produced during the first discharge (lithiation) are nanoscale and highly dispersed. This dispersion within the Li2O matrix has the same effect as beginning with nanoparticles, as mentioned previously, and has been shown to help limit the aggregation of Sn during further cycling.14,15 Nonetheless, the capacity of the thin film SnO2 anodes decays rapidly with galvanostatic cycling unless a narrow potential limit is imposed.27 While there have been many studies on SnO2 anodes in general, investigation of the electrochemical properties of Sn3N4 and subnitrides has been limited,28–31 even though anodes of off-stoichiometry SnNy were used in solid state thin film batteries with LiCoO2 almost two decades ago.32,33 In general, the electrochemical reactions with Li are the same for SnO2 and Sn3N4 with the simple substitution of Reaction 1

for Snଷ Nସ + 12Liା + 12݁ ି ⇄ 4Liଷ N + 3Sn ሺ3ሻ

While the amount of Li consumed per mole Sn during this conversion reaction is the same as for Reaction 1, the Δ‫ܪ‬௙∘ for Li3N or Sn3N4 from their elements is comparatively small (-165 kJ/mol or 0 kJ/mol, respectively34). This should render this conversion reaction more easily reversible than Reaction 1, in addition to the slightly greater theoretical reversible capacity of Sn3N4 (859 mAh/g Sn3N4).


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Furthermore, the formation of a Li3N matrix, which is known to have a high diffusion coefficient for Li+,35 could result in better battery performance at high currents than SnO2 anodes. Additionally, recent reports on the electronic and electrochemical properties of N doped SnO2 electrodes have shown that even N concentrations as low as 3% have a dramatic impact on the performance of SnO2.36,37

Results To study the electrochemical properties of materials in the tin oxynitride system, thin films were grown by ALD using existing processes for SnO2, and by developing new processes for SnNy and SnOxNy using TDMAS and remote N2 plasma (pN2). These latter processes and the resulting films were characterized using various surface and thin film techniques including x-ray photon spectroscopy (XPS), spectroscopic ellipsometry (SE), x-ray reflectometry (XRR), and x-ray diffraction (XRD). Optimized thin films were used as anodes in half-cells with Li metal counter electrodes to study the differences in electrochemical reactions and stability of the anodes by cyclic voltammetry (CV) and galvanostatic cycling (GV). Since the majority of the existing literature has studied only SnO2 in liquid electrolytes, this system is used as the reference for comparing the properties of SnNy and SnOxNy in solid electrolytes as compared to liquid electrolyte systems. Growing Nitride Films To begin optimizing the ALD process for SnNy thin films, SE was used to monitor the growth rate in situ. Figure 1a shows the film thickness and ALD chamber pressure as a function of time during growth at a substrate temperature of 200 °C. As expected of an ALD process, the SnNy film grew in discrete steps following each precursor exposure. During TDMAS pulse the film thickness increased ~1.29 Å. On the subsequent pN2 exposure the overall film thickness decreased as the methyl ligands were removed from the surface, resulting in a net growth per cycle (GPC) of ~0.55 Å/cycle. After an initial nucleation period during the first 20 cycles, the SnNy film was observed to grow linearly (See Supplemental Figure S1a).



Saturation curves for the TDMAS and pN2 are shown in Figure 1b. Very long pN2 exposure times were required to saturate the SnNy surface and completely remove the methyl ligands, which has also been observed for ALD growth of TiN by TDMAT and pN2.25 On the other hand, the TDMAS precursor effectively saturated the surface for pulse times greater than 0.4 s. Repeated pulses of TDMAS without p

N2 exposure produced no significant growth at 200 °C, confirming the self-limiting behavior of the

precursor and the process (see Supplementary Figure S1b.) Effects of Temperature As the substrate temperature was varied between 100-350 °C, the GPC, density, and composition of the SnNy films were affected, as seen in Figure 1c-d. At temperatures below 100 °C, the TDMAS precursor simply condenses on the growth surface, leading to a large GPC and large amounts of N and C relative to Sn. At 100 °C the ratio of Sn/N was measured by XPS to be 0.3, which is very close to the ratio of 0.25 in the TDMAS precursor itself. Further evidence of incorporation of the methyl ligands into the film was the density, which was 63% of the theoretical density of Sn3N4 (6.84 g/cm3) as measured by XRR. With increasing temperature, the ratio of Sn/N increased to 0.76 at 350 °C, which is close to the stable Sn3N4 phase, and the density increased to within 94% of the theoretical density. However, at higher temperature there is increased O incorporated into the film, due at least in part to outgassing from the reactor chamber walls. Selecting SnNy Process Conditions It was reported elsewhere21 that the TDMAS precursor thermally desorbs at ~100 °C, and decomposes at 230 °C. This is consistent with the observed trends in the GPC of the SnNy ALD process, where increasing temperature results in a larger Sn/N ratio, decreased C, and an enhanced GPC above 250 °C suggestive of partial-CVD behavior.38–40 Therefore, to avoid thermal decomposition of the precursor, 200 °C was selected as the optimum growth temperature for a well behaved ALD process. Under these conditions (0.5 s TDMAS pulse time, 20 s pN2 exposure, 200 °C


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substrate temperature) as-grown films were measured to have a Sn/N ratio of ~0.5, with 13 at% C and 4 at% O contamination. Growing Oxynitride Films To grow SnOxNy thin films, the SnNy growth surface was periodically exposed to H2O (0.06 s pulse time), as illustrated schematically in Figure 2a. To prevent a direct reaction between the H2O and TDMAS (which could produce a monolayer of SnO2 imbedded in SnNy rather than a true SnOxNy phase) the H2O dose was only applied directly after the pN2 dose. Tuning the relative concentrations of N and O in the film is accomplished by adjusting the frequency of the H2O exposure from a N/O ratio of 1 (after every N cycle) to 500 (after 500 N cycles), with 0 representing a SnO2 ALD process and no pN2 exposure. Chamber pressure and film thickness over time are shown in Figure 2b for a SnOxNy process with a N/O cycle ratio of 3. As before, the film thickness increased in a step-wise fashion with each TDMAS pulse, and decreased during pN2 exposure. Following a pulse of H2O, the SE model shows no significant change in film thickness or other parameters. Figure 2c shows the GPC and density of the SnOxNy films for different N/O cycle ratios. Both GPC and density change substantially when the N/O cycle ratio was varied from 1 to 10, changing between the values for the pure SnO2 and SnNy films on either end. From the XPS quantification (Figure 2d) this is also the range over which the composition varies most substantially, with the measured N/O ratio in the films changing from 1/3 to 4/1. For frequent oxidation steps (low N/O), the amount of Sn and C was like that of the SnO2 process, indicating that the H2O is more effective than the pN2 at cleaving the methyl ligands. The ratio of Sn to its oxidants (Sn/(N+O), in at%) did not vary significantly over the composition range, remaining at ~0.5. Based on invariance of this ratio, the fact that no change in film thickness was observed in the SE data following an H2O pulse, and that a single chemically shifted Sn 3d doublet is observed in the XPS (see Supplemental Figure S2), it seems reasonable that the brief H2O exposure simply replaces N in the growth surface with O.



Regardless of growth temperature or composition, XRD showed that all films were predominantly amorphous as deposited (see Supplemental Figure S3). Samples of each composition were annealed in flowing Ar at 800 °C for 2 h to induce the growth of a crystalline Sn3N4 phase. However, the annealing caused the growth of randomly oriented SnO2 crystal grains in all compositions, and no other crystalline phases could be identified. Liquid Cell Electrochemistry Coin cells were fabricated by depositing ALD SnO2, SnNy, and SnOxNy anodes of each composition onto stainless steel discs with 1 M LiClO4 in EC:DMC 1:1 by volume, and using Li foil counter electrodes. Figure 3a shows first cycle CV data for SnO2, SnOxNy, and SnNy films, each showing a distinct conversion peak (labeled peak 1) on first discharge at 1.25, 0.98, and 0.90 V vs Li/Li+ respectively. In the SnO2 anodes, the position of peak 1 is consistent with many reports in the literature, but the amplitude of the peak is comparable to the alloying peaks, making it much larger than in those reports.14,27,41–43 These conversion peaks are associated with reactions which break the SnO2 into Sn and Li2O, and similarly break SnNy into Sn and Li3N (reactions 1 and 3, given in the Introduction). It is not surprising that these reactions for SnO2 and SnNy have different energy barriers (and thus occur at different potentials), however the presence of a single peak in the case of a SnOxNy anode is interesting. If the conversion reaction for SnON was a two-step process in which Li2O is formed, followed by Li3N, then there should be two peaks on the first cycle, roughly in the same positions as those of the SnO2 and SnNy. However, the observation of only one peak implies that the breakdown of the O–Sn–N environment occurs in a single step, and that the reaction occurs at an energy that is distinct from that of a pure nitride or oxide structure. After the conversion reaction, the SnO2 anode exhibits several more peaks that are also in agreement with reports in the literature.13,27,41–44 For all three compositions, the primary alloying and dealloying


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reactions occur at the same voltages according to Reaction 2 (labeled peak 2 and peak 3, respectively), although there is a trend of increasing current with increasing N content. Upon recharge, after peak 3 there are small peaks in the CV data that may be associated with the extraction of charge from the Li2O/Li3N matrices, which would be the reverse of Reactions 1-2. A single peak (labeled Y) is observed in the SnNy anode at ~0.8 V vs Li/Li+ as a shoulder on peak 3, while two broad peaks are seen in the SnO2 anode at ~1.2 and ~1.9 V vs Li/Li+ (labeled O1 and O2, respectively). The latter two peaks are generally attributed to the oxidation of Sn, reversing Reaction 1,13,14 but the shoulder in the SnNy CV labeled as peak Y cannot be identified at this time. In the SnOxNy a superposition of these features can be observed as a continuous, low slope between peak 3 and the cutoff at 2 V. As shown in Figure 3b, under GV cycling each composition shows the conversion reaction as a short plateau around 1 V in the first discharge which does not return on subsequent cycles. This leads to an initial capacity loss in all conversion materials as Li becomes bound in the formation of Li2O and Li3N. The first cycle capacity losses were about the same for all three compositions (~27% of the first discharge capacity). However, the SnNy and SnOxNy anodes studied here showed remarkably higher gravimetric capacity than the SnO2 anode, even after this first cycle loss. Additionally, in the SnNy anode much more of the charging occurs below 1 V vs Li/Li+ (in the alloying/dealloying regime), thus giving this material a more energy efficient charge/discharge profile. Evolution of the Reactions Changes in the electrochemical reactions in the tin oxynitride anodes under repeated cycling are shown in Figure 3c-e. Besides the loss of peak 1 after the first cycle, many of the other features remain unchanged after seven cycles. The SnO2 anode showed the most variation, with a shift of peak 2 to higher voltage, the appearance of a second peak on the high voltage side of peak 3, and a continuous decrease in the magnitude of the oxidation peaks at 1.2 and 1.9 V vs Li/Li+. In



contrast, the features of the SnNy and SnOxNy anodes were not shifted during further cycling, implying greater chemical stability brought on by the replacement of O with N. Electrochemical Behavior in Solid-State Cells Arrays of TFSSB half-cells were fabricated as illustrated in Figure 4a, with 30 nm ALD anodes of different compositions. The CV data shown in Figure 4b-c for each composition are consistent with the data from liquid cells. In all cases the same peaks are observed in the solid-state cells as in the liquid, and have been labeled accordingly. The SnNy anode showed the best cycling stability and highest currents during the alloying reactions, while the SnO2 anode changed substantially in the first 20 cycles. As in the liquid cells, the SnO2 anode develops new peaks below 1 V vs. Li/Li+ as the primary Li-Sn alloying reactions decrease in peak current and shift to higher voltage. Unlike in the SnNy and SnOxNy anodes, the SnO2 conversion reaction can be seen to reoccur after the first cycle, but decreases in peak current as the oxidation reactions also decrease. In the SnOxNy CV the oxidation reaction also decreases during cycling, but peak Y at ~0.8 V vs. Li/Li+ does not change, similar to the same peak in the SnNy anode. Long Term Cycle Stability Under GV cycling at a rate of ~3C, the evolution of the SnO2 anode results in significant capacity loss beyond the first cycle, as seen in Figure 5a. The SnO2 anode charge capacity does not stabilize until after 100 cycles, and the coulombic efficiency (CE) continues to change even after that. The SnOxNy anode also shows substantial discharge capacity loss within the first 100 cycles, quickly reducing to 2/3 of the second cycle capacity, while the SnNy anode showed more ideal behavior at this high rate. Both the SnNy and SnOxNy anodes stabilized with a CE of nearly 100%. The CE of the cells was calculated by taking the ratio of the charge capacity (lithium extracted from the tin) divided by the discharge capacity (lithium inserted into the tin). Thus all materials show very low first cycle CE as the irreversible conversion reaction occurs. For the SnO2, the CE rises above 100% for the first 15 cycles as the matrix of Li2O is broken down and not fully reformed on subsequent cycles. The


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Li2O matrix eventually reaches an equilibrium, but in the later cycles the CE of SnO2 drops well below 100% as portions of the active Sn clusters become isolated, preventing Li from being extracted on subsequent charges. The voltage profiles shown in Figure 5b-c are consistent with the CV data. Initially, a significant portion of the reversible capacity of the SnO2 anode is above 1 V, and this capacity quickly decreases in the first 30 cycles, as seen elsewhere in tests with composite electrodes made from SnO2 powder.13 The discharge profiles also develop numerous short plateaus after 30 cycles, which again suggests the growth of crystalline Sn particles. In the SnNy and SnOxNy anodes, however, the voltage profiles accumulate most of their available capacity below 1 V in the alloying reactions with very little evolution occurring over 200 cycles. The SnOxNy anode charging profile does become steeper above 1 V during the first 30 cycles, just as the SnO2 anode profile did, and by 50 cycles is almost as steep as the SnNy profile above 1 V. The loss of capacity here reduces the overall volumetric capacity of the SnOxNy anode to below that of the SnNy. High Rate Capabilities The capacity of the Sn anodes at varying current densities is shown in Figure 6 after the cells had been stabilized by GV cycling 100 times. Because the differences in the capacities of each anode is so large, the current density (rather than a C rate) was used to be consistent across the compositions. There is a sudden drop in the capacity of all anode materials at 5000 μA/cm2, however throughout the test the SnNy anode maintained the highest capacity. The SnOxNy anode was also able to maintain capacities near that of the SnNy, despite containing substantially more O.

Discussion Improvement of the ALD Process Although the SnNy films produced here could be considered high purity in the context of nitrides, it is seen in the XPS that there is a substantial amount of C incorporated. Additionally, in the high resolution spectra shown in Supplemental Figure S2, two components can be



seen in the N 1s region, one which is attributed to N-Sn,45–47 and a higher binding energy component which may be H3CN=CH2 imine ligand fragments.48 The concentration of this higher energy N 1s component decreases as the C contamination decreases as well, and the impact of this N component and the embedded C on the electrochemical properties of the anodes is not known. In the films produced from the oxynitride process, the N/O ratio rapidly increases with decreasing H2O pulse frequency, which implies that the short H2O pulse time does not over saturate the surface and does not penetrate the surface beyond a couple atomic layers. To have more fine control over the N/O ratio, shorter H2O pulse times may be required, though a study of the effects of the H2O pulse time was not conducted. Evolution of the Electrochemical Reactions The appearance of several additional peaks during cycling of the SnO2 anode probably indicates additional steps during the Li-Sn alloying reaction, which is known to occur in several stages in pure Sn. The theoretical maximum lithiation corresponds to the formation of Li4.4Sn, which occurs at 0.2 V vs Li/Li+.12 In the SnNy and SnOxNy anodes the alloying reactions at voltages above 0.2 V are much weaker than the final reaction, and do not become more prominent during cycling. The crystallinity of the films may play a role here. If the Sn in the SnO2 anode aggregates and crystallizes during cycling, then the other alloying reactions would become more prominent, the peak currents would decrease, and the prominence of the oxidation reactions would decrease as the surface-to-volume ratio decreased. However, in the SnNy anode the Sn particles may not crystallize or aggregate, and so the principle electrochemical reactions remain unchanged during cycling. Role of the Matrix The SnNy anodes consistently showed substantially higher reversible capacity compared to SnO2. In the solid-state cells, the SnNy lithiated to ~5.4 Li/Sn on the second cycle (after the conversion reaction), whereas the SnO2 only lithiated to the theoretical alloying capacity of 4.4 Li/Sn. (A similar phenomena was reported by Bagetto et al. in sputtered Sn3N4 films.29) One possibility is that the


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higher electrical and ionic conductivity of the Li3N matrix allows for more better connectivity of the Sn particles, and a better network for conducting Li+ ions and electrons between the Sn and the electrolyte or current collector, respectively. This was the expected effect, and would lead to higher reversible capacity at larger current densities than with SnO2 anodes, as was observed. However, the conductivity of the matrix does not explain why the reversible charge storage of the SnNy anodes is greater than the solubility limit of 4.4 Li/Sn. It is unlikely that the extra capacity in SnNy is due to the breakdown of the Li3N since there is no reaction observed at the conversion potential after the first cycle. (Though the presence of peak Y in the CV data of Figures 3-4 does imply that Li is extracted in two discrete steps from SnNy.) The Li3N matrix may allow some additional, reversible storage either on the surface of Sn particles or within itself. It seems more likely that the additional capacity of the SnNy anodes is due to one of these additional storage mechanisms. Furthermore, the weak bonding in Li3N may give this matrix more elastic qualities than the Li2O, and so the volume expansion of Sn particles during cycling can be more reversibly accommodated. Additional studies on the mechanical properties of the Li3N may provide insight into the improved cycle stability of the SnNy.

Conclusions Thin films of SnNy and SnOxNy were grown by ALD with a tunable N/O ratio. Overall, varying the composition has little or no impact on the crystallinity, density, or growth rate using these process parameters. Using plasma-excited nitrogen (pN2) requires long exposure times to break the precursor ligands at the growth surface, but unintentional C and O contamination is generally low. Control over the N/O ratio is afforded by adjusting the duration of the H2O pulse as well as the number of nitride growth cycles between H2O exposure.



The initial gravimetric capacity and first cycle capacity loss of the tin oxynitride anodes studied here do not depend substantially on the N/O ratio. This is surprising, as the SnNy anodes produced here had an overabundance of N, which should lead to larger first cycle capacity losses than the SnO2. However, it is clear that the Sn anodes containing more N are superior in their long term cycle performance and rate capabilities. The SnNy and SnOxNy anodes behave very similarly in their capacity and capacity retention, although the oxynitride anode presented here contains significant amounts (30 at%) of O. It should therefore be expected that low levels of O contamination in SnNy thin films would not affect the electrochemical performance.

Methods Atomic Layer Deposition Thin films of SnO2, SnNy, and SnOxNy were grown on stainless steel and Si substrates in a Cambridge Nanotech Fiji F200 ALD reactor with a base pressure of ~30 mTorr. Films were typically grown to 30 nm thick at a substrate temperature of 200 °C, with a chamber working pressure of 100 mTorr under Ar carrier gas. TDMAS was used as the Sn precursor with remote pN2 for SnNy growth and H2O for SnO2. SnOxNy films were grown using the SnNy process and periodically pulsing H2O into the reactor after several cycles, following a similar process for TiOxNy reported in the literature.49 SnO2 films were grown using previously optimized parameters based on other reports,21–23,27 by alternating pulses of TDMAS and H2O. Film Characterization During the characterization of the SnNy ALD process, the substrate temperature, TDMAS pulse time, and pN2 exposure duration were all varied to obtain parameters for an optimized GPC. For this optimization, the substrate temperature was first fixed at 200 °C and the TDMAS and pN2 exposure times were varied independently (pN2 duration was fixed at 20 s while testing the TDMAS, and then the TDMAS pulse time was fixed at 0.5 s). Substrate temperature was then varied with the TDMAS and pN2 exposures fixed at 0.5 s and 20 s, respectively.


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Film thickness was monitored in situ by SE (JA Wollam M-2000D) and measured after deposition by both SE and XRR (Panalytical MRD X’Pert Pro), the latter also providing film density and roughness estimates. A single Tauc-Lorentz style oscillator was used to model all the SE data. In fitting the XRR data, the films were assumed to be composed of either stoichiometric Sn3N4, SnO2, or SnON. The fitted values for film density and thickness were found to be largely independent of the exact composition, due to the overwhelming weight percentage of Sn in all films and the similarities in the scattering of C, N, and O. Composition was measured by XPS (Kratos Axis Ultra) without exposing the samples to air, and film crystallinity was measured by XRD before and after heating to 800 °C in a flowing Ar atmosphere. Electrochemical Testing Battery testing was done using a Biologic MPG-2 for coin cells and a Biologic VS200 for solid state cells. Coin cells were constructed using Li foil, tin oxynitride ALD films deposited on stainless steel disks, and 1 M LiClO4 in EC, DMC 1:1 by volume as the electrolyte. An array of SSBs (as illustrated in Figure 3a) was fabricated on Si substrates by first depositing an 80 nm Pt current collector by e-beam evaporation, followed by the ALD tin oxynitride film. A 500 nm thick film of LiPON was deposited by magnetron sputter deposition at Sandia National Lab. Finally, an array of Li dots (1 mm diameter, 2 mm spacing) was deposited by thermal evaporation through a steel mask. Samples were exposed to air between the ALD tin oxynitride and LiPON depositions. One corner of the sample was masked during the tin oxynitride and LiPON depositions, allowing electrical contact to the Pt layer which acted as a common anode current collector for the array of discrete batteries. To normalize the currents and capacities measured for the thin films, the active thin film mass was estimated by weighing several samples of the films deposited on stainless steel disks. In the solid state cells, film thicknesses were taken from fitting the XRR oscillations, and the active area of the anodes was assumed to be defined by the area of the Li pad deposited on top.



Supporting Information Additional figures referenced in the text on the film growth rate and XPS, XRD, and AFM data are available online in the Supporting Information.

Author Information * email: [email protected]

Author Contributions This work was originally conceived by DM Stewart, KE Gregorczyk, GW Rubloff, and SB Lee. DM Stewart lead the project, developed the ALD processes, performed electrochemical measurements on the solid state cells with the help of AJ Pearse, and composed this manuscript. NS Kim assembled and performed electrochemical measurements on the coin cells. EJ Fuller and AA Talin grew the LiPON electrolyte layer for the solid state cells. All authors contributed intellectually to the interpretation of the data.

Funding Sources Acknowledgements Atomic force microscopy images were collected by Angelique Jarry, at the University of Maryland. This work was wholly supported by Nanostructure for Electrical Energy Storage (NEES) II, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (award no. DESC0001160). Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.


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Figure Captions Figure 1. (a) Reactor pressure and film thickness during growth of SnNy thin film, measured by in situ SE, showing discrete ALD pulses, and step wise growth. Dashed lines indicate beginning of each precursor pulse. (b) GPC saturation curves for TDMAS and pN2 pulse times. (c) GPC and film density versus growth temperature, using optimized pulse times from (b). (d) SnNy film composition versus growth temperature, neglecting surface C and O components (See Supplemental Figure S2). Figure 2. (a) Schematic diagram of the SnOxNy ALD processes. (b) Reactor pressure and film thickness versus time during a SnOxNy film growth. (c) Film composition with varying N/O cycle ratio deposited at 200 °C, neglecting surface O and C components (see Supplemental Figure S2). (d) GPC and film density versus N/O cycle ratio. Figure 3. Electrochemical data from coin cells using liquid electrolyte. (a) First cycle CV of three tin oxynitride films of different compositions in liquid electrolyte cycled at 0.1 mV/s from 0.05-2 V vs Li/Li+. (b) GV of the same films done at 100 mA/g, for the first 1.5 cycles. (c-e) First (thick), second (dashed), and seventh cycle (thin) CV data for (c) SnO2, (d) SnOxNy, and (e) SnNy anodes. Figure 4. (a) Diagram of sample set up for electrochemical testing of solid state half cells. (b-d) CV at 1 mV/s from 0.2-2 V vs Li/Li+ of the first (thick), second (dashed) and twentieth (thin) cycles for (b) SnO2 (c) SnOxNy and (d) SnNy half cells. Note that in b-d the first cycle conversion reactions are very deep and so have been cut off for overall graph clarity. Figure 5. (a) Anode charge capacity (solid) and CE (dashed) versus cycle number for 200 cycles at a constant current of 100 µA/cm2. (b-d) Several cell voltage profiles during galvanostatic cycling of (b) SnO2, (c) SnOxNy, and (d) SnNy half cells, where thick black curves are the first discharge and thin curves are subsequent charge/discharge cycles as labeled. Figure 6. Anode discharge capacity for different current densities between 50-10,000 µA/cm2 after the cells were stabilized by cycling 100 times at 100 µA/cm2.


mTorr Thickness (Å)

9.5

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0.6 0.5 0.4 0.3 0.2 0.1 0.0

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C O

80 Atomic %

500

60

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40

3

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0.13 0.12 0.11 10.0

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 c) 15 16 17 18 19 20 21 22 23 24 25 26 27

b)Materials Chemistry of 0

N2 plasma


TDMAS

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20

4.0 0 ACS Paragon Plus Environment 100 150 200 250 300 350 100 150 200 250 300 350 GrowthTtemperature (°C) Growth Temperature (°C)

Sn

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b)Materials Chemistry of mTorr

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Sn 1

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50

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I (A g )

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1.5

Y

Sn Oxidation

0.0 SnO2

-0.5

1st discharge 1st charge 2nd discharge

2. Alloying 0.0

1. Conversion

0.5

1.0

1.0

0.5

SnOxNy

SnNy -1.0

SnN y

2

Sn O 0.5

Sn ON x

y

3. Dealloying

+

-1

I (A g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 c) 16 17 18 19 20 21 22 23 24 25 26 27 28 29

1.0

V vs Li/Li (V)

a)

SnOxNy

1.5

2.0

0.0

0.4

+

0.8

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1.6

SnO2

SnOxNy

3

Capacity (A h g ) e) 3

3 O1

1.2 -1

V vs Li/Li (V)

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SnNy

SnO2

0.0

SnNy

Y

O2

0.0 1

1

-0.5 2

2 1st

0.0

0.5

2nd 1.0

1.5 +

V vs Li/Li (V)

1

2

-1.0 7th 1st 2nd 7th ACS Paragon Plus Environment 2.0 0.0 0.5 1.0 1.5 2.0 +

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I (A g )

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SnO2

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5

O1

O2

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V vs Li/Li (V) d) 1st

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2nd

20th SnOxNy

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SnNy

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3 5

5 -1

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I (A g )

-1

I (A g )

1 2 3 4 5 6 7 8 9 10 11 12 c) 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Chemistry ofb) Materials 10

Y O1

1 2

Y

0 -5 -10

2 2.0 -15 ACS Paragon Plus Environment 1.5 2.0 0.5

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400 SnOxNy 200

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SnO2 96

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