Mechanistic Study of Lithium Aluminum Oxide Atomic Layer

Dec 20, 2012 - ... Jeffrey W. Elam , Xiangbo Meng , Anthony K. Burrell , Chunmei Ban ... Daniel Membreno , Nicolas Cirigliano , Bruce Dunn , Jane P. C...
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Mechanistic Study of Lithium Aluminum Oxide Atomic Layer Deposition David J. Comstock and Jeffrey W. Elam* Energy Systems Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ABSTRACT: There is significant interest in developing lithium conductive thin films that have potential applications as lithium-permeable electrode barrier coatings and as solid electrolytes in thin film batteries. In this work, we demonstrate the atomic layer deposition (ALD) of lithium aluminum oxide (LiAlOx) thin films and provide a thorough characterization of the growth mechanism. LiAlOx thin films were deposited by combining the ALD processes for Al2O3 (trimethylaluminum and water) and LiOH (lithium tert-butoxide and water). The composition of the films was controlled by adjusting the percentage of LiOH cycles. Both the pure LiOH process and the combined LiAlOx process were characterized by a combination of quartz crystal microbalance, Fourier transform infrared spectroscopy, and film deposition studies. These studies revealed a complex growth mechanism that is strongly affected by the hygroscopic and reactive LiOH component. Stable ALD with a constant growth rate as a function of ALD cycles was only achieved at ≤50% LiOH cycles. Within this stable regime, a maximum Li cation percentage of 55% and a growth rate of 1.5 Å/cycle were observed. LiAlOx films with >50% LiOH cycles exhibited greater Li cation percentages and stable growth only for the initial 20−30 cycles. This narrow window of stable LiAlOx ALD may restrict the deployment of this process in battery applications.

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thin films of the solid electrolyte lithium phosphorus oxynitride (LiPON) have been shown to stabilize LiCoO2 cathodes.9−11 However, high quality LiPON thin films are typically prepared by sputtering, which is not well-suited for uniform deposition onto nonplanar surfaces such as electrode particles. Alternatively, all solid-state batteries provide improved stability and safety by eliminating liquid electrolytes entirely but typically exhibit low rate capabilities due to slow solid-state diffusion and reduced capacities due to planar geometries. These problems can be resolved by moving to threedimensional architectures, which enable smaller diffusion lengths and increased areal capacities. A variety of threedimensional architectures have been proposed for such solidstate batteries.12,13 However, a primary obstacle to implementing these architectures remains the lack of conformal deposition strategies for LIB materials, including lithium solid electrolytes. The limited conformality afforded by LiPON sputtering is a clear motivation for developing ALD processes for lithium solid electrolyte materials, and there has been much recent work toward developing these materials. For example, LiOH and Li2CO3 are the most common sources for solid-state synthesis of lithium-containing materials and both have been demonstrated by ALD. LiOH ALD has been demonstrated using lithium tert-butoxide (LiOtBu) and H2O,14 while Li2CO3 ALD has been demonstrated using LiN(SiMe3)2−H2O−CO2,15 LiOtBu−H2O−CO2,14 and Li(thd)−ozone.16 Additionally,

INTRODUCTION Significant effort has focused recently on developing atomic layer deposition (ALD) processes for lithium ion battery (LIB) applications.1,2 ALD is a thin film deposition method that uses iterative, self-saturating precursor exposures to deposit films in a monolayer-by-monolayer fashion. As a result, ALD enables thin film growth with precise thickness and composition control, excellent uniformity over large substrate dimensions, and conformal coverage of complex, three-dimensional structures. With these attributes in mind, ALD is particularly well-suited for two distinct LIB applications: protective barrier coatings on LIB electrodes and thin film lithium solid electrolytes for all-solid-state LIBs. Both of these applications require thin, conformal, lithium-conductive materials deposited over complex, three-dimensional structures, and ALD is an attractive technology for achieving this result. Electrode barrier coatings serve to stabilize electrode surfaces and prevent deleterious electrode/electrolyte interactions and have long been shown to improve capacity retention within LIBs.3 More recently, thin ALD Al2O3 coatings have been demonstrated as effective barrier coatings to improve the capacity retention of both cathodes4−6 and anodes.7,8 However, due to its poor lithium conductivity, there is a trade-off with regards to the Al2O3 thickness. Thicker Al2O3 coatings provide enhanced protection and stability but also impede Li transport into and out of the electrode particles. Thinner Al2O3 coatings will have a higher Li conductivity, but the protection will be minimal. In contrast to the insulating Al2O3, a lithium solid electrolyte film would allow a thicker, more stable barrier coating without sacrificing Li conductivity. With this in mind, © 2012 American Chemical Society

Received: September 5, 2012 Revised: December 11, 2012 Published: December 20, 2012 1677

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deposition onto the windows. Samples for FTIR were prepared by pressing ZrO2 nanopowder (50% LiOH compositions, the Li content was difficult to estimate from the QCM data because of the H2O physisorption. As a result, to estimate Li content, we evaluated just the mass gains associated with TMA and LiOtBu adsorption. For example, at 80% LiOH cycles, the LiOtBu/ (LiOtBu+TMA) percentage was 81% over the first 15 cycles and decreased to 48% over cycles 75−90. This suggests that for thicker films, increasing the cycle ratio of LiOH actually decreases the Li cation percentage, as more Al2O3 is deposited with each cycle because of its reaction with LiOH and less Li is deposited because of the poor LiOtBu adsorption on LiOH. Consequently, for thick films, the maximum Li cation percentage of 55% is achieved using 50% LiOH cycles. Lastly, the morphology and conformality of the ALD LiAlOx thin films were assessed by SEM imaging. A LiAlOx film was deposited using 600 ALD cycles with a 50% LiOH cycle ratio onto a micromachined Si trench wafer with 1.2 μm wide by 13 μm deep structures. As shown in Figure 9, the LiAlOx films were smooth and coated the entire trench structure. The film was 92.6 nm thick at the top of the trench, which corresponds to a growth rate of 1.54 Å/cycle and matched well with the growth rate measured by ellipsometry. The film was thinner at the bottom of the trench relative to the top of the trench. This variation likely results from the low vapor pressure of LiOtBu relative to TMA that makes it more difficult to infiltrate high aspect ratio structures with the LiOH component. It is clear that there is still some Li content at the bottom of the trench though, as the 66.6 nm film is much thicker than the 33 nm thickness expected for 300 ALD Al2O3 cycles alone. Longer LiOtBu exposures or a higher LiOtBu bubbler temperature should alleviate this problem and enable more conformal deposition on such structures.

remains constant into the growth regime (Figure 5b), and this explains why the mass uptake rate remains constant for this composition in Figure 4. In contrast, the QCM step shapes for both the Al2O3 and LiOH components using 50% LiOH cycles change between the nucleation and growth regimes (Figure 5d). A small mass gain with the H2O following TMA appears, which is not observed in the nucleation regime, and there is reduced mass loss with the H2O following LiOtBu, suggesting that the films have become slightly hygroscopic. However, the 50% LiOH films never exhibit the very large mass gains with H2O exposure that are characteristic of the pure LiOH ALD (Figure 1c). Lastly, at 80% LiOH, the QCM step shapes in the growth regime (Figure 5f) deviate substantially from those in the nucleation regime. The average mass gain per cycle becomes much larger in the growth regime because of two factors. First, there is enhanced mass gain with TMA exposure, due to its increased reaction with both physisorbed H2O and deposited LiOH, and second, there is a significant mass gain during the H2O exposures following both TMA and LiOtBu, which is indicative of significant H2O physisorption. As a result, the majority of the increased mass gain observed at 80% LiOH cycles is due to increased Al deposition and increased H2O retention. Ionic conductivity increases with Li content in LiAlOx,21 so higher Li concentrations are desirable for applications in barrier films and solid electrolytes. Unfortunately, the hygroscopic nature of the ALD LiAlOx films prepared using >50% LiOH cycles enhances the Al incorporation and reduces the Li concentration. We evaluated two strategies to mitigate this problem: longer TMA exposures and longer purges following the H2O exposures. Longer TMA exposures were expected to react with excess H2O and passivate the LiOH component within the film, while longer purges were expected to allow H2O desorption. As shown in Figure 6, neither strategy was very effective. Longer TMA exposures resulted in greater mass gain, indicating even greater Al2O3 deposition due to TMA reactions with H2O and LiOH but did not prevent the LiOHdominated hygroscopic growth mechanism at greater LiAlOx ALD cycles. Increasing the H2O purge times from 5 to 20 s reduced the overall mass gain per cycle; however, this was mostly because of desorption of water content and, again, the growth mechanism was not changed at greater LiAlOx ALD cycles. Following the in situ QCM studies, LiAlOx films were deposited onto Si(100) substrates using LiOtBu−H2O and TMA−H2O timings of 5−5−2−5 and 1−5−1−5, respectively, and characterized using ex situ measurements. As shown in Figure 7, the growth rates determined by spectroscopic ellipsometry are consistent with the QCM results. Below 50% LiOH cycles, the growth rate remained constant versus the number of ALD cycles. The growth rate achieved at 50% LiOH was 1.5 Å/cycle, which compares well with the previous results for this composition.20 Moreover, the film thickness was relatively uniform along the length of the 30 cm ALD reactor. At >50% LiOH cycles, the LiAlOx growth rate increased abruptly because of the increasingly hygroscopic nature of the films and stable ALD could not be achieved. The growth rate was no longer linear as a function of ALD cycles, and there was significant variation in the growth rate along the reactor length. We attribute this instability to the time-dependent desorption of H2O from these films. The composition of the LiAlOx thin films, defined by the Li cation percentage, was determined from ICP-MS and QCM

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CONCLUSIONS We have presented a thorough characterization of LiOH ALD using LiOtBu and H2O and have demonstrated its combination with Al2O3 to prepare LiAlOx thin films. Pure LiOH ALD exhibits distinct nucleation and growth regimes, with the 1682

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growth regime characterized by significant H2O physisorption to the hygroscopic LiOH. This physisorbed H2O, as well as the reactivity of the deposited LiOH toward TMA, significantly complicate the LiAlOx ALD. Successful LiAlOx ALD is highly dependent upon the percentage of LiOH ALD cycles. As a result, stable LiAlOx ALD and thick films could only be achieved using ≤50% LiOH cycles, which yielded a maximum Li cation percentage of 55%. However, by staying within the nucleation regime, thinner films with Li cation percentages as great as 82% have been demonstrated.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Center for Electrical Energy Storage: Tailored Interfaces, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Electron microscopy was performed at the Electron Microscopy Center for Materials Research (EMCMR) at Argonne National Laboratory. Use of the EMCMR was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357 operated by UChicago Argonne, LLC.

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