Crystal Orientation-Dependent Reactivity of Oxide Surfaces in Contact

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Surfaces, Interfaces, and Applications

Crystal Orientation-Dependent Reactivity of Oxide Surfaces in Contact with Lithium Metal Justin G. Connell, Yisi Zhu, Peter Zapol, Sanja Tepavcevic, Asma Sharafi, Jeff Sakamoto, Larry A Curtiss, Dillon D. Fong, John W Freeland, and Nenad M. Markovic ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03078 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Applied Materials & Interfaces

Crystal Orientation-Dependent Reactivity of Oxide Surfaces in Contact with Lithium Metal Justin G. Connell†,‡,*, Yisi Zhu†, Peter Zapol†, Sanja Tepavcevic†, Asma Sharafi#, Jeff Sakamoto#, Larry A. Curtiss†, § Dillon D. Fong†, John W. Freeland , Nenad M. Markovic† †

Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Joint Center for Energy Storage Research, Argonne National Laboratory, Argonne, IL 60439, USA # Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, USA § X-ray Sciences Division, Argonne National Laboratory, Argonne, IL 60439, USA ‡

* Email: [email protected]

Abstract Understanding ionic transport across interfaces between dissimilar materials and the intrinsic chemical stability of such interfaces is a fundamental challenge spanning many disciplines, and is of particular importance for designing conductive and stable solid electrolytes for solid-state Li-ion batteries. In this work, we establish a surface sciencebased approach for assessing the intrinsic stability of oxide materials in contact with Li metal. Through a combination of experimental and computational insights, using Nb-doped SrTiO3 (Nb:STO) single crystals as a model system, we were able to understand the impact of crystallographic orientation and surface morphology on the extent of the chemical reactions that take place between surface Nb, Ti and Sr upon reaction with Li. By expanding our approach to investigate the intrinsic stability of the technologically-relevant, polycrystalline Nb-doped lithium lanthanum zirconium oxide (Li6.5La3Zr1.5Nb0.5O12) system, we found that this material reacts with Li metal through the reduction of Nb, similar to that observed for Nb:STO. These results clearly demonstrate the feasibility of our approach to assess the intrinsic (in)stability of oxide materials for solid state batteries, and points to new strategies for understanding the performance of such systems.

Keywords: Solid electrolytes, solid-state batteries, Li metal, buried interface, reactivity, model system, surface science

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Introduction Understanding ionic transport across interfaces that are formed between dissimilar materials, as well as the intrinsic chemical stability of such interfaces, is a fundamental challenge that spans disciplines from solid-state physics to materials chemistry and electrochemistry. Interfacial stability impacts a broad scope of applications including oxygen ion transport in memristors1, efficiency of solid oxide fuel cells2 and the development of new electrochemical energy storage systems that move beyond current Li-ion technologies3,4. Solid-state batteries are a particularly promising technology due to their inherently safe mode of operation5, improved energy density due to the higher voltage window enabled by solid oxide/sulfide electrolyte materials relative to conventional organic liquid electrolytes6,7, and the potential of developing new system architectures that are simply not possible with conventional Li-ion systems8. Nevertheless, significant barriers exist to implementing solid-state batteries, including modest power densities due to the low ionic conductivities of many solid-state electrolytes9 and, more importantly, significant issues with developing robust interfaces that are stable to contact with Li metal10 and capable of withstanding significant stress/strain during extensive cycling11,12. Gaining insight into the interfacial stability of solid-state battery systems is a highly challenging task. Charge/discharge testing and impedance spectroscopy provide useful information about system-level performance, but are ultimately insufficient for understanding the atomic-level processes that govern stability in solid-state batteries. Furthermore, significant ambiguities exist over the extent of surface film formation (e.g., Li2CO3) that takes place during cell assembly and sample preparation, complicating the interpretation of such analyses13. In addition to ambiguities associated with interfacial chemistry, relatively little information exists about the influence of crystallographic orientation/texture on the stability and overall performance of solid electrolyte materials. Although recent efforts have made progress in understanding the influence of grain boundary structure/chemistry on preferential Li migration and eventual cell failure due to dendrite formation along grain boundaries11,12, the fundamental stability of the individual grains remains largely unexplored. As anisotropies in ionic conductivity along various crystallographic orientations and differences in the reactivity of the various possible crystalline surface terminations will ultimately determine the intrinsic, longterm stability of any solid electrolyte material, developing deeper understanding of these properties may make it possible to design more stable and reversible solid-state electrochemical


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interfaces. By first working with well-defined, single crystal model electrodes, it will be possible to develop the atomic-/molecular-level insights necessary to understand the impact of orientation-dependent interfacial stability and transport properties on performance, as has been established in the past for solid-aqueous interfaces14–16. Acquiring such insight is complicated by the fact that many existing in situ and operando analysis techniques are limited in providing meaningful information about the chemistry of interfacial processes in solid-state batteries, as the relevant interfaces are, by the nature of the cell architecture, inaccessible during cycling. In light of these difficulties, new methodologies are needed to understand the electrochemical transformations that take place at buried interfaces in solid-state devices. Significant progress has been made in recent years towards developing spectroscopic means of directly investigating interfacial stability in vacuo, using both X-ray spectroscopic17–20 and transmission electron microscopy (TEM)-based21,22 techniques to analyze the presence or absence of interphase formation in a variety of solid electrolyte systems in contact with Li metal. The biggest advantage of such ultrahigh vacuum (UHV)-based approaches is the ability to verify the absence of interfacial films on both the solid electrolyte and Li metal prior to contact, helping to ensure that the observed interfacial chemistry is representative of the specific electrolyte material under investigation. Although such studies have significantly advanced the understanding of the intrinsic stability of solid electrolyte materials, existing methodologies still do not precisely recreate the conditions “real” materials will experience during cell fabrication and testing. For example, ion beam sputtering processes carried out near the sample surface produce highly energetic (i.e. mobile) Li species, causing transformations in solid electrolyte materials that may not otherwise occur at room temperature. Furthermore, it is unclear what role surface defects, such as those generated during Ga-ion beam thinning for TEM imaging, may play in (de)stabilizing the interface relative to conventional surface preparation techniques such as mechanical polishing. In this work, we establish a surface science-based approach for assessing the intrinsic stability of oxide materials in contact with Li metal in order to understand the impact of crystallographic orientation and surface morphology on the extent of the chemical reactions that take place. Through the use of magnetron sputtering to deposit thin films of Li metal on singlecrystalline, Nb-doped SrTiO3 (Nb:STO) substrates at room temperature, a more representative interface is created for investigating the intrinsic stability of the Li-STO system. Although not a


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solid electrolyte material itself, STO serves as an ideal model system as its surface chemistry has been extensively studied and atomically smooth, single crystal surfaces can be readily prepared with known surface termination23–26. Furthermore, Ti and Nb are well known to be unstable to reduction by Li metal27,28, enabling facile validation of our in vacuo methodology. We clearly demonstrate the ability of our approach to resolve the reduction of surface species after Li deposition, with nominally the same amount of Ti and Nb reduction on STO(001), (111) and (110) surfaces. Surprisingly, despite the similar extent of chemical reduction that takes place, significant differences exist between the morphological stability of the three surfaces. The STO(110) surface exhibits significant changes after the deposition and subsequent removal of Li metal, while the STO(001) and (111) surfaces remain relatively unperturbed. Density functional theory (DFT) calculations reveal that the observed morphological instability of STO(110) can be explained by destabilization of the surface TiOx layer due to incorporation of Li underneath this layer. Finally, we utilize the above approach to investigate the intrinsic stability of the technologically-relevant, polycrystalline Nb-doped lithium lanthanum zirconium oxide (Li6.5La3Zr1.5Nb0.5O12, LLZO) system, which is also found to react with Li metal through the reduction of Nb and a weak, but discernible, interaction with La. These results clearly demonstrate the feasibility of our approach to assess the intrinsic (in)stability of oxide materials for solid state battery systems.

Results Deposition of Li on STO(001). In order to understand the intrinsic chemistry of Li metal in contact with a model oxide surface, Li was deposited directly onto STO(hkl) surfaces and then transferred directly between the sputter and analysis chambers under ultrahigh vacuum (UHV) conditions (≤5x10-9 mbar throughout transfer; see experimental methods for more details). Central to these investigations was the preparation of well-defined, single crystal surfaces in order to identify differences in stability and reactivity based on crystallographic orientation. We begin by summarizing results on STO(001) surfaces to illustrate our general approach, and Figure 1 illustrates structural and chemical characterization of a representative STO(001) surface both before and after Li deposition. From low energy electron diffraction (LEED, Figure 1a), a 1x1 surface reconstruction is clearly resolved on the as-prepared STO(001) surface, and from atomic force microscopy (AFM, Figure 1b), the well-defined crystal terraces of HF-etched and


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annealed23 STO(001) surfaces can be easily seen. X-ray photoelectron spectroscopy (XPS) analysis of the Ti 2p, Nb 3d and Sr 3p core levels indicates that the pristine STO surface is fully oxidized, with only Ti4+, Sr2+ and Nb5+ oxidation states and minimal surface contamination by adventitious carbon (Figure 1d-e, Supporting Information (SI) Figures S1a and S2). AFM imaging of the sample after Li deposition reveals an island morphology (Figure 1c), which is unsurprising given the low temperature used to deposit Li metal (25ºC). The average island size depends on the length of the Li deposition (Figure S3), with typical samples analyzed in this work having islands ~500 nm in size. Although the film morphology shown in Figure 1c should be qualitatively similar to that of the pristine Li deposit, the island size itself is not truly representative as the sample was exposed to air during transfer to the AFM instrument. XPS analysis of the sample surface before air exposure shows significant Li intensity, with signal from the underlying STO substrate still clearly visible as well (Figure S2). After air exposure, however, XPS indicates that the surface is entirely covered by Li2CO3 (Figure S4), with signal from the underlying STO substrate no longer observable – consistent with volumetric expansion of the islands during conversion to Li2CO3 (Figure 2c). By assuming the as-deposited Li metal is entirely converted to Li2CO3 upon air exposure and using the expected volume expansion upon conversion, it is possible to put a lower bound on the average diameter of the pristine Li metal islands of ~90 nm (see SI for details). Regardless of the true size of the as-deposited islands, they are significantly larger than the escape depth of Al Kα-generated photoelectrons through Li metal (≤ 16.5 nm)29. This implies that signal from the substrate observed by XPS must come from the regions very near the edges of the Li islands or from any thinly coated or Li-free regions between islands. Nevertheless, Figure S3 clearly demonstrates that the observed extent of Ti reduction does not change significantly with island size, clearly indicating that the underlying chemistry measured with XPS is representative of the intrinsic reactivity of Li with STO. Analysis of the Li 1s core level on the as-deposited Li-STO sample surface (Figure 1d) indicates that the sputtered Li film is primarily present in the metallic state, with some of the Li also present as oxide. This suggests that a fraction of the Li metal reacts with the underlying STO substrate. Indeed, Figure 1e-f demonstrates that after Li deposition, ~38% of the total Ti signal corresponds to a mixture of 3+ and 2+ oxidation states, and 33% of the total Nb signal corresponds to the 4+ oxidation state. The oxidation state of Sr was unaffected by Li deposition (Figure S1a). These results are consistent with thermodynamic data (Figure S5) that indicate the


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reaction of Li with STO proceeds much easier via Ti reduction than Sr reduction. DFT calculations of the electronic density of states (DOS) for Li adsorbed on the STO (100) surface, discussed in detail below, indicate charge transfer to Ti 3d states, consistent with the observed reduction of Ti and Nb in XPS. The persistence of Ti4+ and Nb5+ species after Li deposition indicates incomplete reduction of the STO substrate, which can be explained by two possible factors: (i) the reaction between Li and STO is self-limiting (i.e., only the first few monolayers of STO react), and/or (ii) the spaces between Li islands on the surface are essentially Li-free, resulting in regions of unreacted STO. As noted above, the deposition of thicker and thinner Li layers does not result in significant differences in the observed extent of reaction (Figure S3), suggesting that there is not a large fraction of uncoated surface regions between Li islands and that the reaction is instead surface-limited. Angle-resolved XPS (ARXPS) of the film yields slight changes in the relative intensities of the various oxidation states of Ti with increasing sample tilt angle (Figure S6). In the most surface-sensitive case (60° sample tilt angle), 55% of the total Ti signal corresponds to Ti3+ and Ti2+, as compared with 42% and 40% with 30º tilt and no tilt, respectively. This further indicates that the reactions are surface-limited, as tilting to higher angles results in an increase in the amounts of Ti3+ and Ti2+ relative to Ti4+. As the island morphology of the Li films makes it difficult to conclusively assign a surface-limited reaction via ARXPS analysis, X-ray absorption spectroscopy (XAS) measured at grazing incidence was also performed to measure the relative amounts of both surface and bulk reduction of STO(001). This analysis also suggests that the reaction between STO and Li is surface-limited, as measurements of the Ti L-edge in total electron yield (TEY) and total fluorescence yield (TFY) modes before and after Li deposition reveal differing amounts of reduction (Figure 2a-b). The TEY spectra, which are sensitive to the top few monolayers (

Crystal Orientation-Dependent Reactivity of Oxide Surfaces in Contact

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