Proton Bulk Diffusion in Cubic Li7La3Zr2O12 Garnets as Probed by

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Proton Bulk Diffusion in Cubic LiLaZrO Garnets as Probed by Single X-Ray Diffraction Caroline Hiebl, David Young, Reinhard Wagner, H. Martin R. Wilkening, Günther J. Redhammer, and Daniel Rettenwander J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10694 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

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The Journal of Physical Chemistry

Proton Bulk Diffusion in Cubic Li7La3Zr2O12 Garnets as Probed by Single X-Ray Diffraction C. Hiebl§, D. Young#, R. Wagner|, H. M. R. Wilkening§ G. J. Redhammer|*, D. Rettenwander§* §Graz

University of Technology, Institute for Chemistry and Technology of Materials, and Christian Doppler Laboratory for Lithium Batteries, 8010 Graz, Austria. #Department

|University

of Materials Science and Engineering, Massachusetts Institute of Technology, 02139, Cambridge, MA, USA.

of Salzburg, Department of Chemistry and Physics of Materials, 5020 Salzburg, Austria.

ABSTRACT: Ceramic electrolytes, characterized by a very high ionic conductivity as it is the case for Al-stabilized cubic Li7La3Zr2O12 (Al:LLZO), are of utmost interest to develop next-generation batteries that can efficiently store electrical energy from renewable sources. If envisaged not as solid electrolyte but as protecting layer in lithium metal batteries with liquid electrolytes, the ceramic should allow Li+ to pass through but block out other species such as H+. Protons, e.g., originating from the decomposition of electrolyte solvent molecules, will form detrimental LiH that severely affects the performance and lifetime of such batteries. Although Li-ion dynamics in Al:LLZO has been the topic of many studies, until today little information is available about macroscopic proton diffusion in LLZO. Here, we used single-crystal X-ray diffraction to study the Li+/H+ exchange rate in AL:LLZO over a period of about 3 years. Rietveld refinements reveal that H solely exchanges on the 96h site. The Li:H portion significantly changes from the anhydrous pristine sample to Li4.21:H0.66 after 17 days of altering in humid air and finally to Li2.55:H2.32 after 960 days. Considering the change of the Li:H portion and the probing depth of X-rays into Al:LLZO, we applied a spherical diffusion model to estimate the proton diffusion coefficient of D0 ≈ 10−17 m2 s−1. Such a proton diffusion coefficient value is sufficiently high to have significant impact on cell performance and safety if Al:LLZO is going to be used to protect the Li-metal anode from reaction with the liquid electrolyte. In particular, during Li plating such a high H+ penetration rate may accelerate the formation of LiH giving rise to safety problems of these types of batteries.



Introduction

New energy storage devices are needed to accelerate the electrification of our society and lower fossil fuel usage. Providing Li-ion batteries with sufficiently high energy density can enable applications such as long-range electric vehicles. Since Li-metal has the highest theoretical capacity (larger than 3,000 mAh g-1) for Li-ion battery anodes and a very low redox potential (− 3.040 V versus the standard hydrogen electrode), it has been widely recognized as an ideal component in next-generation Liion batteries.1,2 Despite of several decades of research, there are still many bottlenecks to overcome such as (i) controlling the irreversible formation of the solid electrolyte interface (SEI) because of the thermodynamic instability of liquid electrolytes against Li metal and (ii) supressing the formation of Li dendrites.3 Recently, Zachman et al. observed the existence of two distinct deposit structures of Li metal: type-I and type-II dendrites. Type-I dendrites are roughly 5 µm across with low curvature and an extended SEI, whereas type-II dendrites are hundreds of nanometers thick and tortuous. They consist of lithium hydride instead of Li metal, and may contribute disproportionately to the loss of battery capacity.4 The hydrogen required for the formation of LiH originates not only from the reduction of

water impurities in the non-aqueous electrolyte but also from the decomposition of electrolyte solvent molecules. This decomposition process strongly depends on the cell voltage.5 To overcome these limitations, solid (ceramic) electrolytes are proposed to protect the Li metal anode by providing a physical barrier against the components of the liquid electrolytes and impurities.6 One of the most promising inorganic electrolytes, which has attracted a worldwide interest due to its high ionic conductivity and electrochemical stability, is Alstabilized cubic Li7La3Zr2O12 (Al:LLZO) garnet (see Figure 1A for structural details). Al:LLZO offers a very high Li+ conductivity and is electrochemically stable against Li metal. Furthermore, Li-bearing garnets attracted great attention because of their relative stability in the presence of humidity and air.7 Although garnets are structurally stable in aqueous solution, significant Li-H exchange can occur.8,9 Uhlenbruck et al. showed that even traces of water, e.g., when LLZO is stored in an argon-filled glovebox, lead to Li+/H+exchange.10 In particular, this finding is important to commercial applications in solidstate Li-ion batteries, which then require special handling conditions during manufacturing of garnet-type electrolytes. In cases when protons contribute to the charge carrier transport during battery operation, type-II dendrites could

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even be formed despite the Li metal being protected by a solid electrolyte.

Figure 1. (A) The crystal structure of cubic Li7La3Zr2O12 garnets (space group: Ia-3d). O2- ions occupy the general crystallographic position 96h located at the polyhedral corners, not shown. This arrangement forms a O2- framework with interstices occupied by La3+ at the eight-fold coordinated position 24c (A-site) and by Zr4+ at the six-fold coordinated position 16a (B-site). Li+ reside on the four-fold coordinated 24d position (C-site) and on the six-fold coordinated 96h site The latter is a split site of the 48g position forming a 3-D pathway leading to high Li-ion diffusivity. (B) SEM pictures of the single crystal glued on a glass capillary and stored in humidified atmosphere for 960 days. (C) Change of the lattice parameter over time. (D) Change of the occupation number and site distribution of Li in Al:LLZO as a function of time. (E) Change of the occupation number of H in Al:LLZO over time. Solid line represents the fit based on the spherical diffusion model used herein to calculate the macroscopic proton diffusion coefficient D0. Black symbols corresponds to sample 1, and the blue symbols to samples 2 and 3 (see SI).

In order to evaluate the ability of H+ to penetrate through LLZO, we performed a long-time single-crystal Xray diffraction (SCXRD) study over a time period of 3 years on Al:LLZO, which was stored in humidified atmosphere. A spherical diffusion model was developed to describe the Li+/H+-exchange kinetics. The model enabled us to estimate a macroscopic proton diffusion coefficient D0 at room temperature. Our results provide critical information on the ability of Al:LLZO to protect Li metal against the detrimental effect of protons.



Experimental

Synthesis. The single crystals used herein were prepared by a solid-state ceramic sintering route as described in detail in previous studies.11,12 Single crystals were selected from the freshly prepared sintered ceramic pellets. Single-crystal X-ray diffraction data were collected on a Bruker SMART APEX CCD-diffractometer. Intensity data were collected with graphite-monochromatized Mo K α X-radiation (50 kV, 30 mA); the crystal-to-detector

distance was 30 mm and the detector positioned at − 30° and − 50° 2Θ using an ω-scan mode strategy at four different ϕ positions (0°, 90°, 180° and 270°) for each 2Θ position. 630 frames with ω = 0.3° were acquired for each run. With this strategy, data in a large Q-range up to minimum d values of d = 0.53 Å could be acquired. This is necessary for accurate determination of anisotropic displacement parameters and to reduce correlation effects between atomic displacement parameters and site occupation numbers. Three-dimensional data sets were integrated and corrected for Lorentz-, polarization and background effects using the APEX2 software.13 Structure solution (using direct methods) and subsequent weighted full-matrix least-squares refinements on F2 were done with SHELX-2012 as implemented in the program suite WinGX 2014.14,15


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The Journal of Physical Chemistry



Results & Discussion

For the (SCXRD) measurements suitable crystals, selected on the basis of their optical properties, i.e., regular shape and homogeneity in colour, were glued on top of glass capillaries (0.2 mm in diameter, see Figure 1B). Excess glue was carefully removed from the surface with acetone to maximize the amount of clean surface on the crystal open to air exposure. After a first diffraction measurement, 6 h after synthesis, the single crystal was stored without removing it from the capillary in a desiccator. Instead of the drying agent, the bottom of the desiccator was filled with water to saturate the environmental humidity at 25 °C over the whole period of observation. Diffraction measurements were performed on the Al:LLZO single crystals at intervals of 17, 189, 575, and 963 days after the initial measurement (sample 1). Additionally, some data from preliminary studies on other Al:LLZO single crystals are shown for comparison (see sample 2 and 3; cf. SI for detailed structural information on fractional atomic coordinates, anisotropic atomic displacement parameters and selected bond length for all samples summarized in Table S1 – S3). All measurements on these crystals over time reveal centric cubic space group symmetry 𝐼𝑎3𝑑. Over the course of this study no evidence for a change in space group was observed. The initial lattice parameter of pristine Al:LLZO, determined shortly after finishing the synthesis, is close to that reported by Wagner et al.16 As shown in Figure 1C, the lattice parameter increases significantly with exposure to humidity before approaching a steady state at long exposure times. The lattice parameter of 12.9746(2) Å, measured after 963 days, is close to the steady state. The initially strong expansion in lattice parameters suggests Li+/H+ exchange even at short exposure times to humidity. A similar observation has been reported by Ma et al.14 As is clearly visible in Figure 1B, additional evidence for the ongoing exchange of Li by H is given by the formation of a secondary surface phase. Under the humid condition, most likely, Li2CO3 is formed by the reaction of the surface Li with moisture in air to form LiOH, which subsequently absorbs CO2 to form Li2CO3.17 The Al3+ content, previously shown to be solely hosted in the 24d position, was fixed during refinement to the value of 0.216 per formula unit (pfu), as determined by energy dispersive X-ray spectroscopy (EDXS).11,12 The amount of Li located at the 24d or 96h position was allowed to vary, which revealed evidence of a Li vacancy content of 45 % and 59 % in the 24d and 96h positions of the pristine sample, respectively. The refinement is in excellent agreement with the ones given by Wagner et al., except for their smaller unit cell dimension.12 This difference can be explained by the fact that their samples has been stored in air before their characterized and therefore Li+/H+ exchange already took place. Assuming that the Al content in the 24d site does not change, it is observed that the remaining electron density at the 24d site increases, while that at the 96h site distinctly decreases. These changes can be interpreted as a migration of Li from the 96h to the 24d position. This change corresponds to 0.2 pfu during the

first 200 days. No significant change was observed afterwards. Modelling the electron density at the 96h site with Li only, the Li content decreases from ~4.9 pfu to 3.2 pfu. The decrease of the Li occupation number in the 96h site in excess of the increase in the 24d site occupancy can be interpreted as a Li+/H+ exchange. In order to analyze the Li+/H exchange the following assumptions were applied. (i) The Al content at 24d was again fixed to the value obtained from EDX, and the remaining electron density was modelled with Li+ only as no change in scattering power over time was observed. (ii) The amount of vacancies at 96h was assumed to be the same as in the virgin sample. Moreover, the electron density was modelled as a mixture of Li+ and H+, and a fixed amount of vacancies of 59 %. Finally, according to the refinement we estimate that, according to the above-mentioned assumptions, up to 2.31 formula units H+ are located within the 96h position. Worth noting, this change in occupation numbers best explains the changes in total electron density. Our findings support results from earlier studies suggesting that an extensive Li+- H+ exchange leads to a decrease of the mobile Li ions on the 96h position. This decrease in occupation number is also associated with a strong decrease in ionic conductivity.8,16,18 Based on our analysis, the occupation of Li and H in the 96h position significantly changes from the anhydrous pristine sample to Li4.21H0.66 (17 days in humid air) and finally to Li2.55H2.32 (960 days), see Figure 1D and Figure 1E. By considering the evolution of H content over a certain length of time and approximating the single crystal as spherical, a diffusion coefficient D0 can be estimated based on a spherical diffusion model (Eq. 1).19 𝐶 ― 𝐶1 𝐶0 ― 𝐶1

=1+

2𝑎

―1𝑛

𝜋

𝑛

∑∞ 𝑛=1

𝑛𝜋𝑟 ― 𝐷 𝑛2𝜋2𝑡/𝑎2 𝑜 𝑎 𝑒

𝑠𝑖𝑛

(1)

Here, C is C(r, t), the concentration of the species of interest (H+ for reasons explained below), r is a radial location in the sphere, t is time, and a denotes the radius of the sphere. C0 is the concentration at the surface assumed to be at some constant concentration, whereas the initial H concentration of the sphere C1 is assumed to be zero throughout. An expansion out to 100 terms of the summation was applied to numerically calculate C. C was then integrated over the volume of the sphere with a = 75 µm corresponding to the approximate average dimension of the LLZO crystal. Eq. 1 is then numerically fitted to the data shown in Figure 1E to derive both C0 and D0. The resulting fit aligns with our data very well with a mean square root error of 0.93. Based on our calculations, we obtained a C0 of 0.20 formula units and D0 turned out to be of the order of 2 × 10-17 m2/s. This value is in reasonable agreement with results from Kilner and co-workers (10−16 m2 s−1) who measured the chemical diffusion coefficient of H+/Li+ exchanged Gastabilized LLZO samples. The samples had been immersed in water at 100 °C and were then analyzed using focused ion beam secondary ion mass spectrometry.20 The authors observed that H+ extended as far as 1.35 μm into the



Li6.55Ga0.15La3Zr2O12 garnet pellet after treatment for 30 min in H2O. This treatment significantly affected ion dynamics in Ga:LLZO. The authors report on a decrease of the bulk ionic conductivity by an order of magnitude. Here, the proton bulk diffusion coefficient D0 is within an order magnitude comparable to the bulk Li+ diffusion coefficient of tetragonal LLZO. Compared to Li+ diffusivity in cubic LLZO:Al (D0(Li+) ~ 10−13 m2 s−1), D0(H+) is by several orders of magnitude lower.21,22 Therefore, in this case, we can safely that H+ diffuses much slower than Li+ in LLZO. When LLZO is used as a Li metal protecting layer (see Figure 2A), it is important to be aware of the time t it takes for H to penetrate through LLZO and to reach the Li metal.

 ASSOCIATED CONTENT Supporting Information. Fractional atomic coordinates, anisotropic atomic displacement parameters and selected bond lengths.

 AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

 ACKNOWLEDGMENT The research was supported by the Austrian Science Fund (FWF) project number (P31437-N36).

REFERENCES 1. 2.

Figure 2. (A) Illustration of the anode side of a Li metal battery using a protecting layer to avoid contact between Li metal and the liquid electrolyte. (B) Penetration depth of H into LLZO over time.

For a given protecting layer thickness x, t can be calculated according to Eq. 2. 𝑥 = 2 𝐷0𝑡 (2) As an example, assuming a 100 nm thick LLZO layer the time for H to reach the Li metal side turns out to be within minutes (see Figure 2B for illustration). This value refers to a situation without any externally applied electric force. The results herein suggest that the degradation of the Li metal anode is potentially not avoidable by LLZO-based protecting layers. In particular, during Li plating the H penetration rate may accelerate LiH formation leading to high interfacial resistances, significant capacity loss, and type-II dendrites formation.4



Conclusion

We have applied single-crystal X-ray diffraction to study the nature of Li+/H+ exchange in Al-stabilized LLZO. Based on the H+ concentration change over time, a spherical diffusion model was used to estimate the proton diffusion coefficient to be of the order of 2 × 10-17 m2/s. This diffusion coefficient is high enough to have significant impact on cell performance and safety when LLZO is used as a protecting layer for Li metal anodes in batteries relying on liquid electrolytes.

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Proton Bulk Diffusion in Cubic Li7La3Zr2O12 Garnets as Probed by

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