Enhanced Li Ion Conductivity in LiBH4−Al2O3 ... - ACS Publications

Article Cite This: J. Phys. Chem. C 2017, 121, 26209-26215

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Enhanced Li Ion Conductivity in LiBH4−Al2O3 Mixture via Interface Engineering Yong Seok Choi,†,‡ Young-Su Lee,*,† Dong-Jun Choi,† Keun Hwa Chae,§ Kyu Hwan Oh,‡ and Young Whan Cho† †

High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea § Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea ‡

S Supporting Information *

ABSTRACT: A new solid-state Li ion conductor composed of LiBH4 and Al2O3 was synthesized by a simple ball-milling process. The element distribution map obtained by transmission electron microscopy demonstrates that the LiBH4 and Al2O3 are well mixed and form a large interface after ballmilling. The ionic conductivity of the mixture reaches as high as 2 × 10−4 S cm−1 at room temperature when the volume fraction of Al2O3 is approximately 44%. The ionic conductivity of the interface between LiBH4 and Al2O3 was extracted by using a continuum percolation model, which turns out to be about 10−3 S cm−1 at room temperature, being 105 times higher than that of pure LiBH4. This remarkable rise in conductivity is accompanied by the lowered activation energy for the Li ion conduction in the mixture, indicating that the interface layer facilitates Li ion conduction. Near-edge X-ray absorption fine structure analysis reveals the presence of B− O bondings in the mixture, which was not detected by X-ray diffraction. This disruption of the chemical bondings at the interface may allow an increase in carrier concentration and/or mobility thereby resulting in the pronounced enhancement in conductivity. This result provides a guideline for designing fast Li ion conductor through interface engineering. formation to the orthorhombic phase and reduces to ∼10−8 S cm−1 at room temperature (RT). Since the high transition temperature is an unavoidable obstacle to adopting LiBH4 in commercial batteries, many researchers have made an effort to achieve high ionic conductivity at low temperature through tailoring the chemical structure by doping halides,25−27 hydration,28 and combining complex anions.29−31 As an another route to increase conductivity, our previous study demonstrated that ionic conductivity of LiBH4 at RT can be enhanced up to 4 orders of magnitude higher by mixing with SiO2 nanoparticles.32 Liang and colleagues first discovered such a phenomenon in the LiI−Al2O3 composite, where the addition of Al2O3 increased the ionic conductivity of LiI by 2 orders of magnitude.33 These kinds of mixtures, i.e., ionic conductor with insulating fine particles, have been termed dispersed ionic conductors and are characterized by the formation of defective and highly conducting space-charge layers at the interface between the conductor and insulator. Similar enhancement in ionic conductivity has been found when insulating matters, such as SiO2, Al2O3, and B2O3, are mixed with Li ion

1. INTRODUCTION All solid-state lithium ion batteries with solid electrolytes have drawn much attention as an alternative to batteries using organic liquid electrolytes, due to their high power density and great thermal/mechanical stability.1−4 However, it still remains a challenge to design suitable solid electrolytes with sufficient chemical and electrochemical stability, and also with high lithium ion conductivity. Among the various solid electrolytes discovered thus far,5−16 lithium borohydride (LiBH4) is considered one of the most promising candidates. LiBH4 possesses excellent properties including high electrochemical stability of up to 5 V (Li+/Li), high temperature durability, and negligible electronic conductivity.17,18 Matsuo et al.19 discovered that LiBH4 displays high Li ionic conductivity of ∼10−3 S cm−1 above 110 °C, which is the phase transition temperature from low temperature phase (orthorhombic structure, Pnma space group) to high temperature phase (hexagonal, P63mc). Several researchers have fabricated all solid-state batteries with LiBH4 as a solid electrolyte and successfully demonstrated the charge and discharge of the battery with active electrodes such as S, LiCoO2, and TiS2 at high temperatures above 110 °C.20−24 However, the ionic conductivity of LiBH4 drastically decreases by several orders of magnitude after phase trans© 2017 American Chemical Society

Received: September 6, 2017 Revised: October 29, 2017 Published: November 10, 2017 26209

DOI: 10.1021/acs.jpcc.7b08862 J. Phys. Chem. C 2017, 121, 26209−26215

Article

The Journal of Physical Chemistry C

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of LA mixtures before and after the ball-milling process. All LA mixtures before ball-

conductors, highlighting the importance of interface engineering.32,34−36 Most recently, Blanchard et al.37 suggested that nanoconfinement of LiBH4 into a mesoporous scaffold (MCM41) can maximize the interface area between two mediums, and the Li transport is much faster compared to bulk LiBH4. Such interface control is worthwhile because it can be applicable to any kinds of ionic conductors, but the main mechanism of fast Li transport at the interface has not yet been elucidated. In order to develop fast ionic conductors in the future, it is crucial to identify the key factors that govern Li mobility at the interface. Herein we characterize the interface of a composite Li ion conductor, LiBH4−Al2O3, and attempt to identify the mechanism of conductivity enhancement. In order to improve the ionic conductivity through interface engineering, we selected the γ-Al2O3 nanoparticle (primary particle diameter of 5 nm) as an additive insulator that is expected to form a large interface area with LiBH4. It is found that the ionic conductivity of the LiBH4−Al2O3 mixture can be raised to 2 × 10−4 S cm−1 at RT by optimizing the volume fraction of Al2O3. Comparison with the interface of the LiBH4−SiO2 mixture reveals superior ion conduction at the LiBH4−Al2O3 interface.

2. EXPERIMENTAL METHODS Commercial LiBH4 powder (purity 95%, Acros) and γ-Al2O3 (purity 99.99%, US research nanomaterials) were used as starting materials. In order to reduce the particle size, commercial LiBH4 powder was ball-milled using a planetary mill (Fritsch P7) operated at 600 rpm for 2 h. As received γAl2O3 was dried under vacuum at 350 °C for 7 h to remove the adsorbed water and oxygen. Then, premilled LiBH4−Al2O3 (LA) mixtures with five different volume fractions (19, 33, 44, 54, and 73 vol % of Al2O3; see Supporting Information for details) were ball-milled for uniform mixing. A ball-milling process was conducted using the planetary mill operated at 200 rpm for 30 min. All handling of the samples was performed in a glovebox filled with argon (p(O2), p(H2O) < 1 ppm). The X-ray diffraction (XRD) patterns of LA mixtures were measured using a Bruker D8 Advance diffractometer with Cu Kα radiation operating at 40 kV and 40 mA at RT. The exposure time and step size were 1 s step−1 and 0.03°, respectively. To prevent air exposure, samples were sealed with polyimide thin-film (7.5 μm) tape in the glovebox. A high resolution transmission electron microscope (TEM, FEI Titan) operating at 200 keV was used to observe the microstructure of the solid electrolyte. A TEM sample with a thickness of ∼80 nm was prepared using a focused ion beam (FIB, FEI Quanta 3D FEG). All sample transfer processes were conducted using a mobile air-lock holder. Digital Micrograph software was used for analysis of selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS). The ionic conductivity of the LA mixtures was measured by AC impedance spectroscopy using a Solartron impedance analyzer (SI 1260). A detailed description of the impedance measurement can be found in the Supporting Information. To detect a change in bonding structure after ball-milling, near edge X-ray absorption fine structure (NEXAFS) measurements were conducted at 10D XAS KIST beamline in Pohang Light Source (PLS), Korea. B K-edge NEXAFS spectra were collected in both total-electron yield (TEY) mode and fluorescence yield (FY) mode and are normalized with respect to the incident photon flux.

Figure 1. X-ray diffraction patterns of LA mixtures (vol % of γ-Al2O3 in the legend) (a) before and (b) after ball-milling process.

milling (Figure 1a) display the features of starting materials, including strong peaks of LiBH4 and broad peaks of γ-Al2O3. The peak intensity of LiBH4 is proportional to the amount of LiBH4. The broad peak around 18° comes from the polyimide thin film used to prevent air exposure. After ball-milling, the XRD patterns of LA mixtures (Figure 1b) remain almost identical except for broadening of the LiBH4 peaks. The broadened peaks indicate the reduction in the crystallite size of LiBH4, but any chemical reaction between LiBH4 and Al2O3 during ball-milling is not evident in the XRD patterns. The formation of a large interface between conductor and insulator and its percolation throughout the conductor are important for improving conductivity by interface engineering. Thus, we attempt to visualize the phase distribution in the microstructure of LA 44 vol % mixture after ball-milling using TEM. Figure 2a shows the cross-sectional image of the LA 44 vol % sample. It can be seen that tiny particles exhibiting dark or bright contrast are well mixed at the nanometer scale. EELS element mapping (Figure 2b) clarifies that Li and B signals mainly come from the dark contrast region, while Al signals are stronger at the bright contrast area. Ring patterns in SAED (Figure 2a) match well with the calculated positions of LiBH4 and Pt (deposited for surface protection during TEM sampling), but the pattern for Al2O3 is barely visible because it is nanocrystalline. This result is in good agreement with the XRD analysis. From this microstructure analysis, we can predict that a large area of interface between LiBH4 and Al2O3 is created by ball-milling, and that would possibly provide a fast conduction path for Li ions. Figure 3 is a plot of the ionic conductivities of LA mixtures as a function of temperature, measured by electrochemical impedance spectroscopy. Interestingly, even a small amount of Al2O3 (∼19 vol %) greatly improves ionic conductivity up to 4 × 10−5 S cm−1 at RT, which is almost 3 orders of magnitude 26210

DOI: 10.1021/acs.jpcc.7b08862 J. Phys. Chem. C 2017, 121, 26209−26215

Article

The Journal of Physical Chemistry C

Figure 2. (a) TEM image of the microstructure of LA 44 vol % mixture and corresponding SAED pattern. (b) EELS element mapping of red square area in (a). Red, green, and blue color represent Li, B, and Al, respectively.

ionic conductivity enhancement by mixing oxide compounds (e.g., SiO2, Al2O3, and B2O3) is a universal phenomenon applicable to a wide range of conductors.32,35−38 However, the degree of enhancement varies in the range of 100 to 10000 times higher depending on the chemical composition and size of the insulating materials being mixed. In our LA mixtures, the degree of enhancement appears among the highest, and this suggests that the interface engineering is a very effective route to increase the conductivity of LiBH4. The conductivity data in Figure 3 can be analyzed in a different fashion, i.e., conductivity variation as a function of volume fraction of Al2O3 at a specific temperature, which is presented in Figure 4a. On top of the experimentally measured conductivity, we have simulated the conductivity of a composite (σ(p)) as a function of the volume fraction of an insulator (p) using a continuum percolation model developed by Roman et al.39 Using their formalism, the ratio τ (= σA /σB) between the interface conductivity (σA) and the bulk conductivity of LiBH4 (σB) can be estimated (see Supporting Information for the details). The simulated curves (solid lines) with τ values of 1.5 × 105 (at 25 °C), 5.6 × 104 (at 62 °C), 2.3 × 104 (at 99 °C), and 37 (at 124 °C) reasonably reproduce the trend of the measured values (solid symbols) shown in Figure 4a. We compare the interface conductivity of LA mixtures with that of LiBH4-fumed silica (LF) mixtures in our previous study32 and present the result in Figure 4b. Overall, the interface conductivity is much higher than that of bulk, being ∼10−3 S cm−1 at RT, as can be immediately known from the extremely high τ value. Comparison between LA and LF mixtures reveals that the interface conductivity of LA mixtures is almost twice as high as that of LF mixtures irrespective of the temperature. Therefore, the higher τ value (or interface conductivity) in the LA mixture mainly, but not fully, accounts for the higher total conductivity of LA (see Figure S3) and the rest can be attributed to the increased proportion of the volume

Figure 3. Arrhenius plots of the ionic conductivities of LA mixtures in different mixing ratios (vol % of Al2O3 in the legend). The gray dots are data of pure LiBH4. All the data were obtained from the second cooling run.

higher than that of pure LiBH4. The conductivity increases continuously as the amount of Al2O3 increases and hits a maximum of ∼2 × 10−4 S cm−1 at RT when the volume fraction of Al2O3 reaches 44%. Notably, this is the highest value compared to other studies32,37 that report conductivity enhancement through interface engineering (see Figure S3 for comparison). In addition, an LA 44 vol % mixture displays a very tiny change in conductivity at the phase transition temperature, at which the conductivity of bulk LiBH4 suddenly rises by 3 orders of magnitude. This implies that the Li ion conduction is dominated by the interface layer with a minor contribution by bulk LiBH4. After hitting the maximum point, the ionic conductivity decreases as the volume fraction of Al2O3 increases because the large amount of insulating materials disrupts the percolation of the interface layer. Based on previous literature and our own studies, we conclude that the 26211

DOI: 10.1021/acs.jpcc.7b08862 J. Phys. Chem. C 2017, 121, 26209−26215

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The Journal of Physical Chemistry C

Figure 4. (a) Comparison between the measured (solid symbols) and simulated (solid line) normalized conductivities for the LA mixtures at 25, 62, 99, and 128 °C. (b) Arrhenius plots of the ionic conductivities of LA and LF interface layer calculated by a continuum percolation model.

fraction of interface due to the smaller particle size of γ-Al2O3 (∼5 nm) compared to fumed silica (∼7 nm).32,36 The difference in τ values suggests that the interface conductivity can be varied depending on the chemical composition of the oxide particles, and the LiBH4−Al2O3 interface might be more efficient in promoting the diffusion of the Li ion than the LiBH4−SiO2 interface. Suwarno et al. observed a similar phenomenon that the chemical nature of the scaffolds (carbon and silica scaffolds in their study) can affect the hydrogen mobility and interface layer thickness by changing interfacial energy.40 In their study, the interfacial thickness of LiBH4 with SiO2, deduced from the decreased enthalpy change of the phase transition, is found to be larger than that with carbon. In our analysis, the interfacial thickness was arbitrarily fixed to 1 nm for both LA and LF mixtures, and therefore the relatively high τ value of LA mixtures may also reflect the increased interfacial thickness in LA mixtures. Further study is now underway to understand how the chemical composition of an insulator alters the interface conductivity and to find out whether other nonoxide compounds can deliver the conductivity enhancement to a similar degree. Since the ion conduction in solid takes place by jumping of the ions to the adjacent vacancies or interstitial sites, the structural evolution at the interface strongly influences the activation energy required for Li migration. The activation energy (Eσa ) of mixtures was determined from the slope of Arrhenius plots of the ionic conductivities below the phase transition temperature in Figure 3. Figure 5 shows the activation energies as a function of the volume fraction of

Al2O3 in the LA mixtures. The reported activation energies of LF mixtures32 are coplotted for comparison. The activation energies of LA mixtures range from 0.44 to 0.56 eV below the phase transition temperature. Those values are much smaller than the reported value of bulk LiBH 4 (∼0.76 eV, orthorhombic phase).41 This result indicates that the interface between LiBH4 and Al2O3 serves as a pathway for facile diffusion of Li ions. Such a decrease in activation energies was consistently found for LF mixtures. Nonetheless, the volume fraction of the insulating material at the activation energy minimum is smaller for the LA mixture than the LF mixture. It is likely that the smaller particle size of Al2O3 than SiO2 shifts the volume fraction at the activation energy minimum to the lower value. At this point, we have demonstrated that the interface between LiBH4 and Al2O3 significantly raises the ionic conductivity. However, identifying the physical and/or chemical origins which underlie such conductivity enhancement is challenging because it requires the structure analysis in the subnanometer scale. NMR spectroscopy has been the most common tool to probe the interface, but the focus so far has been on the ionic mobility change instead of the chemical bonding change.37,42−44 We employed NEXAFS spectroscopy to investigate the structural changes at the interface by analyzing the chemical bonding of the constituting elements. Figure 6 shows B K-edge NEXAFS spectra of pure LiBH4 and LA mixtures with a different volume fraction of Al2O3. A very tiny peak appears around 194 eV in the TEY spectrum of pure LiBH4 while no distinct peak exists in the FY spectrum. As the amount of Al2O3 increases, the intensity of the peak at 194 eV increases in both FY and TEY spectra, indicating that the bonding character of B has changed upon ball-milling with Al2O3. The peak at 194 eV is known to be the transition of B 1s electrons to the unoccupied π* state of [3]B with O (planar BO3),45,46 and it applies to our study, as the intensity correlates well with the amount of Al2O3. The tiny peak seen in the TEY spectrum of pure LiBH4 is likely to originate from the oxidized surface, which would be buried in the FY spectrum due to the reduced contribution by the surface. The new chemical bonds between B and O probed by the NEXAFS spectra reveal that Al2O3 is not completely inert, which was not evident in the XRD pattern in Figure 1. Such an interfacial reaction possibly contributes to the formation of a defective interface layer, thereby increasing the number of mobile Li ions and/or promoting the migration of Li ions. The formation of B−O

Figure 5. Activation energy of Li conduction in ball-milled LA and LF mixtures as a function of volume fraction of oxides (Al2O3 or SiO2). 26212

DOI: 10.1021/acs.jpcc.7b08862 J. Phys. Chem. C 2017, 121, 26209−26215

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Figure 6. B K-edge NEXAFS spectra of pure LiBH4 and LA mixtures. Total electron yield and fluorescence yield for left and right panel, respectively.

LiBH4−Al2O3 having different volume fractions of Al2O3 was simply prepared by high energy ball-milling. TEM images and EELS element mapping data exhibit that LiBH4 and Al2O3 are well mixed and create a large interface area during the ballmilling process. The ionic conductivity of mixture reaches an extremely high value of ∼2 × 10−4 S cm−1 at RT, which is 104 times higher than that of pure LiBH4. This enhancement in ionic conductivity is probably due to the formation of a highly defective interface between LiBH4 and Al2O3, supported by the formation of B−O bondings detected in the NEXAFS spectra, which would decrease the activation energy for Li migration and/or increase the mobile Li ions. By analyzing the conductivity of the LA mixtures employing a continuum percolation model, we have found that Al2O3 is even more effective in making a highly conducting interface than the previously investigated SiO2. The remarkable conductivity enhancement found in this study underscores the utility of the interface engineering as a simple and promising pathway for exploring new electrolytes for all-solid-state energy storage and conversion devices in the future.

bondings signifies the perturbation of chemical bondings at the interface, but Li ions do not appear to migrate through the region enriched with B−O bondings since the conductivity is not simply proportional to the amount of B−O bondings which increases with the amount of Al2O3. Instead, the Li ion migration is likely to occur through the interfacial LiBH4, possibly defective, directly contacting the region rich with B−O bondings. Therefore, even though the amount of B−O bondings increases, the conductivity inevitably starts to decrease when the amount of conducting phase, LiBH4, drops below a certain level. To probe any variations in the interfacial structures between LA and LF mixtures, we compare the NEXAFS spectra of LA 54 vol % and LF 55 vol % in Figure S4. When a similar volume % of oxide material is ball-milled with LiBH4, LA generates a larger amount of B−O bondings than LF does. As previously discussed, the larger number of B−O bondings cannot be directly interpreted as higher conductivity. However, the result at least indicates that γ-Al2O3 is more reactive or forms a larger interface area compared to fumed silica, which indirectly supports the higher conductivity of LA mixtures. Another interesting point is the BO3 peak position (∼194 eV) shift to higher energy in the case of LF. The shift to higher energy is caused by shorter B−O bondings, different next nearest cations, etc.47 Therefore, the LA and LF mixtures have chemically distinct interfacial bondings which can result in the different interfacial conductivities. A recent first-principles study proposed that the low ionic conductivity of orthorhombic LiBH4 is not due to lower mobility but due to lower carrier density.48 Thus, a higher number of mobile Li ions, i.e. increased carrier concentration, at the interface can significantly promote the ionic conductivity. Such a mechanism was manifested in the LiF−Li2 CO3 composite in which the carrier density is enhanced by the migration of Li interstitials from bulk LiF to the LiF/Li2CO3 interface, leading to significantly improved ionic conductivity.49 In our case, the formation of the B−O bondings reflects a partial destruction of the inherent crystal structure of LiBH4 at the interface, but we still need to clarify why mobile Li ion density and/or mobility consequently becomes higher. Although the exact mechanism is subject to further investigation, our study highlights the interface engineering as an efficient route to increase ionic conductivity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08862. Analytical details, Table S1, and Figures S1−S4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +82-2-958-5412. ORCID

Young-Su Lee: 0000-0002-3160-6633 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the Innovation Fund Denmark via the research project HyFill-Fast and by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Korea (Grant Number 2015M1A2A2074688).

4. CONCLUSION In this study, we have characterized Li ion conductors composed of LiBH 4 and Al 2 O 3 and optimized their conductivity by means of interface engineering. A mixture of 26213

DOI: 10.1021/acs.jpcc.7b08862 J. Phys. Chem. C 2017, 121, 26209−26215

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