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Sep 28, 2016 - Synopsis. The rock-salt-type LiBH4 phase emerging under high pressure is a promising solid electrolyte for all-solid-state lithium batt...
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Li-Ion Conductivity and Phase Stability of Ca-Doped LiBH4 under High Pressure Takeya Mezaki,† Yota Kuronuma,‡ Itaru Oikawa,† Atsunori Kamegawa,§ and Hitoshi Takamura*,† †

Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan ABSTRACT: The effect of Ca doping on the Li-ion conductivity and phase stability of the rock-salt-type LiBH4 phase emerging under high pressures in the range of gigapascals has been investigated. In situ electrochemical measurements under high pressure were performed using a cubic-anvil-type apparatus. Ca doping drastically enhanced the ionic conductivity of the rock-salttype phase: the ionic conductivity of undoped and 5 mol %Cadoped LiBH4 was 2.2 × 10−4 and 1.4 × 10−2 S·cm−1 under 4.0 GPa at 220 °C, respectively. The activation volume of LiBH4−5 mol % Ca(BH4)2, at 3.2 cm3·mol−1, was comparable to that of other fast ionic conductors, such as lithium titanate and NASICONs. Moreover, Ca-doped LiBH4 showed lithium plating-stripping behavior in a cyclic voltammogram. These results indicate that the conductivity enhancement by Ca doping can be attributed to the formation of a LiBH4−Ca(BH4)2 solid solution; however, the solid solution decomposed into the orthorhombic LiBH4 phase and the orthorhombic Ca(BH4)2 phase after unloading the high pressure.

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INTRODUCTION While lithium secondary batteries are widely used in portable electric devices and electric vehicles, they contain a flammable liquid electrolyte and are therefore considered a potential fire hazard. All-solid-state lithium batteries use a solid electrolyte instead of a liquid electrolyte and are notable for their high safety, high energy density, and design flexibility. In general, however, the Li-ion conductivity of solid-state Li-ion conductors is lower than that of liquid electrolytes. To fabricate the all-solid-state lithium batteries with high energy density, it is necessary to develop Li-ion conductors with high Li-ion conductivity. To date, various types of Li-ion conductors have been reported as candidate solid electrolytes.1−5 Lithium borohydride, LiBH4, and related materials have been given significant attention as a new family of fast Li-ion conductors.6−13 The Li-ion conductivity of LiBH4 is high, in the order of 10−3 S·cm−1, and is characterized by a phase transition from the orthorhombic to hexagonal structure at approximately 115 °C.6 The advantages of this material include its low weight (0.67 g·cm−3), high formability, and chemical compatibility with Li metal anodes with a high capacity. To ensure good contact between the electrolyte material and electrode active materials at the electrode−electrolyte interfaces, it is necessary for the solid electrolytes in all-solid-state batteries to have high formability. As shown in Figures 1 and 2, the crystal structures of LiBH4 differ with pressure and temperature: the structure can be hexagonal, orthorhombic, tetragonal or a rock-salt type.14−18 It has been well-reported that the hexagonal phase which emerges under ambient pressure at high temperatures is a fast Li-ion conductor.6,17,19,20 The charge−discharge cycle performance of electrochemical cells using a hexagonally structured LiBH 4 -based solid electrolyte has been re© XXXX American Chemical Society

Figure 1. Pressure−temperature phase diagram of LiBH4.14−18

ported.21−25 In other NMR studies and first-principles molecular-dynamics simulations, it has been suggested that Li ions in the hexagonal phase diffuse mainly along its a-b plane.6,26,27 This anisotropic Li-ion conduction forms an interface where no electrochemical reactions between a Li ion and electron occur. On the other hand, the Li ions in the rocksalt-type phase which emerge under high pressure in the range Received: July 19, 2016

A

DOI: 10.1021/acs.inorgchem.6b01678 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Ca(BH4)2 → CaLi• + 2BH4BH4 × + VLi′

(1)

where CaLi• refers to a Ca ion on a Li site with a positive charge, BH4BH4× refers to a BH4 ion on a BH4 site with a net charge of zero, and VLi′ refers to a vacancy on a Li site with a negative charge. As described in eq 1, the Li-ion conductivity and phase stability including the electrochemical and structural stability of Ca-doped LiBH4 depends on the concentration of Ca(BH4)2. The purpose of this study is to investigate the Li-ion conductivity and phase stability of Ca-doped LiBH4 under high pressure.

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EXPERIMENTAL SECTION

LiBH4 (Sigma-Aldrich, purity >95%) and Ca(BH4)2 (Sigma-Aldrich) were mixed by hand at given molar ratios and pressed into a pellet (thickness ≈ 1.0 mm, diameter = 2.8 mm) by uniaxial pressing. The nominal composition was LiBH4−x mol %Ca(BH4)2 (x = 0, 2, 5, and 10). Because LiBH4 and Ca(BH4)2 are highly reactive with moisture and oxygen, sample preparation was conducted in an Ar-filled glovebox. The pellets were set into a high-pressure electrochemical cell. Li metal and Mo metal were used as the reversible electrode and irreducible electrode for Li-ion conduction, respectively. A detailed setup of the cells is described elsewhere.29 In situ electrochemical measurements were performed under high pressure using a cubicanvil-type apparatus under 2.0 to 5.0 GPa at a constant temperature of 220 °C. Ionic conductivity was measured by AC impedance spectroscopy for symmetric cells using Li electrodes and Mo electrodes with an impedance analyzer (NF Corporation; FRA5095). The frequency range was from 1 MHz to 0.1 Hz, and the applied voltage was 20 mV. The electrochemical stability of Ca-doped LiBH4 was evaluated by cyclic voltammetry for an asymmetric cell using a reference Li electrode and working Mo electrode with a potentiogalvano stat (Solartron; Model 1287). The potential range was from −0.5 to 5.0 V versus Li+/Li0, and the scan rate was 1.0 mV·s−1. Highpressure measurements were taken using a compression process. The crystal structures of Ca-doped LiBH4 were identified by X-ray diffractometry before and after taking high-pressure measurements at room temperature with an X-ray diffractometer (Bruker AXS; D8 advance) by the parallel beam method using a glass capillary. The scan range was from 15° to 50° and the X-ray source was Cu Kα radiation (wavelength = 1.54 Å). Prior to taking high-pressure measurements, the pressure was calibrated as a funtion of the load applied to the cells by the pressure-induced phase transitions of Bi, Tl and Ba at room temperature.30 Temperature calibration was also conducted using Ktype thermocouples.

Figure 2. Hexagonal (top) and rock-salt-type (bottom) structures of LiBH4.14−18

of gigapascals at high temperature diffuse isotropically.14−18 To enhance the efficiency of the electrochemical reactions and obtain high power density and high cycle performance, it is ideal if the solid electrolytes have isotropic Li-ion-conducting paths. For this reason, LiBH4 with the rock-salt-type structure can be considered a promising solid electrolyte. However, the Li-ion conductivity of the rock-salt-type phase is approximately 1 order of magnitude lower than that of the hexagonal phase,17,19 which has been attributed to its lower carrier concentration.19 The use of aliovalent doping has the potential to increase the carrier concentration and therefore enhance the Li-ion conductivity of the rock-salt-type phase. In this study, the effect of Ca doping on the Li-ion conductivity and phase stability of the rock-salt-type phase was investigated. In the hexagonal phase with higher Li-ion conductivity, it has been shown that Li ions diffuse by the interstitialcy mechanism; lithium vacancies and interstitial Li ions formed by the Frenkel-type defect contribute to Li-ion conduction.27 In the rock-salt-type phase, however, the vacancy mechanism due to the Schottky-type defect may well play a role in Li-ion conduction. The Schottky-type defect is often observed in typical alkali halides with a rock-salt-type structure.28 The substitution of Ca divalent cations for Li sites is expected to increase the amount of lithium vacancies to fulfill charge neutrality and therefore enhance the Li-ion conductivity of the rock-salt-type phase. By using the Kröger− Vink notation, the defect equilibrium equation corresponding to the Ca substitution is described as follows:

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RESULTS AND DISCUSSION Li-Ion Conductivity of Undoped and Ca-Doped LiBH4. As shown by eq 1, the substitution of Ca divalent cations for Li sites is expected to increase the amount of lithium vacancies and therefore enhance the Li-ion conductivity of the rock-salttype LiBH4 phase. First of all, the Li-ion conductivity of LiBH4−x mol %Ca(BH4)2 (x = 0, 2, 5, and 10) was measured under high pressure by AC impedance spectroscopy. Figure 3 shows the typical Nyquist plots of the symmetric cells using LiBH4 and LiBH4−5 mol %Ca(BH4)2 measured under 4.0 GPa at 220 °C, where the rock-salt-type phase emerges. Note that the Li metal (melting point ≈180 °C under ambient pressure) is in its solid phase with a bcc-type structure at 220 °C under high pressure.31 As shown in Figure 3a, only a semicircle attributed to the bulk contribution to electrical resistance was observed for Li | LiBH4 | Li. Meanwhile, Mo | LiBH4 | Mo showed a semicircle and linear response at higher and lower frequencies, respectively. This difference in the low-frequency response between the cells using different electrodes indicates that the major carrier in the rock-salt-type phase is the Li ions. B

DOI: 10.1021/acs.inorgchem.6b01678 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Pressure dependence of the bulk Li-ion conductivity of LiBH4−x mol %Ca(BH4)2 measured using Mo electrodes (x = 0, 2, 5, and 10).

LiBH4, the clear drop in the Li-ion conductivity in the order of approximately 1 order of magnitude occurs with the phase transition from the hexagonal to rock-salt-type structure,17,19 and can be attributed to a decrease in the carrier concentration as the phase transition takes place.19 In the case of Ca-doped LiBH4, no significant conductivity enhancement was observed in the hexagonal phase. It has been reported that M(BH4)2 (M = Mg, Ca) doping does not enhance the Li-ion conductivity of the hexagonal phase due to limited solubility into the phase and the sufficiently high carrier concentration in the absence of dopants.27,34 In the case of the rock-salt-type phase, Li-ion conductivity was markedly improved, as shown in Figure 3. It should be noted that the maximum conductivity of Ca-doped LiBH4 with the rock-salt-type structure was dependent on the Ca(BH4)2 concentration. The highest Li-ion conductivity was obtained in LiBH4−5 mol %Ca(BH4)2; LiBH4 and LiBH4−5 mol %Ca(BH4)2 showed 2.2 × 10−4 and 1.4 × 10−2 S·cm−1 under 4.0 GPa at 220 °C, respectively. This conductivity behavior has been observed in fast ionic conductors where mobile ions diffuse through a vacancy such as rare-earth-doped ceria with a fluorite-type structure and lithium lanthanum titanate with a Perovskite-type structure.35,36 In the case of rareearth-doped ceria, two competing effects related to defect association develop as the dopant concentration increases: paths with a low activation enthalpy appear and strong traps form due to multiple association.35 In the case of lithium lanthanum titanate, fast Li-ion conduction occurs when clusters consisting of Li ions and lithium vacancies percolate through the system because La ions are an obstacle for Li-ion conduction.36 As a result, the maximum conductivity depends on the total concentration of Li ions, lithium vacancies, and La ions, in accordance with the percolation theory.36,37 Considering these factors affecting ionic conductivity, it can be concluded that the Li-ion conductivity of the LiBH4−Ca(BH4)2 solid solution is affected by the defect association between a lithium vacancy and Ca ion and site percolation, which is dependent on the Ca(BH4)2 concentration. Another factor affecting ionic conductivity is the size of the bottleneck for ionic conduction. Its influence is evaluated by the activation volume associated with the change in volume for

Figure 3. Typical Nyquist plots of (a) Li | LiBH4 | Li, Mo | LiBH4 | Mo (top) and (b) Li | LiBH4−5 mol %Ca(BH4)2 | Li, Mo | LiBH4−5 mol %Ca(BH4)2 | Mo (bottom) measured under 4.0 GPa at 220 °C.

The bulk resistivity of LiBH4 calculated from the diameter of the semicircles with a capacitance of the order of 10−10 F was approximately 5.0 × 103 Ω·cm. As shown in Figure 3b, the resistivity of LiBH4−5 mol %Ca(BH4)2 was approximately 2 orders of magnitude lower than that of LiBH4. Furthermore, the clear linear response of Mo | LiBH4−5 mol %Ca(BH4)2 | Mo is similar to that observed in Mo | LiBH4 | Mo. These results suggest that Ca doping drastically enhances the Li-ion conductivity of the rock-salt-type phase. This conductivity enhancement is presumably attributed to the formation of a LiBH4−Ca(BH4)2 solid solution and an increase in the amount of lithium vacancies through which the Li ions diffuse. Because LiBH4 and Ca(BH4)2 are wide-bandgap materials,32,33 the amount of electronic conductivity in the solid solution should be negligible. At lower frequencies, Li | LiBH4−5 mol % Ca(BH4)2 | Li showed a depressed semicircle, but with time, the response became linear and the bulk resistivity calculated from the cell gradually increased (not shown). These results imply that a chemical reaction occurs between Ca(BH4)2 in the LiBH4−Ca(BH4)2 solid solution and Li electrodes. Ca(BH4)2 was reported to react with Li metal to form CaH2 under ambient pressure up to 100 °C.34 Assuming that a similar reaction occurs at the interfaces between Ca-doped LiBH4 and Li electrodes under high pressure, the decrease in Li-ion conductivity and the depressed semicircle attributed to the interfaces suggest a decrease in the amount of lithium vacancies, as indicated by eq 1, and the appearance of CaH2, which is an insulator, respectively. Factors Affecting the Li-Ion Conductivity of Ca-Doped LiBH4. Figure 4 shows the pressure dependence of the bulk Liion conductivity, σ, of LiBH4−x mol %Ca(BH4)2 (x = 0, 2, 5, and 10) at 220 °C measured using Mo electrodes. The decrease in Li-ion conductivity at each phase of LiBH4 with increasing pressure can be attributed to the narrowing of the bottleneck for Li-ion conduction under high pressure. In the case of C

DOI: 10.1021/acs.inorgchem.6b01678 Inorg. Chem. XXXX, XXX, XXX−XXX

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conductors, La0.52Li0.35TiO2.96 and Na1+xZr2SixP3−xO12, and typical ionic conductors, as shown in Table 1, LiBH4−5 mol %Ca(BH4)2 with the rock-salt-type structure can be classified as a fast Li-ion conductor.17,19,20,38−40 The activation volume of the typical ionic solids (i.e., NaCl and KCl) in their extrinsic regions was also listed in Table 1.40 On the other hand, the activation volume of LiBH4 −10 mol %Ca(BH 4 ) 2 was significantly larger, at 6.3 cm3·mol−1. This larger activation volume implies a strong defect association between a lithium vacancy and Ca ion and a small bottleneck for Li-ion conduction. Because stable association pairs need to be disassociated to create lithium vacancies which contribute to Li-ion conduction, it can be inferred that a strong defect association can contribute to the activation volume and decrease the Li-ion conductivity. Moreover, the heavy doping of immobile Ca ions with a larger ionic radius likely results in the narrowing of the bottleneck for Li-ion conduction. Electrochemical and Structural Stability of Ca-Doped LiBH4. To verify that the major carrier in Ca-doped LiBH4 with the rock-salt-type structure is Li ions, not Ca ions, and evaluate its electrochemical stability, in situ cyclic voltammetry under high pressure was performed. Figure 6 shows a cyclic

defect formation and ion migration. Figure 5 shows the Ca(BH4)2-concentration dependence of the bulk Li-ion

Figure 5. Ca(BH4)2-concentration dependence of the bulk Li-ion conductivity under 4.4 GPa (left axis) and the activation volume (right axis) of the rock-salt-type LiBH4 phase at 220 °C.

conductivity under 4.4 GPa and the activation volume in the rock-salt-type structure at 220 °C. The activation volume was calculated from the conductivity in the pressure range from 4.2 to 5.0 GPa. The rock-salt-type LiBH4 phase showed an activation volume of 5.7 cm3·mol−1. This value was larger than that of the hexagonal phase calculated in this study, at 4.6 cm3· mol−1, and in previous studies, at between 2.8 and 6.4 cm3· mol−1.17,19,20 It should be noted that the most recently proposed activation volume of 6.4 cm3·mol−1 is likely to have been due to large sample deformation during the compression process.17 The larger activation volume of the rock-salt-type phase can be attributed to the lower carrier concentration and the stronger contribution of ion migration to the change in volume. In other words, the Li-ion conduction in the rock-salttype phase is likely to require larger lattice expansion for defect formation and ion migration. LiBH4−2 mol %Ca(BH4)2 and LiBH4−5 mol %Ca(BH4)2 showed smaller values of 3.9 and 3.2 cm3·mol−1, respectively. This decrease in activation volume can likely be attributed to the small contribution of defect formation because of the large amount of lithium vacancies introduced by Ca substitution. Furthermore, these results suggest that, near the optimum Ca(BH4)2 concentration of 5 mol %, the clusters consisting of Li ions and lithium vacancies are effectively formed with the small effect of defect association. Compared with the reported activation volume of fast ionic

Figure 6. Cyclic voltammogram of Li | LiBH4−5 mol %Ca(BH4)2 | Mo measured under 4.0 GPa at 220 °C.41

voltammogram of Li | LiBH4−5 mol %Ca(BH4)2 | Mo measured under 4.0 GPa at 220 °C. Because cathodic and anodic peaks corresponding to Li plating and stripping behaviors were observed near the open circuit voltage of 0.55 V versus Li+/Li0, it can be determined that Ca-doped LiBH4

Table 1. Activation Volumes for Ionic Conduction of LiBH4−5 mol %Ca(BH4)2 with the Rock-Salt-Type Structure and Other Ionic Conductors17,19,20,38−40 ionic conductor

activation volume/cm3·mol−1

LiBH4−5 mol %Ca(BH4)2 LiBH4 La0.52Li0.35TiO2.96 Na1+xZr2SixP3−xO12 (x = 1.8−2.3) NaCl KCl

3.2 4.6 (in this study), 6.4,17 2.8−3.2,19 320 1.6−1.738 1.6−3.039 740 840 D

DOI: 10.1021/acs.inorgchem.6b01678 Inorg. Chem. XXXX, XXX, XXX−XXX

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orthorhombic Ca(BH4)2 phase once the high pressure was unloaded. By using in situ structural characterization methods, it will be possible to clarify the phases present in Ca-doped LiBH4 and the solid solubility of Ca(BH4)2 in LiBH4 with the rocksalt-type structure under high pressure.

with the rock-salt-type structure is a pure Li-ion conductor. The other small peaks near 0.3 and 1.3 V versus Li+/Li0 may be attributed to Mo oxides with the ability to work as anodes for lithium batteries.41 However, a broad anodic peak near 3.0 V versus Li+/Li0 implies that Ca doping degrades the electrochemical stability of LiBH4, which originally has a wide potential window.42 The decomposition products during the anodic polarization are being clarified. Lastly, to evaluate the structural stability of Ca-doped LiBH4, X-ray diffraction measurements were taken before and after the high-pressure measurements. Figure 7 shows the X-ray

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81 22 795 3938. Fax: +81 22 795 3938. Present Addresses ‡

(Y.K.) Steel Research Laboratory, JFE Steel Corporation, Fukuyama 721-8510, Japan. § (A.K.) Research Center for Environmentally Friendly Materials Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported in part by JSPS KAKENHI Grant Number 26249103.

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REFERENCES

(1) Kanno, R.; Murayama, M. Lithium Ionic Conductor ThioLISICON The Li2S−GeS2−P2S5 System. J. Electrochem. Soc. 2001, 148, A742−A746. (2) Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A. A lithium superionic conductor. Nat. Mater. 2011, 10, 682− 686. (3) Inaguma, Y.; Liquan, C.; Itoh, M.; Nakamura, T.; Uchida, T.; Ikuta, H.; Wakihara, M. High Ionic Conductivity in Lithium Lanthanum Titanate. Solid State Commun. 1993, 86, 689−693. (4) Murugan, R.; Thangadurai, V.; Weppner, W. Fast Lithium Ion Conduction in Garnet-Type Li7La3Zr2O12. Angew. Chem., Int. Ed. 2007, 46, 7778−7781. (5) Lapp, T.; Skaarup, S. Ionic Conductivity of Pure and Doped Li3N. Solid State Ionics 1983, 11, 97−103. (6) Matsuo, M.; Nakamori, Y.; Orimo, S.; Maekawa, H.; Takamura, H. Lithium Superionic Conduction in Lithium Borohydride Accompanied by Structural Transition. Appl. Phys. Lett. 2007, 91, 224103. (7) Matsuo, M.; Takamura, H.; Maekawa, H.; Li, H.; Orimo, S. Stabilization of Lithium Superionic Conduction Phase and Enhancement of Conductivity of LiBH4 by LiCl addition. Appl. Phys. Lett. 2009, 94, 084103. (8) Maekawa, H.; Matsuo, M.; Takamura, H.; Ando, M.; Noda, Y.; Karahashi, T.; Orimo, S. Halide-Stabilized LiBH4, a Room-Temperature Lithium Fast-Ion Conductor. J. Am. Chem. Soc. 2009, 131, 894− 895. (9) Matsuo, M.; Remhof, A.; Martelli, P.; Caputo, R.; Ernst, M.; Miura, Y.; Sato, T.; Oguchi, H.; Maekawa, H.; Takamura, H.; Borgschulte, A.; Züttel, A.; Orimo, S. Complex Hydrides with (BH4)− and (NH2)− Anions as New Lithium Fast-Ion Conductors. J. Am. Chem. Soc. 2009, 131, 16389−16391. (10) Ley, M. B.; Ravnsbæk, D. B.; Filinchuk, Y.; Lee, Y.; Janot, R.; Cho, Y. W.; Skibsted, J.; Jensen, T. R. LiCe(BH4)3Cl, a New LithiumIon Conductor and Hydrogen Storage Material with Isolated Tetranuclear Anionic Clusters. Chem. Mater. 2012, 24, 1654−1663.

Figure 7. XRD patterns of LiBH4−5 mol %Ca(BH4)2 measured before (bottom) and after (top) the high-pressure measurements.33,42

diffraction (XRD) patterns of LiBH4−5 mol %Ca(BH4)2. After the high-pressure measurements, three additional peaks were observed; they can be attributed to the orthorhombic LiBH4 phase, which is thermodynamically stable under ambient pressure, the orthorhombic Ca(BH 4 ) 2 phase, and an unidentified phase.33,42 The conductivity measurements suggest these peaks are the result of the decomposition of the LiBH4− Ca(BH4)2 solid solution after the high pressure is unloaded. That is, the unidentified phase may well be a decomposition product which appears during decompression. In order to clarify the phases present in Ca-doped LiBH4 and the solid solubility of Ca(BH4)2 in LiBH4 with the rock-salt-type structure under high pressure, in situ structural characterization needs to be carried out.

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CONCLUSION In situ electrochemical measurements under high pressure indicate that Ca doping drastically enhances the Li-ion conductivity of the rock-salt-type LiBH4 phase, with LiBH4 and LiBH4−5 mol %Ca(BH4)2 showing 2.2 × 10−4 and 1.4 × 10−2 S·cm−1 under 4.0 GPa at 220 °C, respectively. The smaller activation volume of LiBH4−5 mol %Ca(BH4)2, at 3.2 cm3· mol−1, is comparable to that of other fast ionic conductors such as lithium titanate and NASICONs. The conductivity enhancement by Ca doping can be attributed to the formation of the LiBH4−Ca(BH4)2 solid solution, which is a pure Li-ion conductor. It was shown, however, that the solid solution decomposed into the orthorhombic LiBH4 phase and E

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Inorganic Chemistry (11) Ley, M. B.; Boulineau, S.; Janot, R.; Filinchuk, Y.; Jensen, T. R. New Li Ion Conductors and Solid State Hydrogen Storage Materials: LiM(BH4)3Cl, M = La, Gd. J. Phys. Chem. C 2012, 116, 21267−21276. (12) Miyazaki, R.; Maekawa, H.; Takamura, H. Synthesis of Rock-Salt Type Lithium Borohydride and Its Peculiar Li+ Ion Conduction Properties. APL Mater. 2014, 2, 056109. (13) Takano, A.; Oikawa, I.; Kamegawa, A.; Takamura, H. Enhancement of the Lithium-Ion Conductivity of LiBH4 by Hydration. Solid State Ionics 2016, 285, 47−50. (14) Pistorius, C. W. F. T. Melting and Polymorphism of LiBH4 to 45 kbar. Z. Phys. Chem. 1974, 88, 253−263. (15) Filinchuk, Y.; Chernyshov, D.; Nevidomskyy, A.; Dmitriev, V. High-Pressure Polymorphism as a Step towards Destabilization of LiBH4. Angew. Chem., Int. Ed. 2008, 47, 529−532. (16) Dmitriev, V.; Filinchuk, Y.; Chernyshov, D.; Talyzin, A. V.; Dzwilewski, A.; Andersson, O.; Sundqvist, B.; Kurnosov, A. PressureTemperature Phase Diagram of LiBH4: Synchrotron X-ray Diffraction Experiments and Theoretical Analysis . Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 174112. (17) Yamawaki, H.; Fujihisa, H.; Gotoh, Y.; Nakano, S. Phase Boundaries and Molar Volumes of High-Temperature and HighPressure Phase V of LiBH4. J. Phys. Chem. Solids 2015, 76, 40−44. (18) Nakano, S.; Fujihisa, H.; Yamawaki, H.; Kikegawa, T. Structural Analysis of Some High-Pressure Stable and Metastable Phases in Lithium Borohydride LiBH4. J. Phys. Chem. C 2015, 119, 3911−3917. (19) Takamura, H.; Kuronuma, Y.; Maekawa, H.; Matsuo, M.; Orimo, S. Lithium Ion Conduction in Lithium Borohydrides under High Pressure. Solid State Ionics 2011, 192, 118−121. (20) Sundqvist, B.; Yao, M.; Andersson, O. Ionic Conductivity in Three Crystalline Phases of LiBH4 under Pressure. High Pressure Res. 2013, 33, 141−151. (21) Takahashi, K.; Hattori, K.; Yamazaki, T.; Takada, K.; Matsuo, M.; Orimo, S.; Maekawa, H.; Takamura, H. All-Solid-State Lithium Battery with LiBH4 Solid Electrolyte. J. Power Sources 2013, 226, 61− 64. (22) Takahashi, K.; Maekawa, H.; Takamura, H. Effects of Intermediate Layer on Interfacial Resistance for All-Solid-State Lithium Batteries Using Lithium Borohydride. Solid State Ionics 2014, 262, 179−182. (23) Unemoto, A.; Yasaku, S.; Nogami, G.; Tazawa, M.; Taniguchi, M.; Matsuo, M.; Ikeshoji, T.; Orimo, S. Development of Bulk-Type All-Solid-State Lithium-Sulfur Battery Using LiBH4 Electrolyte. Appl. Phys. Lett. 2014, 105, 083901. (24) Unemoto, A.; Chen, C.; Wang, Z.; Matsuo, M.; Ikeshoji, T.; Orimo, S. Pseudo-binary Electrolyte, LiBH4−LiCl, for Bulk-Type AllSolid-State Lithium-Sulfur Battery. Nanotechnology 2015, 26, 254001. (25) Unemoto, A.; Ikeshoji, T.; Yasaku, S.; Matsuo, M.; Stavila, V.; Udovic, T. J.; Orimo, S. Stable Interface Formation between TiS2 and LiBH4 in Bulk-Type All-Solid-State Lithium Batteries. Chem. Mater. 2015, 27, 5407−5416. (26) Ikeshoji, T.; Tsuchida, E.; Ikeda, K.; Matsuo, M.; Li, H.; Kawazoe, Y.; Orimo, S. Diffuse and Doubly Split Atom Occupation in Hexagonal LiBH4. Appl. Phys. Lett. 2009, 95, 221901. (27) Ikeshoji, T.; Tsuchida, E.; Morishita, T.; Ikeda, K.; Matsuo, M.; Kawazoe, Y.; Orimo, S. Fast-Ionic Conductivity of Li+ in LiBH4. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 144301. (28) Mehrer, H. Diffusion in Solids Fundamentals, Methods, Materials. Diffusion-Controlled Processes; Springer: Berlin, 2007; p 467. (29) Takamura, H.; Kobayashi, J.; Takahashi, N.; Okada, M. Electrical Conductivity of Ceria Nanoparticles under High Pressure. J. Electroceram. 2009, 22, 24−32. (30) Bean, V. E.; Akimoto, S.; Bell, P. M.; Block, S.; Holzapfel, W. B.; Manghnani, M. H.; Nicol, M. F.; Stishov, S. M. Another Step toward an International Practical Pressure Scale 2nd AIRAPT IPPS Task Group Report. Physica B+C 1986, 139-140, 52−54. (31) Schaeffer, A. M. J.; Talmadge, W. B.; Temple, S. R.; Deemyad, S. High Pressure Melting of Lithium. Phys. Rev. Lett. 2012, 109, 185702.

(32) Miwa, K.; Ohba, N.; Towata, S.; Nakamori, Y.; Orimo, S. FirstPrinciples Study on Lithium Borohydride LiBH4. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 245120. (33) Miwa, K.; Aoki, M.; Noritake, T.; Ohba, N.; Nakamori, Y.; Towata, S.; Züttel, A.; Orimo, S. Thermodynamical Stability of Calcium Borohydride Ca(BH4)2. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 155122. (34) Sveinbjörnsson, D.; Blanchard, D.; Myrdal, J. S. G.; Younesi, R.; Viskinde, R.; Riktor, M. D.; Norby, P.; Vegge, T. Ionic Conductivity and the Formation of Cubic CaH 2 in the LiBH 4 −Ca(BH 4 ) 2 composite. J. Solid State Chem. 2014, 211, 81−89. (35) Kilner, J. A. Fast Oxygen Transport in Acceptor Doped Oxides. Solid State Ionics 2000, 129, 13−23. (36) Inaguma, Y.; Itoh, M. Influence of Carrier Concentration and Site Percolation on Lithium Ion Conductivity in Perovskite-Type Oxides. Solid State Ionics 1996, 86-88, 257−260. (37) Stauffer, D.; Aharony, A. Introduction to Percolation Theory; revised 2nd ed.; Taylor and Francis: United Kingdom, 1994. (38) Inaguma, Y.; Yu, J.; Shan, Y.; Itoh, M.; Nakamura, T. The Effect of the Hydrostatic Pressure on the Ionic Conductivity in a Perovskite Lanthanum Lithium Titanate. J. Electrochem. Soc. 1995, 142, L8−L11. (39) Samara, G. A. High-Pressure Studies of Ionic Conductivity in Solids. Solid State Phys. 1984, 38, 73−74. (40) Yoon, D. N.; Lazarus, D. Pressure Dependence of Ionic Conductivity in KCl, NaCl, KBr, and NaBr. Phys. Rev. B 1972, 5, 4935−4945. (41) Auborn, J. J.; Barberio, Y. L. Lithium Intercalation Cells Without Metallic Lithium. J. Electrochem. Soc. 1987, 134, 638−641. (42) Matsuo, M.; Orimo, S. Lithium Fast-Ionic Conduction in Complex Hydrides: Review and Prospects. Adv. Energy Mater. 2011, 1, 161−172.

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DOI: 10.1021/acs.inorgchem.6b01678 Inorg. Chem. XXXX, XXX, XXX−XXX