Complex Hydrides: New Solid-state Conductors

Introduction

Research and development of solid-state lithium fast-ion conductors is crucial because they can be potentially used as solid electrolytes in all-solid-state batteries, which may solve the safety and energy-density related issues of conventional lithium-ion batteries that use liquid (farmable organic) electrolytes. So far, a wide variety of inorganic lithium fast-ion conductors of oxides,^{1 ,2} nitrides³ and sulfides^{4, 5} have been extensively studied because they have an advantage in high ion conductivity [>10^-3 S/cm at room temperature (RT)] as well as high lithium ion transport number (~1). Further efforts continue to overcome various barriers to practical realization, for example very high grain boundary resistance.

Lithium fast-ion conduction was discovered in LiBH₄ (686026) in 2007. LiBH₄ has not only high ion conductivity but also the following advantages for the solid electrolyte in lithium-ion batteries: (1) extremely low grain boundary resistance, (2) high electrochemical stability up to at least 5 V (vs Li⁺/Li) at 390 K, (3) extremely low polarization to metallic electrodes, (4) commercial availability and (5) suitability for various material processing techniques, including mechanical milling, heat treatment, impregnation and vapor deposition. This article reviews the fast-ion conduction in LiBH₄ and its derivatives (LiBH₄-LiI and LiBH₄-LiNH₂ systems), as a new category of solid-state lithium fast-ion conductors.

Lithium Fast-ion Conduction in LiBH₄

Complex hydrides are generally expressed as M(M′H_n), where M is a metal cation and (M′H_n) represents a complex anion such as (NH₂)^-, (BH₄)^- and (AlH₄)^-. LiBH₄ is a representative complex hydride, and has recently attracted great interest as a potential candidate for advanced hydrogen storage materials because of its high hydrogen density. LiBH₄ undergoes a structural transition at about 390 K from the orthorhombic low-temperature (LT) phase to the hexagonal high-temperature (HT) phase. Our study of microwave irradiation-induced rapid hydrogen desorption suggested that LiBH₄ might become conductive in the HT phase, although it is an insulator in both the LT phase and the HT phase with large band gap of about 7 eV.

Therefore, we systematically examined the lithium ion conductivity of LiBH4 by an arc complex impedance method and ⁷Li NMR measurement.⁶

Figure 1(a) shows the temperature dependence of the electrical conductivity of LiBH₄ as determined from the impedance plots shown in Figure 1(b). The impedance plots of both the LT phase and the HT phase show only a single arc, indicating that the response arising from the grain boundary is not observed, although the pelletized sample used for the impedance measurement was prepared simply by pressing powder LiBH₄ without subsequent sintering. The conductivities of the LT phase are very low, in the range of 10^-8 to 10^-6 S/cm, and the value linearly increases with increasing temperature. At about 390 K, the transition temperature, the conductivity drastically increases by three orders of magnitude. As a result, the HT phase exhibits high conductivity of the order of 10^-3 S/cm. The activation energies for conduction are evaluated to be 0.69 eV and 0.53 eV for the LT and HT phases, respectively.

In order to confirm whether the high electrical conductivity of LiBH₄ in the HT phase is due to the lithium fast ion mobility, ⁷Li NMR was measured. Figure 2 shows the ⁷Li NMR spectra at selected temperatures.

Figure 2. ⁷Li NMR spectra of LiBH₄ at selected temperatures.

The shapes of the spectra drastically change at the structural transition temperature. In the LT phase (<385 K), only broad and small peaks are observed. On the contrary, in the HT phase (>388 K), each spectrum shows a central sharp line and two satellite lines. The decrease in the line width of the central line indicates the motional narrowing caused by the lithium fast ion motion in the HT phase. Furthermore, Figure 1 also shows the NMR-derived conductivity estimated by the Nernst- Einstein equation using the correlation times obtained by the temperature dependence of the spin-lattice relaxation time T1. Clearly, the NMR data show a fairly good agreement with that measured by the impedance method. Thus, we concluded the charge carrier must be Li⁺ ions; the HT phase is a lithium fast-ion conductor.

As shown in Figure 3, the characteristic feature of the HT structure is that both Li⁺ ions and (BH₄)^- ions line up along the a axis and the b axis such that there is no (BH₄)^- ion between Li⁺ ions and vice versa. This arrangement may enable Li⁺ ions to migrate along these directions. Detailed studies using first-principles molecular dynamics simulations⁷ and high-pressure impedance measurements⁸ are now under way.

Enhanced Conductivity of the Complex Hydrides Derived from LiBH₄

The lithium fast-ion conduction in LiBH₄ could potentially aid the development of solid electrolytes in all-solid-state batteries. From an application point of view, however, it is highly desirable to enhance the conductivity at room temperature (RT). For that purpose, next, we tried materials design of fast-ion conductors of complex hydrides. As a typical example, we have investigated the LiBH₄-LiI and LiBH₄-LiNH₂ systems with different concepts.

Stabilization of the HT Phase in the LiBH₄-LiI System^9-11

We assumed that the stabilization of the HT phase should significantly improve the conductivity at RT, and the substitution of (BH₄)^- ion by I^- ion, of which ion radius (2.20 Å) is larger than that of (BH₄)^- ion (2.05 Å), might be effective for that purpose because it has been reported that alkali-metal borohydrides MBH₄ (M = Na, K, Rb and Cs) with longer distances between neighboring (BH₄)^- ions exhibit the lower transition temperatures.¹²

Figure 4(a) shows the x-ray diffraction (XRD) profiles of (1-x)LiBH₄ + xLiI synthesized by mechanical milling. The profiles gradually change from the LT phase to the HT phase with increasing amounts of LiI (518018) . For x = 0.25 and 0.5, only the peaks corresponding to the HT phase are observed with being shifted to lower angle, indicating the substitution of (BH₄)^- ion by I^- ion. The transition temperatures determined by differential scanning calorimetry (DSC), shown in Figure 4(b), clearly show the stabilization feature of the HT phase depending on the molar ratio of LiI. The vibrational properties measured by spatially resolved Raman spectroscopy demonstrated the importance of anharmonic effects in the structural transformation in this system.¹³

Figure 1 also shows the result of the ion conductivity measurements for x = 0.25. The conductivity follows the Arrhenius behavior throughout the measured temperature range without the abrupt change observed for LiBH₄ due to the structural transition. As expected, we achieved the increase in the conductivity at RT by 3 orders of magnitude without high conductivity of the HT phase decreased. We also confirmed that the substitution of (BH₄)^- ion by Cl^- ion leads to the enhanced conductivity.¹⁴

Double Complex Anions in the LiBH₄-LiNH₂ System¹⁵

The LiBH₄-LiNH₂ system has been studied from the viewpoint of developing hydrogen storage materials, and two phases, Li₂BNH₆ and Li₄BN₃H₁₀, have been known to exist. As shown in Figure 5, both Li₂BNH₆ and Li₄BN₃H₁₀ have different configuration of anions from LiBH₄: Li⁺ ions are tetrahedrally coordinated by combinations of (BH₄)^- and (NH₂)^-. The LiBH₄-LiI system showed the ion conductivity is affected by configuration of anions to a great degree, which made us get interested in these local structures.

The conductivity measurements indicate that Li₂BNH₆ has fast-ion conductivity of 2×10^-4 S/cm at RT, which is 4 and 5 orders of magnitude higher than those of the host hydrides LiBH₄ and LiNH₂, respectively, and the conductivity monotonically increases upon heating (Figure 1). The activation energy significantly decreases at around 368 K from 0.56 eV (303-348 K) to 0.24 eV (above 368 K) due to melting. The total ion conductivity reaches up to 6×10^-2 S/cm at the highest temperature measured 378 K. Li₂BNH₆ could be a good ionic liquid electrolyte as well as a solid electrolyte. Li₄BN₃H₁₀ also exhibits high ion conductivity of 2×10^-4 S/cm even at RT. The activation energy is evaluated to be 0.26 eV. It is noteworthy that this value is less than half those in Li₂BNH₆ before melting and LiBH₄, indicating that Li₄BN₃H₁₀ has higher lithium ion mobility. This feature was also confirmed by ⁷Li NMR, reflecting the different configuration of anions.¹⁵

Future Perspective

In this short review, we have introduced the lithium fast-ion conduction in LiBH₄ and its derivatives. We have recently elucidated that (NH₂)^-based and (AlH₄)^-based complex hydrides are also lithium ion conductors. The further research and development should provide new lithium ion conductors and new phenomena characteristic for complex hydrides.

Additionally, our group is currently focusing on the following subjects in order to demonstrate all-solid-state batteries using complex hydride solid electrolytes: 

1. Search for non-oxide cathode materials
2. Synthesis of thin-film Li₄BN₃H₁₀

Among the complex hydrides introduced in this review, Li₄BN₃H₁₀ is the most promising candidate for solid electrolyte because it has the highest conductivity and the lowest activation energy. Thin-film Li₄BN₃H₁₀ would allow the fabrication of thin-film lithium ion batteries, such as graphite/thin-film Li₄BN₃H₁₀/above-mentioned new cathode, for micro electromechanical system (MEMS).

Acknowledgments

Support for this work by KAKENHI (22760529, 18GS0203, 21360314 and 21246100) is gratefully acknowledged.