Synthesis of rock-salt type lithium borohydride and its peculiar Li⁺ ion conduction properties

State-of-the-art battery research is on All-Solid-State-Batteries. In applications where safety is considered a first order consideration, chemically stable batteries with no liquid electrolytes but with high energy density need to be developed.

To get completely solid batteries into functional use, there are two challenges that researchers need to meet. The first is to make a solid electrolyte where the ions migrate easily. While clear steps forward have been made,^1,2 the actual application of these materials has been hampered by their poor sinterability and contact with Li metal.^3–6 The other challenge researchers face is to construct a good interface between the electrolyte and electrode active materials. The simultaneous formation of electronic and ionic conduction paths within electrodes is essential for a continuous electrochemical reaction. This is not an issue in liquid electrolytes because the penetration of the electrolyte in active materials provides both electronic and ionic conduction paths.

Our research suggests that there is one solid electrolyte which has both of these required characteristics, and that these features have the potential for further improvement. LiBH₄ undergoes a phase transition from an orthorhombic structure (hereafter referred to as the L.T. form) into a hexagonal form (the H.T. form, or high temperature form) at 115 °C.⁷ Matsuo et al. reported super Li⁺ ion conduction for the H.T. form of LiBH₄.⁸ Furthermore, it was confirmed that LiI doping stabilized the H.T. phase.^9,10 Since almost the same conductivity values were obtained from the AC impedance method and the ⁷Li NMR T₁ measurement, a dense green pellet without any grain boundary resistance can be obtained by uniaxial pelletizing.⁸ In other words, a material with good contact between LiBH₄ and the electrode materials has been developed without the need for sintering.

Takahashi et al. recently fabricated a cell composed of LiBH₄ and LiCoO₂ thin film covered with Li₃PO₄ layer and reported a charge-discharge performance at 120 °C.¹¹ An electrochemically fresh contact between LiBH₄ and Li metal was anticipated due to its strong reducing character. In order to fully utilize this characteristic, a LiBH₄ - Li composite electrode was fabricated with the potential of enhancing tolerability under high rate conditions.¹² 

Since it is well known that the reaction field of an electrochemical reaction is limited to the interface between the electrolyte and electrode active materials, electrolyte materials should preferably have an isotropic diffusion path to eliminate any ineffective surface area which does not contribute to electrochemical reactions. It follows, then, that a cubic structure would allow for a more effective interface with electrode materials than the hexagonal form of LiBH₄, for which two dimensional conduction has been proposed.^13,14 Fortunately, LiBH₄ undergoes a phase transition above 4 GPa to form a rock-salt type structure, referred to as the high pressure form (H.P. form).¹⁵ 

Takamura et al. reported that the electrical conductivity for the H.P. form measured in the 4–6 GPa range indicated a lower carrier concentration than the H.T. phase.¹⁶ The most interesting point is that almost the same activation energy for Li⁺ ion conduction is obtained despite the completely different crystal structure (wurtzite and a rock-salt type structure for the H.T.⁷ and H.P. phases,¹⁵ respectively).¹⁶ With this in mind, it is reasonable to speculate that when combined with its expected isotropic conduction path, the H.P. form after the doping of a considerable number of cation vacancies would be superior to the H.T. form.

Nevertheless, in order to put it into practical use, the H.P. form would need to be stable at ambient pressure, and stabilizing the H.P. form has proven to be quite a challenge. Typically, a given phase is stabilized by the doping of other compounds in the matrix of the base material, as has already been reported for the LiBH₄ - LiI system.^9,10 Another method to synthesize chemically modified H.P. form would be doping LiBH₄ with KI, which also has a rock-salt type structure. In our study, we synthesized the H.P. form of LiBH₄ by the second of these methods and verified the structure both macroscopically and microscopically. The thermodynamic and ion conduction properties were also investigated.

LiBH₄ (Aldrich, >90% purity) and KI (Aldrich, >99.998% purity) were mixed in a mortar at a given molar ratio and pelletized under 1 t/cm². The obtained pellet was sealed in a silica glass tube and sintered under vacuum conditions. Samples with a high LiBH₄ content were sintered at 180 °C, while LiBH₄ poor samples were sintered at approximately 250 °C. After sintering the samples for 1 h, they were ground and then re-pelletized for the subsequent 8 h of sintering. To identify the crystal structure of the samples, Cu - Kα XRD measurements were carried out using BrukerD8 Advance Diffractometer. The microstructure of the sample was observed by SEM (JSM-6360LA, JEOL). The fracture surface of the sintered sample was observed without polishing. The composition analysis was made by the EDX method and the distribution morphology of the secondary phase was determined by atomic mapping for K and I. For AC impedance measurements, Li foils were fixed on both side of the pelletized sample (diameter 10 mm; thickness ∼1 mm). The measured temperature range was from 30 °C to 160 °C and heating/cooling cycles were repeated. Potentiostatic measurement was performed using Li or stainless steel electrodes. All measurement procedures were done in Ar atmosphere.

Fig. 1(a) shows the XRD patterns of KI - LiBH₄ systems. While diffraction peaks derived from KI were observed for all samples, none were detected for either the H.T. or L.T. form of LiBH₄. In the LiBH₄ rich sample (e.g., LiBH₄ 75 mol. % KI), extra peaks were observed and are indicated by the solid black spots. The calculated lattice parameters from these XRD patterns are presented as a function of the amount of LiBH₄ doping in Fig. 1(b). Values on the a axis decreased along with the amount of LiBH₄. A comparison of the ionic radii of the constituent ions of KI and LiBH₄ indicates K⁺ (1.38 Å) and I⁻ (2.20 Å) are larger than Li⁺ (0.76 Å) and BH₄⁻ (2.05 Å), respectively. The contraction of the lattice parameters of KI occurs with the formation of solid solution between KI and LiBH₄ via the substitution of each cation and anion. This confirms that the H.P. form of LiBH₄ was successfully synthesized under ambient pressure by incorporating LiBH₄ into the rock-salt type KI lattice.

The microstructure of the synthesized H.P. form of LiBH₄ was not quite what we expected, however. The SEM image of the fracture surface of sintered 3KI·LiBH₄ (25 mol. % LiBH₄) is shown in Fig. 2(a) and the results of the EDX analysis at two different measurement points are listed in Table I. The most prominent difference between the two points was the potassium content. Because KI was used as the base material for LiBH₄ doping, it is reasonable to assume that the atomic ratio of K and I would be the same if LiBH₄ had dissolved in the KI lattice. At the point marked by the blue symbol, the atomic ratio of K and I was almost unity, suggesting this region was the rock-salt type LiBH₄ integrated in KI. However, a smaller atomic ratio of K compared to I was detected at the point marked by the red symbol, indicating a crystal structure is quite different from that of rock-salt in this domain. The EDX atomic mapping figure on K and I provide a visual image of the distribution morphology of these two regions (Fig. 2(b)). The yellow area in Fig. 2(b) shows that the intensity of K and I were equal, indicating the presence of rock-salt type solid solution. A potassium deficit domain is represented in green between the yellow grains. The area ratio of green and yellow regions from Fig. 2(b) indicates that the solid solution between KI and LiBH₄ exists as a main phase, although small amounts of the secondary phase were detected. The integration of LiBH₄ into KI was also confirmed microscopically, confirming the results of the XRD analysis. Details in the secondary phase and a SEM image of LiBH₄·KI (50 mol. % LiBH₄) are given in Fig. S1 and Table S1 of the supplementary material.¹⁷ The thermodynamic properties of these samples were also described in the supplemental material (Figs. S2–S5).¹⁷ 

The temperature dependence of electrical conductivity for 3KI·LiBH₄ is shown in Fig. 3 with Nyquist plots at each temperature. Only one semicircle derived from bulk resistance is observed without any grain boundary contribution. The microscopic configuration of this sample (Fig. 2(a) and 2(b)) strongly suggests that the migration path of the Li⁺ ion was formed among rock-salt type grains without being blocked by other components. There is no doubt that the observed electrical conductivity behavior represents that of the KI - LiBH₄ solid solution. As such, the phase transition behavior of the secondary phase detected by DSC measurements was not expected to have an influence on the conductivity results (see Fig. S3 in the supplementary material).¹⁷ The slope of the Arrenius plot was clearly changed at approximately 120 °C, indicating that the intrinsic and extrinsic region of carrier formation were observed above and below the knee temperature, respectively. This behavior strongly supports a vacancy mediated conduction mechanism where a mobile ion migrates by using a vacant cation site formed by thermal excitation (the intrinsic region) or charge compensation for aliovalent doping (the extrinsic region). The results of samples with different composition (KI·LiBH₄) can be found in Fig. S6 of the supplementary material.¹⁷ 

Determining which ionic species contributes to the total ion conduction is the most interesting task in solid state ionics especially in the case of the light doping materials in focus in this work. As shown in Fig. 4, almost the same resistance values were obtained by AC and DC methods suggesting the transference number of the Li⁺ ion in 3KI·LiBH₄ was nearly unity. Together with the observed micro electronic current (in the order of 10⁻⁸ A) measured by the stainless steel electrodes, 3KI·LiBH₄ was shown to be a pure Li⁺ ion conductor. With such a low Li⁺ ion content in the main phase (at least below 25 mol. % considering the Li amount in the secondary phase), this is particularly impressive: the Li⁺ ion plays a major role in ion conduction while the K⁺ ion appears to remain stationary at the equilibrium site. We would like to call this new conduction mechanism the “Parasitic Conduction Mechanism.” This Parasitic Conduction Mechanism indicates that light LiBH₄ doping is sufficient to allow this material to function as an isotropic Li⁺ ion conductor. From the slope of the Arrhenius plots at the extrinsic region, an activation energy of 0.62 eV is obtained (Fig. 3), which is equivalent to the energy required for migration from one stable site to another via a cation vacancy. This is slightly larger than the reported value of other pure rock-salt type Li⁺ ion conductors such as the H.P. form of LiBH₄ (0.56 eV)¹⁶ and LiI (0.43 eV).¹⁸ We would like to propose two possibilities to explain these results. When two different cations simultaneously exist in the same lattice, the activation energy for cation conduction is known to increase, as has been reported for β - Al₂O₃.¹⁹ This is known as the mixed cation effect and is also quite common in cation conductive vitreous materials.²⁰ This phenomenon may have contributed to the higher activation energy for the KI - LiBH₄ system. Another possibility is the excess large lattice volume of KI framework for Li⁺ ion conduction. Kummer reported a size effect on the cation mobility at the β - Al₂O₃ lattice, indicating that a large lattice volume is not necessary for small ions like the Li⁺ ion despite its effectiveness in cation conduction due to the expanding bottleneck.²¹ Although we could not obtain the detailed structural information about the Li⁺ site from the XRD patterns due to its small scattering factor, it may well be that the lower coordination numbers favored by the relatively small Li⁺ ion would have kept the Li⁺ in the vicinity of the I⁻ site rather than at the center of the K⁺ site. An investigation of conductivity under high pressure may provide accurate information about the proper framework size for Li⁺ ion conduction.

If the Parasitic Conduction Mechanism occurs within a given Li⁺ ion conductor, a light amount of Li doping would be sufficient for material design, which means a new ion conductor could be fabricated without the limits imposed by the solubility limit of a given system. This relatively uncommon method of material synthesis involving the Li compound doping of the existing crystal lattice has the potential to become a standard procedure in the quest for a new ion conductor and to provide the stabilization of a given phase. We believe our work on the so-called Parasitic Conduction Mechanism will contribute to the further progress of solid state ionics.

In conclusion, we synthesized the H.P. form of LiBH₄ under ambient pressure by doping LiBH₄ with the KI lattice by sintering. The formation of a KI - LiBH₄ solid solution was confirmed both macroscopically and microscopically. That is, the H.P. form was clearly stabilized by chemical modification. The obtained sample was shown to be a pure Li⁺ conductor despite its small Li⁺ content. This conduction mechanism, where the light doping cation played a major role in ion conduction, was termed the “Parasitic Conduction Mechanism.” This mechanism made it possible to synthesize a new ion conductor and is expected to have enormous potential in the search for new battery materials.

H.T. would like to thank the Funding Program for Next Generation World-Leading Researchers (NEXT Program: GR011) by Japan Society for the Promotion of Science.