Phase Behavior in the LiBH₄–LiBr System and Structure of the Anion-Stabilized Fast Ionic, High Temperature Phase

Abstract

The fast ionic, high temperature (HT) phase of LiBH4 can be stabilized by Br¯ substitution. Lithium borohydride bromide compounds, Li­(BH4)1–x Br x , have been synthesized mechanochemically, with and without thermal treatment and the resulting phase behavior determined as a function of composition. Single phase materials exist for 0.29 ≤ x ≤ 0.50 with conductivity 2 orders of magnitude higher than LiBH4 at 313 K. Powder neutron diffraction has been used to resolve the details of the crystal structure of one such compound. These demonstrate that 7Li­(11BD4)0.667Br0.333 retains the HT structure (hexagonal space group P63 mc, a ≈ 4.2 Å, c ≈ 6.7 Å) from 293 to 573 K. The borohydride bromide exhibits considerable static and dynamic disorder, the latter invoking complex rotational motion of the (BH4)¯ anions.

Phase Stability and Fast Ion Conductivity in the Hexagonal LiBH4–LiBr–LiCl Solid Solution

“…(see paragraph 3.1) the solubility limits of the hexagonal solid solution were defined. The calculated mutual molar quantity of phases has been checked with respect to the level rule,61 confirming the reliability of calculations.The solubility limits in the LiBH 4 -LiBr system were determined by Sveinbjörnsson et al35 and confirmed in this study (samples s5-s9). The Li(BH 4 ) 1-α (Br) α hexagonal solid solution is stable at RT as a single phase from = 0.3 to = 0.5.…”

“…In In order to study the changes in the vibrational properties of lithium borohydride, due to the stabilization of the hexagonal phase by halide additions, IR-ATR spectroscopy was performed on AN sample s2 and the result is shown in Figure 3, together with the spectrum for pure o-LiBH 4 Usually, both mechanical and thermal treatments are necessary to obtain a single lithium borohydride-halide hexagonal solid solution. 52,57,58 However, after 24 h of milling, a complete solid solution h-Li(BH 4 ) 0.667 Br 0.333 was successfully obtained by Sveinbjörnsson et al 35 In order to investigate the feasibility to form a 3-anions hexagonal phase only by mechanochemistry, the same composition of the sample s2 was ball milled for different times (sample s3). Figure S6 A small difference in ionic radius is promoting the formation of solid solutions.…”

“…59 Indeed, for LiBH 4 -LiI and LiBH 4 -LiBr systems, that have similar anionic radii, a significant miscibility is expected and confirmed experimentally, with the stability of the hexagonal phase at RT. 32,35,52,57 On the other hand, Clanion is miscible in h-LiBH 4 only at temperatures close to 100 °C. 60 In fact, the radii of BH 4 and Cldiffer significantly.…”

Lightweight complex metal hydrides for Li-, Na-, and Mg-based batteries

“…On the other hand, the higher mobility of Li 1 in Li(BH 4 ) 1Àn I n , compared to LT-LiBH 4 , was assigned to the I À -induced lattice anharmonicity and the increased distance between neighboring [BH 4 ] À units [105,111]. The formation of solid solutions with enhanced Li-ion conductivity was also observed in the LiBH 4 -LiBr and LiBH 4 -LiCl systems [14,16,17]. Additionally, in the case of Li(BH 4 ) 1Àn Br n , the importance of applied synthesis methods (mechanochemical powder processing alone over combination with thermal annealing) on the ionic transport properties was emphasized [16].…”

Molecular-salt hybrids; integration of ammonia borane into lithium halides

“…The HT polymorph can be stabilised at room temperature by partial replacement of (BH 4 ) − with Br − or I − , leading to an increase in conductivity by several orders of magnitude at 30°C. 7,8 Other hydrides of even higher complexity such as amide borohydrides can exhibit Li ion transport into the superionic regime with values of conductivity at ambient and elevated temperatures far exceeding those of the equivalent component binary lithium compounds themselves. 9,10 In fact, from a practical standpoint, complex borohydrides exhibit surprisingly good compatibility with Li (or Na) electrodes and a high electrochemical stability.…”