Lithium-Ion Conduction in LiBH4 Hydrated H2o and D2o

Abstract

Lithium secondary batteries are widely used for mobile electric products because of their high energy density, low self-discharge rate and lack of memory effect. Recently, all-solid-state lithium secondary batteries have been attracted much attention as alternatives to batteries using organic liquid electrolytes. Among the various solid electrolytes studied, lithium borohydride (LiBH4) is attracting much attention. LiBH4 with an orthorhombic phase (space group: Pnma) at room temperature is an insulator; however, the hexagonal phase (P63 mc) emerging above 115°C shows a high lithium-ion conductivity of ≈10−3 S·cm−1 [1]. LiBH4 is also well-known to be easily reacted with water. Hydration reactions of LiBH4 leading hydrogen gas emission have already been studied as a hydrogen storage medium [2, 3]. These reports focus on the hydrogen storage behaviors; however, the relationship between hydration and lithium-ion conductivity has not been clarified yet. In this study, the effects of hydration on the phase stability and the lithium-ion conductivity of LiBH4 were studied. LiBH4 powder (purity > 95%) was enclosed in a glass tube and heat-treated in a vacuum at 100°C for dehydration. Water vapor was then gradually introduced from 0.1P 0 to 0.9P 0 (P 0: saturation vapor pressure) at 25°C by Sieverts apparatus (BEL Japan, Inc.; BELSORP 18) without exposure to air. Distilled water (H2O) and deuterated water (D2O) were used as water sources. Their structural information was obtained by X-ray diffraction (XRD) and micro Raman spectroscopy. A differential scanning calorimetry (DSC) was carried out to evaluate the stability of the hydrated samples. Electrochemical properties were obtained by AC impedance measurements and DC potentiostatic measurement under an Ar atmosphere in hermetically sealed vessels. 1H and 7Li nuclear magnetic resonance (NMR) was also conducted at 7 T. XRD and Raman analyses show LiBH4 hydrated with H2O was monohydrated LiBH4·H2O (monoclinic structure, P21/c) which has Li+, BH4 − tetrahedrons and H2O molecules in the lattice. LiBH4·H2O showed higher electrical conductivity than that of anhydrated LiBH4 around room temperature. The main carrier was found to be Li+ by DC measurement with lithium electrode. A maximum electrical conductivity of 4.89×10−4 S·cm−1 was attained at 45°C. This conductivity is comparable with that of the high temperature phase of LiBH4; however, the conductivity drastically decreased above 50°C [4]. To evaluate the phase stability of LiBH4·H2O, DSC measurement was conducted for the sample in a hermetic aluminum pan. An endothermic reaction was observed at around 55°C in the 1st heating process. This reaction appears to relate the conductivity drop above 50°C. As temperature further increased, a large exothermic peak emerged above 60°C. LiBH4 presumably reacted with desorbed water and decomposed accompanied by H2 gas emission, which was detected by quadrupole mass analyzer. It is also noted that H2O gas emission is slightly observed in whole temperature range. This dehydration presumably affects the electrical conductivity deterioration. 1H and 7Li NMR measurement was conducted to investigate local structure of LiBH4·H2O. 7Li NMR spectra of LiBH4·H2O show motional narrowing indicating fast Li+ conduction. This implies that enhancement in electrical conductivity originates from high Li+ mobility. 1H NMR spectra indicate the existence of 2 different local environments attributed to H2O and BH4 −. Local motion of H2O and BH4 − in LiBH4·H2O might affect Li+ conduction as in the high temperature phase of LiBH4. To further clarify the interactions between Li+ and structural water, LiBH4 hydrated with D2O was also prepared and analyzed. [1] M. Matsuo et al., Appl. Phys. Lett. 91 (2007) 224103. [2] J.P. Goudon et al., Int. J. Hydrogen Energy 35 (2010) 11071–11076. [3] H. Yamawaki et al., J. Alloys Compd. 541 (2012) 111–114. [4] A. Takano et al., Solid State Ionics 285 (2016) 47–50.