Solid-state ion conductors are gaining increasing importance, among other ion conductors, to enable a transition to next-generation all-solid-state Li batteries. However, few lightweight and low-cost materials show sufficiently high Li-ion conduction at room temperature to be used as solid electrolytes. Here, we report the effect of adding nanosized oxides, SiO2, CaO, MgO, γ-Al2O3, TiO2, and ZrO2, to LiBH4 by ball-milling. In all cases, the room temperature Li-ion conductivity was greatly enhanced. For SiO2, which has been reported before as a conductivity enhancing material, the highest conductivity (4.1 × 10–5 S/cm at 40 °C) and the lowest activation energy (0.49 eV) were found at 20 v/v% SiO2. For the first time, ZrO2 and MgO were also added to LiBH4, leading to more than a 4 orders of magnitude increase in conductivity at 40 °C, reaching 0.26 and 0.18 mS/cm, respectively. Based on insights into the effect of structural properties on conductivity, we present a set of general guidelines to maximize the Li-ion conductivity in these nanocomposite solid electrolytes, independently of the type of oxide added. We expect that these results and insights will be helpful for the further development of new room temperature solid-state ion conductors.

This study shows a flexible system that offers promising candidates for Li-based solid state electrolyte. The Brsubstitution for BH 4 stabilizes the hexagonal structure of LiBH 4 at room temperature, whereas Clis soluble only at higher temperatures. Incorporate chloride in hexagonal solid solution lead to increase the energy density of the system. For the first time, a stable hexagonal solid solution of LiBH 4 containing both Cland Br-halide anions has been obtained at room temperature (RT). The LiBH 4 -LiBr-LiCl ternary phase diagram has been determined at RT by X-ray diffraction coupled with a Rietveld refinement. A solubility of up to 30% of Clin the solid solution has been established. The effect of the halogenation on the Li-ion conductivity and electrochemical stability has been investigated by Electrochemical Impedance Spectroscopy and Cyclic Voltammetry. Considering the ternary samples, h-Li(BH 4 ) 0.7 (Br) 0.2 (Cl) 0.1 compositionshowed the highest value for conductivity (1.3 × 10 −5 S/cm at 30 °C), which is about three order of magnitude higher than that for pure LiBH 4 in the orthorhombic structure. The values of Li-ion conductivity at room temperature depend only on the BH 4 content in the solid solution, suggesting that the Br/Cl ratio does not affect the defect formation energy in the structure. The chloride anion substitution in the hexagonal structure increases the activation energy, moving from about 0.45 eV for samples without Clions in the structure, up to about 0.63 eV for h-Li(BH 4 ) 0.6 (Br) 0.2 (Cl) 0.2 compositions, according with the Meyer-Neldel rule. In addition to increasing Li ion conductivity, the halogenation increase also the thermal stability of the system.Unlike for the Li-ion conductivity, Br/Cl ratio influences the electrochemical stability: a wide oxidative window of 4.04 V vs. Li + /Li is reached in the Li-Br system, while further addition of Cl is a trade-off between oxidative stability and weight reduction. The halogenation allow both binary and ternary systems operating below 120 °C, thus suggesting possible applications of these fast ion conductors as solid-state electrolyte in Li-ion batteries. Graphical AbstractKeywords: complex hydride, solid state electrolyte, Li-ion batteries

LiBH 4 has been widely studied as a solid-state electrolyte in Li-ion batteries working at 120 °C due to the low ionic conductivity at room temperature. In this work, by mixing with MgO, the Li-ion conductivity of LiBH 4 has been improved. The optimum composition of the mixture is 53 v/v % of MgO, showing a Li-ion conductivity of 2.86 × 10 –4 S cm –1 at 20 °C. The formation of the composite does not affect the electrochemical stability window, which is similar to that of pure LiBH 4 (about 2.2 V vs Li + /Li). The mixture has been incorporated as the electrolyte in a TiS 2 /Li all-solid-state Li-ion battery. A test at room temperature showed that only five cycles already resulted in cell failure. On the other hand, it was possible to form a stable solid electrolyte interphase by applying several charge/discharge cycles at 60 °C. Afterward, the battery worked at room temperature for up to 30 cycles with a capacity retention of about 80%.