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

Abstract

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

Room-Temperature Solid-State Lithium-Ion Battery Using a LiBH4–MgO Composite Electrolyte

“…The gray line corresponds to the Li-ion conductivity of pure LiBH 4 . 10 (b) Li-ion conductivity at 20, 60, and 120 °C as a function of the pore filling. Dashed lines are a guide for the eyes.…”

“…Li-ion conductivity data for the samples were obtained by collecting the electrochemical impedance spectroscopy (EIS) spectra following the procedure reported in ref ( 10 ). The EIS measurements were performed using an HP4192A LF impedance analyzer and a Novocontrol (BDS 1200) sample cell in the temperature range of 20 < T < 130 °C (every 10 °C).…”

“…A solid solution is formed by substituting BH 4 – with I – , Br – , and Cl – , stabilizing the hexagonal polymorph at RT. 10 , 13 , 14 Mixing other complex anions with LiBH 4 ( e.g. , NH 2 – and NH 2– ) leads to the formation of compounds with different structures, with enhanced ionic conductivity at RT.…”

Combined DFT and geometrical–topological analysis of Li-ion conductivity in complex hydrides

“…4 Different approaches have been applied in order to increase the Li-ion conductivity of these compounds. It can be achieved, for example, with stabilizing of the hexagonal phase by mixing it with halides 5,6 or creating a high conductive interface by mixing or nano-confining it with oxides. 7,8 New compounds can be also formed, as in the case of mixing LiBH 4 with lithium imide and amide.…”

Synthesis, Polymorphism and Thermal Decomposition Process of (n-C4H9)4NRE(BH4)4 for RE = Ho, Tm and Yb

“…Although most of studies focus on examining of the process of hydrogen release, several reports indicate at least some degree of reversibility of such systems, which is crucial for their application, e.g., [ 10 ]. Various borohydrides can also serve as precursors of metal borides [ 11 , 12 , 13 , 14 , 15 ] or boron nitride [ 16 ], and some of them have been explored as prospective solid-state electrolytes in Li + batteries [ 17 , 18 , 19 , 20 , 21 ] or luminescent and magnetic materials [ 22 , 23 ], while the borohydride complexes of rare-earth elements were also tested for organic catalysis [ 24 , 25 ].…”