Dynamics of porous and amorphous magnesium borohydride to understand solid state Mg-ion-conductors

Abstract

Rechargeable solid-state magnesium batteries are considered for high energy density storage and usage in mobile applications as well as to store energy from intermittent energy sources, triggering intense research for suitable electrode and electrolyte materials. Recently, magnesium borohydride, Mg(BH 4) 2 , was found to be an effective precursor for solid-state Mg-ion conductors. During the mechanochemical synthesis of these Mg-ion conductors, amorphous Mg(BH 4) 2 is typically formed and it was postulated that this amorphous phase promotes the conductivity. Here, electrochemical impedance spectroscopy of as-received γ-Mg(BH 4) 2 and ball milled, amorphous Mg(BH 4) 2 confirmed that the conductivity of the latter is ~2 orders of magnitude higher than in as-received γ-Mg(BH 4) 2 at 353 K. Pair distribution function (PDF) analysis of the local structure shows striking similarities up to a length scale of 5.1 Å, suggesting similar conduction pathways in both the crystalline and amorphous sample. Up to 12.27 Å the PDF indicates that a 3D net of interpenetrating channels might still be present in the amorphous phase although less ordered compared to the as-received γ-phase. However, quasi elastic neutron scattering experiments (QenS) were used to study the rotational mobility of the [BH 4 ] units, revealing a much larger fraction of activated [BH 4 ] rotations in amorphous Mg(BH 4) 2. These findings suggest that the conduction process in amorphous Mg(BH 4) 2 is supported by stronger rotational mobility, which is proposed to be the so-called "paddle-wheel" mechanism. Energy storage is one of the grand challenges for present and future generations. In recent years, intermittent renewable energy production has increased worldwide resulting in a high demand for energy storage systems. "Beyond Li-batteries", which are all-solid-state batteries with alternative working ions, including Na and Mg, are considered a promising alternative as they are cheaper and with respect to their natural abundancy more sustainable. However, the transport properties of larger Na + or double-charged Mg 2+ are challenging and directly correlated to the underlying crystal structure and dynamics. Understanding the accompanying structural and dynamic changes as well as finding high-performance cathode materials remain bottlenecks for the improvement of Mg-ion batteries 1. Mg-ion batteries (MIBs) have several advantages compared to Li-ion technology 2. For instance, the low electrochemical potential of −2.4 V (vs. standard hydrogen electrode (SHE)) is close to the one of Li with −3.0 V of Li/Li + , which allows for high cell voltages. Furthermore, Mg metal has a higher volumetric capacity of 3833 mA•cm −3 compared to 2036 mAh•cm −3 of Li metal, and magnesium has a higher natural abundancy of

High Hydrogen Mobility in an Amide–Borohydride Compound Studied by Quasielastic Neutron Scattering

“…Similar findings of hindered rotations have been made for the α , β , and γ phases of . [ 52,54,57 ] These studies also highlight the crucial dependence of motions on crystal structures and temperatures.…”

Metal (boro-) hydrides for high energy density storage and relevant emerging technologies

“…Heere et al recently investigated the structure of the amorphous phase and the results are further discussed in Ref. [215]. The study used X-ray total scattering and pair distribution function (PDF)…”

The Crystal Chemistry of Inorganic Hydroborates

“…− anion [36,37]. The disorder of atomic positions observed from Bragg scattering needs a correct understanding of the superposition of anion point symmetry (Figure 1) with the point symmetry of Wyckoff positions in the crystal.…”