Methylamine Magnesium Borohydrides as Electrolytes for All-Solid-State Magnesium Batteries

Abstract

Solid-state magnesium electrolytes may pave the way for novel types of rechargeable, sustainable, and cheap batteries with high volumetric and gravimetric capacities. There are, however, currently no solid-state magnesium electrolytes that fulfill the requirements for solid-state battery applications. Here, we present the synthesis, structure, and properties of six new methylamine magnesium borohydride compounds, α- and β-Mg(BH4)2·6CH3NH2, Mg(BH4)2·3CH3NH2, and α-, α′- and β-Mg(BH4)2·CH3NH2. The β-Mg(BH4)2·CH3NH2 polymorph displays a record high Mg2+ ionic conductivity of σ(Mg2+) = 1.50 × 10–4 S cm–1 at room temperature. The high Mg2+ conductivity of β-Mg(BH4)·CH3NH2 is facilitated by a one-dimensional chain-like structure interconnected by weak dihydrogen bonds and dispersion interactions, forming a migration pathway across the chains. The oxidative stability of Mg(BH4)2·CH3NH2 is ∼1.2 V vs Mg/Mg2+, and the reversible plating and stripping were confirmed by cyclic voltammetry and symmetric cell cycling, revealing high stability toward magnesium electrodes for at least 50 cycles at 60 °C.

Hemi-methylamine lithium borohydride as electrolyte for all-solid-state batteries

“…1a. 18,23,33,34 tetrahedral units are connected by edge-sharing via bridging BH 4 -groups along the c-axis and via corner-sharing along the aaxis in the sequence ABAB (Fig. 1e), forming two-dimensional layers in the ac-plane, which are stacked along the b-axis.…”

“…[15][16][17][18] Interestingly, this approach has also proven useful for multivalent ionic conductors as reported for Mg(BH 4 ) 2 with different N-and O-based neutral ligands. [19][20][21][22][23][24] The fast cation conductivity in these materials have been suggested to result from a high exibility of the structure and the exible coordination of BH 4 2CH 3 NH 2 . The new compound exhibit a high ionic conductivity, s(Li + ) = 1.88 × 10 −3 S cm −1 at T = 31 °C, making it a promising solid-state electrolyte.…”

“…15–18 Interestingly, this approach has also proven useful for multivalent ionic conductors as reported for Mg(BH 4 ) 2 with different N- and O-based neutral ligands. 19–24 The fast cation conductivity in these materials have been suggested to result from a high flexibility of the structure and the flexible coordination of BH 4 − . 15,17,25…”

Enhanced interface stability of ammine magnesium borohydride by in situ decoration of MgBr2·2NH3 nanoparticles

“…[14][15][16] Regarding the Mg(BH 4 ) 2 -based solidstate electrolytes, breaking the [BH 4 ] tetrahedral cages in Mg(BH 4 ) 2 could improve the ionic conductivity but also lower the stability. For instance, some electrolytes (e.g., Mg(BH 4 ) 2 Á (NH 3 BH 3 ) 2 , 9 Mg(BH 4 ) 2 ÁCH 3 NH 2 17 ) allow Mg stripping/plating only at a low current density r10 mA cm À2 , and decompose on the anode surface at higher current density of e.g., 0.1 mA cm À2 forming a passivation layer and hindering the Mg 2+ migration.…”

“…For instance, some electrolytes ( e.g. , Mg(BH 4 ) 2 ·(NH 3 BH 3 ) 2 , 9 Mg(BH 4 ) 2 ·CH 3 NH 2 17 ) allow Mg stripping/plating only at a low current density ≤10 μA cm −2 , and decompose on the anode surface at higher current density of e.g. , 0.1 mA cm −2 forming a passivation layer and hindering the Mg 2+ migration.…”