Investigation of Mg(BH₄)(NH₂)-Based Composite Materials with Enhanced Mg²⁺ Ionic Conductivity

Abstract

Nowadays, the development of rechargeable Mg batteries remains a challenge, especially due to the difficulty to find a non-corrosive liquid electrolyte with suitable ionic transport properties. A possible improvement could come from a solid electrolyte, such as Mg(BH 4)(NH 2), which has been recently reported as a solid-state Mg-ion conductor. In this study, its synthesis parameters are carefully investigated. The formation of an additional phase is reported, whose amount is decreased by optimizing the synthesis parameters and especially by increasing the ball-milling speed. For the first time, 11 B MAS-NMR spectroscopy is applied to this Mg-B-N-H system: it reveals that an additional phase is always present even in an amorphous state. Interestingly, it strongly influences the ionic conduction properties. Indeed, the as-obtained borohydride-amide composite exhibits a high conductivity of 3.10-6 S.cm-1 at 100°C, one of the highest ever-reported ionic conductivity for a Mg 2+ solid conductor at such low temperature.

Facile Self‐Forming Superionic Conductors Based on Complex Borohydride Surface Oxidation

“…This was explained by the possibility to generate through the Li 2 (BH 4 )(NH 2 ) structure plural Li + occupation sites. Similar findings were also reported for Na(BH 4 ) 0.5 (NH 2 ) 0.5 , 2 × 10 −6 S cm −1 at 27 °C,20 and Mg(BH 4 )(NH 2 ), 3 × 10 −6 S cm −1 at 100 °C 21. By encapsulating LiBH 4 into a silicon dioxide scaffold (MCM‐41) a higher ionic conductivity of ≈10 −4 S cm −1 at 40 °C has also been claimed and this was attributed to fast [BH 4 ] − reorientation at the MCM‐41 wall /LiBH 4 interface 22.…”

“…To date, the best possible ionic conductivity has been reported upon the modification of Mg(BH 4 ) 2 with ethylenediamine leading to an ionic conductivity of 5 × 10 −8 S cm −1 at 30 °C 42. Modification with ammonia enabled an ionic conductivity of 3 × 10 −6 S cm −1 at 100 °C 21. The very low conductivity of pristine Mg(BH 4 ) 2 is attributed to its structure in which the Mg 2+ ions are seating in firm tetrahedral cages composed of four BH 4 − units with strong coulombic interaction 43…”

“…[42] Modification with ammonia enabled an ionic conductivity of 3 × 10 −6 S cm −1 at 100 °C. [21] The very low conductivity of pristine Mg(BH 4 ) 2 is attributed to its structure in which the Mg 2+ ions are seating in firm tetrahedral cages composed of four BH 4 − units with strong coulombic interaction. [43] The actual Li + and Mg 2+ ion transport upon oxidation of their respective borohydride was further confirmed by CV measurements showing peaks related to their relative deposition and stripping (Figures S13 and S14, Supporting Information).…”

“…Similar findings were also reported for Na(BH 4 ) 0.5 (NH 2 ) 0.5 , 2 × 10 −6 S cm −1 at 27 °C, [20] and Mg(BH 4 )(NH 2 ), 3 × 10 −6 S cm −1 at 100 °C. [21] By encapsulating LiBH 4 into a silicon dioxide scaffold (MCM-41) a higher ionic conductivity of ≈10 −4 S cm −1 at 40 °C has also been claimed and this was attributed to fast [BH 4 ] − reorientation at the MCM-41 wall /LiBH 4 interface. [22] The effect of larger size anions including [B 12 H 12 ] 2− , [B 10 H 10 ] 2− , and CB 9 H 10 − was also investigated.…”

Recent Development of Mg Ion Solid Electrolyte

“…It showed the ionic conductivity of 10 −6 S cm −1 at 150 • C and the electrochemical window of ∼3 V in estimation. Le Ruyet and Janot followed the work on Mg(BH 4 )(NH 2 ) in 2019, and studied the influences of the synthesis parameters on the ionic conductivity (Le Ruyet et al, 2019). They carefully investigated the synthesis parameters and the ionic conductivity could reach as high as 3 × 10 −6 S cm −1 at 100 • C, which was three orders of magnitude higher than that reported by Higashi.…”