Room-Temperature Mg-Ion Conduction Through Molecular Crystal Mg{N(SO2CF3)2}2(CH3OC5H9)2

Abstract

Molecular crystals have attracted increasing attention as a candidate for innovative solid electrolytes with solid-state Mg-ion conductivity. In this work, we synthesized a novel Mg-ion-conducting molecular crystal, Mg{N(SO2CF3)2}2(CH3OC5H9)2 (Mg(TFSA)2(CPME)2), composed of Mg bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2) and cyclopentyl methyl ether (CPME) and elucidated its crystal structure. We found that the obtained Mg(TFSA)2(CPME)2 exhibits solid-state ionic conductivity at room temperature and a high Mg-ion transference number of 0.74. Contrastingly, most Mg-conductive inorganic solid electrolytes require heating above 150–300°C to exhibit ionic conductivity. These results further prove the suitability of molecular crystals as candidates for Mg-ion-conducting solid electrolytes.

Organic Crystalline Solid Electrolytes with High Mg-Ion Conductivity Composed of Nonflammable Ionic Liquid Analogs and Mg(TFSA)2

“…The nearest Mg–Mg distances ( d (Mg–Mg)) for 1 and 2 were estimated to be 6.880 and 8.141 Å, respectively. Compared to the recently reported Mg-ion conductive molecular crystal, [Mg­{N­(SO 2 CF 3 ) 2 } 2 (cyclopentyl methyl ether) 2 ] ([Mg­(TFSA) 2 (CPME) 2 ]; d (Mg–O) av = 2.07 Å, d (Mg–Mg) = 9.474 Å), the Mg–O and Mg–Mg distances in 1 and 2 were both shortened.…”

“…The activation energy was calculated to be 117 kJ mol –1 . Organic ionic crystal 2 exhibits higher conductivity and larger activation energy than (Mg­(TFSA) 2 (CPME) 2 ), which exhibits an ionic conductivity of 4 × 10 –7 S cm –1 at 40 °C and activation energy of 69 kJ mol –1 . The large differences in the structure around the Mg ion and the molecular arrangements in crystal lattices found in 2 and Mg­(TFSA) 2 (CPME) 2 may have led to this difference in ionic conductivity.…”

“…Although the value of t Mg2+ of 2 is preliminary data, it is evidently smaller than that of the previously reported Mg-ion-conducting molecular crystal, Mg­(TFSA) 2 (CPME) 2 ( t Mg2+ = 0.74) . In the crystal lattice of Mg­(TFSA) 2 (CPME) 2 , the ionic components that can become diffusion species are only Mg ions and TFSA anions.…”

“…Mg­(TFSA) 2 (CPME) 2 , which is composed of the flammable organic solvent CPME, exhibits a low melting point and moderate ionic conductivity of 10 –6 S cm –1 around the melting point . However, the use of organic ionic species N 1122 TFSA with a relatively high melting point as a component of 2 resulted in an increase in the melting point to approximately 100 °C, indicating that the thermal stability of 2 was improved compared to that of Mg­(TFSA) 2 (CPME) 2 .…”

“…To attain this objective, we focus on molecular crystalline electrolytes consisting of organic molecules and metal salt with organic anions as promising materials. − We recently reported a molecular crystal, Li­{N­(SO 2 F) 2 }­(NCCH 2 CH 2 CN) 2 , with a high lithium conductivity of 10 –4 S cm –1 at room temperature, and also successfully fabricated an all-solid-state battery. , These electrolytes are characterized by the formation of ionic conduction paths in the ordered arrangement of component molecules and ions, similar to those found in ceramic electrolytes. Because of this characteristic feature, the structural control of component anions and organic molecules will lead to modification of the structure of the ionic conduction paths in molecular crystalline electrolytes to improve the ionic conductivity. , This indicates that molecular crystals are a potential candidate for innovative solid electrolytes with high Mg-ion conductivity …”