Construction of nanostructures for selective lithium ion conduction using self-assembled molecular arrays in supramolecular solids

Abstract

In the development of innovative molecule-based materials, the identification of the structural features in supramolecular solids and the understanding of the correlation between structure and function are important factors. The author investigated the development of supramolecular solid electrolytes by constructing ion conduction paths using a supramolecular hierarchical structure in molecular crystals because the ion conduction path is an attractive key structure due to its ability to generate solid-state ion diffusivity. The obtained molecular crystals exhibited selective lithium ion diffusion via conduction paths consisting of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and small molecules such as ether or amine compounds. In the present review, the correlation between the crystal structure and ion conductivity of the obtained molecular crystals is addressed based on the systematic structural control of the ionic conduction paths through the modification of the component molecules. The relationship between the crystal structure and ion conductivity of the molecular crystals provides a guideline for the development of solid electrolytes based on supramolecular solids exhibiting rapid and selective lithium ion conduction.

Keywords: 101 Self-assembly / Self-organized materials; 20 Organic and soft materials (colloids, liquid crystals, gel, polymers); 206 Energy conversion / transport / storage / recovery; 501 Chemical analyses; 504 X-ray / Neutron diffraction and scattering; Ion conduction; lithium; molecular crystals; self-assembly; solid electrolytes; supramolecules.

Graphical abstract

Figure 1.

Schematic diagram of the development of ion conduction paths using supramolecular assemblies.

Figure 2.

The designing concept of molecular crystalline electrolytes.

Figure 3.

Crystal structure of compound 2 determined from the powder X-ray diffraction (XRD) pattern at 100 K. (Reproduced from [37] with permission from John Wiley and Sons.).

Figure 4.

The packing view of compound 2 along the b-axis determined from the XRD pattern at 100 K. (Reproduced from [37] with permission from John Wiley and Sons.).

Figure 5.

Ion conductivities of compound 2 (blue), compound 1+2.5LiTFSA (red) and compound 1+1.8LiTFSA (green). (Adapted from [37] with permission from John Wiley and Sons).

Figure 6.

Molecular structures of compounds 3–5.

Figure 7.

Crystal structures of compounds 3–5 determined from single-crystal X-ray diffraction study at -120 °C. (Li: yellow, C: gray, N: blue, O: red, F: green, S: dark yellow. Hydrogen atoms have been omitted for clarity.) (Adapted from [34] and [36] with permission from John Wiley and Sons and Elsevier.).

Figure 8.

Packing view of compound 3 determined from single-crystal X-ray diffraction study at -120 °C. (a) along the a-axis, (b) along the b-axis, (c) along the c-axis (Li: yellow, C: gray, N: blue, O: red, F: green, S: dark yellow. Hydrogen atoms have been omitted for clarity.) (Adapted from [36] with permission from John Wiley and Sons.).

Figure 9.

Ion conductivities of compounds 3–5 as a function of temperature (compound 3: blue; compound 4: green; compound 5: red).

Figure 10.

Molecular structures of compounds 6–10 (compound 7: R¹=CH₃, R²=CH₃; compound 8: R¹=H, R²=CH₂CH₃; compound 9: R¹=H, R²=CH₃).

Figure 11.

Crystal structures of compounds 6–8 and 10 determined from single-crystal X-ray diffraction study at -120 °C. (Li: yellow, C: gray, N: blue, O: red, F: green, S: dark yellow. Hydrogen atoms have been omitted for clarity.) (Adapted from [35] with permission from The Royal Society of Chemistry.).

Figure 12.

Ordered arrangement of lithium ions in compounds 6–8. (a) compound 6: along a-axis, (b) compound 6: along b-axis, (c) compound 6: along c-axis, (d) compound 7: along a-axis, (e) compound 7: along b-axis, (f) compound 7: along c-axis, (g) compound 8: along a-axis, (h) compound 8: along b-axis, (i) compound 8: along c-axis. (Li: yellow, C: gray, N: blue, O: red, F: green, S: dark yellow. Hydrogen atoms are omitted for clarity). (Adapted from [35] with permission from The Royal Society of Chemistry.).

Figure 13.

Ion conductivities of compounds 6–10 as a function of temperature. (Compound 6: blue; compound 7: orange; compound 8: purple; compound 9: red; compound 10: green.) (Reproduced from [35] with permission from The Royal Society of Chemistry.).