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New solid electrolyte for Li-ion batteries using self-assembling molecular crystals

5 September 2017

A researcher at Shizuoka University in Japan has identified new solid materials that could lead to the manufacture of non-toxic solid-state lithium-ion batteries, according to a study recently published in an open access paper in the journal Science and Technology of Advanced Materials.

Makoto Moriya investigated the development of supramolecular solid electrolytes by constructing ion conduction paths using a supramolecular hierarchical structure in molecular crystals; the ion conduction path is an attractive key structure due to its ability to generate solid-state ion diffusivity.

The construction of supramolecular assemblies and control of their hierarchical structures has attracted considerable attention because of their potential to allow the development of molecule-based materials. In particular, the application of ordered molecular assemblies in supramolecular solid to solid state ionics is an attractive method for the fabrication of innovative electrochemical devices. One of the most important functions of such supramolecular solids is lithium ion conductivity, which would allow their applications in all-solid batteries, providing both flexibility and safety.

To create solid electrolytes using such molecular assemblies, it is necessary to understand the relationship between their structure and function. Crystallization seems to be a feasible method of constructing conduction paths for lithium ion diffusion since molecular crystals (MCs) possess three-dimensional (3D) ordered arrangements of component molecules in their crystal lattices. Furthermore, the 3D ordered structures are easily altered by modifying the component molecules.

However, MCs have received relatively little attention as potential solid electrolytes compared to amorphous polymers with lithium ion conductivity. One of the reasons for this is the difficulty in fabricating conduction paths to allow ion diffusion in MCs. … The development of a method to form and control the structure of ion conduction paths is essential for the use of MCs as solid electrolytes. To achieve this goal, we have focused on hierarchical supramolecular structures provided by the self-assembly of small molecules and counter anions of lithium salts to obtain MCs with lithium ion conductivity.

—Makoto Moriya (2017)

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In Moriya’s strategy, ion channels suitable for lithium ion conduction consisting of small molecules and lithium salts are formed in the frameworks of the self-assembled structures. Then, ion conduction paths are constructed via stacking of the channel structures using crystallization. Click to enlarge.

To obtain the crystals Moriya added an organic compound to lithium salt. The molecules self-assembled to form channels through which lithium ions moved, creating an electric current. 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. Changing the crystals’ structures affected their ion-conducting functions.

Ion conductivity in these materials was not as high as in organic liquids, inorganic ceramics or glass electrolytes, but it was comparable to polymer electrolytes, which have been targeted as potential solid electrolytes.

Further, the amount of flammable organic substances in the molecular crystalline electrolytes was lower than in polymer electrolytes and in conventional liquid electrolytes, making them a potentially safer alternative.

Moriya found it easy to control the crystal structure of these solid materials by making alterations to their molecules. This structural versatility could prove to be a powerful tool for improving ion conductivity in these materials.

The coordination environment of the lithium centers was a critical factor for determining the activation energy of ion diffusion. The magnitude of the ionic conductivity was influenced by the intermolecular Li–Li distances, which were affected by the molecular arrangement, and the number of vacant hopping sites, which was influenced by the molecular structure outside the lithium centers. These observations of the influence of supramolecular hierarchical structure on molecular ionics provide unconventional strategies that could open the door to the design of new solid electrolytes and thus the development of new molecular devices.

—Makoto Moriya

Resources

  • Makoto Moriya (2017) “Construction of nanostructures for selective lithium ion conduction using self-assembled molecular arrays in supramolecular solids” Science and Technology of Advanced Materials 18:1, 634-643 doi: 10.1080/14686996.2017.1366816

September 5, 2017 in Batteries, Solid-state | Permalink | Comments (0)

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