(A) The most stable position of Na+ insertion; (B) The calculated voltage plateaus of Na+ insertion; (C) Trajectories (purple bullets) of Na+ in Na4C48O20H24Zn4 and Na12C48O20H24Zn4 obtained by the MD simulation. (Image by Liu Jianjun)

Recently, a research group from Shanghai Institute of Ceramics, led by Professor Liu Jianjun, designed a 3D wavy-layered structure of a metal-organic compound, Zinc perylene tetracarboxylates (Zn-PTCA) as Na-ion anode materials through cooperation with a research group from Huazhong University of Science and Technology led by Professor Huang Yunhui. It was the first time Na-ion was stabilized at the conjugated rings in organic molecules, generating electrochemical activity for Na-ion storage. It was verified to efficiently improve Na-storage capacity in experiment and theory. The research was reported in a new journal Chem, a sister journal to Cell (https://doi.org/10.1016/j.chempr.2018.08.015).

The Na-storage schematic diagram of Na-PTCA and Zn-PTCA (Image by Liu Jianjun)

Metal-organic frameworks (MOF) with large 3D channel structures are constructed by assembling transition metals (or transition metal clusters) and organic molecules. They have been applied in gas adsorption and separation, catalyst design because of adjustable channels, high surface areas, and rich organic functional groups. However, MOFs were limited to developing as electrochemical materials due to a low specific capacity. Taking Na-ion battery material as an example, Na-storage in MOF is sited in organic functional groups such as C=O and C≡N. The storage mechanism is well recognized as a double bond reorganization mechanism between organic functional groups and conjugation structures. Due to large radii, Na-ions are hardly inserted into the inter-layers of conjugated structures. Even with insertion, Van der Waals' force between inter-layers is destroyed, which further leads to thermodynamic instability and electrochemical inactivity. Therefore, activating conjugation rings for Na-ion storage is of much importance in improving storage capacity, but it faces a major challenge.

The research group found that this 3D wavy-layered structure plays an important role in stabilizing Na-storage in sp2-C conjugated rings through comprehensive studies, including computational electrochemistry, molecular dynamics, and electronic structure analysis. The transition metal ions Zn2+ is coordinated with organic molecules by forming a local structure of [ZnO6]2-, which prevents traditional layer arrangement between organic rings. The open-framework structures are favorable to improving Na-ion diffusion kinetics and the instability factor from Van der Waals' force. The computational design was verified by electrochemical characterization, reaching a significantly high specific capacity of 357 mAh g-1. Zn-PTCA can store Na-ions in functional groups -COO- and conjugated rings sp2-C with two clear insertion-reaction platforms. A series of in-situ characterizations such as XRD, NMR and IR-spectrum indicate the framework structure of Zn-PTCA is well maintained during discharge and charge, therefore exhibiting a good cyclic performance.

For more information, please contact：

Prof. Liu Jianjun

Shanghai Institute of Ceremics, Chinese Academy of Sciences

E-mail: jliu@mail.sic.ac.cn

Source: Shanghai Institute of Ceramics, CAS