Recently, a research group from the Shanghai Institute of Ceramics, led by Prof. Liu Jianjun, designed a 3D wavy-layered structure of metal-organic compound, Zinc perylenetetracarboxylates (Zn-PTCA) as Na-ion anode materials in joint efforts with a research group from the Huazhong University of Science and Technology led by Prof. Huang Yunhui. It is the first realization of Na-ion 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).

Figure 1. The Na-storage schematic diagram of Na-PTCA and Zn-PTCA

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 area, and rich organic functional groups. However, development of MOFs as electrochemical materials was limited 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 interlayers of conjugated structures. Even with insertion, Van der Waals' force between interlayers is destroyed, which further leads to thermodynamic instability and electrochemical inactivity. Therefore, activating conjugation rings for Na-ion storage is of great importance to improve storage capacity, but faces a large challenge.

Figure 2.(A) The most stable positions 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

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 ion 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 improve Na-ion diffusion kinetics and instable 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.

Figure 3. (A) Discharge/charge curves at 50 mA g^-1 from0.01 to 2.0 V versus Na/Na⁺. (B) Cycling performance of Zn-PTCA at 200 mA g^-1 in the voltage range of 0.01 to 2.0 V versus Na/Na⁺. (C) Discharge/charge curves of Zn-PTCA electrode at the current density range from 50 to 1000 mA g^-1. (D) CV curves at a scan rate of 0.02 mV s^-1.

Based on this report, Arumugam Manthiram, a distinguished professor in the field of battery materials, from the University of Texas at Austin, wrote a preview commentary in the journal of Joule. He depicted their work as “This is an innovative work, which can provide an efficient strategy to design high-capacity electrode materials for rechargeable batteries. A large amount of organic aromatic compounds can be explored as metal-organic 3D framework materials for high-capacity electrodes.” (cited from https://doi.org/10.1016/j.joule.2018.10.013).

Article link: https://doi.org/10.1016/j.chempr.2018.08.015

For more information, please contact:

Prof. Liu Jianjun

Shanghai Institute of Ceramics, CAS

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

Source: Shanghai Institute of Ceramics, CAS