Bis-Imino-Acenaphthenequinone Based Covalent Organic Framework for Lithium-Ion Battery Applications
Authors: Mantripragada Bharat SriMitra , Badam Rajashekar , Matsumi Noriyoshi 
Published: July 2021

Covalent organic frameworks (COF) are crystalline and porous, 2 or 3-dimensional structures. The precise structure tunability and high porosity make these materials interesting for various applications like gas adsorption, proton conduction, energy storage etc. Further, COFs synthesized from π-conjugated systems with short interlayer distances could exhibit electron conduction properties[1]. This property makes these materials promising candidates for secondary batteries as well. In this regard, there were reports on imine functional COFs synthesized by condensation of aldehyde and amine[2]. These COFs exhibit uniform porosity and a stable cycling performance as anodes in lithium ion and Sodium ion batteries. Further, the high porosity, the precise structure tunability and possibility of various functional groups presents a unique opportunity to produce heteroatom doped, highly porous and semi crystalline carbon upon pyrolysis. Owing to the drawbacks of graphite in fast charge and discharge, much effort is being directed towards synthesizing heteroatom doped carbonaceous materials among which, N-doped carbon shows promising results. In this regard, various strategies like pyrolysis of carbonaceous material with doping agents like urea, melamine etc., is extensively studied[3,4] However, these N-doped carbon materials suffer with lack of good conductivity and lack of precise control over the amount of heteroatom doping. In this regard, we present the synthesis of novel bis imino acenaphthoquinone (BIAN)-melamine based covalent organic framework and its bifold applications as 1) an organic alternative to graphite anode in lithium-ion battery and 2) a precursor material for synthesis of N-doped carbon with good rate capability.
BIAN-melamine covalent organic framework (BM-COF) was synthesized by simple poly condensation method. Further, BM-COF was pyrolyzed at 800 ℃ to obtain N-doped carbon (Py-BM COF). Both the materials BM-COF and Py-BM-COF were systematically characterized and used to prepare anodes for Li ion batteries. The electrodes were coated using active material (BM-COF/Py-BM-COF), PVDF (binder), and acetylene black (conductive additive) in the weight ratios of 70:15:15 in case of BM COF as active material and 80:10:10 for Py-BM-COF as active material. Coin cells of 2025 type were fabricated using lithium as counter/reference electrode, 1 M LiPF6 in 1:1 EC: DEC as electrolyte, and the coated electrodes as anodes. Cyclic voltammetry was performed at various scan rates in a potential window of 0.01 V to 3.00 V for BM-COF anodes and 0.01 to 2.10 V for Py-BM-COF anodes. Galvanostatic charge discharge studies were carried out in the same potential windows at various current densities and the cycle life of the material was evaluated.
The formation of imine bond in COF was confirmed by FT-IR and X-ray photoelectron spectroscopies. The pore size and surface area were evaluated by nitrogen adsorption-desorption studies. The surface area was found to be 31.6 m2/g and the pore size was found to be 1.7 nm. The XRD spectrum showed sharp peak at 13º corresponding to a d-spacing of 0.68 nm indicating high crystallinity and a stacked structure due to π interactions. The nitrogen content in Py-BM COF was found to be 13 atomic percentage using EDX. The XPS spectra showed the presence of pyridinic, pyrrolic and quaternary nitrogen. TEM micrographs showed macro pores. Li-ion battery with BM-COF anode material showed a reversible capacity of 317 mAh/g at 50 mA/g current density which was higher than that of graphite at 50 mA/g current density and the Py-BM-COF anode showed a capacity of 260 mAh/g at 1 A/g current with a capacity retention over 85% after 500 cycles (Figure-1). We report the synthesis and bifold applications of BIAN-melamine COF. The reported organic material and the N-doped carbon exhibits good electrochemical performance. These results present the material as interesting alternative to graphite anodic material.
References:
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Acknowldgement:
Mbsmitra is thankful for the financial support provided by Ministry of Education, Culture, Sports, Science and Technology.

Figure 1