Lithium-ion batteries are heralded as potential candidates for large-scale energy storage applications. The low specific capacity or poor cyclability of commonly used anodes limit the extensive application of lithium-ion batteries. In this context, organic molecules can offer a potential solution to extend the scope of lithium-ion battery application. In this work, we demonstrate the synthesis and electrochemical properties of a nitrogen-rich, n-type porous organic polymer bearing BIAN and melamine moieties (PBM). The PBM exhibits a porosity of 1.5 nm and excellent electrochemical performance in terms of its rate capability, cycling behavior, and capacity. The anodic half-cell of the PBM active material delivers specific capacities of 850 mAh/g at 400 mA/g, 740 mAh/g at 750 mA/g, and 300 mAh/g at 1000 mA/g current densities with an excellent cyclability over 3000, 2000, and 1100 cycles, respectively, at each current density. Thus, this material is a promising candidate as an anodic material in lithium-ion batteries.
Among several strategies employed to reduce overpotential and achieve reliable reversibility with Li-O 2 batteries, the use of atomically dispersed bifunctional carbon catalysts is very attractive. However, most of the methods used to prepare these bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts require high temperatures and exhibit low yields, and it is therefore difficult to predetermine the active sites qualitatively and quantitatively. Here, we propose the use of atomically dispersed metal centers coordinated to diimine moieties of conjugated polymers as bifunctional catalysts without further modification (pyrolysis or composite formation) for Li-O 2 battery applications. Poly(bisiminoacenaphthenequinone) (BIAN) iron complex (BP-Fe) catalysts showed high OER activities, which enabled 100% coulombic efficiency for 160 galvanostatic charge discharge cycles with a capacity limit of 500 mAh/g at a current density of 250 mA/g. The overpotential corresponding to charging was as low as ~1.0 V and exhibited almost no change in discharge overpotential across 160 cycles. Additionally, it showed a commendable rate capability with only a 170 mV increase in charge overpotential when the charge-discharge rate was increased from 100 to 500 mA/g.
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: [1] Ramees, P., Pradip, K., and Deepak, C., J.Chem.Sci. 130, no. 5 (2018): 51. [2] Zhendong, L., Qinsi, Y., Yi. X., Siyu, G., Weiwei, S., Hao, L., Li-Ping, L., Yong, Z., and Yong, W., Nat.Comm. 9, no. 1 (2018): 1 [3] Li, S., Lei, W., Chungui, T., Taixing, T., Ying, X., Keying, S., Meitong, L., and Honggang, F., R SC .Adv. 2, no. 10 (2012): 4498 [4] Zhen-Huan, S., Lin, S., Jing-Jing, C., Wen-Jing, B., Feng-Bin, W., and Xing-Hua, X., ACS Nano. 5, no. 6 (2011): 4350 Acknowldgement: Mbsmitra is thankful for the financial support provided by Ministry of Education, Culture, Sports, Science and Technology. Figure 1
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